CYCL IC SEDIMENTATION IN THE MIXED S IL IC ICLAST IC-CARBONATE ABO-HUECO TRANSIT IONAL ZONE (LOWER PERMIAN) , SOUTHWESTERN NEW MEXICO 1

GREG H. MACK Department of Earth Sciences New Mexico State University

Las Cruces, New Mexico 88003 AND

W. C. JAMES Department of Geological Sciences

University of Texas at E1 Paso El Paso, Texas 79968

ABSTgAC'r: In southwestern New Mexico, Lower Permian (Wolfcampian) rocks grade southward from nonmarine siliciclastics (Abo and Earp Formations) to marine carbonates (Hueco and Horquilla Formations). A transitional zone between siliciclastic and carbonate facies trends east-northeast across southwestern New Mexico and consists of 64 to 186 m of cyclically mterbedded siliciclastic and carbonate rocks, which were deposited in tidal-fiat and shallow-marine environments. Shallow-marine facies include fossiliferous limestone and olive-gray shale. Tidal-flat facies consist of 1) tipple-laminated sandstone, which was deposited on intertidal sandflats near mean low tide, 2) mixed sandstone-shale, which was deposited on an intertidal flat shoreward of the ripple-laminated sandstone facies, and 3) nodular shale, which is characterized by pedngenic calcareous nodules and was deposited in a supratidal setting. The intertidal facies are truncated by or grade laterally into rare channel sandstones, which represent tidal-creek or estuarine facies. In addition to siliciclastic tidal-flat deposits, a few beds of laminated carbonate also were deposited in the intertidal zone.

Vertical sequence analysis aids in delineating three types of depositional cycles. Asymmetrical cycles display the vertical sequence: basal fossiliferous limestone--olive-gray shale--ripple-laminated sandstone--mixed sandstone-shale--nodular shale, and record shoreline prngradation. The asymmetrical cycle is always overlain by fossiliferous limestone, which indicates a major transgression that inhibited siliciclastic sedimentation. A enmmon symmetrical cycle consists of fossiliferous limestone---olive-gray shale--ripple- laminated sandstone--olive-gray shale--fossiliferous limestone, and indicates systematic seaward and landward migration of facies zones associated with smaU-scale sea-level changes. A less common symmetrical cycle involves laminated carbonate--fossiliferous limestone--laminated carbonate. Cyclic sedimentation in Abo-Hueco transitional strata is most likely the result of glacial eustatic sea-level fluctuations.

INTRODUCTION

Lower Permian (Wolfcampian) sedimentary rocks in southwestern New Mexico display a facies change from red, nonmarine siliciclastic rocks in the north (Abo and Earp Formations) to marine carbonate rocks in the south (Hueco and Horqui l la Formations) (Fig. 1). The sflici- clastics are predominantly siltstone, shale, and fine-grained sandstone, and represent the distal end of a southward- prograding elastic wedge derived from the ancestral Rocky Mountains in Colorado and northcrn New Mexico (Kott- lowski 1965; LeMone et al. 1971; Greenwood et al 1977). Thin l imestone and chert-pebble conglomerates are found in fluvial facies of the Abo Format ion in the Cooke's Range and near Santa Rita and reflect local rel ief within the otherwise low-gradient alluvial plain (Fig. 1). East of the study area, near the present-day Sacramento Moun- tains, coarse detritus of the Abo Format ion was shed westward from the Pedernal Uplift (Fig. 1; Otte 1959; Pray 1961; Speer 1983). The clastic dispersal system on the west side of the Pedemal Uplift had little or no in- fluence on Abo clastics exposed in and westward of the southern San Andres Mountains, however, and will not be considered in this study. South of the transition be- tween nonmarine and marine sedimentary rocks, the Hueco and Horquil la Format ions consist of marine l ime- stone and minor marine shale. In the Frankl in Mountains, south of the study area, the Hueco Format ion is com-

Manuscript received 31 January 1985; revised 20 May 1985.

posed of normal marine wackestone and packstone, algal- plate boundstone, and minor grainstone and shale (Jor- dan 1975).

The transitional zone between siliciclastic and carbon- ate facies trends east-northeast across southwestern New Mexico from the Peloncil lo Mountains to the southern San Andres Mountains (Fig. l). A seaward salient appears to exist in this transit ional zone near the Big Hatchet and Animas Mountains and a landward recess was situated near the San Andres Mountains (Fig. 1). The transit ional zone consists of 64 to 186 m of cyclically interbedded red siltstone and fine-grained sandstone, gray shale, and l imestone. F rom a regional perspective, this interval rep- resents the change from nonmarine to marine facies. Within the interval, as many as seven facies (5 sfliciclastic and 2 carbonate) are interbedded on a scale of 0.1 to 5 m. Mixed sil iciclastic-carbonate sequences present spe- cial problems for the interpretation of deposit ional en- vironment, because siliciclastic sediment, especially sand, generally inhibits carbonate sedimentation. Permo-Car- boniferous rocks throughout the world contain especially good examples of mixed sil iciclastic-carbonate sequences, and have recently received renewed interest among sedi- mentologists (Reynolds et al. 1976; Saunders et al. 1979; Heckel 1980; Rawson and Turner-Peterson 1980; Dfiese and Dott 1984). Equally interesting are the nature and origin of the cyclicity in these sfliciclastic-carbonate se- quences. Abo-Hueco transitional strata contain as many as 40 cycles, both symmetrical and asymmetrical, which reflect relative or absolute changes in sea level. A depo- sitional facies model provides the foundation for under-

JOURUAL OF SEDIMENTARY PETROLOGY, VOL. 56, NO. 5, SEPTEMBER, 1986, P. 635--647 Copyright 1986, The Society of Economic Paleontologists and Mineralogists 0022-4472/86/0056-635/$03.00

636 GREG H. JVL4CK AND H~ C. JAMES

Son~. ) ~ Pe/oncdlo Andres~ ~

SantoRita Mts. ~

Dana Mts.Ana~ 1 Cooke s ~ @~ Sacromenlo

,Peloncillo Range Organ Mts. Mts. Mts. ~t~ Roblec

c.~Animcs ~ Mts, ! t~Mts. FloridaMts. \ ~ppe 1 tj f~ IBig : . . . . ~ ~ Horquirlo Member

Hatchet o km 50 / I ts

I ~ "-F, ~ / ~ ~] Uplift ( / . ~q'~v- ~ / ~ Fit ~-~"~:~-~

Hueco

FIG. 1 .--Upper map shows the location of measured sections (mlid circles) of Al:m-Hueco transitional strata in southwestern New Mexico. The sections in the Cooke's Range and at Santa Rita were found in this study to be north of the transitional zone. The lower map is an Early Permian paleogeographic map of southwestern New Mexico. North of transitional zone, Abo and FEarp Formations are siliciclastic redbeds deposited by fluvial systems. South of transitional zone, Hueco and Horquilla Formations are open-marine limestone and minor shale. The transitional zone consists ofinterbedded silicidastic and carbonate rocks. Palinspastie reconstruction is not attempted, because ofconflieting ideas on the amount of post-Permian crustal shortening in southwestern New Mexico (el. Drewes 1978; Seager 1983).

standing cyclic sedimentation. Although the Wolfcam- plan mixed siliciclastic-carbonate interval is exposed in only eight different areas in southwestern New Mexico, the quality of exposure at seven of the locations is ex- cellent; the measured sections contain less than 10 percent cover. Consequently, the Abo-Hueco transitional zone is well suited for documenting the depositional environ- merits and cyclicity of a mixed siliciclastic-carbonate se- quence.

METHODS

Eight sections of the Abo-Hueco transitional strata were measured with the aid of a Jacob's staff and Brunton compass. In addition, sections at Santa Rita and in the

Anlmo$

O~ Hatct~t Mrs.

' ~ / /

; Hueco lower !

' . . . . . . j Member I , i Hueco

upper I~m~r T'C ' 2"'d

t o meters

Vertical ~ole

Organ Mes.

middle Hueco Member lower Hueco

Member

San ~dres Mrs.

g

mlm

middle Hueo dember

lower Hueco Member

Fro. 2.--Correlation of Abo-Hueco transitional strata in southwestern New Mexico. Shaded intervals were measured for this study. No hor- izontal scale intended.

Cooke's Range were measured as part of this study, but were subsequently interpreted to be north of the transition zone. Over 95 percent of the individual units were sam- pled and covered intervals trenched to expose fresh rock. One hundred twenty carbonate and 25 sandstone/silt- stone thin sections were examined. Twelve shale samples were analyzed with a Norelco-PhiUips X-ray diffractom- eter. Glycolated and unglycolated runs were performed on each sample. Vertical sequence analysis was performed on data from the Peloncillo, Animas, Florida, Robledo, Dana Ana, San Andres, and Organ Mountains. The Big Hatchet section was excluded because of a significantly greater percentage of covered intervals than is present in the other seven stratigraphic sections.

STRATIGRAPHY

Wolfcampian sedimentary rocks in the zone of tran- sition between the Abo and Hueco Formations have been divided into two formations, with the lower locally sub- divided into as many as four members (Fig. 2). In the southern San Andrcs, Organ, and Dona Ana Mountains, the basal Wolfcampian unit is the Hueco Formation, which is subdivided into three members (Fig. 2; Seager et al. 1976, Seager 1981). The lower and middle Hueco mem- bers are predominantly limestone. The middle member is ovcrlain conformably by a transitional sequence of in- terbedded red siltstone, gray shale, and limestone, which is called the Abo-Hueco member. In the Robledo Moun- tains, the lower and middle members are overlain by the Abo tongue, which consists of interbeddcd red siltstone, gray shale, and limestone, and the upper Hucco member, which contains a thick, ledgc-forming limestone and thin- ner red siltstone and gray shale (Fig. 2; Seager et al. 1976). The Abo tongue and upper Hueco member are lithologi- cally similar to and occupy a similar stratigraphic position

CYCL IC SEDIMENTATION, MIXED S IL IC ICLAST IC-CARBONATE DEPOSIT 637

as the Abo-Hueco member. The Abo-Hueco member in the southern San Andres Mountains is overlain by redbeds of the Abo Formation, which lack interbedded marine limestone. The Abo Formation is absent in the Robledo, Organ, and Dona Ana Mountains due to post-Permian erosion. Seager (1981) suggests that the lower part of the Abo Formation in the southern San Andres Mountains is coeval with and grades laterally into the upper portion of the Abo-Hueco member in the Organ Mountains.

In the Florida Mountains the Permian Hueco For- mation is undifferentiated and consists of marine lime- stone, gray shale, and four thin siltstone beds (Clemons and Brown 1983). This Hueco section correlates lithologi- tally with the Abo-Hueeo member of the Hueco For- marion farther to the east (Fig. 2).

In extreme southwestern New Mexico, Arizona strati- graphic names are applied to Permian rocks. These rocks correlate in lithology, age, and depositional environment with the Abo and Hueco Formations of south-central New Mexico. In the Big Hatchet and Animas Mountains, Wolfcampian strata include the upper half of the Hor- quilla Limestone and the Earp Formation (Zeller 1965; Zeller and Alper 1965). The upper Horquilla Limestone consists of thick, massive limestone beds separated by thinner beds of shale and limestone and appears to be lithologically and chronologically equivalent, based on invertebrate fossils, to the lower and middle Hueco mem- bers (Fig. 2; Zeller 1965). The Horquilla Limestone is overlain by the Earp Formation, which includes red silt- stone and shale, as well as a few thin beds of limestone and dolomite near the top of the section in the Big Hatchet Mountains. The Earp Formation does not contain as many marine limestones as the Abo-Hueco member, but oth- erwise is lithologically similar.

The Wolfcampian section in the Peloncillo Mountains consists of the Horquilla and Earp Formations. The Hor- quilla Limestone is mostly Pennsylvanian in age, but the upper part is Wolfcampian (Gillerman 1958; Armstrong et al. 1978; Drewes and Thorman 1980a, b). Conformably overlying Wolfcampian limestones of the HorquiUa For- mation is a sequence of interbedded red siltstone, gray shale, and limestone which has been mapped variably as the lower Earp Formation (Gillerrnan 1958; Armstrong et al. 1978) or as the upper member of the Horquilla Formation (Drewes and Thorman 1980a, b). This inter- val is lithologically equivalent to the Abo-Hueco member (Fig. 2). The uppermost unit of probable Wolfcampian age, the Earp Formation, is composed of red siltstone and shale and is lithologically equivalent to the Abo For- mation.

This study is concerned exclusively with the Abo-Hue- co member of the Hueco Formation and its lithologic equivalents (Fig. 2). The correlations in Figure 2 probably do not represent time lines, but instead represent se- quences of rock of approximately the same age that were deposited in similar environments. The Abo-Hueco member is in effect a lithosome bounded below and southward by manne carbonates and above and north- ward by nonmarine siltstone and shale. The most precise dating of Abo-Hueco transitional strata comes from the

Abo tongue and upper Hueco member in the Robledo Mountains, which are determined to be Late Wolfcam- pian (LeMone et al. 1971, LeMone et al. 1975). The other sections have not been as precisely dated but probably are similar in age.

LITHOFACIES DESCRIPTIONS

Olive-Gray Shale Facies

The olive-gray shale (0.3-16 m thick; 2.3 m, average thickness) is the most common facies within the Abo- Hueco transitional zone, constituting 54 percent of the total thickness. This facies changes color upward from olive-gray, gray or tan to red in some sections and has a range of weathered colors. It is fissile to blocky and con- rains very rare organic debris (mostly plant fragments) and/or ostracods. Locally, burrows are present, and rare- ly, this facies has a marked, mottled (perhaps due to bioturbation) character. The shale is often silty and, very rarely, contains thin siltstone interbeds. It is usually a slope-forming interval. The principal clay mineral is illite. Smectite, chlorite, hematite, dolomite, and calcite are present in some samples. This facies may have sharp or gradational boundaries with the fossiliferous limestone and tipple-laminated sandstone facies.

Ripple-Laminated Sandstone Facies

This facies (0.2-8 m thick; 1.6 m, average thickness) consists of very fine- and fine-grained sandstone to coarse, arenaceous siltstone. It composes 14.6 percent of the Abo- Hueco transitional zone. It is a tan to yellowish brown weathering ledge former.

Ripple laminae (mostly of asymmetrical ripples) are the dominant physical sedimentary structure (Fig. 3a). Climbing ripples occur locally. Ripple cross-laminae are present in 2- to 10-cm-thick sets and often indicate that deposition within any single sandstone bed was from uni- directional currents. However, bi- and polydirectionality of currents are indicated at many localities. Sparse, low- amplitude, some flat-topped, and some interference rip- pie forms are also present. Horizontal laminae in 2- to 20-cm-thick sets and desiccation cracks are not uncom- mon. Convolute laminae are rare. Carbonate rock frag- ments and shell fragments are present locally as are very rare raindrop impressions and minor, planar crossbeds.

Plant debris and fern or other plant impressions are locally common. Burrowing is rare or absent to locally abundant. Vertical burrows are 1 to 2 em long and 0.3 cm in diameter, whereas sinuous horizontal burrows are 0.5 cm wide and several centimeters long. Tracks and trails are also locally common on some bedding surfaces.

The ripple-laminated sandstone facies occurs in beds that are traceable hundreds of meters and in some places grades laterally into the sandstone channel facies. The ripple-laminated sandstone facies may have sharp or gra- dational vertical boundaries with the olive-gray shale and mixed sandstone-shale facies.

638 GREG H. MACK AND I4: C. JAMES

CYCLIC SEDIAIENTATION, MIXED SILICICLASTIC-CARBONATE DEPOSIT 639

Mixed Sandstone-Shale Facies

The mixed-sandstone-shale facies (1--6 m thick; 2.4 m, average thickness) is defined on the basis of interbedding on the scale of 5 to 20 cm (Fig. 3b). It represents 3 percent of the Abo-Hueco transitional zone. Sandstone beds are very fine- to fine-grained, generally micaceous, and grade into coarse siltstone. The facies is dominated by asym- metrical ripple laminations, commonly indicating bipolar current directions. Wavy and convolute laminae are rare. Desiccation cracks are locally present. Most beds are tab- ular and laterally continuous, but some pinch and swell, whereas others are broadly lens-shaped. Plant debris, horizontal burrows, and bioturbated zones are not un- common.

Red, green, or gray micaceous shale is typical of the mixed sandstone-shale facies. Thin, silty, wavy laminae occur locally. The shale weathers fissile to blocky, com- monly contains fine organic debris (mostly plant mate- rial), and locally is mottled or burrowed.

Nodular Shale Facies

The medium-gray, nodular shale facies (0.5--4.0 m thick; 1.5 m average thickness) has a distinctive blocky char- acter both in weathered and fresh exposures (Fig. 3c). It represents 1.7 percent o f the total thickness of the Abo- Hueco transitional interval. The principal clay mineral is illite. Carbonaceous debris is common, and locally the facies possesses a mottled appearance. It is occasionally silty and locally consists of pinkish, weathered, mottled siltstone.

This facies is identified based on the presence of 2- to 10-mm-sized, irregular to spherical calcareous nodules (Fig. 3c). In thin section, the calcareous nodules consist of micrite enclosing floating grains of sand- and silt-sized silicate grains. Similar nodules with downward-tapering calcareous root casts are well developed in fluvial Abo rocks in the Cooke's Range. Both the root casts and the calcareous nodules are found as clasts in interbedded con- glomerates, demonstrating their probable pedogenic or at least their very early diagenetic origin.

The nodular shale facies usually has a gradational lower contact with the mixed sandstone-shale facies and a sharp upper contact with the fossiliferous limestone facies.

Channel Sandstone Facies

The channel sandstone facies, although rare (< 1 per- cent of total thickness) is very distinctive. Broad, shallow channels typify this facies (5 to 50 cm depth; 1 to 12 m

width). However, a large 5 m by 40 m channel was ob- served at the Dona Ana section (Fig. 3d). Within this channel large-scale epsilon(?) foresets are present. Chan- nels usually have a distinct scour base, locally contain rip-up clasts, and can truncate several meters of adjacent strata. There are shale drapes and discontinuous shale beds, generally less than 10 cm thick. Crossbeds and rip- ple laminations with possible reactivation surfaces are also present. Rare composite foresets (> 1 m thick) of internally rippled and bioturbated sandstone grade lat- erally to rippled sandstone. Mottled, bioturbated, or bur- rowed zones up to 30 em thick, along with plant debris, are common within some horizons. The sandstones are usually fine- to very fine-grained.

Fossiliferous Limestone Facies

The gray, weathered, ledge-forming, fossiliferous lime- stone facies (0.1-13 m thick; 1.7 m average thickness) constitutes 25.8 percent of the Abo-Hueco transitional zone. Not uncommonly, individual limestone beds are argillaceous or are separated by shale partings or thin shale layers less than 10 cm thick. The two most common lithologies are bioclastie wackestone/packstone and pel- leted wackestone/packstone, which correspond to stan- dard microfacies 9 and 19, of Wilson (1975), respectively. Bioclastic wackestone/packstone consists of a variety of broken and whole fossils in a micrite matrix (Fig. 3e). Some fossils have well-developed micrite envelopes. Fos- sils include foraminifera, bivalves, gastropods, echino- derm columnals, echinoid spines, ostracods, brachiopod shells and spines, bryozoa, and phylloid algae. Peloids, intraclasts, and detrital silt and sand are uncommon. Bio- turbation has often homogenized the texture of the lime- stone, and burrows are visible on bed tops and, less com- monly, within beds. Burrows range from small-scale, sinuous to branching forms, a few millimeters wide and long, to larger burrows up to several centimeters wide and 30 cm long. Some burrows display an internal scalloped wall lining suggestive of a pelleted texture. A few beds have indistinct wavy laminations and are graded, but most appear to be massive.

Pelleted wackestone/packstone is composed of peloids and a restricted fauna of ostracods and foraminifera (Fig. 3f). The relative abundance of the three principal allo- chems varies. Some beds are dominated by ostracods and peloids, whereas other beds are peloid- and foraminifera- rich. Bivalves and gatropods are minor constituents. Much less common are beds of foraminifera grainstone, intra- clast foraminifera grainstone, and silty micrite with only a few scattered foraminifera.

(.-.

FxG. 3.--Selected facies of Abo-Hucco Wansitionai strata: a) Asymmetrical ripples of the tipple-laminated sandstone fades. Scale equals 10 cm. b) Interbedded rippled sandstone (ledges) and shale (recesses) of the mixed sandstone-shale facies. Scale equals 15 crn. c) Calcareous nodules and blocky weathering in the nodular-shale facies. Scale equals 10 cm. d) Portion of sandstone channel, Dona Aria section. Approximate channel boundaries defined by dashed lines. Maximum thickness of channel is 5 m. e) Photomicrograph of a bioclastic wackestone of the fossiliferous limestone facies. Bar scale is 0.5 mm long. 0 Photomicrograph of an ostracod-rich pelleted wackestone of the fossflfcrous limestone facies. Bar scale is 0.5 mm long. g) Laminated carbonate facies showing laminations, fenestral fabric, and vertical burrows. Pencil is 15 cm long.

640 GREG H. MACK AND W. C. JAMES

TABLE 1,--Cycles ident~fwd in outcrops of Abo-Hueco transition strata. Asymmetrical Cycle (see Fig. 4A); Symmetrical Cycle Type 1 ( ripple- laminated sandstone; see Fig. 4B); Symmetrical Cycle Type H (see Fig

4C)

Percent Number Number Percent Number of Symmet- Symmet- of

Section Asym- Section rical Percent of rical Section Thickness metrical Thick- Cycles Section Cycles Thick-

Section Locations (Meters) Cycles ness Type I Thickness Type i l ness

San Andres Mts. 151 3 30 23 66 6 4 Robledo Mts. 116 2 18 18 75 5 7 Dona Ana Mts. 119 4 30 22 68 3 2 Organ Mrs. 186 2 15 33 82 5 3 Florida Mrs. 125 0 0 22 100 0 0 Animas Mts. 167 0 0 16 100 0 0 Peloncillo Mts. 64 0 0 8 100 0 0

The fossi l i ferous l imestone has a sharp lower contact w i th the nodu lar shale facies. The upper and lower con- tacts are gradat iona l to sharp when assoc iated w i th the o l ive-gray shale.

Laminated Carbonate Facies

The laminated carbonate facies (0.2-1.5 m thick; 0.4 m average th ickness) fo rms a smal l percentage (0.9 per- cent) o f the Abo-Hueco t rans i t iona l strata. The un i ts have tan to gray weather ing. Beds conta in dist inct , wavy lam- inae wh ich resemble LLH s t romato l i tes (Fig. 3g). Calcite- fil led vugs 0.1 to 1 cm in d iameter are present in some beds, impar t ing to such beds a fenestral fabric. There are also local in t rac last or brecc ia hor izons , as well as irreg-

ular, cr ink ly laminat ions that may be due to des iccat ion or mic ro topography deve loped on algal mats . Add i t ion - ally, the upper few cent imeters o f some beds are bur- rowed. A l though most ly l imestone, th is facies is local ly do lomi t ic .

STATISTICAL ANALYSIS OF VERTICAL SEQUENCE

In terbedded sequences o f the prev ious ly descr ibed fa- cies appear on outc rop to be ar ranged in three d is t inct types o f cycles. The most common is a symmetr ica l cycle invo lv ing fossi l i ferous l imestone, o l ive-gray shale, and r ipp le - laminated sandstone (Table 1). A var ia t ion o f th is cycle invo lves the a l te rnat ion o f fossi l i ferous l imestone and o l ive-gray shale (55 percent o f the cycles). A th icker, asymmetr ica l cycle, invo lv ing fossi l i ferous l imestone , ol- ive-gray shale, r ipp le - laminated sandstone , mixed sand- stone shale, and nodu lar shale is also vo lumetr i ca l ly im- por tant . The asymmetr ica l cycle is a lways observed in outc rop to be over la in by fossi l i ferous l imestone. The least common cycle ident i f ied in outc rop cons ists o f lam- inated carbonate and fossi l i ferous l imestone (Tab le 1).

To test these field observat ions we have col lected ver - t ical sequence in fo rmat ion f rom seven wel l -exposed st rat igraphic sect ions (452 facies t rans i t ions) conta in ing a wide var ie ty o f facies types. For the purpose o f stat ist ical analysis , the channe l sandstone- fac ies is g rouped wi th the r ipp le - laminated sandstone facies, because o f the i r com- mon assoc iat ion and the pauc i ty o f channe l sandstones .

Based on facies d i s t r ibut ion and abundance , Markov

TABLE 2.-- Transition count, expected, x 2 and normalized differences (Z) matrices, Group I (A-D). Transition count matrix, Group IH (E). B-D matrices constructed following Powers and Easterling (1982)

A, Transition Count Matrix B. Exl~ccted Matrix

OGSH NSH MSSH RLS FLS LC OGSH

OGSH - 0 0 52 83 11 146 NSH 0 - - 0 0 l0 0 l0 MSSH l I0 - - l 0 0 12 RLS 39 0 12 -- i 1 53 FLS 93 0 0 1 -- 8 102 LC 13 0 0 1 6 -- 20

146 l0 12 55 100 20 C. z Matrix

OGSH NSH MSSH RLS FLS LC

OGSH - - 6.3 7.7 5.0 NSH 6.4 -- 0.2 1.0 MSSH 5.8 480.0 -- 0 RLS 0. l 1.0 97.2 -- FLS 1.8 2.1 2.6 10.9 LC 0 0.3 0.4 0.6 E. Transition Count Matrix

FLS OGSH RLS

FLS -- 29 0 OGSH 30 - 25 RLS 0 25 --

0.1 0.3 29.7 0.4 2.6 0.4

10.4 0.5 - - 3.5 0.7 --

NSH MSSH RLS FLS LC

OGSH - - 6.3 7.7 38.2 80.2 13.0 NSH 6.4 -- 0.2 1.0 2.1 0.4 MSSH 7.7 0.2 -- 1.2 2.6 0.4 R~ 36.9 1.0 1.2 -- 12.3 2.0 FLS 81.0 2.1 2.6 12.8 -- 4.4 LC 13.0 0.3 0.4 2.1 4.3 --

D. Normfl izedDif ferencesMa~x

OGSH NSH M~H R~ FLS

OGSH -- -2.5 -2.8 +2.2 +0.3 +0.6 NSH -2 .5 - - -0 .4 +1.0 +5.5 -0 .2 MSSH -2.4 +21.8 -- -0.1 -1.6 -0.6 RLS +0.3 -1.0 +9.8 -- -3.2 -0.7 FLS +i.3 -1.4 -1.6 -3.3 -- +1.7 LC 0.0 -0.5 -0.6 - 1.5 +0.8 --

A) Matrix of observed facies transitions. Abbreviations: OGSH = olive-gray shale; NSH = noduar shale; MSSH = mixed sandstone- shale; RLS = ripple-laminated sandstone; FLS = fossfliferous limestone; LC = laminated carbonate.

B) Estimated expected cell frequencies. C) X2ro, = 678. Calculated for 19 degrees of freedom. D) Normalized differences (Z) matrix. Positive cell values considered

only in construction of Markov diagrams. E) Same as A above.

CYCLIC SEDIMENTATION, ~IIXED SILICICLASTIC-CARBONA TE DEPOSIT 641

chain statistics have been compiled for two groupings of data: 1) Group I-four sections with all facies transition types represented (San Andres Mrs., Dona Ana Mts., Or- gan Mrs. and Robledo Mts.), and 2) Group II--composite section (pooled data from all seven stratigraphic sections). A third grouping (Group III--Animas Mts., Florida Mts., and Peloncillo Mrs.) could not be treated separately by Markov analysis due to a low number of facies transition types (D. Powers, per. comm., 1984). Differences in facies abundance and distribution suggest, for purposes of re- lating depositional process to vertical sequence, that the data be discussed in terms of the first and third groupings mentioned above. Data tables for Groups I and III are presented (Table 2; all other data tables are available from the authors).

The Group I data base consists of 343 facies transitions arranged in a 6 x 6 matrix (Table 2). Following Powers and Easteding (1982), an expected data matrix, X z matrix (approximately Chi-square distributed), and a Z matrix (normalized differences matrix) are tabulated. The quasi- independence model for x 2 = 678 is much beyond the 99.9 percentile of the Chi-square distribution for 19 de- grees of freedom. Hence, there is considerable evidence against the hypothesis of quasi-independence. Stated another way, there are statistical grounds in addition to outcrop observational information strongly indicating there was vertical sequence dependence (memory) within the depositional system.

Markov diagrams have been constucted based on data mainly from Group I (Fig. 4). These diagrams, coupled with outcrop observations, strongly support the existence of three types of cycles within the Abo-Hueco transitional strata. The most statistically significant sequence is the asymmetrical cycle (Fig. 4a). From a statistical stand- point, the weakest step in this cycle is the transition from the basal fossiliferous limestone to the olive-gray shale. This in part may be due to the olive-gray shale having been very widespread within the depositional setting. Per- haps the boundaries of the olive-gray shale facies with the fossiliferous limestone facies were very gradational or there may have been isolated patches of carbonate de- position within the shale facies. All other transitions in this asymmetrical cycle are very strongly supported in a statistical sense.

The symmetrical cycle, involving fossiliferous lime- stone, olive-gray shale, and ripple-laminated sandstone (Fig. 4b), does not show up well in a statistical sense for Group I data probably because of the influence of facies transitions forming asymmetrical cycles at these strati- graphic locations. However, the observed distribution of these three lithologic states has an almost perfectly sym- metrical distribution based on data from Group III (Table 2). Unfortunately, the Powers and Easterling (1982) ap- proach does not differentiate between perfectly symmet- rical cycles and a random facies distribution for the spe- cbSc case of 3 x 3 (1 degree of freedom) data sets (D. Powers, per. comm., 1984).

Finally, the carbonate-dominated, symmetrical cycle defined by the fossiliferous limestone and laminated car- bonate facies is also supported by vertical sequence anal-

ysis. However, this type of cycle appears statistically less significant than the asymmetrical cycle (Table 2; Fig. 4c). Repetition of fossiliferous limestone and olive-gray shale is also common, but appears statistically less significant than the cycles discussed above (Table 2).

DEPOSITIONAL MODEL

The Abo-Hueco transitional zone occupies a paleogeo- graphic position between meandering-fluvial facies of the Abo and Earp Formations to the north and shallow-ma- rine carbonate facies of the Hueco and Horquilla For- mations to the south (Fig. l; Jordan 1975; Kottlowski et al. 1975; Broadhead 1983; Cappa and MacMillan 1983). Therefore, the Abo-Hueco transitional zone sediments were deposited within nearshore and/or shoreline envi- ronments. The presence in the Abo-Hueco transitional zone of fossiliferous marine limestone and siliciclastic rocks, some displaying evidence of subaerial exposure, supports this conclusion. Interbedded sandstone and shale, the abundance of ripple laminations, bi- and polymodal paleocurrent data, shale drapes, reactivation surfaces, desiccation features, restricted faunas, channel features, and fining-upward siliciclastic sequences suggest that the shoreline environment was a tidal flat (Fig. 5; Reineck 1972).

Fossiliferous limestone is interpreted to be the most seaward facies in the Abo-Hueco depositional system (Fig. 5). Bioclastic wackestone/packstone has a diverse fauna, including filter feeders such as bryozoa, brachiopods, and cnnoids or blastoids, and indicates normal marine con- ditions (LeMone et al. 1971; LeMone et al. 1975). Pelleted wackestone/packstone contains a more restricted, gen- erally ostracod-rich fauna, indiccating a brackish-water en- vironment (LeMone et al. 1971; LeMone et at. 1975). The predominance of micrite in both types of limestone reflects quiet-water deposition. Although the fossiliferous limestone is the most seaward facies, it was probably deposited in water less than 10 m deep (LeMone et al. 1975).

The olive-gray shale facies contains few features di- agnostic of depositional process. Perhaps the best indi- cator of its proper place within the depositional model is the fact that the olive-gray shale commonly directly over- lies or underlies either the fossiliferous limestone facies or siliciclastic rocks that display evidence of subaefial exposure (Fig. 4a, b). This stratigraphic position suggests that the olive-gray shale is either marine or shoreline in otigin. Because of the lack of evidence of subaerial ex- posure, the olive-gray shale is interpreted to have been deposited in a low-energy, shallow-marine setting shore- ward of the offshore limestone and seaward of the tidal flat (Fig. 5). The olive-gray shale resembles lagoonal shale, but there is no evidence that the Abo-Hueco shoreline was protected by either carbonate or detrital islands or shoals.

The siliciclastic tidal fiat is represented by tipple-lam- inated sandstone, mixed sandstone shale, nodular shale, and channel sandstone facies (Fig. 5). In addition, tidal- fiat carbonate sediment also occurs, but rarely. The ripple-

642

A. Asymmetrical Cycle

Meters 0 i

I0

8

6

4

2

GREG H. MACK AND W. C. JAMES

B. Symmetrical Cycle I 0 -

Fossiliferous Limestone 8-

+21.8 6 bJ

J ~ Mixed Sandstone -

Shale

T E t~ Ripple-

~ Laminated Sandstone

+2.2 I z

I Ol~e-iray ~nale + J.~ Meters 0

Fossi liferous Limestone

F f Fossiliferous

I ~ F ) Limestone f lg l ; i~!o

o, ie rO, w I Rip le-

meon high

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decreases landward, because wave energy is greatest near mean low tide (Reineck and Singh 1975, p. 358). The presence of probable tidal-flat structures and the strati- graphic association with the olive-gray shale and fossil- iferous limestone facies suggest that the ripple-laminated sandstone facies was deposited near mean low tide (Fig. 5). It may be possible that some beds of tippled or cross- bedded sandstone were deposited in a subtidal setting, but the presence of desiccation cracks prohibits subtidal deposition for most of the beds of this facies.

The mixed sandstone-shale facies always directly over- lies the rippled-laminated sandstone facies (Fig. 4a). The mixed sandstone-shale facies is similar to the tipple-lam- inated sandstone facies except for abundant interbedded shale. Sedimentary structures, lithologies, and facies as- sociations indicate that the mixed sandstone-shale facies was deposited on an intertidal flat shoreward of the ripple- laminated sandstone facies, a depositional subenviron- ment designated here as an intertidal mixed flat (Fig. 5).

The ripple-laminated sandstone and mixed sandstone- shale facies are truncated by the channel-sandstone facies, and locally the ripple-laminated sandstone facies grades laterally into the channel sandstone facies. The structures and facies associations suggest that the channel sandstone represents tidal creek and/or estuarine deposition (DeRaaf and Boersma 1971; Terwindt 1971; Van Beek and Koster 1972). The small channels probably represent tidal creeks that headed on the tidal fiat, whereas the largest channels probably are estuarine, although they could not be traced shoreward into fluvial facies. The channel-sandstone fa- cies is rare, indicating that the Abo-Hueco tidal flat was traversed by very few tidal creeks and estuaries.

The nodular-shale facies always directly overlies the mixed sandstone-shale facies and is interpreted to have been deposited in a supratidal setting (Figs. 4a, 5). The distinguishing feature of this facies is carbonate nodules, which resemble stage II caliche nodules (Gile et al. 1981). Similar nodules, interpreted to be pedogenic, are found in fluvial Abo rocks near Socorro, New Mexico (Broad- head el at. 1983), and in the Sacramento Mountains (Del- gado 1977; Speer 1983). An intertidal mudflat origin is ruled out for the nodular-shale facies, because pedogenic carbonate nodules require long periods of subaerial ex- posure to form.

Intertidal deposition is also represented by the lami- nated carbonate facies. Stromatolitic laminations and fe- nestral structures are common features of intertidal car- bonates (James 1979). Carbonate tidal-flat sediment may have been deposited during periods of low siliciclastic influx or in isolated pools or along bays within the sili- ciclastic tidal flat (KeUerhals and Murray 1969).

The tidal-flat environment of the Abo-Hueco transi- tional zone has characteristics that are similar and dis- similar to modern tidal flats along the margins of the North Sea (Evans 1965; Tcrwindt 1971; Van Beck and Kostcr 1972; Reineck 1972, 1975) and along the north- western margin of the Gulf of California (Thompson 1975). Indeed, the Abo-Hueco tidal flat can be viewed as a hybrid of these two modern analogs. The Abo-Hucco tidal flat is similar to North Sea tidal flats in that both

CYCLIC SEDIMENTATION, MIXED SILICICIL4STIC-CARBONATE DEPOSIT 643

Fig. 5.--Interpretation of facies distribution for Abo-Hueco transi- tional strata. Facies names (lowercase letters) and depositional envi- ronment interpretations (capitalized) arc indicated on the diagram.

display a landward decrease in grain size, which results in fining-upward progradational sequences. Segregation of grain size on the Gulf of California tidal flat is much less pronounced, perhaps because of the limited size range of sediment brought to the shoreline from the Colorado River delta. The interchannel regions of both Abo-Hueco and North Sea tidal flats are characterized by ripple lam- inations, which are uncommon along the northwest coast of the Gulf of California. The Abo-Hueco tidal flat differs from North Sea tidal flats by being finer-grained and hav- ing no coarse-grained, high-energy subtidal facies. These differences suggest that Abo-Hueco sediment was derived from source areas that were either of lower relief or were more distal than source areas for the North Sea tidal flats, and that the coastline of the Abo-Hueco transitional zone experienced lower wave energy than North Sea tidal flats. The dominance of tidal energy over wave energy may reflect widespread Early Permian cratonic submergence (Klein 1982). There also is no evidence in the Abo-Hueco interval of offshore islands, bars, or shoals, which are common along the eastern margin of the North Sea. Fine grain size and the paucity of tidal channels in the Abo- Hueco tidal flat are similar to the Gulf of California tidal flat, where the coarsest sediment is very fine-grained sand and the depositional surface is virtually undissected by tidal channels. Finally, the supratidal facies of the Abo- Hueco transitional zone appears to be intermediate be- tween North Sea and Gulf of California supratidal sedi- ments. The North Sea supratidal zone, in response to temperate climatic conditions, is heavily vegetated, and the sediment contains abundant organic matter and root mottling, which are not common in the Abo-Hueco in- terval. Although pedogenic caliche nodules in the Abo- Hueco supratidal facies indicate arid or semiarid paleo- climate, similar to the Gulf of California, the Abo-Hueco

644 GREG H. MACK AND W. C. JAMES

interval apparently lacks evaporite minerals that are com- mon in the supratidal sediment of the Gulf of California.

Abo-Hueco transitional strata compare unfavorably with beach/barrier and delta depositional models. The Abo-Hueco sections contain too much shale for a beach/ barrier environment and display fining-upward progra- dational sequences rather than coarsening-upward pro- gradational sequences common to beach/barrier systems. The Abo-Hueco interval also lacks laminated and cross- bedded foreshore and upper shoreface facies, which are diagnostic of beach/barrier sediment. A deltaic model can also be ruled out because of the thinness of the siliciclastic sediment and because of the lack of a subaqueous dis- tributary mouth-bar sandstone facies in the Abo-Hueco interval.

Although the depositional model in Figure 5 applies to all of the measured sections in the Abo-Hueco transitional zone, the location of the section within the transitional zone controls the relative abundance of each facies (Table 2). Those sections which were located near the seaward boundary of the transitional zone are dominated by fos- siliferous limestone, olive-gray shale, and tipple-lami- nated sandstone. In contrast, the sections located along the landward edge of the transitional zone have a much higher elastic to carbonate ratio, and have more sandflat, mixed-flat, and supratidal sediment. Most seaward are the Peloncillo Mountain and Florida Mountain sections, whereas the most landward sections are found in the An- imas and Big Hatchet Mountains. Sections in the Rob- ledo, Dona Ana, San Andres, and Organ Mountains con- tain the full range of facies.

ORIGIN OF CYCLIC SEDIMENTATION

Cyclic sedimentation in the Abo-Hueco interval, illus- trated in Figure 4, can be understood within the context of the depositional model (Fig. 5). The symmetrical cycles involve the superposition of facies that are interpreted to have been adjacent (Figs. 4b, 5). Vertical stacking of fos- siliferous limestone, olive-gray shale, and ripple-lami- nated sandstone facies indicates seaward progradation of the facies zones. Conversely, ripple-laminated sandstone overlain by olive-gray shale and fossiliferous limestone suggests landward shifting of the facies zones. The asym- metrical cycle also reveals a vertical facies change com- mensurate with seaward progradation of the shoreline (Fig. 4a). However, the asymmetrical cycle is always overlain by fossiliferous limestone, the most offshore fa- cies, indicating the transgression bypassed siliciclastic de- position. Finally, the symmetrical carbonate cycle, which involves the vertical stacking of laminated carbonate- fossiliferous limestone-laminated carbonate, reflects sea- ward followed by landward movement of the carbonate facies zones (Fig. 4c).

The total number of cycles per stratigraphic section ranges from 8 to 40 and averages 25 (Table 1). Each symmetrical cycle, as well as the asymmetrical cycle and the overlying fossiliferous limestone, records a transgres- sion and a regression. An estimate of the average duration of each transgression and regression will provide an im-

portant constraint on the interpretation of the origin of the cycles. The best chronologic control is for the Robledo Mountains section, where the Abo-Hueco member was determined to be upper Wolfcampian (LeMone et al. 1971, LeMone et al. 1975). The Robledo section has a total of 50 transgressions and regressions (25 cycles). I f Late Wolfcampian is assumed to span 8 million years (half of the Wolfcampian) (Harland et al. 1982), then the average duration of each transgression and regression is 160,000 years. Although this number is speculative at best, it does provide a first-order estimate of the scale of cyclic changes in the Abo-Hueco interval.

There is a variety of different mechanisms that produce cyclic sedimentation. Allocyclic mechanisms include in- terrnittent tectonic uplift or basin subsidence, climatic fluctuation, and eustatic sea-level change. Furthermore, lateral shifting of facies zones, independent of changes in outside variables, may result in autocycles (Beerbower 1964). A single control on Abo-Hueco cyclic sedimen- tation is not demonstrable; instead, the relative merits of each potential mechanism will be discussed.

Autocyclic shifting of facies zones may be a viable mechanism for the origin of symmetrical cycles, espe- cially those involving only two facies, such as fossiliferous limestone and olive-gray shale. Irregularities in the boundaries between facies zones or patches of carbonate within the shale zone could result in vertical facies changes independent of outside variables. However, vertical changes involving three or more facies, such as fossilif- erous l imestone--ol ive-gray shale--r ipple- laminated sandstone or the asymmetrical cycle, are less likely to be autocyclic, because of the significant change in elevation and water depth implied by the facies changes. An au- tocyclic origin seems unlikely for deposition of the fos- siliferous limestone facies above the nodular shale, be- cause of the exclusion of three intermediate facies. An absolute change in sea level appears to be necessary to explain these cycles. Furthermore, the Abo-Hueco tidal flat does not seem to be the type of environment to undergo rapid autocyclic lateral shifts in facies zones or in the locus of clastic influx. The Abo-Hueco tidal flat was pre- sumably of low relief, had few tidal channels and estu- aries, and experienced relatively low rates ofterrigenous elastic influx, as indicated by sediment accumulation rates on the order of l0 m/m.y. Thus, allocyclic variables were probably more important as controls on cyclic sedimen- tation than autocyclic variables.

Among the allocyclic variables, intermittent tectonism seems the least likely to have controlled Abo-Hueco de- position. The Abo-Hueco transitional zone was an area of low refief and tectonic stability. Fine grain size indi- cates that the Abo-Hueco shoreline was far removed from upliRs in northern New Mexico and Colorado and from the Pedernal Uplift in south-central New Mexico (Fig. 1). Furthermore, the overall trend in the Abo-Hueco strati- graphic interval is for grain size to decrease upsection. A systematic fining-upward trend is also evident in the Abo Formation in the Sacramento Mountains, which reflects tectonic quiescence and gradual erosion of the Pedernal Uplift (Speer 1983). The Abo-Hueco transitional zone

CYCLIC SEDIMENTATION, MIXED SIL IC1CLASTIC-CARBONA TE DEPOSIT 645

was also subsiding at a relatively slow rate, indicated by a sediment accumulation rate on the order of 10 m/m.y., a rate more characteristic of cratonic and miogeoclinal basins than to more tectonically active basins (Schwab 1976). Published isopach maps of Lower Permian rocks suggest that the areas of maximum subsidence were southeast of the transitional zone, in the Orogrande basin, and southwest of the transitional zone, in the Pedregosa basin (Kottlowski 1965; Greenwood et al. 1977). Even if subsidence were episodic, it is questionable that it could account for depositional cycles on the scale of 105 years (Clifton 1981). Intermittent subsidence due to sediment loading also seems an unlikely mechanism for Abo-Hue- co cyclic sedimentation. The load necessary for mantle response corresponds to a precompaction sediment thick- ness of about 50 m, whereas most of the Abo-Hueco cycles are less than 15 m thick (postcompaction) (Mat- thews 1974).

An allocyclic variable that may have influenced sedi- mentation in the Abo-Hueco interval is paleoclimatie fluctuation. Hays et al. (1976) suggest that variations in the earth's solar orbit in the late Pleistocene and Holocene resulted in climatic cycles that range from 22,000 to 100,000 years in duration. The latter value is similar to the calculated "average" cycle for the Abo-Hueco inter- val, and this mechanism has been called on to explain cyclic sedimentation of Miocene oceanic sedimentation offthe west coast of Africa (Dean et al. 1977) and middle Miocene shoreline sediment in California (Clifton 1981). However, there is no evidence of climatic fluctuations in the sedimentary rocks of the Abo-Hueco transitional in- terval. The characteristics of the caliche nodules, which provide the best paleoclimate indicators, appear constant throughout the sections.

The allocyclic variable that is probably the most viable as the control on Abo-Hueco deposition is glacial-eustatic sea-level changes. The 160,000-year "average" transgres- sion and regression of the Abo-Hueco interval is well within the realm ofglacial-eustatic sea-level change (Don- ovan and Jones 1979). Carboniferous and Permian gla- ciation was widespread in Gondwanaland (Crowell 1978) and has recently been cited as the driving force for cyclic sedimentation of Carboniferous rocks in the United States and Europe (Saunders et al. 1979; Heckel 1980; Driese and Dott 1984). A small rise or fall in sea level could have resulted in a major shift of the shoreline, because of the low-gradient of the Abo-Hueco tidal flat. A glacial eustatic model may not be independent of small-scale climatic fluctuations discussed above. A slight change in the Earth's heat budget due to orbital eccentricities could have caused partial melting or net growth of polar ice caps and eustatic sea-level changes (Hays et al. 1976).

Glacial eustacy may be the best model to explain the origin of the asymmetrical cycle and the overlying fos- siliferous limestone (Fig. 4a). During periods of sea-level fall or stillstand the shoreline prograded seaward, result- ing in the asymmetrical cycle. Dunng a major sea-level rise, shoreface erosion, the trapping of terrigenous sedi- ment in drowned estuaries, and the decrease in erosion due to higher base level combined to reduce drastically

the amount ofdetfital sediment brought to and deposited at the shoreline. Consequently, the first sediment to be deposited during the transgression would be the offshore fossiliferous limestone (Ryer 1977). Glacial eustacy may also explain the symmetrical cycles, which involve fos- siliferous limestone, olive-gray shale, and ripple-lami- nated sandstone (Fig. 4b). These cycles reflect smaller- scale sea-level changes than the asymmetrical cycles, be- cause the most landward facies in the symmetrical cycle is the ripple-laminated sandstone. During stillstand or minor sea-level fall, the tidal flat prograded seaward, pro- ducing the regressive part of the symmetrical cycle. A minor sea-level rise would not greatly inhibit the supply of detrital sediment brought to the shoreline, and thus, the transgressive part of the symmetrical cycle did not bypass detrital facies. Support for this model may come from thickness differences between the asymmetrical and siliciclastic-dominated symmetrical cycles. If the mag- nitude of sea-level rise is proportional to the thickness of the asymmetrical cycle and trangressive part of the sym- metrical cycle, then the thicker asymmetrical cycle sup- ports the idea of a greater sea-level rise.

Although glacial-eustatic sea-level change appears to be a reasonable mechanism for cyclic sedimentation in the Abo-Hueco interval, a true test will be future com- parisons of Abo-Hueco cycles with other upper Wolfcam- pian cyclically deposited sediment in North America and on other continents. This approach, recently employed by Saunders and others (1979) for Carboniferous rocks in Arkansas and England, not only provides strong evi- dence in favor of glacial-eustatic control on cyclic sedi- mentation but also provides insight into the number of eustatic changes and how they affect various depositional environments.

CONCLUSIONS

The transitional zone between the nonmarine, siliei- clastic Abo Formation and the marine, carbonate Hueco Formation illustrates an excellent example of mixed silic- iclastic-carbonate strata deposited in large measure under cyclical depositional conditions. The main conclusions of this study are as follows:

1) There are seven major facies present within this tran- sition zone. These facies include olive-gray shale, rip- pie-laminated sandstone, mixed sandstone-shale, nod- ular shale, channel sandstone, fossiliferous limestone, and laminated carbonate.

2) These seven facies were deposited within areas ranging from shallow-marine, carbonate- and shale-dominat- ed settings to tidal-fiat and supratidal environments.

3) Sedimentation was dominated by cyclical patterns of deposition. Three types of cycles are recognized: 1) asymmetrical cycle (from base to top): fossiliferous limestone--olive-gray shale--ripple-laminated sand- stone--mixed sandstone-shale--nodular shale; the nodular shale is in turn always overlain by the fossfl- iferous limestone of the next cycle; 2) symmetrical cycle: fossiliferous limestone--olive-gray shale--rip-

646 GREG H. MACK AND ~: C. JAMES

pie-laminated sandstone--olive-gray shale--fossilif- erous limestone; and 3) symmetrical cycle: fossilifer- ous l imestone-- laminated carbonate--fossiliferous limestone.

4) Although autocyclic shifting of facies areas may be a viable mechanism for the origin of some symmetrical cycles, allocyclic-related processes appear much more likely for vertical changes involving several facies or asymmetrical cycles. Of the several possible allocyclic mechanisms, glaeial-eustatic sea-level changes appear most plausible.

ACKNOWLEDGMENTS

We are grateful to Dennis Powers for his assistance with Markov chain analysis. W. R. Seager and R. E. Clem- ons provided helpful information on outcrop locations.

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