Sedirnentology (1981) 28, 573-597

Carbonate ramp-to-deeper shale shelf transitions of an Upper Cambrian intrashelf basin, Nolichucky Formation, Southwest Virginia Appalachians

J. R. M A R K E L L O * and J. F. R E A D ? * Chevron U S . A., Inc. and t Department of Geological Sciences, Virginia Polytechnic Institiitc

and State University, Blacksburg, Virginia 24601, U.S.A.

ABSTRACT

The Nolichucky Formation (0-300 m thick) formed on the Cambrian pericratonic shelf i n a shallow intrashelf basin bordered along strike and toward the regional shelf edge by shallow water carbonates and by nearshore clastics toward the craton. Lateral facies changes from shallow basinal rocks to peritidal carbonates suggest that the intrashelf basin was bordered by a gently sloping carbonate ramp.

Peritidal facies of the regional shelf are cyclic, upward-shallowing stromatolitic carbonates. These grade toward the intrashelf basin into shallow ramp, cross-bedded, ooid and oncolitic, intraclast grain- stones that pass downslope in to deeper ramp, subwave base, ribbon carbonates and thin limestone conglomerate. Ribbon limestones are layers and lenses of trilobite packstone, parallel and wave-ripple- laminated, quartzose calcisiltite, and lime mudstone arranged in storm-generated, fining upward sequences (1-5 cm thick) that may be burrowed. Shallow basin facies are storm generated, upward coarsening and upward fining sequences of green, calcareous shale with open marine biota; parallel to hummocky laminated calcareous siltstone; and intraformational flat pebble conglomerate. There are also rare debris-flow paraconglomerate (10-60 cm thick) and shaly packstone/wackestone with trace fossils, glauconite horizons and erosional surfaces/hardgrounds. A 15-m thick tongue of cyclic car- bonates within the shale package contains subtidal digitate algal bioherins which devcloped during a period of shoaling in the basin.

Understanding the Nolichucky facies within a ramp to intrashelf basin model provides a framework for understanding similar facies which are widely distributed in the Lower Palaeozoic elsewhere. The study demonstrates the widespread effects of storm processes on pericratonic shelf sedimentation. Finally, recognition of shallow basins located on pericratonic shelves is important because such basins influence the distribution of facies and reservoir rocks, whose trends may be unrelated to rcgional shelf-edge trends.

INTRODUCTION

The major aim of this paper is to describe a car- bonate ramp to shallow basin transition associated with a n intrashelf depression on a pericratonic plat- form. Particular emphasis is placed on the description of the facies and possible processes (especially storin- related types) involved in their formation.

The Late Cambrian Nolichucky Formation and time-equivalent rocks in the Appalachian Valley and Ridge Province of Virginia developed on the regional carbonate shelf o r Appalachian miogeocline (Palmer, 1971a; Williams, 1978) within and peripheral to a shallow basin upon the shelf (Fig. I). The regional

ic) 1981 International Association of Sedinientologists 0037-0746/8 1 /OSOO-0573 $02.00

carbonate shelf passed south-east into deep water pelitic sediments of the Piedmont (Fig. 1) (Rodgers, 1968; Reinhardt, 1974; Keith s( Friedinan, 1977). The regional shelf has been compared to an Atlantic- type continental shelf (Bird & Dewey, 1970), peri- pheral to a marginal or back-arc basin. This inarginal basin may have been ensialic and located behind an Andean-type magmatic arc (Glover ct a/., 1978).

The intrashelf basin was the site of fine clastic and carbonate deposition and was bordered to the north- east (along regional depositional strike) and to the south-east (toward the regional shelf edge) by shallow water carbonates, and to the north-west by nearshore clastics of the craton (Fig. I ) . This intrashelf basin

5 74 J. R. Markello arid J . F. Read

NEARSHORE C L A S T I C S OF

P R E C A M B R I A N / CAMBRIAN ARC

P E L I T I C FACIES OF P I E D M O N T

NOLICHUCKY I N T R A S H E L F

B A S I N 200 KMS 1

Fig. 1. Late Cambrian regional palaeogeography of the southern Appalachian miogcocline showing Nolichucky intrashclf basin as a depression upon the regional car- bonate shelf. Inset maps locate study area.

appears to be located over a persistent Cambro- Ordovician depocentre (characterized by thickening of the Cambro-Ordovician carbonates; Colton, 1970). This depocentre later evolved into a deep foreland basin in the Middle Ordovician (Read, 1980). The Nolichucky intraself basin appears to have many similarities to the inshore basins described from the Cambrian of Westcrn Canada (Aitken, 1978), the Grcat Basin of the western United States (Palmer, 1971b) Lohmann, 1976), the Mesozoic of eastern Canada (Eliuk, 1978) and intrashelf basins and de- pressions of Holocene continental shelves (Ginsburg & James, 19741, niany of which are sites of fine clastic deposition and are bounded to seaward by shallow water carbonates.

The transition from per-itidal shelf carbonates into Nolichucky intrashelf basin facies appears to have many of the characteristics of a carbonate ramp (after Ahr, 1973). That is, slopcs were extremely low, which inhibited devclopnicnt of sediment gravity flows, but apparently favoured storm-generated sequences over wide areas. Secondly, grainstones developed in up-dip positions peripheral to peritidal facies, and linear build-up trends are absent. Facies comprising the

transition include oolitic and oncolitic grainstones (seaward of tidal flats), ribbon carbonates of the deep ramp, and shale, siltstone and limestone con- glomerates of the shallow basin.

Ribbon carbonates are widespread in Lower Palaeozoic sequences (Reinhardt & Hardie, 1976; Cook & Taylor, 1977). Some are developed within cyclic, upward shallowing units and are interpreted as shallow subtidal t o intertidal facies on the basis of sedimentary structures and position in cycles (Reinhardt & Hardie, 1976). However, others that occur in non-cyclic units seaward of peritidal facies, and grade downslope into deep water facies, may be deep ramp/slope facies (Cook & Taylor, 1977). Evidence is presented indicating that Nolichucky ribbon rocks have characteristics of the deeper water types.

Deeper water shale and limestone conglomerate sequences adjacent to Cambro-Ordovician shelves are described from New York-Vermont (Keith & Friedman, 1977), and Quebec (Hubert, Lajoie & Leonard, 1970; Hubert, Suchecki & Callahan, 1977; St Julien & Hubert, 1975), and are off-shelf slope and basin deposits that contain sediment gravity flow units. However, thin conglomerate, siltstone and shale sequences in the Nolichucky Formation largely appear to be storm-generated sequences (cf. Sepkoski, 1978) that formed in relatively shallow, subwave base settings on a gently sloping ramp and on the shallow basin plain of the intrashelf basin. These and other features in Nolichucky sediments are common in other Canibro-Ordovician rocks including those from Alberta (Aitken, 1966, 1967, 1978), Montana (Sep- koski, 1975, 1978), Nevada-Utah (Lohmann, 1976), and Texas (Ahr, 1971). Although these fdcies may on superficial examination appear to be similar to tidal-flat deposits, they lack features typical of emergence and show abundant evidence of deposition below fair weather wave base.

The sequence illustrates the strong influence that the intrashelf basin had on facies distribution on the regional shelf and the effects of storm processes on sedimentation. It also illustrates how the intrashelf basin may control distribution of potential reservoir facies, resulting in facies distributions that locally are unrelated to regional shelf trends.

S T R A T I G R A P H I C S E T T I N G

Upper Cambrian rocks in Virginia are exposed in imbricate thrust sheets that moved from south-east

to north-west. Regional biostratigrapliic irelatioris of the Cambrian units were establishcd by Kesscr ( 1938), Lochman-Balk & Wilson (1958) and Derby (1965) who defined the stratigraphic position of several Dresbachiari trilobite zones and used thtse to cor- relate the Cambrian units.

The Nolichucky Forination (Late Cambrian, Dresbachian) in Virginia, is a shale and limestone unit (0-285 m thick) that interfingers with Elbrook- Honaker carbonates to the north-east (along strike) and to the south-east (toward the regional shelf edge) (Fig. 2). These units rest on the Cambrian Rome Formation and are overlain by Late Cambrian

Copper l i idge~~onococl icague Formations, 330- 520 m thick (Rodgcrs, 1953; Harris, 1964) (Fig. 2).

The Honaker Dolomite (300-350 ni thick) con- tains stromatolitic dolomites and a massive upper unit of oolitic dolomite 20-30 in thick (Fig. 2). The Elbrook Formation is up to 520 ni thick, and consists of cyclic stromatolitic limestone and dolomite units overlain by oolitic dolomite (30-40 m thick), and a n upper member, the Widener Limestone (46 m thick) composed of ribbon rock and ooid and intraclast limestones (Fig. 2).

The Upper Cambrian (Franconian and Trempe- aleauan) Conococheague and Copper Ridge Forma-

A' ACROSS STRIKE SECTION A NW

B' ALONG STRIKE SECTION B

r T

I

,, ~ I TIDAL FLAT CYCLIC CARBONATES B' a OOlD ONCOLITE INTRACLAST GRAINSTONE x x / /

wy 0 RIBBON ROCK LIMESTONE f SHALE SILTSTONE CONGLOMERATE ? ?

OOlD PACKSTONE

/ol ALGAL BIOHERMS - A' 100 KM -

PALINSPASTIC BASE

GLAUCONITE

HARDGROUNDS

SEOIMENTARY DIKES

TRILOBITE ZONAL BOUNDARIES

Fig. 2. Nolichucky Formation stratigraphy, regional cross-sections. A palinspastic base map modified from Dennison RC Woodward (1963) and Dennison (1970) was used to construct stratigraphic cross-sections, and palaeogeographic maps. Inset map of south-west Virginia (palinspastic base) shows locations of cross-sections. AA' is normal to the regional shelf trend, BB' is parallel to the shelf trend. Vertical arrows represent measured sections. Dotted lines on cross- sections are trilobite zonal boundaries (Derby, 1965); from base to top, zones are Bolaspidellu, Ceduriu, Crepicephalus and Aphelurpis.

576 J. R. Markello and J. F. Read

STAGE 1 NOLICHUCKV LOWfR SHALE LOWER L S I MEMBERS STAGE 2 NOLICHUCKV MIDDLE L S I MfMBER STAGf 3 NOLICHUCKV UPPER SHALL MEMBER IMIOOLE CEOARlAl [EARLY CREPICEPHALUSI [LATf CllfPlCEPHALUSl

Fig. 3. Geographic distribution of Upper Cambrian lithofacies in Virginia during three phases of Nolichucky deposition.

to the south-east (Fig. 2) and consists of cyclic, flat- L L O W S U B T I D A L pebble conglomerate, oolitic and stromatolitic lime-

The Copper Ridge Formation (up to 330 m thick) is the north-western equivalent of the Conococheague Formation (Fig. 2 ) and consists of stromatolitic and

stone and dolomite, and minor quartz sandstone. FAIR W E A T H E R WAVE B A S

\ - ,

oolitic dolomite with minor quartz sandstone.

(Fig. 2):

Fig. 4. Schematic profile of peritidal platform-carbonate ramp to intrashelf shale basin transition. Characteristics of lithofacies shown in Table 1. The Nolichucky Formation contains five members

TOP Limestone Member, and the Maynardville Lime- Maynardviffe Limestone4 > 50 m thick; lime- stone-dolomite ribbon rock, and minor oolitic, intraclast and stromatolitic carbonates. Upper Shale Member--0-56 m thick, shale, silt- stone, interbedded skeletal and pellet limestone and limestone conglomerate. Middle Limestone Member4-20 m thick, cyclic shaly limestone and algal bioherms. Lower Shale M e m b e r 4 1 2 5 m thick; litho- logically similar to Upper Shale Member. Lower Limestone Member (or Maryville Lime- stone, usually mapped separately from the Nolichucky Formation)4-77 m thick; lime- stone-dolomite ribbon rock, ooid, intraclast and pellet limestones that pass up into shaly pellet, skeletal and conglomerate limestone.

Base The Nolichucky and adjacent formations (Fig. 2) contain four lithofacies suites which make up the ramp-to-basin transition. These are: ( 1 ) cyclic car- bonates (El brook-Honaker, Conococheague-Copper Ridge Formations and Nolichucky Middle Lime- stone); (2) ooid and oncolitic carbonates of the shallow ramp (Upper Honaker-Elbrook, and part of the Maryville and Nolichucky Lower and Middle Limestones); (3) ribbon carbonates of the deeper ramp (Maryville Limestone, Nolichucky Lower Limestone and Lower Shale, the Elbrook Widener

stone) and; (4) shale facies of the intrashelf basin (Nolichucky Lower and Upper Shale). The geo- graphic distribution of facies during several phases of NoIichucky deposition are illustrated in Fig. 3. Inferred lateral facies relations are shown in Fig. 4 and lithofacies are summarized in Table 1.

CYCLIC CARBONATE FACIES

These include the cyclic stromatolitic facies of the peritidal carbonate platform and the cyclic algal bioherm facies of the intrashelf basin (Fig. 2). Because the focus of this paper is on the facies com- prising the ramp-to-intrashelf basin transition, these cyclic carbonates are not described in detail.

Cyclic stromatolitic facies of the peritidal carbonate platform

Cyclic platform carbonates (Table I ) of the Elbrook- Honaker and Copper Ridge-Conococheague Forma- tions occur adjacent to Maryville and Maynardville oolitic and ribbon carbonates (Fig. 2). The Elbrook- Honaker beds are partly time equivalent to Noli- chucky strata (Figs 2 and 3). Copper Ridge-Cono- cocheague rocks that overlie Nolichucky beds have grossly similar cycles to those of the Elbrook- Honaker Formations, with occasional quartz-rich units at tops of cycles. The cycles are generally 1-5 m

Tab

le 1

. Su

mm

ary

of li

thof

acie

s

Shoa

l wat

er p

hase

of

Intr

ashe

lf b

asin

Sh

allo

w-r

amp

sand

Pe

ritid

al c

arbo

nate

Cyc

lic s

trom

atol

itic

E

nvir

onm

ent

intr

ashe

lf b

asin

D

eep

ram

p sh

oals

pl

atfo

rm

Ooi

d an

d sk

elet

al

Thi

n oo

id/o

ncol

ite

Thi

ck u

nits

of

faci

es

Alg

al b

iohe

rm c

ycle

s Sh

ale

faci

es

1st.

of s

hale

faci

es

Rib

bon

carb

onat

e fa

cies

gr

ains

tone

oo

id g

rain

ston

e fa

cies

Occ

urre

nce

Lith

olog

ies

Geo

met

ry a

nd

thic

knes

s

Bed

ding

and

st

ruct

ures

Bio

ta

Dia

gene

sis

~

Mid

de 1

st. m

embe

r N

olic

huck

y, lo

wer

L

ower

and

upp

er

Mar

yvill

e, N

olic

huck

y R

ibbo

n ro

cks

of

Elb

rook

-Hon

aker

E

lbro

ok-H

onak

er,

Con

o-

and

uppe

r sh

ale

shal

e m

embe

rs a

nd

low

er Is

t., m

iddl

e 1s

t. an

d M

ayna

rdvi

lle, M

ary-

co

chea

gue

and

Cop

per

mem

bers

m

iddl

e 1s

t. m

embe

r M

ayna

rdvi

lle 1

st vi

lle, N

olic

hurc

hy

Rid

ge f

ms

low

er 1

st

Cyc

lic u

nits

(1-7

m t

hick

) Sh

ale,

cal

care

ous

Ooi

d pk

st. a

nd

Rib

bon

carb

onat

es: c

on-

Thi

n oo

id g

rst.

and

Thi

ck o

oid

grst

., 1-

5 m

cyc

lic u

pwar

d-

of:

(top

) (5)

ooi

d pk

st,

quar

tz si

ltsto

ne, 1

st.

skel

etal

grs

t/pk

st.,

tain

ske

leta

l 1st

: fin

ing-

on

coli

te in

trac

last

do

lom

itize

d. R

are

shal

low

ing

sequ

ence

s of

: in

terb

eds

of l

amin

ite

(top

) thi

ck la

min

ites

and

(4)

shal

y ri

bbon

car

bon-

co

nglo

mer

ate

and

coar

se-g

rain

ed

upw

ard

stor

m-g

ener

ated

gr

st/w

kst

and

ribb

on c

arbo

nate

cry

ptal

gal l

amin

ites;

Ilh

ates

/con

glom

erat

e, (

3)

skel

etal

1st.

. arr

ange

d gl

auco

nitic

la

yers

(few

cen

timet

res

digi

tate

alg

al b

iohe

rms,

in

upw

ard-

coar

seni

ng

thic

k) c

ompo

sed

of s

kele

tal

stro

mat

olite

s, c

olum

nar

(2)

shal

y ri

bbon

car

bon-

an

d up

war

d-fi

ning

Is

t., c

alci

silti

te,

and

argi

lla-

stro

mat

olite

s an

d th

roni

- at

es/c

ongl

omer

ate,

(1)

se

quen

ces

(up

to 4

m

ceou

s lim

e m

udst

one

caps

; bo

lites

; rib

bon

carb

onat

es,

basa

l sha

le

thic

k)

and

burr

ow-m

ixed

laye

rs

intr

acla

st l

ags

and

cal-

ca

reni

tes.

Dol

amit

i/at

ion

of c

ycle

s par

tial (

uppe

r pa

rts)

to c

ompl

ete

Reg

iona

lly e

xten

sive

T

ens t

o ov

er 1

50 k

m

Shee

ts, 0

.3-8

m t

hick

, Sh

eets

up

to 4

0 m

thic

k,

Thi

n sh

eets

, 1-1

0 m

E

lor.

gate

she

ets

10 to

E

xten

sive

she

ets.

few

cy

cles

; mai

nly

shee

t-lik

e w

ide,

0-1

50 m

thic

k,

som

e sm

all l

ense

s 10

to 1

00 k

m w

ide,

per

i-

thic

k an

d le

nses

ov

er 5

0 km

wid

e, 1

0 hu

ndre

d m

etre

s th

ick,

tens

un

its;

bio

herm

s ra

nge

mos

t sed

imen

t typ

es

and

thin

she

ets,

10-

ph

eral

to

emba

ymen

t se

vera

l met

res

wid

e to

50

m th

ick,

per

i-

to fe

w h

undr

eds

of k

ilo-

from

dis

cret

e co

lum

ns to

ar

e in

she

ets;

som

e co

ales

cent

str

uctu

res;

co

nglo

mer

ates

in le

nses

co

nglo

mer

ates

. F

orm

thin

an

d so

me

skel

etal

1st

. sh

eets

and

lens

es

in m

egar

ippl

e an

d ri

pple

lens

es

Shal

es fi

ssile

. Nod

ular

Sh

ales

fiss

ile, l

ocal

T

abul

ar to

ripp

le

Nod

ular

to th

in-b

edde

d.

Mas

sive

to fa

intly

C

ross

-bed

ded,

som

e Sh

allo

w m

udcr

acks

. sc

ours

. th

in la

yeri

ng. S

mal

l-sc

ale

sedi

men

tary

dik

es.

cros

s-be

dded

. Meg

a-

Man

y fi

ning

-upw

ard

laye

rs l

ayer

ed. S

ome s

cour

ed g

rave

l lag

s st

arve

d ri

pple

s and

silt

-mud

pa

ralle

l, hu

mm

ocky

and

Si

ltsto

nes h

ave

par-

ri

pple

d to

ps (

a 5-

15

(few

cen

timet

res

thic

k).

basa

l con

tact

s co

uple

ts in

thic

k la

min

ites.

w

ave-

ripp

le l

amin

atio

n in

al

lel

hum

moc

ky a

nd

cm;

up to

2 m

). So

me

high

ly b

urro

wed

C

rypt

alga

lam

inite

s ha

ve

ribb

on 1

st. B

iohe

rms

have

w

ave-

ripp

le l

amin

a-

Bas

es c

omm

only

la

yers

. So

me

cros

s-be

ddin

g pl

anar

to

crin

kly

lam

ina-

de

licat

e di

gita

te, f

aint

ly

tion

(1-2

cm

am

pli-

sc

oure

d an

d ra

re g

radi

ng in

ske

leta

l tio

n, d

eep

pris

m-c

rack

s,

lam

inat

ed s

h-fi

nger

s. O

oid

tude

, up

to 3

0 cm

1s

t in

trac

last

ic la

yers

. pal

isad

e pk

sts.

hav

e ta

bula

r cro

ss-

wav

elen

gth)

. E

rosi

on

stru

ctur

e. T

hrom

bolit

es

lam

inat

ion

surf

aces

com

mon

. ha

ve f

aint

sh-

lam

inat

ion:

pher

al t

o em

baym

ent

met

res

wid

e 30

cm

thic

k

Som

e pa

rtin

g ca

sts.

Com

mon

trac

e fo

ssils

(Cru

rian

a fac

ies)

. C

ongl

omer

ates

are

no

n-gr

aded

to

poor

ly

grad

ed; m

ainl

y cl

ait-

su

ppor

t. M

egar

ippl

es

and

ripp

les on

ske

leta

l 1s

t

digi

tate

str

uctu

res

have

sh

bran

chin

g fi

nger

s. B

urro

ws

pres

ent

in r

ibbo

n ro

cks.

C

ycle

s ha

ve e

rosi

onal

bas

es

Ope

n m

arin

e: tr

ilobi

tes,

O

pen

mar

ine:

trilo

- O

pen

mar

ine:

ech

ino-

O

pen

mar

ine:

trilo

bite

s,

Mai

nly

tran

spor

ted

Mai

nly

tran

spor

ted

Res

tric

ted:

blu

e gr

een

alga

l ec

hino

derm

s, s

pong

es,

bite

s, e

chin

oder

ms.

de

rms,

tril

obit

es,

echi

node

rms,

spo

nges

, ec

hino

derm

and

trilo

- sk

elet

al d

ebri

s m

ats.

Som

e bu

rrow

ers

calc

areo

us a

lgae

sp

onge

s, p

hosp

hati

c sp

onge

s ra

re R

enal

cis.

Com

mon

bi

te d

ebri

s. G

irva

nclla

(R

enal

ris,

Gir

vane

lla)

brac

hiop

ods

burr

ower

s en

crus

ts R

enal

cis

Har

dgro

unds

, gla

ucon

ite

Gla

ucon

ite

and

hard

- V

aria

ble

late

dol

omi-

G

lauc

onit

e an

d ha

rd-

Var

iabl

e dol

omiti

za-

Wid

espr

ead

late

(?

) E

arly

dol

omiti

zatio

n Of

m

inor

dol

omit

izat

ion

grou

nd fo

rmat

ion.

ti

mio

n. A

bund

ant

grou

nd fo

rmat

ion.

Abu

n-

tion

. Har

dgro

unds

do

lom

itiza

tion

uppe

r pa

rts

of c

ycle

s. L

ate

dolo

miti

zatio

n of

bas

al

Muc

h co

mpa

ctio

n in

ha

rdgr

ound

and

da

nt co

mpa

ctio

n, p

ress

ure-

co

mm

on. M

icri

tiza-

sh

ales

gl

auco

nite

form

atio

n so

luti

on a

nd a

ssoc

iate

d ti

on o

f gra

ins c

omm

on

part

s of

cyc

les

dolo

miti

zatio

n

578 J. R. Markello and J. F. Read

thick and commonly contain, from top to base: (4) thick laminated and cryptalgal laminated dolomite; (3) cryptalgal thrombolite and digitate stromatolite heads, and LLH structures; (2) ribbon carbonates (wave-rippled pellet silts with mud drapes); and (1) ooid and pellet/intraclast grainstone/intraclast con- glomerate.

Elbrook-Honaker carbonates are cyclic, shallow- ing-upward sequences that developed on the regional carbonate shelf north-east (along strike) and south- east (towards the regional shelf edge) of the Noli- chucky ramp and intrashelf basin (Figs 1 and 3). They are similar to Recent and other ancient tidal flat sequences described by Mazzullo & Friedman (1975), Reinhardt & Hardie (1976) and James (1977) and result from repeated rapid submergence and sub- sequent progradation of shallow subtidal and tidal facies.

Cyclic algal bioherrn facies of the intrashelf basin

Cyclic carbonates that are markedly different from the above make up the Middle Limestone tongue of the Nolichucky Formation (Fig. 2). These cycles are 1-7 m thick and consist of, from top to base: (5) glauconitic ooid packstone; (4) shaly, ripple-lamin- ated pellet limestone and flat pebble conglomerate; (3) digitate algal bioherms (Renalcis, Girvanella) up to 1.5 m high and 10 m wide-these rest on flat pebble conglomerate or skeletal intraclast packstone, which also forms thin sheets between heads; (2) shaly pellet limestone and conglomerate similar to (4); and ( 1 ) basal shale-overlies ooid packstone of previous cycle.

These cyclic Middle Limestone beds are a shallow- water phase of the intrashelf basin (Figs 2 and 3). They show an upward transition from quiet water shale to rippled pellet silts and high-energy con- glomerate channel and storm lags (cf. Reinhardt & Hardie, 1976; James, 1977; Friedman & Sanders, 1978). The bioherms are high-energy deposits related to shallowing, although the biotas of the mounds still indicate normal marine salinities and open, subtidal settings. Widespread development of the mounds created low-energy settings which favoured subsequent deposition of rippled silts and thin storm- related conglomerates. Capping ooid packstones may reflect shallowing to tide level or they may indicate initial deepening (and more frequent agitation) prior to deeper submergence and shale deposition of the next cycle (cf. Lohmann, 1976).

O O I D AND ONCOLITIC CARBONATES OF THE SHALLOW RAMP

Ooid and oncolitic carbonates comprise the upper Honaker Dolomite, the Upper Elbrook Formation just beneath the Widener Limestone Member, and part of the Maryville Limestone, Nolichucky Lower and Middle Limestone Members and Widener Lime- stone (Fig. 2) . They overlie Elbrook-Honaker cyclic stromatolite sequences and interfinger with and underlie ribbon rocks (Fig. 2).

Thick ooid grainstone

Thick (2040 ni) oolitic dolomites (Table 1) occur in the Upper Elbrook and Honaker Formations peri- pheral to the intrashelf shale basin; they overlie Elbrook-Honaker cyclic stromatolitic carbonates and underlie (or grade seaward into) ribbon rocks (Figs 2 and 3). They have rare thin interbeds of flat pebble conglomerate, cryptalgalaminite or thick laminite. The oolitic dolomites are dark grey, massive, fine to coarse, crystalline dolomite, in which faint large- scale cross-stratification is evident on weathered out- crops. The oolitic dolomites have relict grainstone texture in which the ooids (0.3-0.6 mm diameter) consist of fine to medium crystalline dolomite in medium-grained dolomite mosaic, or have faint circular outlines in medium crystalline dolomite.

Interpretation

Thick ooid grainstones (and oncolite intraclast grain- stones) are interpreted as transgressive lime sand sheets, bars or beach ridges that form on the shallow ramp between tidal-flat, cyclic carbonates and sub- tidal fine-grained ribbon limestone and shales (des- cribed later). Similar up-dip lime sands have been described from the Holocene of the Persian Gulf (Loreau & Purser, 1973), the Yucatan Shelf (Logan eta/. , 1969), and Shark Bay (Hagan & Logan, 1974), and the Jurassic ramp of Texas and Alabama (Ahr, 1973). The 20-40 m thick Elbrook-Honaker dolo- mitized ooid grainstone resembles facies of modern ooid shoals (to 5 n i depth) which contain rippled and megarippled sediments, are commonly unburrowed, and composed of small ooids (less than 0.5 mm), minor skeletal debris, pellets and fibrous cements (Ball, 1967: Loreau &Purser, 1973; Hine, 1977). The thick ooid units formed where the ooid lithotope occupied the same geographic position for a long period of time (i.e. rate of sedimentation equalled

Shale shelf transitioris iri the Appalachiaris 579

subsidence/sea-level rise). This stillstand appears t o mark the transition from transgressive t o regressive conditions (Fig. 2). Thick ooid sands did not develop platformward of the Middle Limestone cyclic car- bonates, o r in the offlap (Maynardville-Copper Ridge) sequence (Fig. 2) because of low-energy conditions that reflect the very low gradients on the ramp at these times.

Thin ooid grainstone and oncolite intraclast limestone

These are 1-10 ni thick, massive to faintly layered and are interbedded with ribbon rocks and skeletal limestone of the Maryville, Nolichucky Lower Lime- stone and Widener Limestone (Figs 2 and 5, Table I ) . Hardgrounds are common.

MARYVILLE LIMESTONE

The ooid grainstones (Fig. 6A) are composed mainly of ooids (0.3-0.6 mm diameter), intraclasts (up t o 1 cni rounded clasts of ooid grainstone, algal boundstone and lime mudstone), oncolites, and small amounts of abraded pelmatozoan debris. Some re- worked ooid and skeletal grains have adhering mud/ cement and most grains have micritic rims. Inter- granular cements are fine, equant and bladed calcite and sediments are commonly partly dolomitized.

Oncolite intraclast limestones are 1-10 m thick sheets and rare thin (5-30cm) layers and lenses (several metres across with scoured bases). They have alternating fine- and coarse-grained laminae (Fig. 6B), and rare tabular cross-stratification. Sediments are mainly grainstones (partially dolomitized) com- posed of granule to pebble size, spherical to discoidal

LOWER LIMESTONE MEMBER

THRUST ' 1 2 3.

WIDENER MAVNAROVILLE LIMESTONE LIMESTONE

5 ' 9. , CRYPTALGALAMINITE - OIGITATE/THROMBOLITE

0010 GRAINSTONE

0NCOLITE;INTRACLAST ~~

CONGLOMERATE

RIBBON ROCK

SHALE

STROMATOLITE

GRAINSTONE

NW SE

COVERED INTERVAL

Fig. 5. Columnar sections: ribbon rock and grainstone sequences. Sections I , 2, Maryville Limestone; Section 3, Nolichucky Lower Limestone Member; Section 4, Widener Limestone; Section 5 , Maynardville Limcslonc. Sections are located on stratigraphic cross-section (insel).

37 $ I I ) 2s

580 J. R. Markello and J. F. Read

Fig. 6. Ooid and oncolite carbonate rocks of the shallow ramp. (A) Photomicrograph of ooid grainstone, Maryville Limestone. (B) Photomicrograph of cross-laminated oncolite intraclast grainstone. Note oncolites (arrows) with dark Renulris in cores and lighter outer coats of Girvnnelh. (C) Photomicrograph of oncolite packstone/wackestone. Note erosion surfaces (arrows) and doloniitiLed oncolite (D).

oncolites and intraclasts (composed of admixtures of Some asymmetric oncolites are in place, with thicker ooids, skeletal grains, pellets and intraclasts), platy coats on upper sides of oncolites, but most have been clasts of Gir~*aml lu and Rc/iulcis limestone, and small rotated. amounts of ooids, pellets, and abraded and reworked Oncolitic packstones and wackestones (Fig. 6 C ) echinoderm and trilobite fragments (Fig. 6B). Cores occur as rare thin layers and lenses (1-5 cni thick) in of oncolites contain branching Rr/io/ci.\ clusters ribbon rocks. Packstones commonly have scoured (locally encrusted on shell fragments) and have and erhsional bases with basal, coarse, oncolitic and asyninietric and less concentric coats of Girrtnrrlltr. skeleta'l lags that grade up into wackestone. They are

Shale shelftransitions in the Appalachiaiis 58 1

composed of large (up to 7 mm) concentrically coated Girvanella oncolites (cores of lime mudstone intra- clasts or trilobites), together with intraclasts (lime mudstone), trilobites, echinoderms, pellets and lime mud. Many grains have micritized rims, and are partially to completely dolomitized. Cements are fine equant and fine columnar calcite.

Inrerpretatiori

Thin ooid grainstone and interbedded oncolite- intraclast grainstones are similar to modern ooid and intraclastic sand ribbons and shoals (0-3 m deep) on high-energy sublittoral platforms (Hagan & Logan, 1974), in that sediments have well-defined layering, hardgrounds, small ooids (average 0.3 niin diameter), reworked ooid and skeletal grains, intra- clasts of ooid grainstone and skeletal limestone, and calcareous algae attached to clasts.

Some oncolite intraclast sands interbedded with fine-grained ribbon limestones, and which contain marine fibrous cement, interstitial mud/micrite cement and asymmetric oncolite coats of algal intra- clasts, may have formed in slightly deeper parts of, or peripheral to active shoals (cf. Hine, 1977). Alter- nating coarse and fine layers, basal erosional surfaces with oncolitic and intraclastic lags and rotated asynimetric oncolites mixed with concentrically coated oncolites, suggest that these sediments were intermittently reworked during storms. Cornnion fibrous cements in the Cambrian lime sands indicate submarine lithification of sands that were immobile for long periods (Ball, 1967; Shim, 1969; Hagan & Logan, 1974). Periodic reworking of these formed intraclasts. Some Reriulcis algal clasts formed in place by encrustation of skeletal grains, but others may be boundstone fragments transported from shallow water biohernis.

R I B B O N C A R B O N A T E FACIES OF T H E DEEPER RAMP

Ribbon carbonate facies make up parts of the Mary- ville Limestone, Nolichucky Lower Limestone and Widener Limestone, are common in the Nolichucky Lower Shale and Middle Limestone Meiiibel-s, and comprise the Maynardville Limestone (Figs 2 and 5 ) . They onlap Elbrook -Honaker grainstone and stroma- tolitic carbonates to the south-east and north-east, and interfinger with and underlie Nolichucky shale facies to the north-west and south-west (Figs 2

and 5). They also overlie Nolichucky shales and inter- finger with and underlie Conococheague and Copper Ridge Formation stromatolitic carbonates (Fig. 2). The ribbon carbonates occur in sequences u p to 40 ni thick, interbedded with ooid and oncolite grain- stone, carbonate conglomerate and shale (Fig. 5). Stromatolitic carbonates are locally associated with Maynardville ribbon facies (Fig. 5). The ribbon car- bonates (Fig. 8A) consist of nodular skeletal lime- stones, fining-upward layers (skeletal limestone- pellet limestone-lime mudstone) and dolomottled layers, that form a gradational sequence from undis- turbed primary sedimentation units to burrow- homogenized units. Regionally correlative glaucon- itic beds and hardgrounds (Fig. 2) occur in the ribbon rocks, and parallel regional ooid packstone sheets and biostratigraphic zonal boundaries of Derby (1965).

Skeletal limestones of ribbon carbonates

Skeletal wackestone, packstone and grainstone that are locally glauconitic, are common lithologies in shaly ribbon carbonates of the Nolichucky Lower Limestone; similar skeletal beds also occur in shale facies cf the Nolichucky Formation. The skeletal beds are layers and lenses of liniestone (4-30cm thick) separated by shale partings, stylolitic dolomite seams and hardgrounds. Internally, the limestones are burrowed, massive to well-layered and, locally tabular cross-bedded. Well-lsyered units contain centimetre-thick horizontal and inclined layers out- lined by hardground surfaces and glauconite-silt concentrations. The hardgrounds are planar to irregular surfaces impregnated. with black opaque material, and generally truncate fabrics of under- lying sediments (Fig. 7A and B), although some hard- grounds developed on wackestone/miidstone lack truncated fabrics (Fig. 7A). Layers above hard- grounds are generally graded with coarse, grain- supported basal sediments passing up into fine wackestone/m udst one.

The skeletal limestones are fine- to coarse-grained grainstone, packstone and wackestone that are inter- layered and locally burrow-mixed. They include trilobite-dominated to echinoderm-dominated sedi- ments that also contain spicules, intraclasts (lamin- ated pellet limestone, lime mudstone, skeletal and ooid limestone and Rcmlris boundstone), variable amounts of pellets, quartz silt and clay/mica niin- erals, and locally abundant lime mud. Glauconite is abundant in soaie packstones. Intraclasts (sand-size

i l - 2

582 J. R. Markello and J. F. Read

Fig. 7. Skeletal limestone lithofacies of the deeper ramp. (A) Photomicrograph of close-spaced, micrite-cemented hardgrounds in packstone. (B) Photomicrograph of hardground that truncates grains and cement in grainstone. Dark grains are glauconite. (C) Photomicrograph of multigeneration cl st (arrow) with four erosion sufaces. (D) Photo-

of picture (H). micrograph of in-place, upward branching Renulcis colonies (arrow I ) encrusting intracIast. Note hardground near top

to 4 cm diameter) are dominantly single-generation clasts but some are ,multigeneration clasts which show evidence of successive cementation, erosion and abrasion (Fig. 7C). Rare, in place, upward-branching algal (Renalcis) colonies encrust some intraclasts (Fig. 7D). Cements include fine columnar, fine equanl, coarse blocky and syntaxial calcite cements.

Interpretation

Compaction and pressure solution have played an important role in the development of ribbon rock fabrics in which argillaceous dolomite seams contain close-packed stylolites. Burrows, argillaceous lam- inae, and lime mudstone layers are common loci for pressure solution and dolomitization. Similar fabrics interpreted as the products of pressure solution and dolomitization have been described by Logan & Semeniuk (1976) and Wanless (1979). However,

primary sedimentary fabrics are sufficiently preserved as to allow environmental analysis.

The skeletal limestone ribbon carbonates are inter- preted as subtidal deeper ramp facies because they lack fqatures indicative of tidal flat deposition, occur downhlope froni cyclic stromatolitic facies and oolitic sands, pass seaward into basinal shale facies (Figs 2 and 3), and contain open marine biotas (including calcareous algae) and glauconite.

Wackestones/packstones are low energy, subwave base deposits, in which some of the fine carbonate may have been carried from adjacent shallow water areas to accumulate together with inplace skeletal carbonate. Cross-stratified packstone/grainstones are higher energy winnowed ramp sediments that may have formed above fair weather wave base, or in subwave base settings subject to periodic reworking of bottom sediments.

Abundant hardgrounds, intraclasts and multi-

Shale shelftransitions in the Appalachians 583

generation clasts indicate periods of marine cementa- tion and dissolution on the sea-floor, possibly during times of decreased sedimentation, as in subwave base ramp settings in the Persian Gulf (Shinn, 1969) and Shark Bay (Hagan & Logan, 1974). Locally abun- dant glauconite in Nolichucky sediments may also indicate periods of relatively slow sedimentation, when clay-rich fecal pellets were diagenetically altered on the sea-floor (Degens, 1965). Locally, hardened sediments were subjected to one or more periods of cementation and reworking and some hardgrounds were overlain by storm-reworked graded sands.

Fining-upward sequences

Repetitive fining-upward layers (1-5 cm thick) in ribbon rocks consist of basal skeletal lags, laminated quartzose calcisiltites, lime mudstone and argilla- ceous stylolitic dolomite caps arranged in thin beds (Fig. 8B). Fining-upwards layers may contain all four lithologies or lack one or more units.

Basal units have planar to irregular micro-scoured bases (1-5 mm relief) and are mainly trilobite- or echinoderm-dominated packstone/grainstone or wackestone. They contain abundant, subhorizontally aligned, convex-up trilobite and lesser echinoderm fragments, pellets and lime mud; interparticle and shelter voids beneath fossils are filled with columnar and equant calcite (Fig. 8C).

Laminated calcisiltites (1-2 cm thick) overlie basal skeletal packstones or rest with sharp contact on underlying fine argillaceous caps of the underlying sequences (Fig. 8B and D). They consist of single or multiple sets of laminae(Fig. 8D) arranged in parallel, horizontal, small-scale hummocky (Harms et a/., 1975) (up to 1 cm amplitude, 5-10 cni wavelength) and wave-ripple ( I cm amplitude, 1-5 cm wave- length) sets. Horizontally laminated sediments com- monly occur in lower parts of laminated units and are overlain by hummocky laminated and wave- ripple-laminated sets. Bases of sets are conformable or are erosional, and laminae generally parallel basal set boundaries (Fig. 8D). Within sets, laminae both thin and fine upward. Laminated units are locallydis- rupted by burrows (Fig. 8D). The calcisiltites consist of pellets, variable amounts of silt-size quartz/ feldspar, opaques (including pyrite), glauconite and scattered flat-lying platy fossils (trilobite and phos- phatic brachiopod fragments); mica plates are con- centrated in thin laminae that alternate with mica- poor layers which outline the layering.

Laminated calcisiltites fine up into 1 cm thick caps of lime mud, that commonly grade up into argillaceous carbonate containing short (1-2 mm) wispy stylolites (Fig. 8C, D and E). Caps have sharp erosional tops, rare flame structures, and abundant horizontal and vertical burrows. Lime mudstones (Fig. 8E) consist of 2-20 ym lime mud/microspar, pellets, silt-size quartz and glauconite, micas, and locally common trilobite detritus. The argillaceous carbonates consist of fine dolomite and clay minerals with numerous wispy, anastomosing stylolites.

Interpretation

Fining-upward layers in the ribbon rocks are inter- preted as storm-reworked, deeper-ramp carbonates that formed below normal wave base. This is indi- cated by the strafigraphic position of the ribbon rocks between shallow-ramp ooid grainstones and intra- shelf basin shales, by the lack of features indicative of exposure, and by the similarity of the fining- upward sequences to ancient storm deposits described by Brenner & Davies (1973), Bowen, Rhoads & McAlester (1974, p. 96, fig. 4), Ager (1974), Kelling & Mullin (1975), DeRaaf, Boersma & van Gelder (1977) and Kreisa (1979). All these occur in sub- tidal settings adjacent to or interbedded with shales and other fine sediments, have basal erosion/scour surfaces, skeletal concentrations, parallel and small- scale hummocky lamination and mud caps.

Such fining-upward layers are generated by erosion and redeposition of sediments during and following storms. Basal erosion surfaces form as high-energy storm waves scour and suspend bottom sediments, and redeposit winnowed material as storm lags (Brenner & Davies, 1973; Bowen et al., 1974; Kreisa, 1979), whereas the calcisiltites and their lamination types form under waning storm-energy conditions as suspended fines settle from suspension (Reineck & Singh, 1972; Harms eta/., 1975; Hamblin &Walker, 1979; Kreisa, 1979).

The transition from parallel t o hummocky and wave-ripple cross-lamination within the calcisiltites may relate to waning energy conditions. The parallel- laminated calcisiltites may form above basal scours/ shell lags by deposition from storm-generated density currenls while storm water levels are still high (Hamblin &Walker, 1979); by suspension deposition of storm-suspended sediment under the influence of wave-, tide- or wind-driven currents (Reineck & Singh, 1972); and/or by deposition of sediment under

584 J . R. Markello arid J . F. Rcad

Fig. 8. For legend see opposite.

Shale shelf transitions in the Appalachians 585

MO - MAYNARDVILLE

MARYVILLE I I

147

145

141

139 anz M

SKELETAL LIMESTONE

CONGLOMERATE

CALCAREOUS SILTSTONE 0 /PELLETAL CALClSlLTlTE -SHALE

-SCOUR BASE

* GLAUCONITE

Fig. 9. Columnar sections : shale-siltstone-conglomerate sequences, Nolichucky Lower and Upper Shale Members. Positions and widths of arrow stems at index column represent stratigraphic locations and thickness of shale intervals magnified in adjacent columns.

the influence of high, wave-induced, oscillatory bottom-current velocities (Allen, 1970, p. 170).

Hummockycross-stratification (Harmset a/., 1975; Kreisa, 1979) has features that are intermediate between horizontal parallel lamination and wave- current ripple lamination (e.g. laminae are gently undulating, laminae thicken over crests, crests of laminae are in-phase, and sets locally truncate under- lying sets), and may be a transitional bedform between the two end members. The in-phase char- acter of hummocks in part reflects high suspended sediment supply. Hummocky cross-stratification may be caused by strong wave action, with surges of greater displacement and velocity than those required to form ripples (Harms et al., 1975) but lower than those required to form plane lamination (Allen, 1970, p. 170). Alternatively, hummocky lamination may be produced in sediments that are transported by storm-generated density currents, and reworked and deposited under the influence of storm waves (Hamblin & Walker, 1979), possibly as water levels

decreased during storm dissipation. Finally, wave- ripple cross-lamination forms as a traction lamina- tion, commonly produced by oscillatory waves asso- ciated with tidal currents (Harms et a/., 1975) and probably reflects lower suspended load in the water column, lower currents (Allen, 1970, p. 170) and possibly decreased water depths, than the planar and hummocky types.

Lime mudstone and argillaceous dolomite caps of fining-up sequences are low-energy suspension deposits formed by deposition of storm-suspended fine sediment (Bowen et a/., 1974; DeRaaf et al., 1977; and Kreisa, 1979), and accumulation of fines either generated in situ or carried on to the deeper ramp between storms. The common occurrence of lime mud layers overlain by argillaceous dolomite caps may result from settling of lime mud before platy, lower density clay minerals. It may also be caused by influx of terrigenous fines on to the deeper ramp during and following storms, when rivers in flood debouched into the basin.

Fig. 8. Ribbon rock lithofacies. (A) Outcrop photograph of ribbon rock (light layers are limestones; dark layers are dolomitic). (B) Polished slab of fining-upward sequences. Scale in millimetres. (C) Photomicrograph of fining-upward sequences showing basal scours (arrows), skeletel packstones and dolomitized mud caps (dark). 1 cm bar scale on left. (D) Photomicrograph of lamination in pellet limestones of fining-upward sequences. Note erosional boundary at base of upper set (arrow). (E) Photomicrograph of burrowed lime mudstone. Note geopetal filling of burrows with sediment and cement. (F) Polished slab of dolomottled ribbon rock.

586 J . R . Markcllo a i d J . F. Rcad

Dolomottled ribbon rocks

Doloniottled ribbon rocks range from slightly bur- rowed, fining-upward layers t o burrow-disrupted irregular layers and mottles of lime mudstone, cal- cisiltite, and skeletal packstone to wackestone. Anastomosing thick stylolites and stylolitic seams are common.

Within slightly burrower'. fining-upward layers (Fig. 8F), basal skeletal packstone, laminated cal- cisiltite and mudstone caps are present, but scoured bases are burrowed, skeletal fragments lack preferred orientation, lamination is burrow-disrupted and lithologies are mixed. Large burrows traverse entire sequences and are filled with host sediment, cement or stylolitic dolomite.

Highly burrow-disrupted ribbon rocks consist of splierical, elliptical to irregular limestone nodules (0.5-2 cni thick, 1-5 cm long) enclosed by horizontal to vertical anastomosing stylolitic argillaceous dolo- mite seams. Burrows preserved within nodules are filled with host sediment and cement, and disrupt primary layering.

Iiiterpretation

Dolomottled ribbon rocks, that contain slightly burrowed, fining-upward sequences to burrow homogenized units, may have formed by burrowing of storm deposits. Burrowers may have been unable to mix sedimentswhere intervals between storrnswere short. Highly burrowed layers mayreflect longer times between storms or periods when numbers of infaunal organisms were high. Consequently, all gradations from little-burrowed, fining-upward sequences to burrow-homogenized limestones are present.

S H A L E F A C I E S O F T H E INTRASHELF BASIN

Intrashelf basin shale facies comprise the Nolichucky Upper and Lower Shale Members, and are enclosed by r ibbm rock and grainstone ramp facies to the nirth-ea5t and south-east (Fig. 2). They consist of shale, calcareous quartz siltstone, limestone con- glonieratc, and lesser glauconitic skeletal and ooid packstone/gr-dinstone. The units are comnionly arranged in upward-coarsening sequences, 30 t o 40 cm thick (Fig. 9) that consist of shale-siltstone- conglomerate (or skeletal limestone). Less commonly, they form fining-upwards sequences (typically u p t o

50 cni thick) of liniestone conglonierate (or skeletal 1 i niest one)-si I t st one-shale.

Shale units

Shale units (Fig. 10A) are up to 4 m thick in the Lower and Upper Shale members, and extend as tongues into ribbon carbonate facies (Fig. 2). The shale is green to dark grey, fissile, calcareous, and contains scattered layers of trilobite, phosphatic brachiopod and rare pelmatozoan debris. The shales consist of elongate flakes of biotite, rare muscovite, chlorite and other clay minerals, fine quartz/feldspar silt, lime mud and euhedral dolomite (20-200 uni rhombs).

Compacted sedimentary dikes occur in two region- ally extensive shale layers (3.3 and 1 in thick), which occur 3-10 ni above the Middle Limestone Member (Fig. 10B). The dikes can be traced upward into para- conglorrerate and siltstone beds which overlie shale units. They taper downward, and are highly com- pacted to 40';; of original length. Dike fills, which are similar to overlying paracongloinerates to which dikes connect, contain clast-supported quartzose limestone conglomerate with subrounded intraclasts (up to 2.5 cm across) of laminated calcisiltite, skeletal limestone and lime mudstone and inter-clast quart- zose skeletal wackestone.

Itztcrprctatiorl

Shale facies of the Nolichucky Formation are inter- preted as shallow basin facies formed below normal wave base because they are stratigraphically furthest from peritidal facies, are enclosed by subtidal ribbon rocks, are fine grained, and lack features suggestive of shallow-water deposition or emergence. These facies are similar to shale sequences from intrashelf basins in the Cambrian (Aitken, 1978) and Mesozoic (Eliuk, 1978). Depositional slopes were very low, indicated by horizontal regional time markers (zonal boundaries, glauconite and hardground horizonr, ooid packstone sheets, sedimentary dike layers and Middle Limestone Member; Fig. 2), and by the general lack of turbidites, slump structures and intra- formational truncation surfaces which are associated with steeper platform margins. Estimates of water depths during deposition of shale lithofacies range from a few metres (estimated from Middle Limestone upward-shallowing cycles containing basal shales) to a few tens of metres (estimated from thickness of the Upper Shale/Maynardville Limestone

587

Fig. 10. Shale and siltstone lithofacies. (A) Cutcrop photograph of Nolichucky shale and interbedded siltstone. Staff (bottom centre) is 1.5 m long. (B) Outcrop photograph of small compacted sedimentary dyke in shale. (C) Polished slab photograph of planar to hummocky laminated siltstone. Scale in cenlimetres. 1.

interval from maximum transgression to Copper Ridge peritidal facies). Aitken (1978) suggested water depths greater than 8-1 2 ni for similar shale sequences in Canada.

Studies of mud deposition on modern continental shelves suggest that most fine-grained sediments, once introduced into the marine realm by river jets a t deltas, are transported to shelf environments in turbid nepheloid or bottom boundary layers by wind and tidal currents, and accumulate by settling from suspension (Howard & Reineck, 1972; McCave, 1972; Drake, 1976; Swift, 1976). Storms maintain suspension of, or cause resuspension of, muds and assist in further transport (Drake, 1976).

Sedimentary dikes in Nolichucky shales are similar to those described by Waterson (l950), Peterson (1968), Truswell (1972) and Williams (1976) in that they are stratigraphically (vertically) restricted, re- gionally extensive, composed of coarser silts, sands and gravels, and occur in shales. The major suggested mechanism of emplacement is remobilization of water-charged sediments (both shales and sands) by liquefaction, triggered by seismic shock (Sims, 1974, 1975; and Rymer & Sims, 1976) or slope failure and slumping. Other ancient sedimentary dikes (Hoffman, 1975) are interpreted as injected fills of subaqueous syneresis cracks. Friedman & Sanders (1978, pp. 408-409) suggest that dikes are emplaced along parallel syneresis joint systems in slightly compacted shales, and are later contorted as shales compact further.

Calcareous quartz siltstone

These occur as 1-10 cm thick beds (Fig. IOC) in 0.3- 2 m thick siltstone intervals of coarsening-upward sequences (Figs 9 and 10A). Siltstones are sharp based, thinly laminated with shale partings and, in coarsening-upward sequences, siltstone beds thicken and shale partings thin upward. Locally, siltstones have thin conglomerate interbeds.

Siltstone beds (Fig. 1OC) contain parallel, hum- mocky and rare wave-ripple lamination (1-2 cm amplitude, 10-30 cm wavelength). Single beds may consist of one lamination set or of several sets separ- ated by basal erosion surfaces. Laminae are 1-3 mni thick, continuous, parallel, basal erosion surfaces thicken over crests of hummocks and thin into troughs. Parallel lamination, the most abundant lamination type, comprises complete siltstone beds or occurs in lower parts of beds and grades up into hummocky lamination. Wave-ripple lamination is

rare in the siltstones, occurring at tops of units whcre rippled bedforms are preserved by clay drapes. Many parallel sets have laminae which thin upward, but some have laminae which are thickest in the middle of sets.

Some siltstone layers have parting casts (Ksiazkie- wicz, 1958; Dzulynski & Walton, 1965) which create polygonal patterns on bedding planes. They are confined to siltstones and appear to be absent from shales. In vertical section, the partings are a few millimetres wide and up to 1.5 cm deep. They widen then thin upward, typically have non-matching walls, and some are highly compacted (to 5 O o 0 original length). They have fills of fine mud or silt, with a crude vertical flow layering outlined by silt stringers or vertical, platy fossils. Fills appear to have been injected both from above and below.

Siltstones also have abundant trace fossils which include curvilinear crawling and resting traces, rare tracks, meandering trails and shallow oblique and vertical burrows similar to the Critziurra facies of Seilacher (1967) and Crimes (1975).

Siltstone laminae are outlined by aligned mica/ clay minerals, ca!cite cement-rich layers, and stylo- litic concentrations of densely packed quartz silt. They are composed dominantly of angular quartz/ feldspar silt, minor mica/clay minerals, rare glau- conite, pyrite, detrital heavy minerals, rare skeletal grains (trilobite, echinoderms and phosphatic brach- iopod fragments), and calcite, lesser dolomite and quartz cement.

Iiiterpretatioii

The siltstones of the Nolichucky shale-siltstone-con- glomerate sequences might be considered to be tidal flat deposits, on the basis of the association of flat pebble conglomerates and parting casts that super- ficially resemble mudcracks. However, a tidal flat origin is not likely because: ( I ) The mudcracks or parting casts differ from

desiccation cracks, because in tidal flat silts and mud it is typically the mud layers that are cracked (cf. Hardie & Ginsburg, 1977). In the Noli- chucky beds, the siltstones are cracked and the fine muddy layers or shales lack cracks. Similar features have been described by Ksiazkiewicz (1958) and Dzulynski & Walton (1965) from flysch sequences, and by Pfeil & Read (1980) from carbonate slope facies. These structures may be due to creep or slumping on a slope (Dzulynski & Walton, 1965; Pfeil &Read, 1980).

Shale shelf transitions in the Appalachians 589

They might also reflect compaction and volume reduction due to dewatering; this hypothesis is supported by the highly compacted nature of some of the filled cracks.

(2) The sediments have open marine biotas. (3) The stratigraphic relations and lithofacies pre-

clude a tidal flat setting. The siltstones of the shale facies are separated from peritidal carbonates of the Elbrook-Honaker Formations by the sub- tidal ooid sands and ribbon carbonates of the shallow ramp. There is no evidence of cryptalgal sediments in limestone beds associated with the siltstones, neither do the siltstones contain char- acteristics of clastic tidal flat deposition such as herringbone cross-stratification, flaser bedding and reactivation surfaces.

The shales and siltstones do not resemble tur- bidite sediments because they lack graded bedding, Bouma sequences, flutes, tool marks and slumps. They are interpreted on the basis of regional relations and sedimentary features, as intrashelf basin sedi- ments, deposited below subwave base by storm processes.

Parallel, hummocky and wave-ripple laminated siltstones of the Nolichucky Formation resemble silt- stones interbedded with shales in the Holocene (Reineck & Singh, 1972), Cretaceous (Masters, 1967; Harms et al., 1975) and Ordovician (Kreisa, 1979), and storm-deposited laminated calcisiltites in the Nolichucky deep-ramp ribbon rock facies. Holocene parallel-laminated silts occur in shales of the Gulf of Mexico and North Sea shelves at depths up to 40 m and as far as 45 km from shore (Reineck & Singh, 1975). Calcareous silts in the Nolichucky intrashelf basin and deep ramp may have been generated in-place by winnowing of skeletal sediments, and may have been carried into the basinfromsurroundingshallower water carbonate areas. Quartz silts in the basin must have been transported considerable distances, over 300 km, from the north-western clastic belt (Fig. 1).

Hayes (1967) considered that sediments of hurri- cane-generated storm sequences in water depths of 20-30 m were transported seaward by turbidity flow. Hamblin & Walker (1979) also propose storm- generated turbidity currents as the mechanism for transporting shelf sediments in the Mesozoic of Western Canada. However, Howard & Reineck (1972) favour transport of storm-suspendedsediments by tide- or wind-driven turbid water masses.

Reineck & Singh (1975) note that deeper offshore storm-generated laminated sequences pass shoreward into shallow, wave-ripple-laminated sediments. In a

similar manner, Nolichucky siltstones have more abundant parallel lamination, lower amplitude and longer wavelength hummocky lamination, and less wave-ripple lamination than laminated calcisiltites of deep-ramp ribbon rocks. This may reflect the action of large storm waves in the basin compared to that of damped waves in shallower, ramp settings (cf. Allen, 1970, pp. 171-172). A basin setting also is supported by the presence of Cruziana trace fossils on siltstone beds.

Carbonate conglomerates Conglomerates occur throughout the Nolichucky Formation, but are best developed in the Lower and Upper Shales (Fig. 9) where they are interbedded with shales and shaly ribbon rocks, cap coarsening-up cycles or, less commonly, occur at bases of upward- fining sequences. Conglomerates are 2-30 cm thick and rarely up to 60 cm thick. They are single and rare composite thin sheets and broad lenses (tens of metres across). Some units thicken and thin or form discontinuous boudin-like layers, and others pinch out into shales or fine laterally into calcisiltites or siltstones. Systematic regional variation in bedding thickness is not apparent. Bases of conglomerate units are planar to irregular and scoured, and tops are draped by finely laminated lime mud or rippled pellet limestone. Rare units are capped by hardgrounds.

Most conglomerates are clast-supported, unsorted and non-graded to poorly graded. Rare paracon- glomerates occur in some thick shales. Clasts are spherical to platy and discoidal, are well rounded to angular, and have random, subhorizontal, imbricate, edgewise, and fanned orientations (Fig. 1 IA). Clasts in conglomerates within shale units lack edge- wise arrangements and are generally smaller, less platy and better rounded than those of conglomerates that rest on limestone beds.

In most conglomerates, clasts are dominantly laminated pellet limestone, lime mudstone, lesser skeletal wackestone, and some multigeneration clasts of laminated pellet limestone/skeletal packstone (Fig. 1 I B) and limestone conglomerate (Fig. 1 1 0 Rare multigeneration clasts contain first-generation pebbles, accretionary sediment and erosion surfaces, all of which indicate multiple depositional, cementa- tion and erosional events. In many conglomerates, clast lithologies are similar to immediately underlying or laterally equivalent limestones. Many clasts have red-stained borders, and are cut by burrows and borings (?) filled with mud or interclast sediment (Fig. 1 1 B). Some clasts have been bent and cracked

590 J. R. Markcllo and J. F. Rcad

Fig. 1 I . Limestone conglomerate lithofacies. All photographs are of polished slabs and all scales in centimetres. (A) Slab of conglomerate that overlies limestone. Note edgewise and fanned orientation of some clasts, and platy clast shapes. (3) Conglomerate with multigeneration clasts of pellet liniestone/skeletal packstone (M). Note burrows preserved in clasts (B). (C) Slab ofconglomerate (20 cm thick) that is enclosed i n shale and occurs north-west (basinward) ofshallow- ramp, thick, ooid grainstones. It contains large multigeneration conglomerate clasts(M), and sediments of oolitic skeletal sand. (D) Slab of paraconglomerate with lime mudstone clasts in shaly lime mud matrix.

by compaction, and V-shaped brittle fractures are filled by inter-clast sediment or cement.

Inter-clast sediment ranges from lime mudstone to grainstone, but in most conglomerates is skeletal intraclast packstone composed of poorly sorted, rounded to angular, skeletal material (trilobites, echinoderms, phosphatic brachiopods, and rare sponge spicules), intraclasts, lime mud, rare ooids, glauconite, quartz silt, and columnar and equant calcite cement. Inter-clast lime mud is commonly dolomitized and locally is iron oxide stained.

Rare mud-supported paraconglomerates (up to 45 cm thick) occur in the Upper Shale Member 5-10 m above the Middle Limestone Member, where they are enclosed in thick shales and are associated with compacted sedimentary dikes. These conglom- erates have planar scoured bases overlain by thin basal laycrs of coarse skeletal packstone, and grade into overlying shales. They consist of unoriented, unsorted, poorly rounded t o angular clasts of bur-

rowed lime mudstone, lesser laminated pellet lime- stone, and skeletal wackestone in lime mud matrix (Fig. 1 1 D).

Inteupretatioii

Most Nolichucky conglomerates interlayered with basin shale and siltstone facies are not earthquake- triggered sediment gravity flow deposits (Cook et al., 1972) because units are thin (a few tens ofcentimetres), clasts are small and locally derived, and depositional slopes are low.

Nolichucky conglomerates are very similar to ancient subtidal shallow-shelf conglomerates de- scribed by Sepkoskr (1975, 1978), Chudzikiewicz (1975), Jones & Dixon (1976) and Kazmierczak & Goldriiig ( 1978), and which are believed to have formed by storm processes. These conglomerates occur in limestone-shale sequences in beds 30-60 cm thick, are dominantly clast-supported with inter-

Skakc shelf trawsitioirs iir the Appuluchiaiw 59 I

particle skeletal-intraclast packstone to mudstone, and contain clasts that are platy to discoidal, up to 10 em long, in subhorizontal and imbricate to edge- wise arrangements.

Clasts in the conglomerates have similar litho- logics to underlying units (Chudzikiewicz, 1975 and Kazinierczak & Goldring, 1978). They form during high-energy storms when semi-lithilied bottom sedi- ments areeroded; currentsarereinforced by thestorni and eroded sediments are transported and later redeposited under waning encrgy conditions (Kelling & Mullin, 1975). Many Nolichucky clasts are bored, and have grains and cement truncated at borders, which indicates that some sediments were highly lithified when reworked, a conclusion supported by the abundance of hardgrounds and marine fibrous cements in Nolichucky sedinicnts. Other clasts may have been fragments of fine lime sediment that was sufficiently coherent to be reworked as pebbles.

Rounded to angular shapes of clasts indicate variable lateral transport during resedimentation. Transport is neither unidirectional nor from a point or linear source because a systematic gradient in conglomerate bedding thickness and clast size is absent. Oligomictic conglomerates which rest on limestones and contain large, angular to subrounded platy clasts, may have undergone little lateral trans- port as evidenced by proximity to source sediments (stibjacent limestones) and angular shapes of many clasts. However, wave reworking of sediments was capable of round iiig highly I i t hified 1 imestone frag- ments such as iiiultigeneration clasts (Fig. 11C). Clasts in grain-supported conglonierates that overlie shale beds may have undergone greater lateral transport, because clasts were eroded from limestone/ siltstone lithotopes and deposited on shales; these clasts tend to be smaller and better rounded than clasts in conglomerates that overlie limestone.

Kandomly developed conglomerates and locally developed upward-fi ni ng sequences wit I1 congl omer- atic basal lags probably developed as a result of recurrent storms (Jones & Dixon, 1976; Kreisa, 1979). Although also probably storm generated, the more common upward-coarsening sequences of shale-siltstone-conglomerate have some similarities to basal portions of progradational sandy shoreline sequences i i the North Sea and Gulf of Gaeta (Keincck & Singh, 1975, pp. 314 and 330). These modern sequences which consist of interlayered silty muds, storm-reworked parallel and hummocky laminated silts, and thick beds of shoreface cross- bedded sands, occur as progradational sheets and

lobes that fine seaward, and prograde during storms when sediments are transported and deposited in deeper water environments. In a similar manner, the Nolichucky coarsening-upwards sequences may have formed by progradation of siltstone sheets and lobes over basin shales. Capping conglomerates may have been deposited as the prograding lobes, with surficial conglomerate-filled channels, migrated out over silt- stones that formed fans in front of the channels. The conglomerates may be analogous to the storm-surge, subtidal, coquinoid sandstones in shelf sequences described by Brenner & Davies (1973). Some con- glomerate caps might also have formed during subse- quent storms by erosion of previous storm-deposited siltstones that had become cemented at or near to the sediment-water interface.

Rare paraconglomerates interbedded with thick shales and composed of subangular, poorly graded clasts of lime mudstone, and lesser laminated pellet limestone in mud matrix, contain allochthonous debris. They resemble debris flow deposits (Cook ct a/., 1972) in that they are laterally extensive, enclosed in shales, massive to poorly graded, and have mud support fabrics, planar bases, locally irregular tops and subangular clasts. It is possible that slopes in the Nolichucky basin were locally sufficient to sustain debris flow. Perhaps storin-induced sediment trans- port from limestone lithotopes may have initiated debris flow following rapid deposition upon water- saturated, uncompacted shales.

Skeletal limestone

Skeletal limestones (up to 30 cm thick) interbedded with shale Iithofacies occur as thin beds and starved ripple/megaripple lenses within shale, as basal skeletal lags in upward-fining sequences, as skeletal caps in upward-coaisening shale-siltstone-skeletal limestone sequences, and as beds interlayered with conglomerates. Caps of skeletal limestones may be rippled or niegarippled. Some have scoured and load- casted bases. Units are massive to thin-layered, to ripple-cross-laminated, and some contain hard- grounds. They are mainly glauconitic skeletal grain- stonelpackstone that resemble skeletal sands in ribbon carbonates, and lime sand matrix of con- glomerates in shale sequences. They mainly consist of trilobite and echinoderm fragments, intraclasts, pellets, interstitial lime mud and variable amounts of glauconite. These are similar to skeletal beds of the ribbon carbonates (Fig. 7).

592 J. R. Markello and J. F. Read

Interpret ation

Probably all of the skeletal sands are storm-reworked deposits, indicated by association with upward- fining and upward-coarsening sequences, interlayer- ing with conglomerates, presence of abundant intra- clasts, starved ripples and megaripples. Some sands may have undergone considerable lateral transport during storms, especially those that rest with sharp basal contacts on shales, and some may even have been transported from ribbon carbonate lithotopes. Others may have resulted from storm reworking of locally produced skeletal sediment.

Ooid packstone

Ooid packstone sheets (0.3-8 m thick) occur between shaly ribbon limestones and the overlying shales in the Nolichucky Lower Limestone/Lower Shale Mem- bers and in thin layers and lenses in shaly limestones and shales beneath the cyclic Middle Limestone (Fig. 2). They also cap cycles in the Middle Limestone (Fig. 2). Basal contacts of ooid beds are commonly erosional, and upper contacts are sharp with preserved symmetrical megaripples (5-15 cm ampIitudes, 0.5- 2 m wavelength; Fig. 12A). Thick oolitic units have thin tabular cross-stratified sets. Hardgrounds and

Fig. 12. Ooid packstone facies of the intrashelf basin. (A)Ooid packstone, Middle Limestone Member. Note megarippled top overlain by shale and the centimetre scale (arrow). (3) Photomicrograph of ooid packstone with sediment-filled bedding plane parting (arrows) that separates cemented ooid packstone beds. Coarse crystalline ooids are dolomitized. (C) Photomicrograph of ooid packstone; platy ooid nuclei are mainly trilobite fragments.

Shale shelf transitions in the Appalachians 593

marine-cemented layers are common and include cryptocrystalline calcite-cemented layers separated by bedding-plane partings filled with infiltrated ooid sediment (Fig. 12B).

The ooid packstone (Fig. 12C) consists of mod- erate- to well-sorted, medium sand- to granule-size ooids, intraclasts (lime mudstone, quartzxe calcisil- tite, and skeletal limestone), locally abundant echinoderm and trilobite fragments, spicules, glau- conite, and abundant interstitial lime mud/pelletal mud. Ooids have well-developed radial fabric and nuclei that include pellets and skeletal grains (Fig. 12C). Thin, fine-columnar cement partially to corn- pletely fringes ooids, but most interparticle void space is filled with lime mud/pellet mud and minor equant cement. Dolomitization of ooids is partial (only nuclei replaced) to complete, and commonly is localized within layers.

Interpretation

Ooid packstones are interpreted on the basis of stratigraphic setting as subtidal sand sheets that accumulated in shallow basin settings, s-award of nearshore grainstone shoals and deeper-ramp, ribbon carbonate environments. The ooid packstones differ from the other ooid sediments in that they occur in extensive thin sheets overlain by shales, have pre- served megarippled bedding surfaces, and are pack- stones with large ooids (1-4 mm) with pronounced radial fabrics, open marine fossil asemblages and glauconite.

Holocene oolitic sediments similar to Nolichucky ooid packstones are forming in protected Persian Gulf lagoons where ooids are moved only during storms, and sustained low-energy periods allow for precipitation of thick radial oolitic cortexes (Loreau & Purser, 1973). Similar ooid packstones occur in intrashelf basin shale facies of the Mesozoic shelf, Canada, where they also have pronounced radial fabrics (Eliuk, 1978) and in the Cambrian inshore basin facies of Canada (Aitken, 1978). The radial fabrics may be partly diagenetic, although they probably reflect primary radial fabrics that were either aragonitic or high Mg calcite. Megaripple bedforms on subwave base oolitic sediments are generated during periodic high tides and large storms, and are preserved because of the subwave base setting and possible stabilization of sediments by subtidal algal films (Bathurst, 1975).

COMPARISON WITH OTHER INTRASHELF BASIN SEQUENCES

The Nolichucky intrashelf basin has facies that are similar to those in intrashelf basins in the Cambrian of the United States and Canada (Palmer, 1971b; Aitken, 1978) and the Mesozoic of Canada (Eliuk, 1978). They are bordered toward the regional shelf edge by shallow-water carbonate platforms that periodically shoaled to tidal levels. Consequently these rimming platforms commonly have interbeds of carbonate tidal flat facies. Width of the rimming platform for the Appalachians is not known. How- ever, rimming platforms may have ranged from 20 to 400 km wide in the Cambrian of Western Canada (Aitken, 1978), and up to 30 km wide in the Mesozoic of eastern Canada (Eliuk, 1978).

Inshore basins may be extremely large. Aitken (1978) suggests the Cambrian basin of Western Canada was 1900 km long by 700-1 10 km wide. In the Appalachians, the basin may have been 800 km long by 300-400 km wide. The Mesozoic intrashelf basin in eastern Canada appears to be smaller (500 km long by 70 km wide; Eliuk, 1978).

Slopes into the intrashelf basins appear to be low, probably of the order of a few metres per kilometre. Consequently, facies transitions from the platform into the basin are typically ramp-like and lack the turbidites and megabreccias that seem to characterize steep shelf edges. Instead, the facies grade from lime sands into muddy carbonates and then into shale sequences. An important feature associated with the transition is that thick ooid grainstone bodies may be located irnmediately.behind the rimming platform.

lntrashelf basins appear to be bordered toward the craton by nearshore clastics which are locally intro- duced via delta-systems and redistributed by marine currents. Intrashelf basin fills common:y are olive- green shale, calcisiltite, quartz siltstone, open marine skeletal limestone and locally, radial-ooid packstone. Flat pebble conglomerates, glauconite and hard- grounds are common, particularly in the Cambrian examples. Finally, and of great importance, is that basin fills appear to be dominantly subtidal facies that may have formed below fair weather wave base.

CONCLUSIONS

( I ) Upper Cambrian (Dresbachian) facies of the Nolichucky and adjacent formations accumulated within and peripheral to a shallow intrashelf basin on

594 J. R. Markello andJ. F. Read

the Appalachian pericratonic carbonate shelf. The intrashelf basin was bordered to the west by the North American craton. It was bordered by a shallow ramp-tidal flat complex to the north-east (along strike) and to the south-east (toward the regional shelf edge). The location of the shelf basin above a pet sistent Lower Palaeozoic depocentre suggests a genetic relationship. Similar basins occur in the Cambrian of western Canada (Aitken, 1978) and the Mesozoic of eastern Canada (Eliuk, 1978).

(2) Lithofacies occur in broad bands which inter- grade laterally and vertically. Four lithofacies suites that make u p the ramp-intrashelf basin transition are: (I) cyclic carbonate peritidal facies; (2) shallow- ramp, high-energy ooid and oncolitic grainstones; ( 3 ) deeper-ramp ribbon carbonates (skeletal lime- stones, storm-generated fining-up sequences, and burrow-mixed sequences); (4) shallow-basin shale- siltstone-carbonate-conglomerate sequences and open marine skeletal limestone and ooid packstone, believed to have formed below fair weather wave base, but above storm wave base. Shale lithofacies appear to be on-shelf deposits and differ from typical deeper water facies (Wilson, 1969) in lack of pelagic sed i nient s , abundance of Cruzium-type fossils, presence of storm sequences and interbedded shallow water and oolitic facies, and by proximity of tidal flat carbonates.

( 3 ) Slopes oil the ramp were very low. This is indi- cated by horirontal time lines that parallel litho- stratigraphic markers (regionally extensive hard- grounds, glauconite and sedimentary dike horizons, ooid packstone sheets, and the Middle Limestone Member) and by general lack of turbidites, slumps and intraformational truncation surfaces. Further, limestone conglomerates in basin shales, which con- sist of basin facies clasts, lack clasts of shallow plat- form rocks and other evidence of widespread, down- slope debris flow deposition typically associated with steeply sloping shelf margins. Estimates of basin water depths during deposition range from a few metres to perhaps a few tens of metres maximum.

(4) Storms were important in the formation of many rock types in the sequence including fining- upward sequences of deep-ramp ribbon rocks; parallel/liiimmocky laminated siltstone/calcisiltites and conglomerate lenses and sl-.eets in deep ramp and.basin facies. Storm origin of most conglomerates is sugg:sted by lack of regional gradients in bedding thickness and clast size, by dominance of locally derived clasts, and by abundance of conglomerates throughout the basin. Storm-initiated sediment

gravity flows may have been involved in deposition of rare paraconglomerates interbedded with shales.

( 5 ) Recogition of intrashelf basins on continental shelves is important because such basins strongly influence distribution of associated facies (including potential reservoir rocks), whose trends may largely be unrelated to regional shelf-edge trends.

A C K N O W L E D G M E N T S

This paper is based on an M.S. Thesis by J. R. Markello, under the supervision of J. F. Read. Thanks are given to W. D. Lowry, C. G . Tillman, George A. Grover, Jr, Ronald D. Kreisa and J. D. Aitken for helpful discussion and for ci-itical review of the manuscript. Technical assistance was provided by Bryan Roberts and Greg Lumpkin (field work), by Sharon Chiang, Martin Eiss and Carol Markello (drafting), by Sue Bruce and Gordon Love (photo- graphy), and by Susan Roth and Donna Williams (typing). Financial assistance was provided by Earth Sciencss Section, Nat ional Science Foundation grants DES 75-15015 andEAR75-15015 to J. F. Read,and by grants from the Department of Geological Sciences, Virginia Polytechnic Institute and State University; a Grant-in-Aid of Research from Sigma Xi, the Scientific Research Society of North America; and a Grant-in-Aid from the American Association of Petroleum Geologists. The senior writer wishes to ex- press much appreciation to his wife, Carol, who aided in preparation of illustrations and provided abundant emotional support throughout the project.

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