Retrospective Theses and Dissertations Iowa State University Capstones, Theses andDissertations

1976

Late Cenozoic sedimentation in the Allia Bay area,East Rudolf (Turkana) Basin, KenyaHoyt Nealy AcuffIowa State University

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Recommended CitationAcuff, Hoyt Nealy, "Late Cenozoic sedimentation in the Allia Bay area, East Rudolf (Turkana) Basin, Kenya " (1976). RetrospectiveTheses and Dissertations. 5723.https://lib.dr.iastate.edu/rtd/5723

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i i

TABLE OF CONTENTS

Page

INTRODUCTION 3

Objectives 3

Method of Study 4

Review of Previous Work 7

GEOLOGIC SETTING 12

STRATIGRAPHY 19

Lake Rudolf Basin 19

East Rudolf 22

Nomenclature 23

Koobi Fora area 24

Koobi Fora Formation 24

Gal ana Boi beds 25

lleret area 25

Guomde Formation 26

Allia Bay area 26

Kubi Algi Formation 26

Koobi Fora Formation 33

PETROLOGY 38

Conglomerates 38

Sandstones 39

Minerology 39

Texture 44

i i î

Page

Mudrocks 46

Minerology 46

Texture 47

Carbonates 47

Tuffs 48

FACIES AND ENVIRONMENTS OF DEPOSITION 52

KoobJ Fora Area 52

Laminated siltstone faciès 52

Arenaceous bioclastic carbonate faciès 53

Lenticular fine-grained sandstone and lenticular-bedded siltstone facies 53

Lenticular conglomerate, sandstone and

mudstone facies 54

Allia Bay Area 55

Laminated siltstone facies 55

Arenaceous bioclastic carbonate facies 60

Lenticular fine-grained sandstone and > lenticular-bedded siltstone facies 62

Lenticular conglomerate, sandstone and mudstone facies 64

Synthesis 66

TECTONIC AND DEPOSITIONAL HISTORY 68

SUMMARY AND CONCLUSIONS 73

APPENDIX 76

Description of Measured Sections 76

i v

Page

Kubi Algi Formation, Allia Bay area 76

Jarigole exposure 76

Type exposure 82

Koobi Fora Formation, Allia Bay area 89

Bura Hasuma Hill exposure 89

SELECTED REFERENCES 96

ACKNOWLEDGMENTS 105

V

LIST OF TABLES

Page

Table 1. Percentages of sandstone minerals 40

Table 2. Percentages of heavy minerals 43

Table 3. Statistical measures of selected sandstones 45

v i

LIST OF FIGURES

Page

g. 1. Area photographs 2

g. 2. Geologic map of the Allia Bay area, East Rudolf, Kenya 6

g. 3. The Afro-Arabian rift system 11

g. 4. Fault pattern and cross section in Northern Kenya 14

g. 5- Tectonic map of the Lake Rudolf area 18

g. 6. Lake Rudolf depositional basin map 21

g. 7. Photographs of prominent geomorphic features in the Allia Bay area 29

g. 8. Photographs of structural features in the Allia Bay area 32

g. 9. Photographs of fossils in the Allia Bay area 35

g. 10. Graphic sections of the Upper Cenozoic sediments,

Allia Bay, East Rudolf 37

g. 11. Photographs of the Kubi Algi Formation 51

g. 12. Lithofacies map of the Allia Bay area at 3.9 myBP 57

g. 13. Geologic map of the Allia Bay area 59

Fig. 1. Area photographs

A. Lake Rudolf basin taken from ERTS-I satellite Feb. 1, 1973

B. Taken southeast of Kubi Algi looking east toward the Kiriu Sogo fault zone and the rift

2

3

INTRODUCTION

Since the mid-1960's the Lake Rudolf basin has become an area of

intense interest and research in geology, paleontology and anthropology

(Patterson, 1966; Patterson e^ , 1970; Howell, T968 ; Arambourg et al.,

1969; Butzer e^ aj_. , 1969; Butzer, 1971; Vondra e^ aJL, , 1971; Bowen and

Vondra, 1973; Vondra and Bowen, 1976; Maglio, 1970, 1972; Leakey et al.,

I97O; Leakey, 1971, 1972, 1973; and Isaac et ajy, 1971). Late Cenozoic

sediments, Miocene through Holocene, are well exposed throughout the

basin. They have yielded a rich and varied fossil fauna that consists

of fish, reptile and mammalian remains, including hominids. Over one

hundred and fifty hominid specimens and an abundance of stone artifacts

have been collected from PIio-Pleistocene sediments along the northeast

shore of the lake (Leakey et al., 1970; Leakey, 1971, 1972, 1973; Leakey

and Wood, 1974; Day ej^ a2_., 1975; and Isaac e^a2_., 1971).

Recently the name of Lake Rudolf was officially changed to Lake

Turkana by the Kenyan government. The research and the illustrations for

this paper were largely completed by the time of the change, so the name

of Lake Rudolf will continue to be used in this report.

Objectives

The objectives of this report are: (I) to provide a geologic map of

the Allia Bay area, which lies within 03°39' and 03°50' N latitude and

36^10' and 36°32' E longitude; (2) to provide a basis for the documenta­

tion of the fossils and artifacts collected; (3) to describe the strata in

detail and determine their environment of deposition; and (4) to determine

4

the provenance of the Allia Bay sediments. The geologic map can be used

by geologists, archeologists and others to locate the outcrops of the Kubi

Algi and Koobi Fora Formations. Correlated sections with dated horizons

are needed by paleontologists and archeologists to place their find in the

proper time sequence. Objectives three and four can be used by members of

the East Rudolf expedition to help determine the overall paleoenvironment

and natural history of the area.

Method of Study

Field work for this study was accomplished during June through August

of 1972. Rock units established by Bowen and Vondra (1973) were mapped on

aerial photographs at a scale of 1:24,000. Twelve critical outcrops were

measured, described and sampled in order to determine the lithologie and

stratigraphie relationships of the various units exposed.

Sandstones and volcanic rocks were sampled and thin-sectioned. The

sandstones were selected for widespread horizontal and vertical distribu­

tion; the volcanics were basalts and ignimbrites, and were collected from

Shin to Jarlgole in the highlands bordering the basin. Minerals in all of

the thin-sections were determined by using a pétrographie microscope, and

the 100-point count method was used to determine the mineral percentages

of the sandstones.

In addition to pétrographie determination, mechanical analysis (sieve

and pipette) was carried out on the sandstones to determine particle-size

distribution. The samples were treated with HCL to remove the cal ci te

cement before they were sieved and then with Calgon to disperse the

clays before they were analyzed by the pipette. Statistical measures

Fig. 2. Geologic map of the Allia Bay area, East Rudolf, Kenya

6

03°50'N

QTkfu

QTkfl

(TURKANA) LAKE RUDOLF

F

SibUot

QTvun

03°39'N

QTkfl

OTkfl

/ -w

QTkfl

QTkfl

A Z. V .^ V

u > i /\ S- > V - '* ^

•A 4 V 7,s QTvun

-.A ' r ^ ' < y^? V

7 ""j^ -1 ^ -y> A A -J •*

J-,3 r p; r V -i - 7 >- A - • '5 - j.. ji r

':; •>-vi* V..Nv V r w V /% V

..«\vw r V ^ \ - "^L./.

M:; -AYv" \ ' il/ r KubiAigi , A » a t- m

t - A I J ' V v A ^ > . - i - I " ^ .

} :V,

S/»//or

((

V /% < -, 7 r ?< . >; ' ? y V J > rj ' y LV T?-

36°32'E

QTkfl

QTkfl

Kubi Atgi

%

03°50'N

, V- :—l*"-

> A / ^ A > I V r ' * A ( . 7 v > v V A •«••v-*,-^—t.*" » " 7.^^ 4 r /» <

J" "•••J. > V -» «•

•» > » ^ •' V < -V r V V A V J f- v'v* A V A i r 7 % r A ^ r ,

#:## >< "c? ' " )

V ^r- < 1 y ^ f - 7 < , A ^ ^ < t '

L- ^ < V

^ 1- > V c. A f ? - ^ 7 < 1 < V V ( " < < t r QTvun < < 7>/V

•' '- ' > A <

Unconsoli cobb and fine shor

Koobi Fo lami

fine-lenti bed lime met

Lower m si I

Oligomict* fine thin limo thin

36°32'E

03°50'N

Â

n' :Tvv ;, [ •' 1 1 r- -> < \ iQTvun u h < A ^

r -» u jk/.

V -, ^

-X V

-J > J 'V 7 > ' r ?

' A t. 7 V 7 >1 -A *1 r < ? > <

> u >1 V A > 7 ^ 4 r L :<;;r .

j y ^ V J V Y " u/.' % rV

.6.7 -I <

' r < 7 ' • - > ^ '

1 A < /. 7 V -. >

Il ^

U V S A 6.

" r 7 V 1

. U' J 7

7 > < ^ L ' ^ T < > J- u 7 r ' 'X" " > r

7 -I > V ^ < V V -1 fv >*';S

:-y {<;XÇr\A

i ^ r

V ^ A <

r > V A A rr% r 7' / ' fv 1, -1 /

7 i < V> <'; V ' f e. ^ i-1 < r E v - 7 > ' v > ^ v > j A V '"•

I .9

EXPLANATION

Alluvium, Beach Sands

Unconsolidated deposits of silt, sand, gravel, and cobbles along streams. Includes alluvial fans and terraces and unconsolidated deposits of fine sand in beach ridges along the present shoreline.

Koobi Fora Formation

Koobi Fora Formation consists of a series of laminated claystones, siltstones and fine-grained sandstones that are overlain by lenticular conglomerates, mudstones, thin beds, of algal stromatolites, fossiliferous limestones and tuffs. Thickness, 180 to 200

meters.

QTkfu

M i l Upper member, fluvial deposits of polymictic

conglomerates, subarkoses and mudstones that grade laterally into fine-grained sandstones, siltstones, thin beds of algal stromatolites, fossiliferous limestones and tuffs. Thickness, 35 to 80 meters. In lleret area, the basal part consists of fine-grained molluscan limestones that grade upward into conglomeratic sandstones, siltstones, claystones, and tuffs. Thickness, 45 to 65 meters.

_ QTkfl_

Lower member, limonitic, gypsiferous laminated

siltstones, claystones, fossiliferous limestones, fine-grained sandstones intercalated with polymictic conglomerates and laminated tuffs. Thickness, 80 to 120

meters.

Kubi Algi Formation

Oligomictic conglomerates at base grade upward to fine-grained sandstones, cross-bedded tuffs,

thin fossiliferous limestones and laminated limonitic siltstones. Sequence is capped by thinly laminated Sureaei Tuff Comolex.

D

I 3 Z > 30

m 33 H > 30 <

F

, , ^ Sibiht ^ ^ ^ <

\ f ^ , > >

ÈMêM --mm-

QTvun

03°39'N

36°10'E

1°46'

i Z c

o Z

SCALE

1 2

MILES

10 12

KILOMETERS

GEOLOGIC MAP OF THE ALLIA BA

QTvun

Sibilot

tS Tka

1

SCALE

1 2

MILES

0 ~ 1 2

KILOMETERS

Geology mapped by H. N. Acuff and B. E. Bowen from field observation and aerial photographs in 1972, assisted by C. F. Vondra.

ETHIOPIA

KENYA

Map Location

P THE ALLIA BAY AREA, EAST RUDOLF, KEN

36°32'E

03°39'N

napped by H. N. A cuff and 'en from field observation photographs in 1972, f C. F. Vondra.

ETHIOPIA

KENYA

S-.9

1

I

"cbftpomerates, subarkd that grade laterally sandstones, siltstones, stromatolites, fossilifen tuffs. Thicl<ness, 35 to| area, the basal part coi| molluscan limestonesth conglomeratic sand| claystones, and tuffs, meters.

, QTkfl_

Lower member, limonitic, g| siltstones, claystof limestones, fine-gtj intercalated with polyq

and laminated tuffs, meters.

Kubi Algi Forn

Oligomictic conglomerates at I fine-grained sandstones!

thin fossillferous limesfl limonitic siltstones. Se

thinly laminated Surej Thickness, 80 to 100 me

QTvun I-

' /I -I Volcanics, und#

Lava flows, ignimbrites, tuf basaltic to rhyolitic cor with sediments and pale| Pliocene age.

Contact

Axis of arm

Map Location

AST RUDOLF, KENYA

A T T V •

V A V

> /I

U A

A ^ 7 ^ V A

^ ^ J ^ : < 1 7 t- -? <

< ' ^ < ^ c ^ T L A? < 7 uV

I^ J 7 r (. 7 %*, 7 <

t 7 x"Kv> '> > r 1 > V ^

.UTpX:, < '< :::/^ ' '

QTvur. , < 7 >1

am# ^ v <

• r V > A , <

A

< V > <

A r <1- • ^ V > ^ V ^ J

"1 J A V

» V ^ V r

V» A. t-

>

A y T > > >

U -> r A

-A «- V ^ J f Av

Î

36°32

03°39'N

I'E I .2

I

QTkfu

Upper member, fluvial deposits of polymictic conglomerates, subarkoses and mudstones that grade laterally into fine-grained sandstones, siltstones, thin beds of algal stromatolites, fossiliferous limestones and tuffs. Thickness, 35 to 80 meters. In Herat area, the basal part consists of fine-grained molluscan limestones that grade upward into conglomeratic sandstones, siltstones, claystones, and tuffs. Thickness, 45 to 65 meters.

^QTkfi_

Lower member, limonitic, gypsiferous laminated

siltstones, claystones, fossiliferous limestones, fine-grained sandstones intercalated with polymictic conglomerates and laminated tuffs. Thickness, 80 to 120 meters.

Kubi Algi Formation

Oligomictic conglomerates at base grade upward to fine-grained sandstones, cross-bedded tuffs,

thin fossiliferous limestones and laminated limonitic siltstones. Sequence is capped by thinly laminated Suregei Tuff Complex. Thickness, 80 to 100 meters.

t-> •

QTvun •» r

Volcanics, undivided

Lava flows, ignimbrites, tuffs and intrusives of basaltic to rhyolitic composition intercalated with sediments and paleosols of Miocene and Pliocene age.

Contact Fault

m 30 H > 3) -<

Axis of anticline

SlYA

7

of graphic mean, inclusive graphic standard deviation, inclusive graphic

skewness and graphic kurtosis were calculated.

Heavy minerals were segregated by using bromoform, methylene iodide

and Clerici's solution. Samples of the heavy and light mineral fraction

of the sandstones, plus grains of the tuffs collected, were mounted on

glass slides to be studied with a stereo microscope. The light minerals

were stained with sodium cobaltinitrite to determine the potassium feldspar

content. Very fine-grained sand, silt and clay samples were studied by

x-ray defraction to determine the minerals present.

A geologic map of the Allia Bay area was drafted and is presented in

this report at a scale of 1:100,000 (Fig. 2) and the rock beds correlated

(Fig. 10). All of the data collected in the field and laboratory were

utilized to draw conclusions regarding the paleoenvironment of the basin

at the time of the emergence of early man.

Review of Previous Work

The European discovery of Lake Rudolf and Lake Stefanie in 1888 is

credited to Count Teleki and Lt. von Hohnel (Hohnel, 1894; Smith, 1900;

Harrison, 1901; and Athill, 1920). As a result of the scientific data

collected by this expedition, Suess (Hohnel, 1894) proposed the tectonic

continuity of the East African Rift System and suggested the possibility

of a Nile-Rudolf connection on the basis of similarities in the aquatic

faunae. During the subsequent forty years, little geologic data were

collected by the expeditions led into the region by Smith (I896, 1900);

Harrison (1901); Maud (1904); Athill (1920); and Holland (1926).

8

Fuchs led an ill-fated geological expedition into the Turkana Province

west of Lake Rudolf in 1934 (Champion, 1937; Fuchs, 1939). He made a brief

survey east of the lake before the expedition was prematurely terminated by

the loss of two members. In 1932 Arambourg initiated a geological and

paleontological expedition to the Omo River Valley to the north (Arambourg

et aj[., 1969). This led to the 19^7 International Paleontological Research

Expedition to the Omo River Valley (Leakey e^ a_l_., 1970; Butzer and Thurber,

1969; and Howell, 1968).

During the summers of 19^3 to I968 Harvard University outfitted expe­

ditions to the southwestern part of the basin to collect paleontological

and geological data (Patterson, 1966; Patterson 1970). As a result

of the International Omo and Harvard University expeditions, much scientific

data have been collected and paleoenvironmental interpretations have been

presented (Leakey e_t a2_., 1970; Butzer et ajj, 1969; de Heinzelin et al.,

1971; Patterson, I966; Patterson et aJU, 1970).

Partly due to the success of these expeditions, R. E. Leakey led a

reconnaissance expedition organized by the National Museum of Kenya to the

East Rudolf area in I968 to assess the fossil content and potential of the

strata. Although the search for hominids was the main concern of the sub­

sequent expeditions, the total paleoenvironment was of great interest and

was to be studied and interpreted.

Geological investigation of the East Rudolf basin was initiated by

Behrensmeyer in I969 (Leakey e^aj_.5 1970). In 1970 a survey to determine

the regional geology was undertaken by a team from Iowa State University

headed by C. F. Vondr? (Vondra e^ , 1971; Bowen and Vondra, 1973;

9

Vendra and Bowen, 1976). The interpretation of the geology has been

continued to date by geologists from England and the United States.

Fig. 3- The Afro-Arabian rift system [adapted from Gass and Gibson (1969) and Baker et al., (1972)]

1 1

u 60° E

L I B Y A

-20° N

E G Y P T ^ \ -STSN ARABIAN

\ / \

C H A D

\

S U D A N

\ / '\ ,// m V \ P E N I N S U L A !

/Y/ 20°/IM-I

i E T H I W ^ ^ P I A

1

3 0 M A L I >

-no

*r >-z

Z A I R E

0°-

/ n d i a n

O c e a n

A N G O L A

Z A M B I A

RHODESIA

SOUTHWEST

AFRICA B O T S W A N

S O U T H A F R I C A .

20° E I

N O R T H

40° E I

20° S-

:=:-:-a=il Rif t Valley Fault

500 loop km s c a l e

60° E

1 2

GEOLOGIC SETTING

Lake Rudolf lies în the Baringo-Suguta graben in part of the Turkana

depression where its margins become Increasingly strongly faulted until it

merges into the main rift to the east (Fig. 3) (Baker e^ aj_., 1972). The

Afro-Arabian rift system extends 6,500 km from Turkey to Mozambique

(Fig. 4) and is part of a world-wide rift system (mid-ocean ridges) that

encircles the earth. The eastern segment of the East African portion of

the rift system begins in the north at the Afar triple junction where it

joins the Red Sea and Gulf of Aden and extends southward across Ethiopia,

Kenya and Tanzania (Suess, I89I; Krenkel, 1922; Gregory, I896; Willis,

1936; Dixey, 1956; King, 1970; Baker e^£l_., 1972).

Elements of the eastern rift began in late Cretaceous-Eocene with the

uplifts that formed the Afro-Arabian swell and the eruption of flood basalt

and ignimbrite in Ethiopia (Gass and Gibson, 1969; Fitch and Vondra, 1976).

During the Miocene further arching of the Ethiopian and Kenyan domes with

related subsidence of the intervening areas occurred, accompanied by a

great outpouring of volcanics (Fitch and Vondra, 1976). Following this

period of essentially vertical movement from the late Cretaceous to early

Miocene, the dominant displacement in the rift zones became horizontal

(Gass and Gibson, 1969). The African rift valleys are believed to be a

divergent plate boundary along which crustal spreading and thinning has

and Is occurring (Gi rdler e_t aj_. , 1969; McKenzie e^ £]_., 1970).

The Turkana depression began to form in the Miocene between the

Ethiopian and Kenyan domes (Saggerson and Baker, 1965; Berry and Whitman,

1968). A thick sequence of basic and acid volcanics with Interbedded

Fig. 4. Fault pattern and cross section in Northern

Kenya [adapted from Baker e^ aj_., (1972)]

1 4

Omo R L. Stefonie

Lotokipi

Basin

h# ij elekech

Muruonochc

'f' Lothidok

Muruosigor

•7 Lodwor^

% ,

Cholbi Desert

K A R A M O J A Moroto

KongeteV Kyogo Plain Sekerr Ronge

S A M B U R U

Cherongoni Mt. Elgon Lerogi

100 KM

U A S I N

GISHU scale

Nondi \

L A K I P A rfTtrïTllTniIltBïTrir N O R T H

37°E R i f t C V

M#l#r# 4000

Ugondo Lotokipi Escarpment

S. Loburr

'3M5 N -

Loke Rudolf

Gof OuKana

3000-

2000-

1000-

Precambrian gneisses

Miocene volconics and sediments

Plio-Pleistocene volconics and sediments

1 5

fluvial and lacustrine sediments accumulated in the shallow basin at this

time (Patterson et al_., 1970; Fitch and Vendra, 1976).

Faulting continued in the Pliocene, dividing the Turkana depression

into a number of major westward-tilted, gently-warped, north-south struc­

tural blocks or half-grabens that were partially filled by intermittent

volcanics and sediments (Baker e;t al_., 1972; Fitch and Vondra, 1976).

The last major tectonic events took place in the early to mid-Pleistocene

when the Kinu Sogo fault zone was formed (Figs, lb, 4, and 5) and faults

extended northward to connect the Sugata Valley at the southern end of

Lake Rudolf to the southern extension of the Ethiopian Rift south of Lake

Stefanie (Figs. 4 and 5) (Howell, 1968; Butzer and Thurber, 1969; Fitch

and Vondra, 1976). Tectonically, the area continues to be active with

faulting and volcan ism which have produced many local unconformities;

and along the eastern edge of the basin the volcanic flows are often

interbedded with sediments.

The Suregei Cuesta forms the northeastern and eastern margin of the

East Rudolf basin. The volcanics which have been dated vary in age from

Miocene to late Pliocene. Basalts exposed in the Suregei Cuesta yield

average K-Ar apparent ages between 11.6 t 0.5 and 14.1 t 1.4 myBP (Fitch

et al_., 1974). These basalt flows and associated ignimbrites and tuffs

form the basement upon which the PIio-Pleistocene sediments were deposited.

The basalts capping the volcanic terrain and entering the basin near Kubi

Algi have apparent K-Ar dates of 3.8 t 0.4 and 3.8 t 0.38 myBP (Fitch

e^£^., 1974). Kubi Algi (Figs. 2 and 7d) and Shin, peralkaline rhyolite

plugs, have an average K-Ar apparent dates of 7-5 t 0.8 and 11.8 Î 0.9 myBP

1 6

respectively (Fitch e_t aj_., 1974). ignimbrite flows are common near

Jarigole (Figs. 3 and 7f); and Sibilot (Figs. 2 and 7e), a mesa, is com­

posed of faulted ignimbrites and associated agglomerates. Two tuffs in

the Kubi Algi Formation have been dated at 3.9 and 4.5 myBP (Fitch and

Miller, 1971). The tuff dated at 4.5 myBP is 59.2 m above the volcanics

which would make the oldest sediments of the Kubi Algi Formation to be

nearly 5 million years old.

In the past Lake Rudolf was much larger than at present and covered

much of the area that constitutes the Allia Bay area today. The alkaline

lake (9.0 PH) and its catchment basin form one of the largest inland

drainage basins in East Africa (Figs, la and 6). The total area of the

basin (146,000 square km) is relatively small in relation to the size of

the lake (7,500 square km surface area and about 50 km deep) (Butzer,

1971) (Fig. 6). Today the Omo River is the only perennial stream flowing

into the lake and accounts for 80% to 90% of the annual influx (Butzer,

1971).

Fig. 5. Tectonic map of the Lake Rudolf area [adapted

from Bowen (1974), Baker et aj^., (1972), Fitch and Vondra (1976)]

18

36» E

SUDAN\ E T H I O P I A Lake

(Sfe fa nie,

K E N Y A ..V 40 N 4°N

3 en

w (/) • y

3° N

Key

Fault — Volcanoes a

20 km. ; 20

s c a l e

19

STRATIGRAPHY

Lake Rudolf Basin

The Lake Rudolf basin has been a sediment trap from the Miocene to

the present. These sediments represent a complex of alluvial fan, chan­

nel, fluvial, deltaic and lacustrine environments of deposition. Local

tectonic activity has apparently been a major factor in the transgressive

and regressive sequences at the different localities.

On the west side of Lake Rudolf near Lothidok (Fig. 6) are coarse

clastic sediments that reach a thickness of 200 m and are called the

Turkana Grits. These lower sediments do not contain any volcanic compo­

nent, indicating that they preceded the earliest Tertiary volcanic

activity in the region, which began about 23 myBP (Walsh and Dodson,

1969). Miocene vertebrate faunae are known from localities near Lothidok

and Loperot (Fig. 6) (Arambourg et^ aj_., 1969; Joubert, 1966). Walsh and

Dodson (1969) consider the Turkana Grits to represent deposits of fluc­

tuating lakeshores and deltas in a lacustrine basin. But fine-grained

laminated deposits of wide lateral extent, which are typical of the

PIio-Pleistocene lacustrine depositional environments, are absent. Thus,

there Is no strong evidence for Lake Rudolf being present in early Miocene.

At Lothagam Hill (Fig. 6), near the southwest side of Lake Rudolf,

720 m of sediments and a basalt sill are included in the Lothagam group

(Patterson e^ a]_., 1970). These deposits lie on Miocene volcanics with

some intercalated sediments which consist of conglomerates, sands and

silts of fluvio-deltaic origin, and laminated clays and silts of lacus­

trine origin. These indicate local variations in source area and rates

Fig. 6. Lake Rudolf depositional basin map [adapted from Butzer (1971)]

21

OMO BASIN

Lake y/ / //'Stefanié

KENYA ETHIOP

Ileret

EAST RUDOLF

Koobi Fora

Allia Bay

U G A N LOTHIDOK

Lodwar*

LOTHAGAM

EKORA N O R T H /

KÀNAPOl LOPEROT Key

I ILake Rudolf Basin ' Y/À Peripheral Highlands \

20 40 60 80 100 km. s c a

22

of sedimentation, and record periodic uplift of local horst blocks and

rapid subsidence of adjoining basins (Dennis Powers, personal communica­

tion, 1975). The sediments in the Lothagam group have been radiometrically

dated (K-Ar method) as 8.3 to 3-7 myBP and faunally dated as 7*0 to

3.7 myBP (Behrensmeyer, 1974).

At Kanapoi and Ekora (Fig. 6) southwest of the lake the sediments

range from coarse-grained fluvial clastics to lacustrine silts, clays,

and tuffs (Patterson et aj[., 1970). The faunal dates of these deposits

range from 3.5 to 4.0 myBP (Maglio, 1970).

The Omo basin at the north end of Lake Rudolf (Fig. 6) consists of

sediments that range in age from 4.5 myBP to the present (Butzer, 1971).

The lower sediments range from conglomerates to clays and are described

as alluvial, deltaic, littoral and lacustrine in origin. Butzer (1971)

and de Heinzelin e_t aj[., (1971) have presented evidence that indicates

there were two periods of primary lacustrine deposition, between 4.5 to

4.2 myBP and from 3.9 to 3.8 myBP. Fluvial processes were dominant

between 3.9 and 4.2 myBP and after 3.8 myBP. Pleistocene sediments at

Omo consist of cyclic and noncyclic fluvial deposits and grade upward to

deltaic-lacustrine facies (Butzer, 1971).

East Rudolf

The sediments in the East Rudolf basin range in age from approxi­

mately 5 myBP to the Holocene (Fitch and Miller, 1971). The total Plio-

Pleistocene sequence consists of 325 m fluvial, deltaic transitional

lacustrine and lacustrine sediments in contact with Miocene and Pliocene

volcanics (Bowen and Vondra, 1973). The sediment outcrops occur in a

23

band, 7 to 40 km wide and 80 km long, along the lakeshore from 03°39' to

04°19' N latitude. The basin is bordered by volcanic highlands to the

east, which are often breached by ephemeral streams draining the area to

the north or northeast. Outcrops are small, of low relief, scattered,

discontinuous and mostly located along the edge of rock-cut terraces or

against the volcanics.

Because of the lack of good continuous outcrops and the difficulty

of establishing secure correlations, tentative mappable stratigraphie

units were originally developed for each of three areas of somewhat con­

tinuous exposure - Allia Bay, Koobi Fora and lleret (Vondra aj_-> 1971).

The flood plain of Laga Bura Hasuma separates the Allia Bay area from the

Koobi Fora area to the north, and further north, the Koobi Fora area is

separated from the lleret area by volcanics of the Kokoi horst and a

large Holocene alluvial plain east of the Kokoi (Bowen and Vondra, 1973).

Nomenclature The stratigraphie nomenclature reviewed below and

used in this report was formalized by Bowen and Vondra (1973). The

Kubi Algi Formation consists of the strata which lie below the base of

the Suregei Tuff Complex and upon the Miocene-Pliocene volcanics. The

Koobi Fora Formation is defined as the sequence of strata between the

base of the Suregei Tuff Complex and the upper contact of the Chari and

Karari Tuffs. The Formation is divided into two members - the Lower and

Upper. The Lower Member extends from the base of the Suregei Tuff Complex

to the base of the first channel sandstones which occur stratîgraphically

above the KBS Tuff. The Upper Member is defined as the strata occurring

between the base of the channel sandstones and the top of the Chari and

24

Karari Tuffs. The Guomde Formation lies between the top of the Char! Tuff

and a 1 m tuff and is confined to the lleret area. The term Galena Boi

beds, an informal name, was proposed for the gray, tuffaceous siltstones

which cap the Guomde Formation in the lleret area and the Koobi Fora

Formation in the Koobi Fora area.

Koobi Fora area The descriptions of the stratigraphie units in

the Koobi Fora and lleret areas are here condensed from Bowen and Vondra

(1973), Bowen (1974, unpublished Ph.D. dissertation) and Vondra and

Bowen (1976). The Koobi Fora Formation comprises most of the strata

exposed in the area, but there are isolated outcrops of the underlying

Kubi Algi Formation and the overlying Galana Boi beds.

Koobi Fora Formation The Koobi Fora Formation is a heter­

ogeneous sequence of boulder to granule-size conglomerates, coarse- to

fine-grained sandstones, variegated siltstones and claystones, bioclastic

carbonates, and tuffs. It ranges in thickness from 135 m along the Karari

escarpment to 175 m near the Koobi Fora spit to 47 m northwest of Derati.

Most of the strata comprising the unit are highly lenticular and grade

vertically or interfinger laterally with each other. However, certain

tuffs are good marker beds and can be traced over great distances. The

KB S Tuff (2.61 ± 0.26 myBP) (Fitch and Miller, 1971) is well-exposed along

the Karari escarpment and Koobi Fora ridge and is the most valuable bed

in the area for establishing secure local correlation. The strata com­

prising the Koobi Fora Formation range in age from over 3 myBP to 1.28 t

0.23 myBP (Fitch and Miller, 1971). The Tulu Bor Tuff, which occurs

25

55 m above the base of the Formation, has been dated by Fitch and Miller

(1971) at 3.18 t 0.09 myBP.

The Lower Member consists of loosely consolidated subarkoses near the

base that interfinger with bioclastic carbonates with occasional lenses of

thin conglomerates and siltstones. Vertically these beds give way to

medium-grained subiitharenites and lenticular-bedded, mud-cracked silt-

stones which contain calcareous root casts. This coarse clastic sequence

is overlain by the Tulu Bor Tuff and limonitic siltstones with lenses of

cross-bedded sandstones. It contains numerous vertebrate fossils, includ­

ing cranial and post-cranial hominid remains.

The Upper Member is characterized by large-scale trough cross-bedded

subiitharenites, basalt cobble conglomerates, lenticular-bedded siltstones

and thin tuffs. The disconformable relationship between the Upper and

Lower Members is marked by a complex of channel sandstones and conglomer­

ates.

Galana Soi beds The Holocene (9360 t 135 yBP) Galana Boi

beds lie disconformably on the Koobi Fora Formation in the Koobi Fora area.

These beds consist of diatomaceous siltstone which contain artifacts, algal

stromatolites, gastropods and pelecypods in a sequence of tuffaceous

mudstones and coarse-grained subarkoses. The beds range in thickness from

0.10 m to 32 m and in one location they have been faulted 120 m above the

present lake level.

Ileret area In the lleret area the Koobi Fora Formation and the

Galana Boi beds are essentially the same 1ithological1y as in the Koobi

26

Fora area. The major difference is the presence of the Guomde Formation

between the Koobi Fora Formation^and the Galana Boi beds.

Guomde Formation The Guomde Formation lies dîsconformably on

the Koobi Fora Formation and consists of laminated siltstones and inter­

calated thin bioclastic carbonates, lithic arkoses and lenticular tuffs.

The formation contains numerous large crotovina and thin-bedded molluscan

carbonates. The beds range in thickness from 32 m to 37 m.

Allia Bay area The Allia Bay area is at the southern end of the

East Rudolf basin. It is bordered by thé volcanic highlands to the east

and south and by Lake Rudolf to the west. Outcrops in the area (the area

included in the map, Fig. 2) are mostly of the Kubi Algi Formation, but

across the northern part the Upper and Lower Members of the Koobi Fora

Formation are present. The outcrops occur in a band from 7 km wide In the

south to 21 km wide in the north with an average width of 17 km.

Kubi Algi Formation The Kubi Algi Formation consists of the

Pliocene strata which lie between the Miocene-Pliocene volcanics and the

base of the Suregei Tuff Complex (Bowen and Vondra, 1973). The Formation

is exposed in many discontinuous outcrops along the edge of terraces or

against the volcanics. Strata of the various exposures were correlated by

utilizing key tuff horizons and by stratigraphie position and sequence

(Fig. 10). The formation varies in thickness from 98 m southwest of Kubi

Algi to 154 m near Jarigole. A total of eight exposures (Fig. 10) were

described and sampled in order to gain a clear understanding of the unit;

the two most significant exposures are discussed here in some detail (see

map. Fig. 3 and Appendix).

27

The type exposure was measured along a terrace from a point 4 km

south of Kubi Algi to within 4 km of Laga Bura Hasuma. The center point

of this exposure is 03°43' N latitude and 36°19' E longitude. At the

type locality, the thickness of the formation is 98 m. The unit is cyclic

in nature consisting of 10 fining upward cycles ranging from conglomerates

to claystones.

At its type locality, the Kubi Algi Formation consists of sediments

ranging from conglomerates to claystones with many interbedded tuff

horizons (see Appendix). Parallel to lenticular bedded, grayish orange

(10YR7/4) to pale yellowish brown (10YR6/2), basalt granule and pebble

conglomerates mark the base of each cycle, and often grade into feldspathic

litharenite (see Appendix). Fine-grained very pale orange (10YR8/2) to

dark yellowish brown (10YR4/2) feldspathic litharenites are the most

common sandstone in the sequence. Most are crossbedded (Figs. Be and f)

with large scale trough crossbeds giving way upward to planar crossbeds.

They often contain abundant fossil remains of ostracods, gastropods,

pelecypods, mammal and fish as well as trace fossils such as root casts

and burrows (Figs. 9a-d and h). The litharenites are very fine- to

medium-grained, very pale orange (lOYRB/l) to grayish orange (10YR7/4) and

display large scale (Omikron) to small scale planar (Alpha) crossbeds.

The litharenites are overlain by yellowish gray (5Y7/2) to pale yellowish

brown (Î0YR6/2) lenticular-bedded siltstones which contain many calcareous

root casts and concretions. The sequence is capped by brown (10YR6/6) to

yellowish gray (5Y7/2) claystones. Calcareous root casts and concretions

are very common throughout the cycle and several beds are tuffaceous. The

Fig. 7- Photographs of prominent geomorphic features in the Allia Bay area

A-B. Photographs of the volcanic highlands (l) in the background with the sediment basin (2) in

the foreground

C. Derati, a peralkaline volcanic plug

D. kubi Algi, a peralkaline volcanic plug

E. Sibilot, a mesa of ignimbrite flows and agglomerates

F. Allia Bay, Jarigole (1) and the volcanic highlands across the south in the background with Bura Hasuma hill (2) in the foreground

29

30

înterbedded tuffs are light gray (N-7) to pale yellowish brown (10YR7/2)

and consist of angular to subrounded silt-sized glass shards. Two have

been radiometrically dated by Fitch and Miller (1971) at 4.5 and 3.9 myBP.

The exposure near Jarigole was measured along the south edge of a

terrace 1 mile north of Jarigole. The center point of the outcrop is

03°4l' N latitude and 39°14' E longitude (see map, Fig. 2 and Appendix).

The Formation attains a thickness of 154 m here. The conglomerates near

Jarigole contain a greater admixture of ignimbrite granules and pebbles

than the type section, reflecting a higher percentage of acidic volcanics

present along the southern margin of the basin.

The fining upward sequence consisting of (1) a lower unit of gravelly

large-scale trough cross-stratified sandstone overlain by small scale

trough cross-stratified sandstone; (2) interbedded claystone and siltstone

units; and (3) an upper unit characterized by a basal scoured surface

overlain by gravels with mud drapes suggests fluvial deposition in a

braided stream system (Allen, 1970). Some of the conglomerates near

Jarigole, the type locality and other exposures are very poorly sorted

consisting of lenses of sand to boulder-sized particules in a random open

framework arrangement surrounded by a clay matrix similar to alluvial fan

deposits described by Bull (1972). Some interbedded medium- to coarse­

grained conglomeratic sandstones are cross-bedded and contain root casts,

burrows, ostracods, gastropods and pelecypods suggesting a prograding

shoreline (Clifton, 1969; Pettijohn e^£l_., 1972; and Harms e^al_., 1975).

The tuffs are thicker (one 19.7 m thick, Fig. 11f) in the southern

part of the basin, and the shards are smaller and more rounded than at the

Fig. 8. Photographs of structural features in the Allia Bay area

A. Fault at Kubi Algi

B. Fault at Sibilot

C-F. Crossbedding

32

33

type locality. Fossils are generally rare in the Kubi Algl Formation

throughout the basin and extremely rare in the Formation near Jarigole.

Ostracods are the most common fossil, but gastropods, pelecypods, verte­

brates and petrified wood are present (Fig. 9). Hominid fossils have not

as yet been found anywhere in the Kubi Algi Formation.

Koobi Fora Formation In the northern part of the study area from

Bura Hasuma hill to the eastern edge of the basin, the lower 117.4 m of

the Lower Member of the Koobi Fora Formation are exposed (see map, Fig. 2

and Appendix). Here the basal beds are the Sliregei Tuff Complex overlain

by thick beds of alternating limonitic siltstones and claystones with

sandstone lenses. The sandstones are conglomeratic and often ripple-

laminated, and vertically grade into a 38 m channel sandstone sequence

that is weakly ripple-laminated. This sequence is capped by algal stro­

matolite beds and bioclastic carbonates interbedded with siltstones.

The Upper Member of the Koobi Fora Formation is exposed west of Bura

Hasuma hill to the present lakeshore (see Fig. 2). Here the sequence is

composed of lenticular conglomerate and medium-grained trough cross-bedded

lithic arkoses interbedded with tuffaceous lenticular-bedded siltstones

and parallel-laminated tuffs.

Fig. 9. Photographs of fossils in the Allia Bay area

A. Ostracod sands

B. Gastropods

C. Pelecypods (Etheria)

D. Vertebrates

E. Algal stromatolite mats

F. Algal stromatolite spheroids

G. Petrified wood at Sibilot

H. Crotovina (1)

Fig. 10. Graphic sections of the Upper Cenozoic sediments, Allia Bay, East Rudolf

OF THE KUBI ALGI FORMATION, EAST RUDO

v, .L.

EXPLANATION

LITHOLOGY

Conglomerate

Sandstone

Sikstone

Claystone

Mudstonc

Carbonate

1 I Tuff

|, # I Locution of outcrop section

I -| Corrciacion line

SCALE f—80

—10

5 >— 0

Meters

COLOR

I

Light olive gray 5Y6/2

Light olive brown 5Y5/6

Yellowish gray 5Y7/2

Very light gray N8

Light gray N7

Very pale orange 10YR8/2

Color Lithology

SCALE

Pale yellowish brown l()YR6/2

Dark yellowish orange 10YR6/6

Grayish orange 10YR7/4

Pale brown 5YR5/2 Moderate yellowish brown 10YR5/4

2 3 4 Kilometers

EAST RUDOLF

Color Lithology

iY6/2

5Y5/6

7/2

0YR8/2

bwn l()YR6/2

tangc 10YR6/6

:0YR7/4.

I5/2 sh brown 10YR5/4

260

/

—

Allia Bay

•200

206 ' .204 \ \ ;

pi 240 V.^

"248

l /

> \ ,- '250>

-! \\Z60\ \ v,j \ VÀSv W . .

LOCATION OF SECTIONS SCALE

0 1 2 3 4 Kilometers

LITHOLOG!

Conglo

Sandsto

Siltstont

Clayscot

Mudsto!

't ' I'l Carbons

~n Tuff

I—» Locatioi

I \ Corrclaci

200

SUREGEI TUFF COMPLEX

202

3.9 my BP TUFF

4.5 my BP TUFF w

LITHOLOGY

•pîl'Sâ Conglomerate

txxd

" COLOR

Sandscone

Siltstone

Claystonc

Mudstonc

Carbonate

Tuff

Location of outcrop section

I 1 Correlation line

—10

5

0 Meters

I

Light olive gray 5Y6/2

Light olive brown 5Y5/6

Yellowish gray 5Y7/2

Very light gray N8

Light gray N7

Ver)' pale orange 10YR8/2

Pale yellowish brown l()YR6/2

Dark yellowish orange 10YR6/6

Grayish orange 10YR7/4

Pale brown 5YR5/2 Moderate yellowish brown 10YR5/4

Color Lithology

m

roior Lithology

38

PETROLOGY

The PIîo-P1eistocene sediments in the Allia Bay area of the East

Rudolf basin consist of conglomerates, sandstones, siltstbnes, claystones,

mudstones,- limestones and tuffs. The rocks analyzed are from the Kubi

Algi Formation and the Lower and Upper Members of thé Koobi Fora Formation.

Conglomérates

Extraformational and intraformational conglomerates are present in

the Allia Bay sediments. Extraformational conglomerates are of the poly-

mi ctic and oligomictic types.

The polymictic conglomerates occur mostly in the center and western

part of the basin. The conglomerates consist predominately of poorly

cemented volcanic clasts with a minor fraction of plutonic, metamorphic

and sedimentary rock clasts in a clay matrix. The rock fragments are

mostly subrounded granules to pebbles with a small component of cobbles.

These conglomerates are mostly channel deposits with a smaller amount

representing prograding deltic deposits. The basalt and ignimbrite clasts

are derived from the surrounding volcanic highlands, and the plutonic and

metamorphic clasts have been transported through the Suregei Cuesta by

streams draining the Ethiopian highlands (Vondra and Bowen, 1976).

The oligomictic conglomerates are channel and fan deposits composed

of basalt and ignimbrite clasts. These clasts are mostly granules and

pebbles, but the alluvial fan conglomerates along the eastern side of the

basin contain a large component of cobbles and boulders. The channel

deposited conglomerates are composed of subrounded volcanic clasts in a

39

clay matrix often cemented by calcite. The alluvial fan conglomerates

consist of subangular clasts in a clay matrix cemented by calcite. All of

the conglomerates contain both ignimbrite and basalt clasts; but in the

south, near Jarigole, ignimbrite is dominant while in the remainder of the

basin, basalt is dominant.

Intraformational conglomerates occur throughout the basin. They are

composed of limonite nodules and fragments In a calcareous fine-grained

lltharenite to feldspathic litharenlte matrix. These conglomerates are

discontinuous and are thin-bedded and lenticular.

Sandstones

Minerology

The composition of sandstones in the Allia Bay area displays only a

small degree of variability laterally and vertically. The average com­

position is 35% quartz, 25% feldspar, 24% rock fragments and 16% accessory

minerals (Table 1). Using Folk's (1968) classification, the sandstones

are 1itharenites, feldspathic 1itharenites, lithic arkoses and arkoses.

The lithic arkoses and arkoses are dominant in the north and the feld­

spathic litharenites and litharenites are dominant In the southern part of

the basin. There is a greater concentration of feldspathic litharenite

and litharenite in the basal units and lithic arkose and arkose in the

upper units. From north to south the amount of rock fragments and plagio-

clase grains increase 10% and 3% respectively, while quartz, orthoclase

and heavy minerals decrease 4%, 2% and 3% respectively, and microcline

remains the same. From the base to the top of the sequence, rock fragments

40

Table 1. Percentages of sandstone minerals

Sample Number Quartz Feldspar

Rock Fragments

Accessory Minerals

(/)

3 (U U > o

0) 0) U) 0) <0 0) l/l c lU 4-1 <0 -u c ID u Q. 4-1 c m fO

o c o <u in O u o 4-> -Q c o <u 4-> 0) o 0 E o E E 3 0) J: 1- cn m 4J m c 4-> CT u 4-1 u (0 tn c 3 •w TJ o (D (A U m oi 0) O o a. o z Û. CQ — Q- s (/) 3: CQ o z

ER-72-122-0102 20 13 5 11 10 8 1 1 4 12 3 10 2 -0104 33 9 6 12 11 7 - 1 3 5 2 9 2 -0106 36 8 2 8 14 10 - - 2 8 2 8 2

ER-72-123-0104 53 10 2 14 2 2 - - - 9 2 6 -

-0105 35 9 3 11 5 4 - - 1 18 3 11 -

ER-72-125-0104 47 6 5 12 6 4 1 - 2 9 2 2 4 -0105 30 9 5 7 31 1 - - 1 11 4 1 -

ER-72-200-0104 40 12 3 18 7 2 - - 1 6 2 7 2 -0105 34 7 3 6 29 5 - - - 10 2 1 3

ER-72-201-0104 37 6 - 24 18 1 - - 1 7 2 1 3 ER-72-202-0101 42 6 - 25 22 - - - • - 2 2 1 -

-0102 34 12 7 17 8 7 2 1 5 1 3 1 2

-0303 34 11 3 12 12 1 - - 4 10 1 10 2 ER-72-204-0102 39 8 2 17 10 3 - - 4 8 4 3 2

-0105 30 4 - 14 45 - - - - 1 1 4 1

ER-72-250-0106 22 6 2 13 26 4 - - - 13 3 8 3 -0123 41 7 5 19 15 1 - - - 3 2 5 2 -0125 28 4 4 16 15 5 5 1 4 8 2 6 2 -0127 20 15 15 6 23 1 1 1 3 8 1 4 2

ER-72-260-0111 32 6 2 7 20 26 - - 1 3 1 1 1 -0121 40 10 2 10 6 18 - - - 6 1 6 1

^Opaque minerals are magnetite, hematite ilmenite and leucoxene.

''Miscellaneous minerals are olivine, augite, zircon, rutile and tourmaline.

41

decrease 10% while quartz and feldspar grains increase and heavy minerals

remain the same.

Most of the quartz grains show strong undulose extinction, but slight­

ly undulose and straight extinction are common, indicating that some of the

quartz is of volcanic origin (Folk, 1968; Petti john e^ aj_., 1972). Also,

many grains possess embayments with straight sides and rounded corners

which suggest a volcanic source (Folk, 1968). The volcanic quartz con­

tains few, if any, inclusions, but the plutonic and metamorphic quartz

contain vacuoles and microlites.

The 25% feldspar fraction averages 7% orthoclase, 4% microline and

14% plagioclase. Sanidine and anorthoclase were observed, but an accurate

point count was not obtained. The K-feldspar content is much lower and

the plagioclase content is higher than in the Koobi Fora and lleret areas

(Bowen, 1974, unpublished Ph.D. dissertation). This indicates the strong

influence of the plutonic and metamorphic source in Ethiopia of the Koobi

Fora and lleret sediments. The transitional lacustrine sandstones have a

much higher content of K-feldspars indicating longshore movement of sand

from other source areas.

The plagioclase feldspars are dominant all over the basin, but

increase in percentage near the east side. Labradorite and some of the

sodic plagioclases were identified with the pétrographie microscope and

by x-ray defraction. By a point count labradorite is the predominant

plagioclase present and appears to be the same as observed in the thin-

sectioned volcanics bordering the Allia Bay area.

42

The rock fragments are composed of 68% basalt, 22% ignimbrîte, 7%

sedimentary (chert included), and 3% plutonic and metamorphic. From north

to south the basalt fragments increase from 12% to 17%, the ignimbrite

fragments Increase from 5% to 9%, and the plutonic and metamorphic

fragments decreased slightly. The rock fragments are medium- to coarse­

grained and subrounded (3.5 on the Power's roundness scale).

Accessory minerals were identified by using the pétrographie micro­

scope, heavy liquids and the stereo microscope. The fine-sand fraction

was separated using heavy liquids and a point count was taken (Table 2).

Heavy minerals (densities greater than 2.8) comprise 2% to 28% of the

samples analyzed. The heavy minerals include magnetite, hematite,

Ilmenite, leucoxene, hornblende, auglte, olivine, rutile, apatite, zircon

and tourmaline. These minerals were derived from volcanic, plutonic and

metamorphic terranes, with volcanic being predominant.

Percentages of the individual heavy minerals vary in different size

fractions, but the greatest concentration occurs In the fine-sand fraction.

The heavy minerals are subangular (2.5 on the Power's roundness scale) com­

pared to subrounded (3.6) for the light fraction. The arid climate retards

chemical rounding of the grains (Folk, 1968; Pettljohn e^ aj_., 1972).

Hornblende (mostly oxyhornblende) is the dominant mineral averaging

63% of the heavy mineral suite. Other averages are magnetite 7%, hematite

3%, 11 menite-leucoxene 12%, augite 6%, olivine 4%, and rutile, apatite,

zircon and tourmaline about 5% of the total. Biotite was not counted as

part of the heavy mineral fraction due to the difficulty of separating it

from the light minerals, but from the point count of the thin-sections.

It constitutes about 6% of the accessory minerals.

43

Table 2. Percentages of heavy minerals

0) a) X o o

Sample Number <u (U <u

0) +J

OJ +->

"O c

— 0) +J

OJ +-> 0) m

<0 E L. 3

<u c U) m z

E 0) 3=

c

1 _o c

c

> 4-> •M (0

o o u

<0 E L. 3

<u c U) m z

E 0) 3= 3= < o a: < N t-

ER-72-122-0101 13 2 25 43 4 8 4 1 -0102 11 3 8 66 4 2 1 2 1 2 -0105 12 - 19 55 2 1 6 2 1 2 -0106 11 3 5 56 11 3 - 6 2 3

ER-72-123-0102 7 - 12 73 - - 6 2 - -

-0104 5 10 18 51 4 - 2 4 2 4 ER-72-125-0102 4 4 12 71 - - - 7 - 2

-0104 1 - - 75 5 9 3 3 2 2 ER-72-200-0104 8 2 16 74 - - - - - -

-0105 1 4 11 56 20 8 - - - -

-0106 5 8 14 47 8 6 4 2 3 3 ER-72-201-01Û1 3 3 10 58 4 10 4 2 3 3

-0104 3 3 8 62 12 5 1 3 - 3 ER-72-202-0101 3 6 10 54 10 8 - 4 3 2

-0102 - 6 12 58 12 4 - 4 2 2 ER-72-204-0102 3 - 13 54 12 8 2 3 2 3

-0105 2 4 9 70 14 - - - 1 -

ER-72-250-0106 11 - 5 59 10 15 - - - -

-0107 11 3 20 59 4 1 1 - - 1

-0127 7 - 8 66 6 8 1 1 1 2 ER-72-260-0104 3 6 17 68 3 1 1 - 1 -

-0111 3 4 10 81 2 1 - - - -

-0120 13 - 18 66 2 - - • - 1 -

-0121 14 - 10 67 5 2 - - 2 -

44

The silt and clay fraction is from 2% to 50% with an average of 19%

by weight for all samples analyzed. The silts contain the same minerals

as the sand fraction except no rock fragments are present. The clays are

montmori1lonite, mixed layer montmori1lonite-i11ite and vermiculite.

These clays are derived from ash falls and the weathering of volcanic and

sedimentary rocks (Folk, 1968).

Calcite is the most common cementing agent, but iron oxide cements

a small portion of some units. All but a few units are very poorly

cemented, and a large majority are friable and loose with a large silt-

clay matrix.

A large majority of the Allia Bay sandstones are immature as they

contain more than 5% clay, are poorly sorted and have subrounded grains

(Folk, 1968). The dry climate has greatly retarded the chemical decay

of the feldspars and many of the accessory minerals. The few sandstones

that are submature have less than 5% clay, are moderately well sorted and

are rounded. These are beach and channel deposits where a large part of

the sediments were transported from the highlands of Ethiopia and

deposited by streams or longshore currents in the lake.

Texture

The sandstones vary in grain size from very fine (3.45 0.09 mm)

to medium (1.30^, 0.4 mm) (Table 3). From the north to the south the

grain size varies from 2.34(6 to 1.93«5. The grain size in the Allia Bay

area is slightly smaller than in the Koobi Fora and lleret areas (Bowen,

1974, unpublished Ph.D. dissertation).

45

Table 3. Statistical measures of selected sandstones

Inclusive Graphic Inclusive

Graphic Standard Graphic Graphic Sample Mean Deviation Skewness Kurtosis

(Mz) (<5|) (SK,) (Kg)

ER-72-122-0101 2.57 1.02 +0.26 0.72 -0102 2.26 0.69 -0.11 1.09 -0105 3.45 0.68 -0 .29 2.64

-0106 1.30 1 .42 +0.44 0.83 ER-72-123-0102 2.30 1.26 +0.33 0.71

-0104 2.07 0.85 -0 .17 1.34 ER-72-125-0102 2.67 0.89 -0.13 0.79

-0104 2.10 0.54 -0.14 1.04 ER-72-200-0104 1.92 1.22 +0.27 1.08

-0105 2.66 0.78 -0.09 0.96 -0106 2.30 1.44 +0.51 1.09

ER-72-201-0101 2.20 1.19 +0.24 0.77 -0104 2.42 0.65 -0.03 1.75

ER-72-202-0101 2.12 1.34 +0.39 0.84

-0102 1.93 1.10 +0.21 1.08

ER-72-204-0102 1.73 0.98 -0.01 0.98 -0105 1.60 1.58 +0.44 0.75

ER-72-248-0109 2.4o 0.82 -0.04 0.90 ER-72-250-0106 1.28 0.62 -0 .18 0.91

-0107 1.43 0.88 -0.11 1.05 -0118 1.73 1 . 1 5 +0.22 0.87 -0122 3.03 0.91 -0.41 1.39 -0127 1.40 1.20 +0.23 1.08

ER-72-260-0104 1.75 1.57 +0.40 0.82

-0111 2.40 0.80 -0.12 1.16

-0120 1.78 1.12 +0.26 1.05 -0121 2.60 1.12 +0.38 0.98

= (416+^50+484) 61 = 484-416 + 495-45

3 — -TT"

SK, = 416+484-2450 + 45+495-2450 K„ = 495-45 2(484-416) 2(495-45) ^ 2.44(475-425)

46

The inclusive graphic deviation varies from 0.54(6 (moderately well

sorted) to 1.56j5 (poorly sorted) with an average of 1.03(0 (poorly sorted).

Many of the beds are bimodel and reflect the large silt and clay content,

which suggests the sediments have traveled a short distance from the

source area (PettiJohn et 1972).

The inclusive graphic skewness varies from +0.51 (strongly fine-

skewed) to -0.41 (strongly coarse-skewed) with an average of +0.10 (fine-

skewed). The fine-skewed units reflect the lack of winnowing of the fluvial

sandstone deposits. The graphic kurtosis varies from 0.72 (platykurtic) to

2.62 (very leptokurtic) with an average of 1.06 (mesokurtic). This indi­

cates that the centers are better sorted than the extremes, but several

units are platykurtic, which indicates that the extremes are better sorted

(Griffiths, 1961).

The average sandstone could be described as fine-grained, poorly

sorted, fine-skewed mesokurtic indicating a texturally immature sandstone

of fluvial and fluvial deltaic origin (Folk, 1968; Pettijohn et a]^., 1972).

The few sandstones which were winnowed by the lake along the beaches are

better sorted and submature (Folk, 1968).

Mudrocks

Mi nerology

The mudrocks consist of siltstones, claystones and mudstones. The

silt in the mudrocks contains most of the mineral suite of the sandstones

with quartz, plagioclase feldspars and heavy minerals predominant. The

type of quartz, feldspar and heavy minerals give a strong indication of

volcanic highland provenance (Folk, 1968).

47

Montmori 1 lonite is tiie dominant clay with mixed-layer montmoriIlonite-

iI lite and vermiculite occurring in minor amounts. The clays reflect the

volcanic source, subaerial and subaqueous weathering of volcanic ash and

weathering of the sedimentary rocks in a hot, dry climate (Folk, 1968).

The clays contain glass shards, pumice, volcanic fragments, plagioclase,

biotite, other mafic minerals, zircon and apatite to strongly indicate the

volcanic source (Folk, 1968).

Texture

Pipette analysis of the siltstones, mudstones and claystones was

conducted. The mean grain-size of the siltstones is 5-72# (medium silt)

and the mean grain-size of the mudstones is 7.07# (very fine silts). The

clay size fraction of the mudstones varies from 36% to 65% with an average

of 47% for the samples analyzed. The shales are over 90% clay with no

cement and are undisturbed by slumping or burrowing organisms.

Carbonates

Most of the carbonates in the Allia Bay area are located in the

northern part in the Koobi Fora Formation. The carbonates are biolithites

and arenaceous bioclastic carbonates.

Fossils of the arenaceous bioclastic carbonates are ostracods,

gastropods (Melanoides sp., Cleopatra sp., Mutela sp.) and pelecypods

(Corbicula sp., Nyassunio sp.) (Vandamme and Gautier, 1972). These fossils

comprise from 30% to 80% of the rock. The carbonates are classified as

biosparites, biosparudites, packed blosparites and packed biosparudites

(Folk, 1968).

48

The biolithites are composed largely of algal stromal!tes. Johnson

(1974) doing micro-stratigraphy 10 km north of the study area identified

the following algal structures: polygonally desiccated algal mats, con­

centric smooth and nearly flat stromatolitic layers, discontinuous algal

mats, compounded spheroidal and hemispheroidal stromatolites and concentri­

cally stacked spheroidal stromatolites. In the Upper Member of the Koobi

Fora Formation compounded spheroidal and hemispheroidal stromatolites are

present with some of the spheroids measuring over 30 cm in diameter.

Near Bura Hasuma hill in the Lower Member of the Koobi Fora Formation

polygonally desiccated and concentric smooth and nearly flat algal

stromatolites are present while stromatolite encrusted basalt boulders

(oncolites) are present west of Derati. Algal stromatolites were not

observed in the Kubi Algi Formation.

In the Kubi Algi Formation no carbonates were observed although imany

sandstones and siltstones are very calcareous and often contain an abun­

dance of calcareous components such as ostracods with smaller amounts of

gastropods, pelecypods and abraded fish and mammalian bones.

Tuffs

The tuffs in the Allia Bay area (Figs. 11a, b, c, and f) are composed

of glass shards, pumice fragments, quartz, plagioclase, sanidine, horn­

blende, biotite, rock fragments and the highly-weathered tuffs have a

significant content of montmori1lonite clay and secondary calcite.

Sanidine crystals contained in pumice cobbles have been dated by the con­

ventional K-Ar, total degassing and spectrum ^Ar/^^Ar age or fission

track dating methods (Fitch et al_., 1974). The glass shards vary from

49

angular needlelike in the fresh beds to rounded in the highly weathered and

may range from 5 to 200 microns in diameter with an average of 80 microns

(very fine sand).

Some of the tuffs are consolidated ash falls with stratification and

show crude to complete sorting of its component parts. Many of the tuffs

are reworked and deposited as sediments in channels and flood plains.

These display cross-bedding and possess sand lenses and mud drapes. Some

of the tuffs contain ostracod and gastropod fossils, indicating a littoral

environment of deposition. The tuff beds vary in thickness from 0.2 to

19.7 m (Fig. Ilf) and average 3-19 m. Tuff beds become much thicker in

the south near Jarigole probably due to the proximity of intense volcanic

eruptions.

Fig. 11. Photographs of the Kubi Algi Formation

A. Suregei Tuff Complex north of Kubi Algi

B. Floodplain deposits south of

Kubi Algi

C. The 3.9 myBP tuff southwest of Kubi Algi

D-E. Conglomerates near Jarigole

F. A 19.7 m tuff near Jarigole

51

52

FACIES AND ENVIRONMENTS OF DEPOSITION

The facies of the Allia Bay area are described and interpreted to gain

a better understanding of the depositional environment. The facies are

highly interbedded and interfingered, but generally parallel the lake and

reflect the changing shoreline with time. The following description of the

facies of East Rudolf is quoted from Vondra and Bowen (1976):

Four major lithofacies, each consisting of several microfacies, have been recognized in the Upper Cenozoic sediments in the East Rudolf area. These are (1) the laminated siltstone facies; (2) the arenaceous bioclastic carbonate facies; (3) the lenticu­lar fine-grained sandstone and lenticular-bedded siltstone facies; and (4) the intertongued lenticular conglomerate, sand­stone and mudstone facies. These are characterized by properties indicative of four major depositional environments (1) prodeltic and shallow shelf lacustrine; (2) littoral lacustrine-beach and barrier beach and associated barrier lagoons; (3) delta plain-distributary channel, levee, and interdistributary flood basin;

and (4) fluvial channel and flood plain.

Koobi Fora Area

The description of the facies in the Koobi Fora area is condensed from

(Bowen, 1974, unpublished Ph.D. dissertation; Bowen and Vondra, 1973; and

Vondra and Bowen, 1976).

Laminated s 11tstone facies

This facies consists of siltstones with lenses of sandstone. The

siltstones are thin-bedded to laminated, yellowish gray (5Y7/2), limonitic

and often argillaceous. Near the present lake this sequence becomes

lenticular-bedded with isolated, discontinuous, flat lenses of grayish

orange (10YR7/4), packed molluscan biosparudites and laminated light gray

53

(N-7), bentonîtîc tuffs. Fossils are rare, but siltstones contain the

gastropods Cleopatra sp. and Melanoides sp.

Arenaceous bioclastic carbonate facies

This facies is exposed throughout the basin and occurs intermittently

in the entire Upper Cenozoic sedimentary facies. The facies consists of

dark yellowish orange (10YR6/6) to moderate yellowish brown (10YR5/4),

very arenaceous, packed gastropod and/or ostracod biosparudite which may

grade laterally into dark yellowish brown (10YR4/2) to yellowish gray

(5Y7/2) very calcareous and fossiliferous, fine- to medium-grained lithic

subarkose or into grayish orange (10YR7/4) biolithite possessing algal

stromatolite structure. The loose to friable, poorly sorted, lenticular

sandstones show indistinct low-angle, small-scale, planar crossbeds. They

contain mud cracks, load casts, calcareous root casts and numerous verte­

brate fossils. Three basic stromatolite structures - mats, hemispheroids

and spheroids are dominant (Johnson, 1974).

Lenticular fine-grained sandstone and lenticular-bedded siltstone facies

This facies occurs throughout the basin and comprises the greatest

volume of all the facies. Most of the middle portion of the Koobi Fora

Formation is composed of this facies. The facies consists of 1 to 25 m

thick lenticular channels of grayish orange (10YR7/4) fine- to medium-

grained sandstone which grade into pale yellowish brown (10YR6/6) 1imonite

clast intraformational conglomerate, or a thin lenticular ripple-laminated,

light gray (N-7) tuff interrupts this sequence. The channel sandstones

contain calcareous concretions, load casts and veins of selenite and

54

streaks of limon!te. Fossils consist of broken gastropods, abraded mam­

malian bones and locally calcareous root casts and, occasionally, the fresh

water oyster, Etheria.

The siltstones near the channels are coarse-grained, pale yellowish

brown (10YR6/2) to yellowish gray (5Y7/2). The siltstones grade into

claystones away from the channels. The claystones are often interbedded

with light gray tuffs (N-7).

Lenticular conglomerate, sandstone and mudstone facies

This facies occurs along the eastern margin of the East Rudolf basin

primarily in the upper portion of the Koobi Fora Formation. It is the most

heterogeneous of the facies and consists of a complex variety of minor

facies which grade laterally and vertically into one another, wedge in,

thicken and pinch out. Lenses of grayish orange (10YR7/4) granule to

cobble conglomerate and fine- to coarse-grained arkose or feldspathic

litharenite occurring in older deposits, and associated fine-grained, very

pale orange (10YR8/2) to grayish orange (10YR7/4) siltstones, claystones,

mudstones and light gray (N-7) tuffs comprise this facies. The sandstones

contain clay galls, abraded vertebrate fossils, load casts and calcareous

root casts. Laterally the sandstones grade and interfinger with coarse­

grained siltstones, mudstones, and highly lenticular, laminated and

lenticular-bedded to massive tuffs. These sediments contain calcareous

root casts and concretions, caliche, mud cracks and incipient paleosols.

55

Allia Bay Area

The same four facies are present in the Allia Bay area, although the

arenaceous bioclastic carbonate facies is quite limited in scope in the

Kubi Algi Formation. These facies are intertongued horizontally and grade

into each other vertically due to the fluctuating and generally retreating

shoreline through time. The changing shoreline reflected tectonic activi­

ty and changing climatic conditions. The facies occur in belts generally

paralleling the lake (Figs. 12 and 13) and migrated through time to the

west recording a general regression of the lake.

Laminated siItstone facies

This facies is exposed west of Bura Hasuma hill near the present

lakeshore in the Upper and Lower Members of the Koobi Fora Formation and

continues south through the Kubi Algi Formation to the volcanic highlands

(Figs. 12 and 13). The laminated siItstone facies interfingers laterally

to the east and grades vertically with the arenaceous bioclastic carbonate

facies.

This facies consists of sequences of thin- to lenticular-bedded pale

yellowish brown (10YR7/2) to yellowish gray (5Y7/2) siltstones. These

siltstones are often sandy, argillaceous, tuffaceous and limonitic with

numerous calcareous root casts and concretions. Manganese dioxide den­

drites and selenite-fi1 led fractures are common. Most beds are 3 to 4 m

thick but vary from 0.3 to 14.2 m. The gastropods (Cleopatra sp.,

Melanoides sp.) are present but are rare in the Kubi Algi Formation. They

occur in lenses in the laminated siltstones and comprise most of the

fossil content of the biosparudites in the Koobi Fora Formation.

Fig. 12. Lithofacies map of the Allia Bay area at 3-9 myBP

Lake

Rudolf

Volcanic

Highlands

Key

Laminated siltstone fades

uHiMn Shoreline

Arenaceous bioclastic carbonate facias

, Lenticular fine-grained sandstone and lenticular-bedded siltstone fades

Intertounged lenticular conglomerate, sandstone and mudstone fades

0 1 2 3 4 K m jrrrmlîflffPntri»

scale NORTH

Fig. 13. Geologic map of the Allia Bay area

4 . "J : > 1 ;t u

MagneiicNoMh — -j

True North

60

The thin- to medium-bedded tuffs are light bluish gray (5Y8/1) and

consist of coarse silt-sized glass shards and are often ripple-laminated

(Fig. 8d). Numerous calcareous root casts and concretions are present in

the tuffs. The sandstones are fine-grained, ripple-laminated, with root

casts and calcareous and limonitic concretions.

The thin- to lenticular-bedded siltstones suggest deposition in quiet

water of low energy (lower shoreface and transition to offshore depths) and

periodic turbulent water from high winds depositing sand lenses in the rip­

pled slit (Harms et al_., 1975). The sandstone layers probably represent

storm deposits where sand eroded from the upper shoreface to offshore

depths and could be partially due to the prograding nature of the deposits.

The silt below is reworked and mixed into the sand by burrowing organisms.

This plus the presence of gastropods suggest deposition in a prode1 ta or

shallow shelf environment (Allen, 1970; Harms e^al_., 1975).

Arenaceous bioclastic carbonate facies

It is difficult to draw definite boundaries to this facies as it is

small in volume and highly interbedded and interfingered with the other

facies. The arenaceous bioclastic carbonate facies is exposed in the

northwest corner of the study area, in the Koobi Fora Formation, near the

mouth of the Laga Bura Hasuma. This facies continues in a narrow band

through the Kubi Algi Formation to the southern edge of the basin (Figs.

12 and 13).

In the Koobi Fora Formation this facies consists of grayish orange

(10YR7A) arenaceous ostracod, pcîecypod and gastropod biosparudite which

grades laterally into dark yellowish orange (lOYRlO/6), medium-grained,

61

argillaceous feldspathic litharenite that has a very high ostracod and

pelecypod content. Interbedded in this sequence are thin layers of algal

stromatolite mats and spheroids (Figs. 9e and f). The carbonate and sand­

stone layers vary in thickness from 0.2 to 1.0 m and contain planar

crossbeds outlined by heavy minerals. The sandstones often contain layers

or lenses of silt, limonite and thin limonite pebble conglomerates. Clay-

stones in the fades are light olive brown (5Y5/6), tuffaceous, limonitic,

and have numerous fractures filled with selenite or coated with Mn02 den­

drites. Pelecypod and ostracod horizons are present in the claystbnes.

The argillaceous tuffs are very bright orange (10YR8/1), limonitic, and

ostracod horizons are common. Most of the tuffs and claystones are less

than 2 m thick and the tuffs are often ripple-laminated (Fig. 8d).

In the Kubi Algi Formation the arenaceous bioclastic carbonate facies

does not contain carbonate beds but instead consists of highly fossilif-

erous, very calcareous, and ferruginous dark yellowish orange (10YR6/6)

lithic arkoses and feldspathic 1itharenites. These sandstones have a very

high content of ostracods and a moderate amount of pelecypods. The beds

vary in thickness from 0.7 to 3 m and are parallel-bedded to small-scale

planar-crossbedded.

This facies in the southern most part of the basin measures 8.3 m in

thickness and consists of a fine-grained, pale yellowish brown (10YR7/2)

litharenite, between a yellowish gray (5Y8/1) tuff and a pale yellowish

brown (10YR6/2) claystone. The litharenite has a clay matrix, thin

calcareous beds, limonite concretions and is ripple-laminated. The tuff

contains numerous ostracods, and the tuff and claystone are thin-bedded

62

with a high content of Mn02 dendrites. The sandstones, carbonates, silt-

stones and claystone units and their textural and structural features would

suggest shoreface, beach and sand dune environments of deposition (Dickin­

son et a]_., 1973; Davis et al., 1971; and Weide, 1968).

Lenticular fine-grained sandstone and lenticular-bedded si1tstone facies

This facies is located in a wide band from Bura Hasuma hill to

Jarigole (Figs. 12 and 13). This facies comprises most of the upper por­

tion of the Kubi Algi Formation and the basal part of the Lower Member of

the Koobi Fora Formation. The si 1tstone and sandstone beds vary from 0.5

to 23 m thick and represent channel and fluvial-del ta plain deposition.

In the Lower Member of the Koobi Fora Formation the lenticular fine­

grained sandstone and lenticular-bedded si 1tstone facies consists of

grayish orange (10YR7/4) to yellowish gray (5Y8/1) lithic arkose channel

sandstone; pale yellowish brown (10YR6/2) to grayish orange (10YR7/4) silt-

stone; and yellowish gray (SYB/I) tuffs. The sandstones are very fine- to

fine-grained with lenses of conglomerate. Near the base of the beds, the

sandstones are weakly ripple-laminated, and they grade upward to small-

scale planar (Mu) crossbeds. The beds are fossiliferous with the gastropod

Cleopatra sp. and the pelecypod Etheria being most prevalent. The silt-

stones are argillaceous, have sand lenses, and often contain ferruginous

sand concretions, calcareous root casts, and gastropod, pelecypod and

vertebrate fossils. The tuffs are argillaceous and thin- to medium-bedded.

The lenticular fine-grained sandstone and lenticular-bedded siltstone

facies in the Kubi Algi Formation in the northern part of the basin con­

sists of grayish orange (10YR7/4) lithic arkoses interbedded with pale

63

brown (5YR5/2) siltstones and light gray (N-7) tuffs. The sandstones are

very fine- to medium-grained with low-angle small-scale crossbeds. The

beds contain mammalian fossils, calcareous root casts and concretions. The

siltstones are lenticular-bedded and the siltstones and tuffs contain low-

angle small-scale planar crossbeds. The sandstones vary from 1 to 13.5 m

and the siltstones and tuffs from 0.6 to 2.8 m in thickness.

In the southern part of the basin near Jarigole, the sandstones are

pale yellowish brown (10YR7/2) litharenites that contain ripple lamination

and are locally argillaceous and calcareous. The siltstones are very pale

yellowish orange (10YR8/2), with lenses of fine-grained litharenite. The

tuffs are very pale orange (10YR8/1), medium-bedded and measure from 1.5 to

19.7 m thick. The siltstones and tuffs are often very sandy and contain

layers of limonite concretions. Farther away from the channels the silt­

stones have a high content of clay, mud cracks, limonite stains, selenite

veins and thin caliche horizons. The only fossils present in the silt­

stones are abraded mammalian bones and calcareous root casts.

The channel sandstones in the western part of the area contain the

fresh water oyster Etheria, which would indicate these streams were peren­

nial at the time of deposition (Vondra and BoWen, 1976). The other channel

sandstones in the area do not contain fossils of Etheria. The lithologies

and primary structures of the sandstones and siltstones of this facies

would suggest deposition in a fluvial-delta plain environment (Frazier and

Osanik, I96I; Allen, 1965a; Coleman and Gagliano, 1965; Morgan, 1967;

Berryhill e^ aj[., 1969; Kanes, 1970; and others). The sequence of primary

structures that occurs in the interbedded siltstones and sandstones has

64

been described as point bar and levee deposits by Coleman and Gagliano

(1965) and Harms et aj^., (1975).

Lenticular conglomerate, sandstone and mudstone feci es

This facies occurs along the eastern margin of the basin near the

volcanic highlands. It outcrops in a band from the northern to the south­

ern extremes of the basin and comprises much of the lower portion of the

Kubi Algi Formation (Figs. 12 and 13).

In the Koobi Fora Formation it consists of dark yellowish brown

(10YR7/2), disc-shaped, basalt pebble conglomerate; medium-grained grayish

orange (10YR7/4) arkose; and dark yellowish brown (10YR4/2) silty clay-

stone that has numerous thin beds of pale yellowish orange (10YR6/6)

lithic arkose; and very pale orange (10YR8/2) bentonitic tuffs. The con­

glomerate and sandstone beds are from 0.2 to 0.8 m thick, but the clay-

stones measure up to 23.5 m in thickness.

The lenticular conglomerate, sandstone and mudstone facies in the

Kubi Algi Formation located In the northern part of the basin consists of

grayish orange (10YR7/4) rounded granule to pebble basalt conglomerate;

fine-grained grayish orange (10YR7/4) lithic arkose to very pale orange

(10YR8/1) feldspathic litharenite with lenses of basalt pebble conglomer­

ate; yellowish gray (5Y7/2) claystone that grades into pale yellowish

brown (10YR6/2) siltstone; and silt-sized light gray (N-7) tuffs. The

conglomerate grades laterally into lithic arkose that displays homogenous

trough (Pi) crossbeds with 6 cm forsets, and into feldspathic litharenite

that has large-scale planar (Omikron) which grades into small-scale planar

(Alpha) crossbeds. Some of the sandstones are argillaceous, have

65

calcareous root casts and concretions, and contain abraded mammalian

fossils. The sllty claystones contain sand lenses, are often tuffaceous,

and have numerous calcareous root casts and concretions. The claystones

and tuffs are often ripple-laminated and measure from 1.5 to 12 m in

thickness.

In the southern part of the basin this facies in the Kubi Algi Forma­

tion becomes very massive. It consists of light grayish orange (10YR7/4)

granule to boulder Ignimbrite conglomerate in a clay matrix. Some of

these conglomerates are channel lag, and some are alluvial fan deposits

that contain boulders and wedge Into the other components of the facies

(Allen, 1970; Bull, 1972; Reineck and Singh, 1973; Harms aj[., 1975).

The pale yellowish brown (10YR7/2) litharenltes grade upward from trough

(pi) crossbeds to ripple laminations. The pale yellowish brown silty

claystones are tuffaceous, parai lei-bedded with numerous fractures filled

with selenite and llmonlte. The yellowish gray (5Y8/1) tuffs contain

selenite, llmonlte stains and calcareous concretions and root casts.

The fining-upward sequence plus the primary structures of large-scale

cross-stratified coarse or gravelly sand at the base, and grading upward

into ripple cross-stratified fine sand at the top would Indicate a mean­

dering or braided channel deposit (Allen, 1963, 1965a, 1965b, 1970; Harms

and Fahnstock, 1965; Williams, 1968, 1971; Blatt e^ aj_., 1972; Pettijohn

e_t , 1972; Harms e^ al_., 1975; and others). The overbank silt and

clay deposited on the floodplain with evidence of alternating slow and

rapid deposition support the interpretation of this facies (Allen, 1963;

Pettijohn et al., 1972; Harms et al., 1975).

66

Synthesis

In the Allia Bay area the laminated siltstone fades is very similar

to the laminated siltstone fades in the KoobI Fora area. The main dif­

ferences are the lack of limestone and the rarity of fossils in the Kubi

Algi Formation, and the sequence is less massive than in the Koobi Fora

area.

The bloclastic carbonate fades in the Koobi Fora Formation in the

Allia Bay area closely resembles this fades in the Koobi Fora area. In

the Kubi Algi Formation, carbonates are completely absent but highly

fossiliferous sandstones are present. The sandstones in the Kubi Algi

Formation are lithic arkoses to litharenltes instead of lithic subarkoses

as Bowen (1974) reported were present in the Koobi Fora area. Fossils are

rare and algal stromatolites are completely missing In all of the beds In

the Kubi Algi Formation. The extent of the bloclastic carbonate fades In

the Allia Bay area is very restricted.

In the Allia Bay area, the lenticular-bedded siltstone fades is most

dominant; this reflects the many streams and deltas that existed at the

time the sediments were deposited in the area. The large-scale trough (Pi)

crossbeds are not as prevalent as Vondra and Bowen (1976) reported In the

Koobi Fora area. The sandstones show a greater component of volcanics than

in the Koobi Fora area.

The lenticular conglomerate, sandstone and mudstone fades comprise a

great amount of the lower part of the Kubi Algi Formation. The conglomer­

ates contain a high amount of ignimbrite and basalt clasts, and the tuffs

are very thick in comparison to the Koobi Fora area. The alluvial fan

67

deposits comprise a high percentage of all the conglomerates in this

fades in the Kubi Algi area. This reflects a narrow basin with the

close proximity of the volcanic highlands to the lake. This narrow

basin in Allia Bay would explain the more restricted distribution of

fades.

68

TECTONIC AND DEPOSITIONAL HISTORY

Following the Cretaceous-Eocene arching and a period of quiesence,

the Baringo-Saguta graben was formed during the Miocene as part of the

Turkana depression (Saggerson and Baker, 1965; Berry and Whitman, 1968;

Baker e^ £]_•, 1972; Fitch and Vondra, 1976). The Lake Rudolf basin formed

in the Baringo-Saguta graben and has been a sediment trap from the Miocene

to the present (Walsh and Dodson, 1969; Patterson e^ , 1970; Butzer,

1971; de Heinzelin £1»» 1971; Vondra and Bowen, 1976). This is sup­

ported by the fluvial sediments on the west side of Lake Rudolf that have

been dated at over 23 myBP (Walsh and Dodson, 1969); lacustrine sediments

southwest of the lake dated at 7 myBP (Patterson ej^ al_., 1970); and

lacustrine sediments in the Omo area north of the lake dated at 4.5 myBP

(Butzer, 1971; de Heinzelin e^ aj_., 1971). In the Allia Bay area on the

east side of the lake. Fitch and Miller (1971) have dated a tuff at

4.5 myBP that lies 59.2 m above the volcanics, which would give an indi­

cation that the oldest sediments in the area are near 5 myBP. As; the

early sediments on the west side of the lake are fluvial (Walsh and

Dodson, 1969), there is no strong evidence that Lake Rudolf formed until

middle Miocene. The basin has continued development to the present

(Patterson e^ aj_., 1970; Butzer, 1971; de Heinzelin e^ al_., 1971; Vondra

and Bowen, 1976).

Arching of the Ethiopian and Kenyan domes, with the accompanying

major faulting, continued into the middle Miocene (Gass and Gibson, 1969;

Baker e_t aj!_., 1972). These major faults west of the lake produced a

half-graben, and on the east side of the lake a monoclinal flexure formed,

69

accompanied by many minor faults (Baker ei^ aj_., 1972; Fitch and Vendra,

1976). A thick sequence of basic and acidic volcanics with Interbedded

fluvial and lacustrine sediments accumulated in the shallow basin at this

time (Patterson e^ aj_., 1970; Fitch and Vondra, 1976). The last major

tectonic events occurred in the early to mid-Pleistocene when the Kinu

Sugo fault zone was formed to the east of Lake Rudolf (Howell, I968;

Butzer and Thurber, 1969; Fitch and Vondra, 1976).

Basalts in the Suregei Cuesta that form the northeastern and eastern

margin of the East Rudolf basin have been dated at 11.6 t 0.5 and 14.1 ±

1.4 myBP (Fitch and Miller, 1976). These basalt flows and associated

ignimbrites and tuffs form the basement upon which the PIio-P1eistocene

sediments were deposited. Tectonically, the area continues to be active

as many faults in the Allia Bay area extend through both the sediments

and the volcanics indicating the faults are postdepositional; also, some

of the volcanic flows are interbedded with sediments.

The Allia Bay area received most of the early sediments in the East

Rudolf basin. Two tuffs dated at 3*9 and 4.5 myBP (Fitch and Miller,

• /

1971) enclose a I3.5 m complex of channel sandstones, which would Indicate

a sedimentation rate of 2.2 cm per 1000 years. This slow accumulation

rate, plus the much larger clasts being transported and deposited today,

would indicate a Pliocene topography of lower relief than that of today.

Lacustrine beds are present in the lower.sediments near the eastern

margin of the basin, indicating the basin was nearly completely inundated

by the lake when some of the lower sediments of the Kubi Algl Formation

were deposited. The shoreline fluctuated back and forth, but generally

70

retreated to its present position in the early Pleistocene. Alluvial fans

and deltas formed in the narrow basin. These sediments, plus the channel,

floodplain and littoral deposits, constitute the units of the Kubi Algi

Formation. Fluvial conditions have continued, with only minor interrup­

tions, in the Allia Bay area since the upper sediments in the Kubi Algi

Formation were deposited.

When the Kubi Algi Formation was deposited, a perennial stream flowed

in from the north near the present Laga Bura Hasuma. The evidence for

this is the higher content of quartz, orthoclase, microcline and accessory

minerals that are derived from plutonic and metamorphic terranes in

Ethiopia, and the presence of the fresh water oyster. Etheria in the

channel sands. The other streams that came into the basin from the east

and southeast appear to have been ephemeral, as indicated by the alluvial

fans, shallow channels with lenticular bar deposits, and the very poorly-

sorted sandstones. These ephemeral streams do not contain the fresh water

oyster, Etheria.

The sediments in the Allia Bay area represent a complex of alluvial

fans, channel, fluvial, deltaic and lacustrine environments of deposition.

These sediments are divided into four lithofacies that generally parallel

the present lakeshore and the volcanic highlands. The fades are inter-

tongued horizontally and grade into each other vertically due to the

fluctuating and generally retreating shoreline through time. The lami­

nated siltstone facies consists of thin- to lenticular-bedded siltstones

interfingered with fine-grained, ripple-laminated sandstones, thin- to

medium-bedded tuffs and a very limited amount of carbonates. This

71

ripple-laminated siltstone sequence with ostracod and pelecypod fossils

appears to have been deposited in a low energy environment of a prodelta

or shallow shelf (Allen, 1970; Harms e^al_., 1975; Vondra and Bowen, 1976).

The arenaceous bioclastic carbonate facies consists of planar-crossbedded

medium-grained fossiliferous sandstones interbedded with thin carbonates,

siltstones and tuffs which would suggest shoreface, beach and sand dune

environments of deposition (Dickinson e^ £]_•, 1973; Davis et al., 1971;

Weide, 1968; Vondra and Bowen, 1976).

The lenticular fine-grained sandstone and lenticular siltstone facies

is composed of lithic arkose channel sandstone interbedded and inter-

fingered with sandy siltstones and tuffs which would suggest deposition In

a fluvial-delta plain environment (Frazier and Osanik, 1961; Allen, 1965a;

Coleman and Gagl iano, 1965; Morgan, 19671 Berryhill a1_., 1969; Kanes,

1970; Vondra and Bowen, 1976; and others). The lenticular conglomerate,

sandstone and mudstone facies is composed of alluvial fan and channel

conglomerates, fine-grained trough (Pi) crossbedded sandstones, siltstones,

claystones and tuffs that would indicate a fluvial channel and flood plain

environment of deposition (Allen, 1963, 1965a, 1965b, 1970; Harms and

Fahnstock, 1965; Williams, 1968, 1971; Blatt et a2_., 1972; Pettijohn

e^ al_., 1972; Harms et^ , 1975; and others).

There have been several transgressions and regressions of the lake

from the time of the deposition of the oldest sediments of the Kubi Algi

Formation to the present. These fluctuations are recorded In the complex

facies change of the sediments. Butzer (1971) states that the lake has

fluctuated 20 m since 1885 and that many of the fluctuations of the Lake

72

Rudolf shoreline reflect climatic changes in the drainage basin. Walsh and

Dodson (1969), Fitch and Vondra (1976) and others believe that the main

shoreline changes have been largely due to tectonic activity from the

Miocene to the present. Evidence of this is the great vertical displace­

ment along faults at Sibilot and Kubi Algi (Figs. 8a and b), and the

lacustrine Galana Boi beds in the Koobi Fora area laying 120 m above the

present lake level.

Climate has been a major factor in the preservation of minerals and

fossils in the East Rudolf basin. Although there have been fluctuations

in the climate in the past, today the mean annual temperature is 29.5° C

and the mean annual rainfall is 35 cm (Butzer, 1971). This aridity would

greatly retard the chemical decomposition of the feldspars and heavy

minerals which results in sandstones and other rocks with a high content

of these minerals (Folk, 1968). The arid climate plus an abundance of

sodium carbonate in the East Rudolf sediments aids in the preservation of

the bones of mammals including hominids. Therefore, the lake basin

located In the rift valley, not only made an excellent place for early

man to live but because of the conditions mentioned above, left us a

good record in their many well-preserved bones.

73

SUMMARY AND CONCLUSIONS

1) The East Rudolf area has been active tectonically from the late

Cretaceous to the Holocene. The volcanic highlands have been faulted and

uplifted while the basin containing Lake Rudolf has been flexed downward.

Fitch and Miller (1976) have dated these highland volcanics from 14.1 myBP

to 3.8 myBP. Along the eastern and southern border of the basin, volcanic

flows are often interbedded with the lower sediments.

2) The catchment basin in Allia Bay averages 17 km wide and is

26 km long from south to north. A total of 270 m of Plio-Pleistocene

sediments of alluvial fan, channel, fluvial, deltaic and lacustrine

environments of deposition constitute the beds of the Kubi Algi and Koobi

Fora Formations.

3) The sediments consist of conglomerates, sandstones, siltstones,

claystones, mudstones, limestones and tuffs. The conglomerates are mostly

volcanic granule to boulder clasts in a clay matrix. Sandstones vary in

composition from arkose to litharenite in a clay matrix cemented by cal-

cite. These immature sandstones reflect the closeness of the volcanic

source area and the high degree of weathering. Mudrocks consist of quartz,

plagioclase, heavy minerals and a high content of montmori1lonite clay.

The carbonates are restricted to the Koobi Fora Formation and are classi­

fied as biolithites and arenaceous bioclastics. Tuffs are composed of

glass shards, pumice fragments, quartz, plagioclase, sanidine and rock

fragments, and the highly weathered tuffs contain a significant amount of

74

montmori11ionîte clay. Sanidine crystals from the tuffs are analyzed

radiometrically to give reliable dates to the tuffs and associated

sediments.

4) Four major lithofacies of Vondra and Bowen (1976) are recog­

nized in the Allia Bay area. These are (l) the laminated siltstone fades;

(2) the arenaceous bioclastic carbonate facies; (3) the lenticular fine­

grained sandstone and lenticular-bedded siltstone facies; and (4) the

intertongued lenticular conglomerate, sandstone and mudstone facies.

These are characterized by properties indicative of four major depositional

environments (1) prodeltic and shallow shelf lacustrine; (2) littoral

lacustrine-beach and barrier beach and associated barrier lagoons; (3)

delta plain-distributary channel, levee and interdistributary flood basin;

and (4) fluvial channel and flood plain. These facies appear in narrow

bands paralleling the lakeshore.

5) Two tuffs dated at 3 - 9 and 4.5 myBP enclose a 13.5 m complex of

channel sandstones which would indicate a sedimentation rate of 2.2 cm per

1000 years. This slow accumulation would indicate a lower Pliocene relief

than today.

6) There have been several transgressions and regressions of the

lake from the time of the deposition of the oldest Kubi Algi Formation

sediments. These fluctuations are recorded in the complex facies change

of the sediments and reflect the tectonic and climatic changes of the area.

7) Climate has been a major factor in the preservation of minerals

and fossils in the area. Mineralization of bones is very rapid due to the

arid climate and the high content of sodium carbonate in the sediments;

75

therefore, many bones of mammals including early man are fossilized In the

East Rudolf catchment basin.

76

APPENDIX

Description of Measured Sections

Kubi A1gi Formation, Allia Bay area

Jarigole exposure

Measured along the edge of a terrace one mile north of Jarigole. The

center point of the outcrop is at 03°41' N latitude and 39°14' E longitude.

Bed Description Thickness (meters)

Pliocene Kubi Algi Formation Total thickness 153-9 m

34 Tuff; light bluish gray, 5Y8/1; consists of sand-size pumice, glass shards and quartz grains; basal contact is sharp; fines upward to clay-size, ripple-laminated at top; medium-bedded; cal­careous root casts and concretions at 2 inch intervals in basal 0.5 m; moderately well-indurated and resistant 1.8

33 Siltstone; pale yellow brown, 10YR7/2; sandy, tuffaceous; basal contact gradual, transition rapid; thin-bedded; numerous calcareous root casts and concretions; sand lenses; sel enite and MnO^ dendrites on fracture surface; moderately wel1-indurated and nonresistant 14.2

32 Litharenite; pale yellow brown, 10YR7/2; fine-grained; subrounded, poorly sorted; consists of quartz and feldspar grains and basalt fragments; basal contact is sharp, (Kappa-type) laminations to small-scale (Alpha to Kappa 1 type) contorted bedding; silt-filled root casts and clay galls; bands of yellow tuff; loose and moderately resistant 3.3

31 Siltstone; pale yellow brown, 10YR7/2; basal contact sharp; thin-bedded, limonite streaks; root casts; moderately wel1-indurated and moderately resistant 1.0

77

Bed Descr ipt ion Thickness (meters)

30 Litharenite; gray orange, 10YR7/4; fine­grained, subangular and poorly sorted;

quartz and feldspar grains, selenite, basalt and ignimbrite rock fragments, basal contact gradual and transition slow, indistinct small-scale (Nu) lami­nations; 5 cm Jimonite-clay concretions, very tuffaceous; loose and nonresistant ....... 1.0

29 Siltstone; light olive gray, 5Y5/2; basal contact very sharp; sandy at base and grades to slightly sandy at top; cal­careous root casts and sand-calcareous concretions; wel1-indurated and nonre­sistant; grades to pale yellow brown siltstone . 4.1

28 Feldspathic litharenite; dark yellow orange, 10YR6/6; fine-grained, subrounded, moderately well-sorted, disrupted frame­work; basal contact is very sharp, outcrop is 100 m wide and terrace is east to west; center large-scale.(Pi) dominant N 50° W,

on north side large-scale (Alpha) dominant N 70° W, on south side large-scale (Pi) dominant N 30° E, possibly a point bar complex with 7 cycles; friable to moderately wel1-indurated and resistant; exfoliation weathering . . 9.9

27 Mudstone; yellow gray, 5Y6/2; granule-sized grains in a clay matrix, subrounded, very poorly sorted; basal contact is very sharp; calcareous root casts and lenticular-bedded; wel 1-indurated and nonresistant . 5^1

26 Litharenite; pale yellow brown, 10YR7/2; fine-grained; subrounded and moderately sorted; consists of quartz, feldspar and hornblende grains plus basalt and ignim­brite fragments; basal contact is distinct, parallel laminations that grade to small-scale (Pi to Kappa-type) ripple laminations;

calcareous root casts, tuff outlines cross-bed sets, capped by 1 cm silt layer; loose and moderately resistant .......... 6.0

78

Bed Descr ipt ion Thickness (meters)

25 Siltstone; pale yellow brown, 10YR7/2; sandy; basal contact unknown; lenticular-bedded and numerous calcareous root casts, sodium salts throughout; well-indurated and moderately resistant 4.5

24 Covered 1.5

23 Mudstone grading to claystone; yellow gray, 5Y6/2; granule-size grains with clay matrix; rounded grains; basal con­tact unknown, thin-bedded, grading to parallel laminations; limonite root casts; friable and nonresistant ........... 0.5

22 Covered; appears to be litharenite; pale

yellow brown, 10YR7/2 5.7

21 Siltstone; pale yellow brown, 10YR6/2; very sandy and argillaceous; basal con­tact is distinct; numerous calcareous root casts and concretions, lenticular-bedded and sèlenite in fractures; breaks into small irregular blocks; well-indurated and nonresistant 1.1

20 Conglomeratic litharenite grades into feldspathic litharenite; gray orange, 10YR7/4; granules are rounded^ poorly sorted, pumice pebbles have altered to clay; consists of quartz, feldspar, hornblende and mica grains and basalt fragments; basal contact is very sharp, lower one-half composed of altered pumice pebbles in a clay matrix and grades upward to calcareous matrix;

resistant, concretions at top 2.7

19 Claystone; very light gray, N-8; slightly silty at base; basal contact gradual and transition slow, breaks into 5 cm square blocks, Mn02 dendrites and sodium salts throughout; wel1-indurated and nonresis­tant . 2.5

79

Bed Descr ipt ion Thickness (meters)

18 Siltstone; pale yellow brown, 10YR7/2; sandy; basal contact distinct; lenses

of medium-grained sand; calcareous root casts and concretions; friable and nonresistant 0.9

17 Tuff; light gray, N-7; fine sand-size and subrounded quartz grains, glass shards very angular; basal contact Is very sharp, parallel laminations with lenses at small-scale (Beta type) laminations; calcareous root casts, basal 10 cm is very calcareous with calcareous concretions, upward lenses contain large-scale (Pi) crossbeds; friable resistant . 9.0

16 Claystone; very light gray, N-8; slightly silty; basal contact Is gradual and tran­sition slow; calcareous root casts, 11monite mottlings, sodium salts through­out; blocky; wel1-Indurated and moderately resistant 1.9

15 Conglomerate; light olive brown, 5Y5/6; clay matrix, granule-size grains; sub-rounded basalt and sandstone fragments,

quartz and feldspar grains; basal con­tact is gradual and transition Is slow; wel1-indurated and nonresistant; grades upward to silt matrix, pale yellow brown, 10YR6/2; lenticular, root casts, Mn02 dendrites and blocky; wel1-indurated and nonresistant 7.5

14 Siltstones; very pale yellow orange, 10YR8/2; basal contact is gradual and transition Is slow, sand lenses are fine­grained with 1imon Ite-f11 led worm burrows, Mn02 dendrites, selenite crystals and high content of sodium salts; wel1-Indurated

and nonresistant 2.6

80

Bed Descr ipt ion Thickness (meters)

13 Mostly obscured by basalt cobble cover; small exposures are claystone; yellow gray, 5Y7/2 at base and middle, gray orange, 10YR7/4 at top; increase in silt content upward; basal contact is sharp, marked by layer of limonite concretions and selenite veins, thin-bedded, very tuffaceous at base,

locally shows parallel layering, Mn02 dendrites, lenticular at the top, blocky fractures; well-indurated and nonresistant 10.0

12 Claystone; pale yellow brown, 10YR6/2; basal contact is gradual and transition

is rapid, thin-bedded, Mn02 dendrites with selenite veins in joints, high content of sodium salts, blocky frac­tures; wel1-indurated and nonresistant 1.7

11 Litharenite; pale yellow brown, 10YR7/2; fine-grained, subrounded and moderately

well-sorted; consists of quartz and feldspar grains and basalt and ignim-brite fragments in a clay matrix; basal contact is very sharp, lenses with ripple laminations; locally very argil­laceous; locally thin beds very calcare­ous with limonite concretions; friable and moderately resistant 6.0

10 Tuff; yellow gray, 5Y8/1; clay-sized grains; basal contact distinct, weathers fissible; contains numerous ostracods, Mn02 dendrites; well-indurated and resistant 0.5

9 Claystone; yellow gray, 5Y7/2; weathers to medium blue gray, 5B9/1, mottled dark yellow, 10YR6/1; basal contact is very wavy, limonite band 1 cm thick, selenite in bedding planes, siickensides; well-indurated and nonresistant 13.7

81

Bed Descr ipt ion Thickness (meters)

8 Tuff; very pale orange, 10YR8/1, silt-size grains, medium-bedded, bottom 10 cm contains concretions of limonite,

horizontal streaks of limonite and Mn02 dendrites, well-indurated and resistant 19.7

c Tuff; very pale orange, 10YR8/1; silt-sized grains, 10 cm claystone at base banded by 2 cm limonitîc layer, few limonite concretions; thin- to medium-

bedded, MnOg dendrites, well-indurated and resistant 2.5

b Tuff; very pale orange, 10YR8/1; sele­nite in vertical fractures, limonite and selenite veins along bedding planes, Mri02 dendrites; well-indurated and resistant 1.6

a Claystone; basal contact is distinct,

thin-bedded, capped by 2 cm limonite bed; numerous horizontal and vertical selenite and limonite veins, tuffaceous near top; wel 1-indurated and resistant 0.7

7 Claystone; yellow gray, 5Y7/4; tuffa­ceous; basal contact is distinct, 2 cm limonite concretion bands, selenite veins, Mn02 dendrites, s 1ickensides, calcareous concretions, sodium salts; wel 1-indurated and nonresistant • • • 1.2

6 Claystone; yellow gray, 5Y7/2; slightly silty; basal contact is distinct, very tuffaceous, parallel lamination, very small sand lenses, limonite concretions, Mn02 dendrites, sodium salts, brittle; moderately well-indurated and nonresis­

tant 2.5

5 Tuff; very pale orange, 10YR8/2; altered to clay; basal contact is gradual and transition is slow, parallel lamination; numerous ostracods, calcareous concre­tions, weathers to thin flexible layers; friable and nonresistant; grades laterally to claystone 0.5

82

Bed Descr ipt ion Thickness (meters)

4 Siltstone; dark yellow, 10YR6/6; com­posed of weathered tuff; basal contact is distinct, thin-bedded; numerous ostracods, Mn02 dendrites, sand lenses, sodium salts throughout, friable and

nonresistant 0.5

3 Claystone; light olive gray, 5Y5/2; basal contact is very sharp, limonite stain on bedding planes, Mh02 dendrites, sodium salts; friable and nonresistant 1.5

2 Conglomerate; light gray orange, 10YR7/4; granules ellipsoidal and rounded, very poorly sorted; composed of ignimbrite and basalt fragments in a clay and tuff matrix; basal contact is very sharp; friable and resistant 2.5

1 Claystone; light gray 5Y5/2; veins of selenite and limonite; basal contact is very sharp; calcite and limonite con­cretions; moderately wel1-indurated and nonresistant 2.0

The Kubi Algi Formation lies on Pliocene ignimbrite; medium olive brown, 5Y4/4; very fine-grained matrix with quartz fragments 2 to 3 mm; vugs stretched, parallel bedding.

Type exposure

This exposure was measured from 4 km south of Kubi Algi, 3°44' N

latitude, 36°26' E longitude along a terrace trending N 60° W to Laga

Bura Hasuma, 3°48' N latitude 36°18' E longitude.

Bed Description Thickness (meters)

Pliocene Kubi Algi Formation Total thickness 98.0 m

83

Bed Descr ipt ion Thickness (meters)

28 Lithic arkose; dark yellowish brown, 10YR4/2; fine-grading to very fine-grained; sub-rounded, moderately well-sorted grading to very poorly sorted; composed of detrital quartz, feldspar, rock fragments, mica; locally calcareous or ferruginous, argilla­ceous at the top; basal contact is distinct; small-scale crossbeds are present; contains slightly abraded gastropods and pelecypods in middle resistant unit; loose and generally nonresistant . 0.7

27 Siltstone vertically grading to claystone; yellowish gray, 5Y7/2; tuffaceous at base, slightly silty toward top and limonitic throughout; basal contact is sharp; limoni­

tic silt-filled root casts and limonite concretions occur throughout; interbeds of lithic arkose, grade laterally to well-

indurated limonite fine-grained lithic arkose; the unit is blocky, sodium salts, sel enite crystals occur in fractures; the unit is friable and deeply weathered 3.9

26 Lithic arkose; dark yellowish orange, 10YR6/6; composed of very fine-grained, subrounded, very poorly sorted quartz grains and rock fragments; the unit is argillaceous; basal contact is sharp; the unit Is capped by 15 cm layer of selenlte; it is parallel laminated; friable but resistant 0.4

25 Claystone; light olive gray, 5Y5/2 grades to yellowish gray, 5Y6/2; local occur­rences of hematite, selenlte occurs In joints, sodium salts throughout, basal contact is gradual, transition rapid; limonitic silt-filled root casts occur throughout, limonite concretions occur along bedding planes; Mn02 dendrites; wel1-indurated and deeply weathered 3.2

84

Bed Descr ipt ion Thickness (meters)

I k Siltstone; yellowish gray, 5Y5/7 grades to grayish orange, 10YR7/4; sandy in the middle to argillaceous and tuffaceous at the top; basal contact is very sharp; lenticular-bedded with sand lenses less than 0.5 cm thick occur throughout, fine material predominant; limonitic root casts; unit breaks into large blocks; wel1-indurated and deeply weathered 3.4

23 Feldspathic litharenite; grayish orange, 10YR7/4; consists of medium-grained, subrounded, moderately well-sorted, grains of quartz, rock fragments, ortho-clase, hornblende, mica; calcareous at

the top and limonitic throughout; basal contact is very sharp and erosional; very indistinct low-angle small-scale planar crossbeds trending S 10 E; calcareous root casts occur throughout, 2 m long pipelike concretions occur in the middle, loose to wel1-indurated and resistant.

Traced laterally to continue section. Laterally this feldspathic litharenite becomes very fine-grained; basal contact distinct; low-angle small-scale planar crossbeds, horizontal sand-filled burrows, mammalian fossils, fish and gastropods at top 3.0

22 Siltstone; light olive gray, 5Y5/2, same as unit 18 below 0.3

21 Tuff; pale yellowish brown, 10YR7/2; com­posed of fine silt-sized glass shards; basal contact is distinct; unit is thin-bedded with fine layers predominant; contains calcareous root casts, se1 enite in joints; unit is well-indurated and nonresistant 0.4

20 Siltstone; light olive gray, 5Y5/2, same

as unit 18 below 0.3

85

Bed Descr ipt ion Thickness (meters)

19 Lithic arkose; dark yellowish gray, 5Y6/2; consists of medium- to coarse-grained, subangular to subrounded, poorly sorted grains of quartz, rock fragments, clay galls, slightly calcareous; basal contact is gradual and transition is slow; (Kappa-type 1) ripple laminations are present; mammalian fossils occur throughout; friable but nonresistant 0.7

18 Siltstone; yellowish gray, 5Y7/2; very sandy; basal contact is gradual and transition is slow; lenticular-bedded; limonitic root casts are numerous, cal­

careous concretions occur throughout; very blocky; wel1-indurated and nonre­sistant 2.4

17 Conglomerate; grayish orange, 10YR7/4 grades to pale yellowish brown, 10YR6/2 and back to grayish orange, 10YR7/4; consists of granule- to pebble-sized particles, which grade to medium- to fine-grained feldspathic litharenite; grains are subrounded and very poorly sorted; composed of basalt, sandstone and siltstone fragments; parallel bed­ding with coarse material predominant, thin silt beds between coarse material; moderately wel1-indurated and resistant 1.5

16 Siltstone; pale brown, 5YR5/2; slightly sandy; basal contact is distinct; lenticular-bedded; numerous calcareous root casts occur throughout, slicken-

sided "peds" are common; Mn02 dendrites; wel1-indurated and moderately resistant 1.6

15 Tuff; grayish yellow, 5Y8/4 to light gray, N-7 to very pale orange, 10YR8/2 at top; consists of silt-sized glass shards; slightly sandy to slightly argillaceous; basal contact is very sharp, (Kappa-type 1) ripple laminations 10 m occurs laterally; center section breaks into large 15-cm to 25-cm blocks, few root casts occur; friable but resis­tant ........ . 1.5-2.8

86

Bed Descr ipt ion Thickness (meters)

14 Lithic arkose; grayish orange, 10YR7/4 interbedded with siltstones, pale brown, 5YR5/2, composed of very coarse to clay-sized particles; sands are rounded and poorly sorted consisting of quartz, rock fragments, mica, calcareous; basal contact is very sharp; contains low-angle small-scale planar crossbeds; mammalian fossils, calcareous root casts and concretions are common; probably channel, levee and proxi­mal flood basin at end of distributary system, lenticular-bedded silts, concre­tionary weathering, sand-iron oxide and sand-calcite concretions on surface; well-indurated and resistant 13-5

13 Tuff; light gray, N-7; consists of silt-sized grains and pumice granules to pebbles up to 2 cm in diameter; basal contact is gradual and transition is slow; contains low-angle small-scale planar crossbeds with one component trending west; calcareous root casts and concretions occur at base, deeply weathered 0.6

12 Claystone; very pale brown, 10YR6/2, as unit 10 below 2.6

11 Lithic arkose; grayish orange, 10YR7/4 as

unit 9 below 5.1

10 Claystone; very pale brown, 10YR6/2; slightly silty; basal contact is gradual

and transition is slow; contains 4 cm-thick tuff lense at top; calcareous root casts and concretions are common; blocky, well-indurated but nonresistant 3.0

9 Lithic arkose; grayish orange, 10YR7/4; consists of fine-grained, subrounded,

very poorly sorted grains of quartz, rock fragments, mica; unit is argillaceous; basal contact is very sharp and erosional; trough, homogenous (Pi) crossbeds, foresets

are 6 cm long; line of flow and pointing distinct - north to south; calcareous root casts and concretions occur; moderately

well-indurated and moderately resistant 3.2

87

Bed Descr ipt ion Thickness (meters)

8 Claystone; yellowish gray, 5Y7/2 grades to siltstone, pale yellow brown, 10YR6/2;

selenite occurs in joints, sodium salts throughout; basal contact is very sharp; massive to lenticular-bedded; contains calcareous root casts and concretions; moderately well-indurated and moderately resistant 7.1

7 Feldspathic litharenite; grayish orange, 10YR7/4; unit is very fine-grained, argillaceous, very poorly sorted and

consists of rock fragments, quartz, mica; basal contact is gradual and transition is slow; thin-bedded, some sand lenses show ripple lamination; wel1-indurated and resistant 1.1

6 Claystone; yellowish gray, 5Y7/2; silt-

stone, pale yellow brown, 10YR6/2; claystone, pale yellow brown, 10YR6/2; slightly sandy and tuffaceous at. top; basal contact is very sharp; siltstone is lenticular-bedded; calcareous root casts and concretions are common; tuff lenses occur at top, ripple-laminated sand lenses occur in silts; claystones are blocky; wel1-indurated but nonre-s i stant 12.6

5 Feldspathic litharenite; very pale orange, 10YR8/1; very fine-grained with lenses of basalt pebble conglomerates, grains are rounded and consist of quartz, rock fragments, feldspar, very argillaceous; basal contact is very sharp; large-scale (Omikron) grades to small-scale (Alpha) crossbeds, low-angle; calcareous root casts are common; well-indurated and resistant . 3.0

88

Bed Descr ipt ion Thickness (meters)

4 Siltstone; pale yellow brown, 10YR6/2 grades to claystone, grayish yellow, 5Y7/2; unit is argillaceous and sandy and grades to slightly silty; basal . contact is distinct; lenticular-bedded; calcareous root casts and concretions are numerous at top; blocky well-indurated and moderately resistant 4.0

3 Feldspathic litharenite; pale yellow brown, 10YR6/2; grades to very pale orange, 10YR8/2; unit is very fine­grained and argillaceous and at base grades to very coarse-grained with conglomerate lenses at top; consists of subrounded, poorly sorted grains

of quartz, rock fragments, clay, tuff, pumice, glass shards; unit is cal­careous; basal contact is very sharp and down-cutting; basal unit has (Kappa-type 1) ripple lamination; calcareous root casts and concretions,

vertebrate fossils occur throughout; loose but moderately resistant, grades laterally to tuff with sand and pumice lenses .12.0

2 Interbedded claystones, light olive gray 5Y6/1 to moderate yellowish brown 10YR4/2, and tuffs, very pale orange, 10YR8/1; 3 claystone units are 0.15, 3.2 and 0.5 m thick and 3 tuffs are 0.5, 0.1 and 1.0 m thick; unit starts with claystone and ends with tuff; claystones are tuffaceous and silt-stones are argillaceous and silt-sized; basal contact is very sharp, medium-bedded to parai lei-laminated, iron oxide concretions and selenite occur on bedding planes; ostracods occur in

upper tuff; wel1-indurated but non-

resistant .... 5.5

89

Bed Descr ipt ion Thickness (meters)

Claystone; dark yellowish orange, 10YR6/6; basal contact is gradual and transition is slow; weathered basalt; slightly calcareous concretions occur throughout; crumbly, very friable and nonresistant . . 0.05

The Kubt Algi Formation lies on Pliocene basalt; grayish black, N-2; very fine­grained, contains gas vacuoles with glass filling; consists of olivine, plagioclase, pyroxene; unit is well-jointed, highly fractured; wel1-indurated and resistant.

Koobi Fora Formation, Allia Bay area

Bura Hasuma Hill exposure

Measured from north to south through Bura Hasuma Hill 03°49' N

latitude and 36°20' E longitude. This is the basal part of the Koobi Fora

Formation.

Bed Description Thickness (meters)

PIio-P1eistocene Koobi Fora Formation Lower Member

Total thickness 117.4 m

30 Biosparrudite; gray orange, 10YR7/4; very coarse, some sand; composed of calcite and dolomite; basal contact is very sharp; contains gastropod, pelecypod and verte­brate fossils; wel1-indurated and resistant 0.3

29 Siltstone; some sand lenses at the base, very argillaceous at the top; basal contact

is very sharp, massive to thin-bedded at the top, Mn02 dendrites in upper part; calcareous root casts throughout; well-indurated and moderately resistant .... 1.5

90

Bed Descr ipt ion Thickness (meters)

28 Biosparrudite; gray orange, 10YR7/4; very coarse with moderate amount of sand; composed of cal cite and dolomite; basal contact is very sharp; contains gastropod, pelecypod and vertebrate fossils; very wel1-indurated and

resistant 0.2

27 Lithic arkose; very pale orange, 10YR8/2; weathers dark yellow orange, 10YR6/6; very coarse-grained, moder­ately rounded and moderately well-sorted; consists of quartz, feldspar,

hornblende grains and basalt fragments, cal ci te cement and limonite stains; basal contact is sharp, large-scale (Beta) crossbeds at base; well-indurated and resistant 1.5

26 Siltstone; pale yellow brown, 10YR6/2; sandy near the top; basal contact is sharp, sand lenses with ripple lami­nation and thin-bedded near the top; blocky, wel1-indurated but nonresis-tant; limonite streaks 0.8

25 Lithic arkose; very pale orange, 10YR8/2; weathers to dark yellow orange, 10YR6/2; coarse-grained, subrounded and poorly sorted; composed of quartz, feldspar, hornblende, calclte, limonite and rock fragments; basal contact sharp, large-scale (Beta-type) lamination,

grouped large-scale erosional planar crossbeds show flowage from north; well-indurated and resistant 0.5

24 Siltstone; pale yellow brown, 10YR6/2; sandy near the top; basal contact is very sharp; sand lenses, ripple-laminated and thin-bedded near the top; very small calcareous root casts and limonite streaks; wel1-indurated but nonresistant 5.2

91

Bed Descr ipt ion Thickness (meters)

23 Conglomerate; dark yellow brown, 10YR2/2; pebbles are disk shaped, rounded and

poorly sorted; consists of rock fragments with clay matrix and calcite cement; basal contact is sharp, mud cracks and load casts; gastropod fossils at the base; well-indurated and resistant 0.2

22 Siltstone; very argillaceous with some sand lenses; basal contact is gradual,

massive to thin-bedded, Mn02 dendrites near the top capped by hard ferruginous sandstone concretions; root casts; well-indurated and resistant ....... . . 1.5

21 Arkose; gray orange, 10YR7/2; fine-grained with lenses of conglomerate; grains are subrounded and moderately sorted; consists of quartz, feldspar, hornblende, mica grains and basalt and ignimbrite fragments; basal contact is distinct, (Kappa-type) ripple-laminated; silt-filled root casts and calcareous concretions, becomes argil­

laceous with ferruginous concretions at the top; loose and nonresistant 3.2

20 Siltstone; very argillaceous with sand lenses at the base; pale yellow brown, 10YR6/2 at the top and gray orange, 10YR7/4 the bottom half; basal contact is very sharp, massive to thin-bedded at the top, capped by ferruginous sand concretions; calcareous root casts, numerous gastropod and pelecypod fossils with some vertebrates; well-indurated and moderately resistant 4.0

19 Lithic arkose; gray orange, 10YR7/4; conglomeratic, fine- to medium-grained, subrounded and moderately sorted; consists of quartz, feldspar and mica

grains with basalt and ignimbrite frag­ments; basal contact is very sharp, weakly defined small-scale (Mu) cross-

stratification - cosets are 50 cm thick; very fossiliferous, mostly gastropods; load casts, calcareous, 1imonite stains ; well-indurated and resistant; channel deposit .... 23.0

92

Bed Descr ipt ion Thickness (meters)

18 Lithic arkose; yellow gray, 5Y8/1; very fine-grained, becomes argillaceous upward; grains are subangular and poorly sorted,

two-size fractions - sandstone and clay with gravel lenses; consists of quartz, feldspar and mica grains and basalt frag­

ments; basal contact is distinct, clay bands at the base, matrix is thin-bedded to weakly ripple-laminated; contains broken gastropods, limonite and selenite streaks; well-indurated and moderately

resistant 15.0

17 Tuff; yellow gray, 5Y8/1; very argilla­ceous; basal contact is gradual and transition is rapid, medium-bedded and blocky; much selenite and locally limo-nitic; wel 1-induraited but nonresistant 1.8

16 Claystone; gray orange, 10YR7/9; slightly silty, very tuffaceous; basal contact is gradual and transition is rapid, medium-bedded, brittle, Mn02 dendrites and selenite in joints; wel1-indurated but

nonresistant . 1.7

15 Tuff; yellow gray, 5Y8/1; silt-sized glass shards with some mica and quartz; basal contact very sharp, locally medium-bedded; wel1-indurated and resistant; Tulu Bor tuff 2.0

14 Claystone; dark yellow brown, 10YR4/2 interbedded with biosparrudite which is

very sandy, 10YR6/6; very fine- to medium-grained and poorly sorted; con­tains quartz and is very calcareous, limonite and selenite streaks; basal

contact is gradual and transition is slow, thin- to medium-bedded; very highly fossi1iferrous - with pelecypods, gastropods and fish - gastropods are the dominant fossil; limonite concre­tions and selenite crystals are encrusted in sandstone; friable, deeply weathered

and nonresistant; represents nearshore lacustrine deposits with changing shoreline 9.0

93

Bed Descr ipt ion Thickness (meters)

13 Claystone; dark yellow brown, 10YR4/2; slightly silty with sandstone lenses and thin beds of sandstones, pale yellow orange, 10YR6/6; fine- to medium-grained; limonite pods and streaks on bedding planes, selenite crystals and sodium salts on the surface; the clay is highly tuffaceous; basal contact is distinct, sandstone lenses and beds are about 1 m apart vertically; wel1-indurated but nonresistant, claystone weathers to a popcorn surface and the sandstone to flagstones 23-5

12 Arkose; gray orange, 10YR7/4; medium-grained and subangular; consists of quartz, feldspar and basalt fragments, limonite concretions, basal contact is distinct; friable and nonresistant 0.2

11 Claystone; light olive gray, 5Y5/2; very tuffaceous near the base, light gray, N-7; very fine-grained; fissile; contains quartz, limonite streaks and selenite in fractures; basal contact is gradual and transition is slow, thin- to medium-bedded; fish fossils in concretion layer; wel1-indurated and moderately resistant, MnOg dendrites; lacustrine deposit 8.0

10 Tuff; very pale orange, 10YR8/2; very fine-grained bentonitic clay; basal con­tact distinct, low density and blocky; wel1-indurated and moderately resistant, weathers to gray clay 1.0

9 Claystone; light olive gray, 5Y5/2; tuffaceous, very fine-grained; contains quartz, limonite streaks in fractures and selenite crystals; basal contact is

gradual and transition is slow, fine-to medium-bedded and parts highly fissile; wel1-indurated and moderately resistant,

lacustrine deposit . 4.5

94

Bed Descr ipt ion Thickness (meters)

8 Siltstone; yellow gray, 5Y7/2; highly argillaceous with sand lenses, fine- to medium-grained, weathers to gray popcorn surface; contains quartz, limonite streaks

and selenite crystals; basal contact is gradual; contains root casts and numerous worm burrows; moderately well-indurated and nonresistant 4.0

7 Feldspathic litharenite; orange 10YR10/6; medium-grained, subangular and very poorly sorted; contains quartz, feldspar, horn­blende, grains and basalt fragments, highly argillaceous; root casts are common, and numerous pelecypods and ostracods; moder­ately well-indurated and resistant 0.8

6 Tuff; same as 2 below 0.2

5 Claystone; same as 1 below 0.5

4 Tuff; very pale orange, 10YR8/1; very

argillaceous with sand lenses, fine­grained; contains quartz, glass shards and basalt fragments; basal contact very sharp, parallel-laminated; pelecypod and ostracod fossils; well-indurated and moderately resistant 0.4

3 Claystone; light olive gray, 5Y5/2; tuffaceous, medium-grained; has limonite streaks and Mn02 dendrites; ostracods numerous; basal contact is distinct,

layer of limonite concretions; moderately well-indurated and moderately resistant 1.3

2 Tuff; very pale orange, 10YR8/1; contains clay, silt-size grains, limonite and Mn02 stains, clay streaks; ostracods present; basal contact is distinct, parallel-

laminated; moderately well-indurated and

resistant 0.7

95

Description thickness (meters)

Claystone; light olive brown, 5Y5/6; medium-grained with quartz and selenite crystals, limonite and MnOg on the surface; contains sandstone bed 0.1 m thick, dark yellow orange, 10YR6/6; basal contact distinct, pelecypod fos­sils; moderately well-indurated but nonresistant 0.9

96

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ACKNOWLEDGMENTS

This research was supported by National Science Foundation Grants

GA-2568A and GS-37813 to Dr. Carl F. Vondra, Department of Earth

Science, Iowa State University. The project was also aided by The

National Geographic Society, the National Museum of Kenya and the

Kenyan Government. The writer especially wishes to thank Dr. Vondra

for his supervision throughout all phases of the project and to

acknowledge Drs. Keith Hussey and John Lemish of the Department of

Earth Science, and Drs. Harold Dilts and Ray Bryan of the Department

of Education for their guidance and assistance.

Special thanks is given to Bruce Bowen for his assistance in

the field and to Richard Leakey for his support and hospitality.

Appreciation is extended to Russ Bainbridge, Hal Frank, Howard White,

Dan Burggraf and Mark Mathisen of the Iowa State University research

team for their assistance in the lab analyses. Sincere thanks to my

wife, Mary, for her understanding and for typing the manuscript.