Late Paleozoic Sedimentation in West Texas Permian Basin'


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Th e Afneriu n Asscciatio n o f Petroleu m Geologist s Bulleti n
V. 56, No, 1 2 (Decembe r 1972), P. 2303-2322,1 3 Ras,
Late Paleozoic Sedimentation in West Texas Permian Basin'
El Paso, Texas 79999
Abstract Rocks of Permian age have been known in
West Texas for over a century, and in the last 40 years
the drilling of numerous test holes has made subsur-
face information available concerning these rocks.
These data have enabled geologists to correlate beds
of late Paleozoic age across the basin, a distance of
more than 200 mi. Results of these correlations are
presented in a series of eight paleogeographic maps.
In early Paleozoic time this region formed a broad,
generally shallow basin. In the middle Paleozoic the
basin slowly filled with carbonate and siliceous rocks.
In Mississippian time a positive median belt developed
in a north-south direction, separating the larger basin
into the eastern or Midland basin and the western or
Delaware basin. In Pennsylvanian time wide carbonate
shelves developed around these basins, especially the
eastern one. At the south end of the basins tectonic
activity increased and foredeeps developed and were
filled with flysch material.
In the Early Permian, seas spread over much of the
basin region, depositing shale in the low parts of the
sea floor while limestones accumulated on the higher
parts. However, by Middle Permian (Leonardian) time,
marine circulation was restricted and evaporites be-
gan to form. This restriction became increasingly se-
vere during the remainder of the period. Thus carbon-
ate reefs and banks formed on the margins of the
Delaware and Midland basins, as well as on the Cen-
tral Basin carbonate platform, overlying the old me-
dian mountain range.
In the latter part of this period, high limestone reefs
were formed on the basin edges, separating basinal
sediments from evaporltic and clastic lagoonal strata.
The Capitan reef surrounding the Delaware basin Is a
notable example. All these formations show traces of
cyclical deposition which may be attributed to eustatic
changes in sea level, perhaps glacial in origin.
During the closing epoch of the period, thick se-
quences of anhydrite, halite, and potash salts accumu-
lated In the basin areas. After a final marine transgres-
sion, continental redbeds covered the region and Per-
mian sedimentation ended.
The first recognition of the Permian rocks in
the West Texas area was by Jules Marcou (1855)
who identified fossils of this age collected on
Captain John Pope's expedition of 1853. In 1855
Pope returned to the southern end of the Guada-
lupe Mountains with Dr. George G. Shumard as
"surgeon and geologist." Shumard described the
rocks of the Guadalupe Mountains and collected
fossils, which his brother, B. F. Shumard (1858)
identified as Permian. Little more was learned of
these rocks until Girty's visit to the base of Guad-
alupe Point in 1901, where he made the extensive
collections that formed the basis for his classic
paper, "The Guadalupian Fauna" (Girty, 1908).
In the meantime, Permian rocks had been dis-
covered in the Mid-Continent area, and during
World War I and immediately afterward petro-
leum geologists traced them on the outcrop from
Kansas and Oklahoma into north-central Texas.
As the rocks of this region seemed to be entirely
different from those cropping out in extreme
western Texas and south-central New Mexico,
both in lithology and fauna, little more could be
said about them than that both were Permian
(Beede, 1910). Furthermore, changes in facies,
especially on the western side of the basin, made
correlation even over short distances very diffi-
cult. Crandall's (1929) discovery of the remarka-
ble Upper Permian section in Rocky Arroyo
north of Carlsbad, New Mexico, although at first
misinterpreted as containing an unconformity,
was a step forward in understanding the relation
of the evaporite sections to the carbonate rocks.
Later Bates (1942) pointed out the gradation in
this section from bedded dolomite of the back
reef facies to the gypsum and red elastics of the
saline lagoon.
Lloyd (1929), then associated with the Euro-
pean geologists of the Shell group, realized that
the massive limestones forming Guadalupe Point
were parts of a great reef complex similar to that
previously recognized in the Triassic rocks of the
Tyrolean Dolomites. Also during the late 1920s
Sidney Powers discovered a late Permian turritel-
Ud in a core of thick-bedded dolomite from the
Hendrick field (Hills, 1940). This indicated that
perhaps the same upper Permian beds found on
the surface also occurred in the dolomite reef
facies of the subsurface. Further drilling con-
firmed this and also showed that carbonates in
'Manuscript received, November 7, 1971; revised and ac-
cepted, March 17, 1972.
^University of Texas at El Paso.
My colleagues, William S. Strain and Karl W. Klement, have
read this paper critically and made many suggestions. Many
other geologists have, in innumerable discussions over the
years, contributed to my understanding of the late Paleozoic in
this region. To all these I am grateful. I especially acknowledge
the suggestions made by the late John Emery Adams in the
early stages of preparation of this paper.
Sally W. Hills did the prehminary editing; Jerry King and
Martha Thomas typed the manuscript.
© 1972. The American Association of Petroleum Geologists. All rights
John M. Hills
the subsurface graded into redbeds and then into
evaporites, much as they do in Rocky Arroyo.
By 1930, Permian subsurface correlations were
sufficiently advanced so that Cartwright was able
to draw an east-west cross section through the
southern part of the Permian basin. This work
showed that the fine clastic materials of the Dela-
ware basin were facies of the massive Capitan
carbonate reef complex, as well as of the dolo-
mites of the Central Basin platform. The similar,
but older, elastics of the Midland basin also were
facies of the Eastern platform carbonates which
graded eastward into the red shale and gypsum of
the long-known north-central Texas Permian.
From 1934 to 1938 P. B. King studied the
outcrop of the Permian rocks in the Guadalupe
Mountains and adjacent Diablo platform, and
published a paper (King, 1942) in which he com-
bined surface information with subsurface data
to construct a comprehensive overview of the
Permian geology of the region. More details were
given in a professional paper, the publication of
which was delayed by World War II (King,
In 1939 Adams et al, using both surface and
subsurface information, proposed that the Per-
mian of the West Texas region be divided into
the following series in ascending order: Wolf-
N. Mexico
1 Permian \
N, \ Basin J
Chih, \ ^v_^^
\ 4
j Coah*\^
—Index map showing location
Okla. 1
Texas /
of Permian basin region.
camp, Leonard, Guadalupe, Ochoa. This pro-
posal was favorably received and the names now
are generally used.
Thus by 1942 the essential framework of our
knowledge of the Permian basin was complete, as
far as the upper part of the Permian system is
concerned. However, many details of correlation
and sedimentary facies remained obscure. Some
of these have been clarified by later work and
some remain uncertain today.
Very little was known in 1942 about the geol-
ogy of the Lower Permian and almost nothing of
the Pennsylvanian and older Paleozoic rocks in
the central part of the basin, although they had
been studied to some extent on the outcrop along
the eastern and western edges. Classifying the
geology of the older rocks was to occupy the
petroleum geologists of the basin for the next 20
In early Paleozoic time, the region now known
as the Permian basin (Figs. 1, 2) formed a broad,
generally shallow basin which merged with the
Ouachita-Marathon geosynchne in the south and
shoaled on the north in south-central New Mex-
ico and the Texas Panhandle. This area has been
named Tobosa basin by Galley (1958). During
most of the early and middle Paleozoic, deposi-
tion consisted largely of shallow-water shelf-type,
carbonates, but was interrupted by shale sedi-
mentation during the Middle Ordovician, Late
Devonian, and Early Mississippian.
The stratigraphic section from the Precambrian
to the base of the Mississippian is thin (about
6,000 ft) over most of the region. Rocks repre-
senting many parts of this interval are missing
from the stratigraphic column. As there is little
angular discordance at the hiatuses, it is difficult
to determine whether the missing beds are absent
because of erosion or nondeposition.
These rocks contain many zones of chert, no-
vacuhte, and other siliceous rocks. These zones
possibly represent long intervals of extremely
slow deposition of silica from solution and sus-
pension (Wilson and Majewske, 1960). These
time intervals may fit into the hiatuses and re-
duce the length of the intervals considerably.
During the early Paleozoic there seemed to
have been no well-marked platforms within the
basin. However, lines of weakness along strike-
slip faults in the basement probably were present
(Hills, 1970). Along these faults later vertical
movement took place. In Early Mississippian
time a different paleogeographi c regime began.
The ancient Tobosa basin began deepening on
Late Paleozoic Sedimentation in West Texas Permian Basin 2305
FIG. 2—Map of Permian basin showing localities and features referred to in text.
John M. Hills
-- ^ MT 6fi
s.s.a SH,
pow wg!ft
/ GR.
Flo. 3—Correlation of rock units of Permian basin. Arrows indicate horizons used as datums for paleogeographic maps (Figs. 4-9,
either side of a medial zone bounded by some of
the old basement faults.
In response to increasing tectonic activity, be-
ginning in the later Mississippian, broad basins
formed in the east and west parts of the Tobosa
basin. Black shale was deposited in the deep
central parts of the basins while broad carbonate
shelves formed around the margins. The black-
shale deposition probably was slow, and much of
it may have taken place during times of slight
sea-level sinking (Adams et ah, 1951). Peterson
and Ohlen (1963) described a similar relation
between the black shale and carbonate deposits
in the Pennsylvanian of the Paradox basin.
At the beginning of Pennsylvanian time strong
reef and bank growth continued along the edges
of the platforms, resulting in a great complex of
reefs and banks at different levels and of different
ages (Owen, 1962; Van Siclen, 1969). Each period
of reef growth reflects a time of rising sea level
because the reef-forming organisms must be cov-
ered with at least a minimum of water to survive,
and lowering of the sea level will kill these orga-
nisms and stop reef growth. Theoretically a still-
stand would cause horizontal basinward growth
of the reef by overgrowing its own submarine
talus. However, there probably are few stillstands
that last for any appreciable span of geologic
time, so such growth is rare. Marine recession
and a subsequent partial rise of sea level would.
of course, result in new reef growth at a lower
level on the forereef talus, unless a flood of terres-
trial material smothered the reef organisms. Be-
cause most of the reef stages can be traced later-
ally for tens of miles, the changes in sea level
probably were eustatic rather than tectonic.
On the broad carbonate shelves of Pennsylva-
nian time there must have been wide channels
between separate banks, with reefs growing at
some points. This is inferred from the fact that
the backreef sedimentary rocks do not show the
changes to red and green shales and evaporites
that are found in other reef complexes of other
ages and in those of the same age in other re-
The western subbasin of the Tobosa basin is
known as the Delaware. Along its northwestern
edge in Eddy County, New Mexico, Pennsylva-
nian banks developed especially during Missou-
rian and Canyon deposition. At other times
many long stretches of strand were formed, upon
which siliceous sand derived from highlands in
central New Mexico accumulated.
The Pennsylvanian also was a period of in-
creased tectonic activity. South and east of the
Permian basin a range of mountains grew out of
the Ouachita-Marathon geosyncline (Flawn,
1961, p. 56-58), and great thicknesses of flysch
materials—Tesnus and Raymond Formations-
accumulated in the foredeeps in front of the
Late Paleozoic Sedimentation in West Texas Permian Basin
FIG. 4—Paleogeography of Texas and New Mexico in earliest Permian time.
range. At the same time, the median section of
the Permian basin was folded into a mountain
range separating the Delaware basin on the west
from the Midland basin on the east (Hills, 1963).
At approximately the same time the Matador
uplift was rising to mark the northern boundary
of the basin. This tectonic activity resulted in
vertical movement along the ancient strike-slip
faults, with some new faulting taking place in the
recently deposited Pennsylvanian rocks. Most of
the structural petroleum reservoirs in the Paleo-
zoic rocks of the entire Permian basin region took
final shape at that time, especially the deep gas
reservoirs of the Delaware and Val Verde basins.
Thus, in the latter part of the Pennsylvanian,
the general framework of the Permian basin had
been established, with two subbasins separated
by a rapidly eroding mountain range. Broad car-
bonate shelves on the east side of the main Per-
mian basin were backed by coal swamps in what
is now north-central Texas.
Although many wells have penetrated into the
pre-Pennsylvanian rocks, literally tens of thou-
sands of holes have been drilled into the Pennsyl-
vanian and Permian strata of West Texas and
southeastern New Mexico. These tests have made
possible detailed correlations of cutting samples,
cores, and electric logs so that time lines have
been well established. Many of these correlations
are shown on the cross sections published by the
West Texas Geological Society (Jones, 1949; Da-
vis, 1953; Scobey et a/., 1951; Feldman et al.,
1962; Vertrees et al., 1964), the Roswell Geologi-
cal Society Stratigraphic Research Committee
(1956), and The American Association of Petro-
leum Geologists (Maher, 1960). The U. S. Geo-
logical Survey's "Paleotectoni c Investigations of
the Permian System" (McKee er al., 1967) has
made available much information on areas out-
side the main Permian basin.
Figure 3 is a correlation chart which shows the
author's interpretation of the time relations of the
various formations of seven regions in, and adja-
cent to, the Permian basin. In each of these ba-
sins the contemporaneous rocks are generally of
diff"erent facies and have received different
names. Many more names have been applied to
these rocks, but Figure 3 shows only those in
common use at the present time. From this cor-
relation it should be possible to reach certain
conclusions with respect to paleogeographic con-
ditions at specific times in the late Paleozoic of
the region. I made an earlier attempt at such a
synthesis (Hills, 1942).
The present work starts with paleogeographi c
conditions al the close of the Pennsylvanian (Fig.
4). This map is drawn on a time horizon in the
John M. Hills
middle Harpersville Formation, as found on the
east side of the basin. On the west side of the
basin in central New Mexico this time horizon
probably is represented by the base of the Bur-
sum hmestone, whereas farther south it is placed
in the Hueco Limestone at the base of the Pow-
wow Conglomerate Member. In the Marathon
region it falls in the upper part of the Gaptank,
and in Kansas it is in the upper part of the
Waubaunsee Group which grades to the red clas-
tic material of the Pontotoc facies of Oklahoma.
The sea at this time had retreated from its
Middle Pennsylvanian stand. Perhaps this retreat
was mainly eustatic, but it was compKcated by
increased tectonic uplift. Folding and uplift had
left mountainous areas in central New Mexico,
southern Oklahoma, and central Texas, and low
mountainous islands projected above the sea in
the central part of the basin in southeastern New
Mexico and adjacent Texas. In the Texas Pan-
handle a granite ridge formed low-lying hilly is-
lands around which debris accumulated, forming
the arkosic "granite wash" gas and oil reservoir
of the Panhandle field. The "granite wash" is
analogous to, and partly coeval with, the Foun-
tain sandstone of the southern Front Range of
the Rockies. The Abo Redbeds of New Mexico
were derived from the lowlands of the central
upHft where the Precambrian crystaUine rocks
were exposed in limited areas in the core of the
central mountain range and were the source of
small patches of arkose around the mountains.
In spite of the constriction of the seas by eu-
static sea-level change and tectonic activity, there
is no evidence in the sedimentary rocks of this
age that marine circulation was impaired to the
extent of causing hypersalinity of the water and
deposition of evaporites. However, salinity may
have increased slightly on the platforms, resulting
in increased amounts of calcium carbonate avail-
able to the reef- and bank-building organisms.
Thus, on the platforms many reefs and banks
caused limestone deposition over great areas. In
the platform area of northern Lea, eastern
Chaves, and northern Eddy Counties, sedimenta-
tion probably was continuous from Pennsylva-
nian to Permian time (Meyer, 1966). The great
horseshoe atoll in the central part of the basin
was in the latter stages of its growth at this time.
This atoll and other reefs and banks developed
large amoimts of porosity through the leaching
action of circulating meteoric water. This poros-
ity was added to remnants of primary porosity
and to secondary porosity caused by recrystalU-
zation from limestone to dolomite. Porous zones
originating in these three ways now form impor-
tant petroleum traps.
At the same time, in the Delaware and Mid-
land basins between the platforms, thin beds of
shale were laid down. In the Val Verde foredeep
in front of the rising Marathon mountains, thick
layers of sedimentary rocks accumulated, many
of them terrigenous. Over these, thrust sheets
advanced from the mountains.
In extreme eastern Texas and southeastern
Oklahoma a seaway may have existed some time
during the Early Permian and possibly in the
Late Pennsylvanian, in which the Eagle Mills and
related beds were deposited (McKee et ai, 1967).
Subsurface evidence as to the age of these beds is
rather scarce, but is sufficient to justify showing
this feature on Figure 4 and subsequent maps.
After the beginning of Permian time tectonic
activity subsided, but the higher platform areas
remained exposed. However, fine sand, silt, and
mud were deposited in the Delaware and Mid-
land basins. In the Marathon and Val Verde
foredeep this material was interbedded with
sandy calcareous deposits which Ross (1963) has
called Neal Ranch Formation. At about the mid-
dle of Wolfcampian time the final episode of the
Marathon orogeny occurred, during which the
Dugout Creek thrust formed and carried lower
Paleozoic folded rocks out over the Neal Ranch
shale (Feldman et ai, 1962). The writer examined
water-well samples from sites north of Marathon
which show lower Paleozoic rocks lying on yel-
low-green shale of Wolfcamp lithology with little
apparent deformation of the shale. This orogenic
episode resulted in the deposition of the coarser
clastic rocks of the lower Lennox Hills Forma-
tion (Ross, 1963) and a corresponding increase of
sandstone deposition in the eastern part of the
Val Verde basin.
After the erosion of the highlands raised by the
Marathon orogeny in the Big Bend region and of
hills formed by the same episode at the positive
centers in central New Mexico and southwestern
Oklahoma, a slow advance of the seas began.
This flood covered all the basin and resulted in
deposition of fossiliferous limestone and shale,
except on the hilltops at the south end of the
central mountains exposed in the Fort Stockton
high. In the north and west, great aprons of ar-
kose spread out from the exposed crystalline
rocks of central New Mexico and the Amarillo-
Wichita mountains.
In north-central Texas, shale and some lime-
stone were deposited. One of these beds is the
Late Paleozoic Sedimentation in West Texas Permian Basin
FIG. 5—Paleogeography of middle Wolfcampian time.
well-known Coleman Junction Limestone Mem-
ber of the Putnam Formation, which is the bed
on which Figure 5 is based.
At the same time, in the central part of the
basin, fringing reefs and carbonate banks were
growing on the flanks of the old mountain range
and converting it into the Central Basin platform.
The late Wolfcampian was the last time during
the Permian when marine circulation was unre-
stricted and normal marine sediments were de-
posited. Later Permian sedimentation was domi-
nated by the growth of extensive carbonate reefs
and banks which severely restricted marine circu-
lation. This restricted circulation, combined with
cyclic or repeated lowering of sea level, gave rise
to shallow, barred basins (Woolnough, 1937), in
which great thicknesses of evaporites were depos-
Figure 6 shows how concentration of saline
material takes place according to Lang (1937),
whose concept fits the known occurrences in the
Permian basin. Seawater enters a lagoon over a
bank or reef, forming a shallow bar. The barrier
hinders the return flow of the highly saline denser
water that has been concentrated by evaporation
in the shallow lagoon behind the bank. This pro-
Norma l
Penesalint Supersaline
Sand a
Red clays
Free circulflfion-
Area of restricted circulation
1 • _ - Q o J , •_ .< • ' Q A I. J X Anhydrite Halite a Red
Lime a Sand Lime Mg. Lime a Anhydrite g
White a Red
Potash solts beds
Character of
tRote of
Composition of
Color of
FIG. 6—Conditions of evaporile deposition in Late Permian time (modified from Lang. 1937).
John M. Hills
cess requires an evaporation rate far in excess of
the freshwater inflow from any tributary streams,
and if it operates on a large scale, the chmate
must be warm and dry.
This concept fits the known occurrences of
saline material in the Permian basin, therefore,
the Permian climate probably was similar to that
prevailing there today—a horse latitudes desert.
To move this area north more than 5° of latitude
by any reconstruction under the theory of conti-
nental drift and polar wandering would place it
in the belt of prevaihng westerhes and a more
humid regime. To move the region 10° or more
south would bring it into the tropical wet-dry
zone. Neither of these chmatic zones is compati-
ble with the type of sediments laid down during
most of Permian time. The global circulation that
determines the general location of climatic belts
is dependent on differences in insolation from
equator to poles, and on the earth's rotation. As
these relations probably have not changed since
the Permian, the climatic belts were generally the
same then as today. Thus it seems that the Per-
mian basin region must have been near its pres-
ent latitudinal position during most of the Per-
mian Period.
Lang's (1937) reconstruction of evaporite-de-
positing conditions is not far from a later theory
of Landes (1963) called the "long fetch" theory,
in which the necessity of a bar is eliminated by
gradual evaporation and increasing salinity over
a wide shallow shelf However, in the case of the
Permian lagoons, initial concentrations of the
seawater by evaporation at the shelf edge made
available calcium ions in greater than normal
concentration for use by carbonate-secreting or-
ganisms. These organisms in turn built up the
shelf-edge barrier, which further impeded circula-
tion and promoted the formation of evaporites in
the lagoonal area. Porosity which developed in
these shelf-edge carbonates forms many impor-
tant petroleum reservoirs in Middle Permian
Life, largely in the form of calcium carbonate-
secreting sponges and bryozoans, was confined to
the shelf edge and part of the adjoining lagoon,
where biohermal mounds were abundant. Away
from the basin edge the salinity of the water
increased abruptly and the only organic remains
are those which were washed into this environ-
ment by occasional storms. Details of the reef
and bank zone are shown by Silver and Todd
(1969), although their models do not emphasize
the importance of the broad areas of evaporitic
deposition in the backreef zones. In this penesa-
Une zone, concentration of seawater to perhaps a
fifth of its primary volume took place, and depo-
sition of anhydrite began. The increased concen-
tration of magnesium ions in the dense layers
near the bottom probably caused the dolomitiza-
tion of the previously deposited calcareous muds
forming the floor of the lagoon (Adams and
Rhodes, 1960). Still further concentration in the
saline and supersahne zones, probably in water
only a few feet deep, resulted in deposition of
halite. In limited areas evaporation went essen-
tially to completion and potassium salts were
precipitated. Many of the supersahne lagoons
were close to shore, so that red sand and clay
interfinger with the various salts. Local thin lime-
stones are found in association with these clastic
beds, indicating that occasionally there was
enough fresh water draining off" the land to re-
duce the salinity of the lagoonal water considera-
Though Lang's (1937, Fig. 24) diagram shows
only one marine fades, I have shown an addi-
tional facies called "pontic." This term was intro-
duced by Lloyd (1938) to denote black shales and
very fine sandstones, a facies called "euxinic" by
some authors. It is quite distinct from the grayer
shales and coarser sandstones of normal marine
basins, and seems to have been deposited in the
depths of restricted basins where circulation was
too poor to allow oxidation of organic material,
but not restricted enough to make possible the
deposition of evaporites. Some lighter colored
shales and fine sandstones in this facies may be
the result of turbidite flows which carried oxi-
dized material from basin edges to deeper water
The effect of the restricted basins in the saUne
and supersahne zones first becomes apparent in
the lowermost Leonard rocks of the West Texas
region. However, evaporite deposits are promi-
nent in the older Wolfcamp beds of Kansas, indi-
cating that recession of the Permian sea took
place from north to south (Adams, 1963). The
basal Leonard formation of west-central Texas is
the Belle Plains. The Valera Member of this for-
mation contains gypsum on the surface which, in
the subsurface of Concho County, grades into
nearly 200 ft of anhydrite and shale of the saline
facies and then to dolomite of the bank-edge
vitasaline zone.
Beds above this member show increasing
amounts of evaporite in the backreef facies and
thick dolomite on the shelf edges. A few thin
sandstone beds are present, which probably re-
flect occasional eustatic lowering of sea level with
consequent spreading of terrigenous sediments
Late Paleozoic Sedimentation in West Texas Permian Basin
FIG. 7—Paleogeography of middle Leonardian lime. Shelf carbonate deposition is starting to separate the Delaware and Val Verde
over the shelves (Silver and Todd, 1969). Figure 7
reflects conditions just prior to the spread of one
of these sand sheets, the FuUerton-Drinkard
(King, 1945). This sandstone is correlated ap-
proximately with the Tubb Sandstone of Silver
and Todd (1969) and the middle Yeso Formation
on the New Mexico side of the basin, as well as
the Vale Formation on the east side of the basin.
At this stage the seas completely covered the
West Texas-New Mexico basin, and sedimenta-
tional fades were well marked. Brackish water
sediments formed a fringe around the edge of the
sea, and only in Oklahoma and northern New
Mexico do they cover any extensive area. Saline
seas covered most of the central part of the basin,
extending far north into Kansas. This arrange-
ment of facies indicates that the land surrounding
this sea probably was both low-lying and defi-
cient in rainfall.
At the same time the vitasaline or limestone-
depositing sea covered all of the southern Central
Basin platform and most of the shelf area on the
east and northwest side of the basin, probably
extending south into Chihuahua. Black shales
with subordinate amounts of fine sand were de-
posited in the depths of the Midland and Dela-
ware basins. The southern extent of these sedi-
ments is unknown, but they appear to thicken
southward and may once have been continuous
with open-sea beds in South Texas and northern-
most Mexico.
In late Leonardian time there was a consider-
able uplift of the lands in the three positive cen-
ters surrounding the Permian basin. This activity
was reflected in the upper Yeso sandstone of
New Mexico, the sandstones and shales of the
Choza Formation of central Texas and the Har-
per sandstone of Oklahoma and Kansas. Clastic
deposition culminated in the deposition of great
sheets of coarse fan and deltaic material that we
know as the Glorieta Sandstone in New Mexico,
the San Angelo conglomerate in Texas, and the
Duncan sandstone and Chickasha Formation in
The Glorieta, San Angelo, and Duncan are
basal clastic beds of a thick, widespread carbon-
ate unit, the San Andres Limestone, that is pres-
ent from central Texas to Arizona and Utah and
is correlated with the Kaibab and Toroweap. The
San Andres grades northward into anhydrite and
salt in Oklahoma and Kansas where it is known
as the El Reno Group. It reflects a major advance
of the sea and a short-term reversal of the general
secular contraction of the Permian seas. The San
Andres also is important because it contains
2312 John M. Hills
FIG. 8—Paleogeography at time of greatest spread of Late Permian seas (datum is base of Blaine).
some of the most prolific petroleum reservoirs of
the province. Traps in these rocks are structural
and stratigraphic, reflecting both the growth of
carbonate banks and the gradation of porous
dolomites into impermeable evaporites.
The carbonate rocks of the San Andres repre-
sent a shelf facies, whereas the fine sandstones
and black shales of the Delaware Mountain
Group are basin facies (Fig. 8). The Delaware
Mountain Group by definition (Adams et al,
1939) comprises the Guadalupian Series in the
western part of the region. These two facies can
be traced within a few miles of each other, but
their age relations are still doubtful.
Paleontologic evidence accumulated by Lewis
(1941), Clifton (1945), Skinner (1946), and HoU-
ingsworth (personal commun.) indicates that the
San Andres is equivalent to the Word Formation
of the Glass Mountains, as well as the Brushy
Canyon and lower part of the Cherry Canyon
Formations of the Delaware basin. A summary
of the fusuline evidence is given by Silver and
Todd (1969), with the stratigraphic position of
the fossils shown on their cross sections. In the
area south of Carlsbad, New Mexico, Hayes
(1959) interpreted the lower San Andres as Leo-
nardian or earhest Guadalupian, whereas the up-
per San Andres which disconformably overlies
these beds is equivalent to the Cherry Canyon
(Guadalupian). My correlations indicate time
equivalency of the upper San Andres dolomite of
the southeastern part of the Central Basin plat-
form with basinal beds bearing Guadalupian fu-
sulinids in central Crockett County.
Nevertheless, some Leonard-type fossils have
been found in the San Andres of New Mexico
(Beede, 1910), and recently Meissner (1967) has
reemphasized physical evidence from New Mex-
ico, which seems to show that the San Andres
should be correlated with the Leonard. The San
Andres probably is continuous and coeval with
the Kaibab of Arizona, which is definitely dated
as Leonardian (McKee and Breed, 1969). The
solution to the problem of the exact age of the
San Andres has yet to be found.
On the other hand, the correlation of the San
Andres dolomite with the evaporite and terrige-
nous rocks on the landward side of the shelf
away from the basin is relatively good. There, as
Figure 8 shows, the limestone and dolomite of
the vitasaline seas grade through the anhydrite
and salt of the saline zone into nearshore red
shale and sandstone. Adjacent beds of the forma-
tion are lithologically distinct and can be traced
for long distances so that their gradation into
other facies can be seen plainly. The cross sec-
tions by the West Texas Geological Society (T. S.
Late Paleozoic Sedimentation in West Texas Permian Basin
Jones, 1949; Davis, 1953) and The American
Association of Petroleum Geologists (Maher,
1960) show this in detail. Well-to-well correla-
tions on the shelf around the north end of the
Midland basin do not encounter the problem of
basin facies, but yield strong evidence that the
San Andres dolomite of the platform and shelf
zones is continuous and probably contemporane-
ous with the Pease River Group of western and
north-central Texas and the El Reno Group of
Oklahoma and Kansas.
The maximum San Andres flood took place
early, at about the time of basal Blaine deposi-
tion (top of the "Slaughter" pay zone of Hockley
County in the northern part of the basin; Acme
of Oklahoma). Figure 8, a paleogeographi c map
of this time, shows an extensive area of limestone
deposition. In this region carbonate banks were
present in which scattered reef mounds grew on
local sea-floor highs. This sea probably extended
westward to join the Kaibab sea. Although beds
of this age appear to have been removed by ero-
sion in far western Texas and southwestern New
Mexico, the stratigraphic position and hthology
of the Kaibab are similar to those of the San
Andres (McKee and Breed, 1969). This forma-
tion has not been found in Mexico, but seas of
this time probably covered northern Chihuahua.
Pontic black mud, fine sand, and silt accumu-
lated in the Delaware and Midland basins, prob-
ably augmented by turbid flows from sediment
banks on the basin margins. Although wide-
spread sandstone zones are not present in the San
Andres section, disconformities in the formation
probably represent diastems during which terrig-
enous material was swept over the shelf and de-
posited on the basin slope. As previously noted,
Hayes (1959) believed there was evidence for a
major hiatus in the San Andres limestone of An-
drews County—the upper San Andres of the shelf
edge. Supporting this view is the presence of
small seams of coaly material that have been
identified as fragmentary remains of land plants
(A. C. Noe, personal commun., 1937).
In north-central Oklahoma, Fay (1964) has
called attention to the presence of carbonate
rocks, as well as thick red clastic beds in the
upper El Reno Group. These beds indicate a
deepening Anadarko basin that may have con-
nected with a sea on the north throughout most
of San Andres time. This confirms earlier work
by Chfton (1944).
Along the eastern edge of the Permian and
Anadarko basins, the San Andres evaporites
grade into the red shales and sandstones of the El
Reno and Pease River Groups. Because these
sandstones become coarser toward the east, they
probably lay near the San Andres shoreline.
However, because the beds are truncated by ero-
sion, the strand deposits are not present. No very
coarse material has been found, so by this time
the ancient Ouachita Mountains probably had
been worn down to low relief
On the northwest side of the main Permian
basin the hmestone-depositing area adjoined a
redbed zone, partly marine and partly terrestrial,
perhaps including the lower Bernal Formation.
The mountains of north-central New Mexico
probably were very low at this time and shed
largely fine sediments.
Throughout San Andres time the sea retreated
not continuously, but in cycles broken by a num-
ber of small advances. Each of these advances
left a record in a dolomite bed overlain by anhy-
drite, halite, and red shale deposited in successive
stages of retreat and desiccation of the shallow
sea. The brines present in the later stages of the
cycle probably were agents of the dolomitization
of the underlying carbonates through the reflux
action of magnesium-charged waters (Adams and
Rhodes, 1960). A similar process on a smaller
scale has been described by Kinsman (1969) from
the sabkhas or supratidal salt flats of the Persian
Gulf region.
Naturally such cycles of advance and retreat
are shown better in the San Andres rocks of the
northern part of the Permian basin at the farther
edge of the vitasaline zone. In Floyd County,
Texas, nine such cycles can be detected and in
Castro County, 13 cycles. On the south, the lower
dolomite beds abruptly become thicker and
merge into an unbroken brown granular dolomite
600 ft thick. This section may be correlative with
Hayes' (1959) lower San Andres member.
In the upper 800 ft of the formation, the evapo-
rites grade less abruptly southward into lighter
more finely crystalline dolomite, with some vuggy
and intergranular porous zones. Locally, banks
comprised of very light-colored, coarsely crystal-
line porous dolomite are developed, notably at
Hobbs in east-central Lea County and at Penwell
in southern Ector County. The upper member of
the San Andres is missing, apparently by erosion,
from the structurally high area of central Crane
and northern Pecos Counties. This leaves only
about 800 ft of the lower brown granular cherty
member present, although the normal thickness is
1,400 ft.
In the northern and northeastern part of the
basin the San Andres thins by steps from the top,
through abrupt disappearance of the entire upper
part of each dolomite-evapont e cycle, where the
resistant basal dolomite above it grades into red
shale, anhydrite, and halite. Thus, both on the
John M. Hills
FIG. 9~Paleogeography during Grayburg-Marlow deposition.
southern Central Basin platform and in the
northeastern basin, there seems to be evidence at
the top of the San Andres for a widespread un-
conformity which results in the truncation of
several hundred feet of beds. In the Anadarko
basin the evidence does not seem to be as good
for an unconformity at this horizon, although the
top of the Dog Creek Shale may mark a consider-
able hiatus.
Thus the San Andres dolomite appears to be
bounded below by sandstone and deltaic sedi-
mentary rocks and above by a well-marked un-
conformity. Above this unconformity is a succes-
sion of rocks, the lower part of the Guadalupe
Series, which are quite different in aspect from
the San Andres in that these strata reflect a sharp
retreat of the sea from its San Andres expansion.
Dolomite in the beds above the San Andres is
limited to the southern half of the Central Basin
platform and the Midland basin (Fig. 9). Evapo-
rite beds become increasingly important above
the top of the San Andres in the north-central
part of the Permian basin and include halite, red
sandstone, and shale. These nearshore saline la-
goonal deposits contrast markedly with the thick
dolomitic San Andres beds underlying them.
This change in hthology indicates a major
change in physical sedimentary conditions. Thus,
on physical evidence of unconformity and
change in lithology, the San Andres beds might
better be placed in the underlying Leonardian
rather than in the Guadalupian, paleontologic
evidence notwithstanding. If, however, one
wishes to follow the paleontologic correlation
and place the San Andres in the Guadalupian,
one must postulate that the post-San Andres un-
conformity is not present in the Delaware basin.
It may be that most of the deposition of the lower
Cherry Canyon elastics took place during the
hiatus at the end of San Andres deposition while
the carbonate rocks of the Central Basin platform
were being eroded by solution, leaving no coarse
debris to accumulate in the adjacent basin. Such
solution-type erosion is occurring in Bermuda
today (Bretz, 1960). Probably porosity in most of
the San Andres petroleum reservoirs was devel-
oped at this time and added to that formed by
previous dolomitization.
In any case, it is certain that the vitasaline sea
at the end of San Andres time occupied a very
restricted area in the south end of the basin.
Shortly afterward the sea retreated completely
from the region, except for the Delaware basin.
Following the post-San Andres interval there
Late Paleozoic Sedimentation in West Texas Permian Basin
was another limited advance of the sea. This
flood carried the carbonate-depositing waters
over most of the Central Basin platform and the
southern part of the Eastern shelf, laying down
the Grayburg Formation. The basal part of the
Grayburg consists of sandy dolomite filling the
low areas—remnant s of the Midland basin and
the San Simon channel. The upper part of the
formation is a well-bedded dolomite with sandy
interbeds and a few thin bentonites. The top of
this zone is the datum on which Figure 9 is
drawn. On the basin edges the dolomites thicken
into porous bank deposits that form important oil
The saline sea advanced northward and ap-
proached the south flank of the Amarillo uphft
where it merged with red mud and sand depos-
ited in brackish and terrestrial environments. The
shoreline of this time is difficult to delineate, as
most of the strand rocks have been eroded. How-
ever, in southwestern Oklahoma lagoonal gyp-
sum and thin dolomite of the Marlow Formation
probably were deposited behind the Verden
Sandstone offshore bar (Fay, 1964). This indi-
cates that shallow marine conditions prevailed at
least that far north.
Recession of the seas continued throughout the
rest of Guadalupian time with minor advances,
which probably reflected eustatic rises in sea
level. Major Capitan reef growth continued
around the edge of the Delaware basin above the
basinward reef slopes of the Goat Seep reef, thus
constricting the area of the basin. Growth was, of
course, possible only during times of rising sea
level. The great vertical dimension of the massive
reef (King, 1948) indicates relatively rapid rise in
sea level. The reef seems to have grown from
centers between surge channels. Jacka e( al.
(1967) presented evidence that these channels
may have been true submarine canyons. At times
of reduced sea level the top of the reef wall was
brought above high tide and growth ceased. Then
fine sand was swept through the channels or can-
yons into the Delaware basin where it formed
most of the Bell Canyon Formation.
Where local subsidence was active, such as in
the northeastern corner of the Delaware basin in
southwestern Lea County, New Mexico, the reef
was less well developed and the channels larger.
Probably in such localities greater amounts of
sand were swept through the channels during
recessions, and perhaps this sand transport may
have been almost continuous throughout Guada-
lupian time. The general northeast trend of the
petroleum-producing sandstones {e.g., Ramsey
sand) of the upper Delaware Mountain Group
(Bell Canyon) may indicate a source in this direc-
In addition to this local subsidence there also
was apparently some general subsidence in the
basin area, which deepened the water in these
structurally negative centers and adjoining shelf
edges. However, the cyclic nature of the apparent
sea-level changes would require very small-scale
mobihty of the earth's crust to cause them tecton-
ically. Therefore, short lived sea-level fluctuations
probably were superimposed on long term subsi-
dence to produce the stages of reef growth.
It has been postulated (Jacka et al., 1967, p.
169-172; Wanless, 1967, p. 52-53) that such eus-
tatic changes in sea level may well have been
caused by the locking up of water in the ice caps
of recurring glaciations in Gondwanaland during
the closing periods of the Paleozoic. Jacka and
his colleagues pointed out that increased rainfall
connected with such glaciations could account
for textural changes in the exposed reef lime-
stones, as well as making increased amounts of
fine terrestrial sediment available for deposition
in the basins.
The duration of the times of lowered sea level
must have been relatively short, because no ap-
preciable reef growth is found on the basin slope,
although the great accumulation of reef talus on
the slope should form a good substratum for such
growth. With another rise in sea level, reef
growth would again be resumed. Back of and
parallel with the reef, sand accumulated in lentic-
ular bodies waiting to be swept into the basin at
the next sea-level recession. Such bodies can be
seen clearly in the Rocky Arroyo section (Bates,
1942). They also form important petroleum reser-
voirs in Range 37, Lea County, New Mexico, and
extend south through central Winkler and Ward
Counties, Texas, to the vicinity of Fort Stockton
in Pecos County.
During the times of low sea level and reef-top
emergence, sand advanced over the reef and
backreef carbonates; after dolomitization these
carbonates were leached by relatively fresh water.
This resulted in well-crystallized, blue-gray dolo-
mite with vuggy porosity, usually connected and
permeable. These porous rocks commonly are
overlain by sandy, finely crystalline and poorly
permeable dolomite. Such porous carbonate
zones form a row of prolific petroleum reservoirs
between the sandstone reservoirs mentioned
heretofore and the marine reef growth centers at
the basin edge. These porous zones are illustrated
John M. Hills
Dolomit e
Dolomite, blu e gray, coarsel y crystalline, poroa s
m Sandstone, red to yello w
Anhydrit e
Flo. 10—Generalized east-west cross section through Cooper-Jalmat field in northern part of T24S (location shown on Fig. 11),
showing relation of porous zones in lower Capitan and Goat Seep reef dolomites to sandstone zones of backreef Artesia Group and
lagoonal anhydrites. Sand zones and corresponding tops of reef porous zones may represent limes of arrested reef growth and
development of porosity by groundwater action.
by the section of the Cooper field (Fig. 10) in
which seven cycles are shown. From outcrop evi-
dence, Bates (1942, p. 98) suggested pauses in
carbonate deposition while gypsum and red clas-
tic material were deposited in the backreef zone.
Jacka et al. (1967) described closely related
phenomena from outcropping rocks of the same
age. With each episode of renewed reef growth
the basin became more restricted, until the reef
and its associated hmestones became merely a
strip surrounding a shrunken Delaware basin as
indicated in Figure 11, a paleogeographi c map of
late Capitan time. During this time a high and
narrow reef with few channels, except in the
northeast corner, cut off most circulation of ma-
rine water in the Delaware basin. This feature
probably trapped most terrigenous sediment
from the land areas. Ball et al. (1971) have postu-
lated a depositional situation in which the quartz
sand was laid down in lagoons between the dolo-
mites of the backreef and the evaporites of the
landward ponds. This interpretation seems in bet-
ter accord with my observations than does that of
Silver and Todd (1969) in which the sands are
considered to be continuous blankets overlapping
older carbonates. Lagoon sedimentation in the
Delaware basin would be very slow, except near
the base of the reef where thick talus slopes and
mud slides are preserved as wedges with steep
original dips (Newell et al., 1953; King, 1948).
Bottom flows spread the finer fractions of this
sediment over the deeper parts of the basin.
Limited water circulation might have been
maintained through a narrow strait north of the
Marathon uplift. Perhaps at the close of Capitan
time this strait became constricted and saline
waters accumulated in the depth of the basin,
eventually becoming concentrated enough to de-
posit evaporites.
Back of the reef zones the dolomites and sand-
stones grade abruptly into the anhydrites, red
shales, and salts of the upper part of the Artesia
Group. The stages of reef growth are reflected in
the backreef zone by carbonate spreading fol-
lowed by deposition of anhydrite and hahte (Fig.
10), whereas cessation of growth resulted in the
spreading of clastic material. Back of the reef the
eastern part of the basin was covered with a very
shallow evaporite-depositing sea in which the
concentration was great enough in the northern
extension to precipitate polyhahte, the mixed po-
tassium-bearing sulfate, as well as anhydrite and
hahte (Davis, 1953). The local development of
beds of red shale and sandstone interbedded with
the evaporites shows that the shallow saline la-
goons were at times filled with detritus washed or
blown from lands surrounding the basin.
Late Paleozoic Sedimentation in West Texas Permian Basin
1 O R r '^ /'..
BRA^I^H .'
FIG. U—Paleogeography of late Guadalupian time (upper Capitan deposition). This was epoch of intensive, highly restricted reef
The close of Guadalupian time marks a great
change in physical conditions in the Permian
basin. Organic reef building ceased and evaporite
deposition spread over the entire region. These
uppermost evaporite beds have been studied by
Kroenlein (1939), Adams (1944), Anderson et al.
(1972), and others. The paleogeographic condi-
tions in the early and middle part of this epoch
are shown on Figure 12.
Reconstruction of conditions at the close of the
limestone reef deposition of the Guadalupian and
the beginning of Ochoan deposition presents
some difficulties. At the close of the Capitan
deposition there was nearly vertical buildup of
the upper Capitan limestone. A,short hiatus is
marked by the Ocotillo silt which hes 75-100 ft
above the Yates Sandstone. It is possible that
shallowing of the southwest entrance to the Dela-
ware basin by tectonic action or sedimentary
processes (Adams, 1944) may have interrupted
the return flow of dense saline waters. This may
have initiated a buildup of salts in the bottom of
the basin overlying the black shales and lime-
stones of the Lamar Formation (uppermost Dela-
ware Mountain Group). In some deep parts of
the basin 30-40 ft of dark anhydritic limestone
beds overhe the Lamar. Possibly these beds mark
the beginning of evaporitic deposition.
Such conditions would lead to the diminution
of flow of nutriments to the reef around the Dela-
ware basin. A rise in sea level of perhaps 100-200
ft may have enabled the Capitan reef to have a
final, rather short, growth period, building the
uppermost Tansill equivalent. Following this final
growth episode eustatic lowering of sea level
probably exposed the top of the reef which, to-
gether with the concomitant fiUing of the basin
with supersaline waters, finally killed the reef
If this reconstruction is correct, the lowest Cas-
tile strata are contemporaneous with the upper-
most Tansill beds. This correlation was suggested
by Kroenlein (1939), but because the lowest part
of the basin, where the first evaporite deposition
took place, is widely separated geographically
from the reef top, no substantial evidence has
been found as to the time relations of these beds.
The most striking feature of the Castile Forma-
tion is the laminated anhydrite, which is 1,500-
2,100 ft thick and widespread in the Delaware
basin. These laminations were first described by
Udden (1924) and consist of 176,800 laminations,
1/10 in. to 1/20 in. thick. Each contains a thin
brown layer of calcite crystals stained by hydro-
carbons grading downward into a slightly thicker
lamina of anhydrite. The thickness of the calcite
John M. Hills
FIG. 12—Paleogeography of early and middle Ochoan times showing conditions during deposition of two greatest salt formations of
North American Permian. Geography beyond central part of Permian basin is unknown.
layers is relatively constant, but that of the anhy-
drite layers varies greatly. In parts of the forma-
tion there are especially thick anhydrite beds,
which seem to have resulted from incomplete
cycles. Adams (1944) studied these laminae and
Anderson et al. (1972) made an extremely de-
tailed study of a series of cores taken from this
formation in the deepest part of the Delaware
basin in Ward and Winkler Counties, Texas.
The results of these studies show that there is
uniformity of laminae over many hundreds of
square miles. This argues for some sort of geo-
chemical control, rather than a repeated opening
and closing of a channel leading to the Delaware
basin or a short-term fluctuation of sea level.
Udden's (1924) suggestion that these laminae
were caused by seasonal variation in the compo-
sition in the seawater seems to be confirmed by
later workers (Adams, 1944; Anderson et al.,
1S*72). If this hypothesis is correct, the whole
section of nearly 2,000 ft of laminated Castile
beds was deposited in the remarkably short time
of less than 200,000 years. In the deep part of the
basin, through some interruption of the flow of
water into the basin, several zones of halite each
several hundred feet thick, were deposited as part
of the laminated sequence.
Toward the reef, a few feet of the basal banded
Castile grades into laminated limestone and then
into uppermost Capitan reef carbonate (C. L.
Jones, 1954; Newell et al, 1953). Anderson et al.
(1972) stated that the Bell Canyon Formation
which underlies the Castile also is laminated. As
the Bell Canyon is the basinal equivalent of at
least part of the upper Capitan reef, it seems that
the conditions causing the laminations originated
before the close of Capitan deposition and con-
tinued for some time afterward. Correlation of
beds of the upper Castile in the vicinity of the
Capitan reef indicates that some of them may be
equivalent to the lowermost Salado halite below
the Cowden anhydrite (C. L. Jones, 1954; T. S.
Jones, 1949). Anderson et al. (1972) showed that
the lower 400 ft of the Salado in the basin is
laminated hke the Castile.
As can be seen in Figure 12, the area covered
by the Castile evaporites differs considerably
from that covered by the Salado. This can be
explained most easily by northeastward tilting of
the Castile in the Delaware basin through subsi-
dence along the West Platform basement fault,
which forms the east edge of the basin (Hills,
1970). This tilt would bring the western margin of
the Delaware basin above sea level and spiU the
brines over the top of the Capitan reef zone into
Late Paleozoic Sedimentation in West Texas Permian Basin
'- ^ ^ • v
FIG. 13—Paleogeography of late Ochoan time, final marine flood of Permian time.
the central part of the Permian basin (Fig. 13).
This idea was first pubhshed by Kroenlein (1939).
Because the Salado is composed largely of hal-
ite beds with interbedded thin red shale and some
polyhalite, these evaporites probably were laid
down in small shallow lagoons. Evaporation was
very great, but deposition often was interrupted
by freshwater drainage from the land which
sometimes flooded the lagoons with fresh water.
Evaporation often was sufficient to permit the
deposition of the potassium-bearing sulfates.
Thin sandstone beds containing rather small, but
well-rounded, frosted grains are found, indicating
that these lagoons might have been filled occa-
sionally with sands derived from the continental
areas on the north and east and deposited by
wind in coastal dunes.
The water remained deeper in the northeastern
Delaware basin and spread over the top of the
reef in southwestern Lea and southeastern Eddy
Counties, New Mexico, to form Ochoa Lake
(Kroenlein, 1939). In this lake, evaporation went
on at a high rate, uninterrupted by changes in
water depth or invasion of clastic sediments, ex-
cept for relatively small amounts of wind borne
clay. Here the sylvite, langbeinite, and related
salts that now form the commercial potash de-
posits of the Carlsbad area precipitated from un-
usually concentrated brines. In the potash areas
the upper Salado beds continue over the reef
zone, with only slight thinning.
However, to the south in Texas and extreme
southeastern New Mexico the reef seems to have
been less affected by tectonic subsidence; there-
fore, there were no saline-depositing lagoons over
the top of the reef during Salado deposition. In
this part of the reef zone the Salado is composed
almost entirely of anhydrite with thin salt beds.
The whole Salado section is only 500-600 ft
thick, compared with 1,200-1,300 ft in southeast-
ern Eddy County. In parts of this reef trend in
northern Pecos County the Salado contains sev-
eral beds, 10-15 ft thick, of brown crystalhne
dolomite. It is possible that on the bottom of the
shallow sea the Capitan reef mounds formed bur-
ied protuberances, which raised the sea floor to
the layer of fresher water near the surface, result-
ing in the deposition on the high of dolomite by
bacterial or algal action.
On the north and east the Salado gradually
becomes more clastic, with less halite and anhy-
drite. This effect is marked along the eastern edge
of the Permian basin in Irion, Mitchell, and
Scurry Counties where almost the entire saline
content of the formation is lost. Thus, it is very
probable that the present eastern hmit of the
Salado in these counties is very close to the origi-
John M. Hilts
nal depositional limit. On the north the same
increase in clastic material is noted, but the entire
Salado section seems to be lost by truncation
along the south slope of the Matador uplift be-
fore the northernmost depositional limit is
On the south edge of the basin, truncation
again occurs under Cretaceous beds against the
north edge of the Marathon-Ouachit a uplift. The
Salado depositional limit probably was very
much south of its present line, but distinct thin-
ning is noticed in southern Pecos, northern Ter-
rell, and central Crockett Counties. On the west
edge of the Salado area along the Reeves-Culber-
son County boundary and west of Carlsbad, New
Mexico, the saline rocks of the Salado have been
truncated by later Permian erosion. This trunca-
tion has been accentuated by halite solution from
the underflow of the Pecos River and related
groundwater channels in late Cenozoic time.
Thus the western depositional Umit of the Salado
is uncertain. Nevertheless, the increase in clastic
material in the formation and the thinning as far
west as the Pecos River indicates that perhaps
here, too, the present limit of Salado rocks is
close to the original depositional edge.
The time needed for deposition of the Salado
strata is not so clearly indicated as the time re-
quired to deposit the varvelike laminations of the
imderlying Castile. Probably, however, the salt of
the Salado was deposited during a relatively short
interval, and the entire Ochoan epoch covered a
very small part of the total of the Permian period,
perhaps less than a million years.
Following the end of Salado sedimentation
there was a period of erosion and some solution
of the saline deposits, as the Salado beds on the
east and west edges of the basin are truncated by
the basal elastics of the overlying formation, the
Rustler. Rustler deposition began with a final
incursion of the Permian sea into the southwest-
em United States. This sea deposited a covering
of red shale and sandstone from 10 to 100 ft thick
over the truncated edges of the Salado evaporites.
Probably this material was derived mainly from
the clastic material once part of the Salado For-
Adams (1944) has described the Rustler in de-
tail, and Hills (1942) has pubHshed a cross sec-
tion of the best developed subsurface occurrences
correlated with the type section. These sections
show that the basal Rustler elastics are overlain
by evaporites which are generally anhydritic on
the south but become more saline on the north.
Overlying these evaporites are three dolomite
beds which are thickest in the southern part of
the Rustler area and become less prominent in
the north and east, where they grade first into
anhydrite, then salt, and then redbeds. As these
dolomites phase out, their protective function
against later erosion is lost, and the Rustler thins
abruptly from western Andrews County to east-
em Garza County where it is absent (Fig. 13).
Probably, as in the case of the Salado, the present
limit of the Rustler is very close to the original
depositional hmit.
The Rustler incursion of the Late Permian sea
has been indicated on Figure 13 as vitasahne.
Walter (1953) described a good fauna from the
Rustler of the Culberson County outcrop. Many
of these fossil forms are preserved in anhydrite.
The dominance of pelecypods and gastropods
and their resemblance to backreef Guadalupian
facies forms suggest a hypersaUne environment.
Much of the Rustler dolomite is of the honey-
comb texture, which may reflect leaching of pi-
solitic forms. The fauna is definitely Late Per-
mian, but it cannot be correlated exactly with
Late Permian faunas in other parts of the world.
In Figure 13, I have shown the Rustler sea as
advancing from the southwest and retaining an
outlet in that direction to the open sea. The Tes-
sey dolomite of the Glass Mountains may be
correlative with the Rustler (Adams, 1944). If so,
the Tessey forms a thickening of the Rustler do-
lomites in the basin, perhaps because the Tessey
occupied a position closer to the entrance from
the open sea where there was better access to
nutriment and less saline water.
In the Oklahoma and Kansas area no forma-
tion is present which can be correlated with the
Rustler. However, possibly the Doxey of north-
central Oklahoma (Fay, 1965, PI. II) may corre-
late with some of the Ochoa formations and may
have been deposited in a sea advancing from the
north. Otherwise there is no evidence in the
southwestern United States of the presence of
Permian rocks as young as the Rustler. The sea-
way in East Texas probably was not receiving
sediment at this time and was not reopened until
middle Mesozoic time.
Rustler sedimentation probably ended by com-
plete withdrawal of the sea from the southwest
Texas area. Because there is no evidence of tec-
tonic movement at that time, the probabiUties are
that this was another eustatic movement. In the
few lagoons remaining in the southern part of the
basin, anhydrite deposits formed, and the basin
was quickly filled and covered by sand and silty
shale carried in from the emergent shelf areas on
the north and northeast. These orange-red sandy
shales and siltstones, known as the Dewey Lake
Formation, contain many large (0.5mm) well-
rounded and finely frosted grains. Therefore,
Late Paleozoic Sedimentation in West Texas Permian Basin
wind probably had a prominent part in the depo-
sition of this formation. However, the Dewey
Lake is regularly bedded with traces of soft gyp-
sum so that it probably was deposited in playa
lakes. The Dewey Lake Formation usually is
from 100 to 200 ft thick. With the deposition of
these redbeds, Paleozoic sedimentation in the
southwestern United States ceased.
In Late Paleozoic time slow sedimentation in
broad basins was replaced by sedimentation in a
system of smaller basins sepa'rated by tectonically
positive belts. These basins restricted circulation,
so that calcium-carbonate concentration in the
seawater increased. Carbonate shelves formed
along the basin margins, while fine clastic sedi-
ments were deposited in the deeper parts of the
basins. This restriction was accentuated by tec-
tonic action in Late Pennsylvanian and Early
Permian times, contemporaneous with the Mara-
thon-Ouachita orogeny.
Thus, by Middle Permian time restriction of
marine circulation, coupled with eustatic with-
drawal of the sea in a southwest direction, re-
sulted in growth of high reefs and carbonate
banks. Behind these barriers were wide shallow
lagoons containing highly saUne waters. Here
evaporites were deposited, ranging from fine-
grained dolomite, through sulfates and chlorides,
to potash minerals.
Most of this deposition probably was rapid
and cyclic, being dependent on eustatic changes
in sea level, perhaps glacial in origin. The long-
term trend was gradual withdrawal of the sea
toward the southwest. However, this was inter-
rupted by a strong and fairly long-lived advance
of the San Andres seas, and the smaller and
short-lived Rustler flood close to the end of the
During the Ochoan, the closing epoch of the
Permian, deep-water evaporitic deposition took
place in the Delaware basin and the banded lami-
nated Castile anhydrite was laid down. Later, the
potash-bearing salts of the overlying Salado For-
mation were deposited in shallow lagoons.
After Salado deposition ceased, the Rustler
flood covered the evaporites. Then, a final marine
recession initiated a short interval of continental
deposition before the close of the period and era.
Sedimentation was not resumed in the region
until the Dockum continental redbeds were laid
down in the Late Triassic.
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