-
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/$03.00
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636 GREG H. JVL4CK AND H~ C. JAMES
Son~. ) ~ Pe/oncdlo Andres~ ~
SantoRita Mts. ~
Dana Mts.Ana~ 1 Cooke s ~ @~ Sacromenlo
,Peloncillo Range Organ Mts. Mts. Mts. ~t~ Roblec
c.~Animcs ~ Mts, ! t~Mts. FloridaMts. \ ~ppe 1 tj f~ IBig : . .
. . ~ ~ Horquirlo Member
Hatchet o km 50 / I ts
I ~ "-F, ~ / ~ ~] Uplift ( / . ~q'~v- ~ / ~ Fit ~-~"~:~-~
Hueco
FIG. 1 .--Upper map shows the location of measured sections
(mlid circles) of Al:m-Hueco transitional strata in southwestern
New Mexico. The sections in the Cooke's Range and at Santa Rita
were found in this study to be north of the transitional zone. The
lower map is an Early Permian paleogeographic map of southwestern
New Mexico. North of transitional zone, Abo and FEarp Formations
are siliciclastic redbeds deposited by fluvial systems. South of
transitional zone, Hueco and Horquilla Formations are open-marine
limestone and minor shale. The transitional zone consists
ofinterbedded silicidastic and carbonate rocks. Palinspastie
reconstruction is not attempted, because ofconflieting ideas on the
amount of post-Permian crustal shortening in southwestern New
Mexico (el. Drewes 1978; Seager 1983).
standing cyclic sedimentation. Although the Wolfcam- plan mixed
siliciclastic-carbonate interval is exposed in only eight different
areas in southwestern New Mexico, the quality of exposure at seven
of the locations is ex- cellent; the measured sections contain less
than 10 percent cover. Consequently, the Abo-Hueco transitional
zone is well suited for documenting the depositional environ-
merits and cyclicity of a mixed siliciclastic-carbonate se-
quence.
METHODS
Eight sections of the Abo-Hueco transitional strata were
measured with the aid of a Jacob's staff and Brunton compass. In
addition, sections at Santa Rita and in the
Anlmo$
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
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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.
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638 GREG H. MACK AND I4: C. JAMES
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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.
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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
Iomino~ed ~de
meon low limestone
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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