A Publication of the
Gulf Coast Association of
Peter R. Rose
718 Yaupon Valley Rd., Austin, Texas 78746, U.S.A.
In Central Texas, the Balcones Fault Zone separates the Gulf Coastal Plain from the elevated Central Texas Platform,
comprising the Hill Country, Llano Uplift, and Edwards Plateau provinces to the west and north. The youngest geologic formations common to both regions are of Albian and Cenomanian age, the thick, widespread Edwards Limestone, and the thin
overlying Georgetown, Del Rio, Buda, and Eagle Ford–Boquillas formations. Younger Cretaceous and Tertiary formations
that overlie the Edwards and associated formations on and beneath the Gulf Coastal Plain have no known counterparts to the
west and north of the Balcones Fault Zone, owing mostly to subaerial erosion following Oligocene and Miocene uplift during
Balcones faulting, and secondarily to updip stratigraphic thinning and pinchouts during the Late Cretaceous and Tertiary.
This study attempts to reconstruct the burial history of the Central Texas Platform (once entirely covered by carbonates of
the thick Edwards Group and thin Buda Limestone), based mostly on indirect geological evidence:
(1) Regional geologic maps showing structure, isopachs and lithofacies;
(2) Regional stratigraphic analysis of the Edwards Limestone and associated formations demonstrating that the Central
Texas Platform was a topographic high surrounded by gentle clinoform slopes into peripheral depositional areas;
(3) Analysis and projection of regional updip thinning patterns of Upper Cretaceous and Tertiary formations from the Gulf
Coast Basin northwestward along the San Marcos Arch, across the Balcones/Ouachita Downwarp, into the heart of the
Central Texas Platform;
(4) Derived published stratigraphic analyses of the Cretaceous Western Interior Seaway;
(5) Estimation of burial depth from thermal maturity of Eagle Ford organic shales (overlying the Edwards by approximately 150 feet) in the outcrop area around Austin and Comstock, and in the subsurface of Wilson, Karnes, and DeWitt counties; and
(6) Implications as to burial depth of Edwards and associated formations based upon the presence or absence of stylolites,
which form in carbonate rocks under known subsurface conditions, including depth related to pressure.
The Late Cretaceous through Tertiary geologic history of the Central Texas Platform may be summarized as follows:
(a) Over the ~10 million years following the end of the Albian, the vast Edwards carbonate bank was mantled beneath a
covering veneer of thin (<100 feet) early Cenomanian formations (Del Rio, Buda, and Eagle Ford–Boquillas) that did not
eliminate the gentle depositional topography around the bank margins, and also did not cover some local highs along the
(b) The western interior of the Central Texas Platform was covered by 700 to 1100 feet of open marine Austin Chalk
(Santonian), Taylor Clay, and Navarro Marl (Campanian and Maastrichtian), and Midway Clay (lower Paleocene),
which muted but did not obliterate depositional topography of the covered bank margins. The low-lying muddy bank
was periodically exposed during this ~28 million year period, and meandering streams developed along its margins with
surrounding very shallow pelagic seas.
(c) Upper Paleocene, Eocene, and Oligocene formations pinched out preferentially westward and northward onto the Balcones/Ouachita Downwarp, which coincided with the underlying Ouachita Thrust Belt and the future Balcones Fault
Zone. Throughout this period (~37 million years), the exposed, low-lying bank (adjacent to coastal plain and fluvialdeltaic depositional tracts) began to be gently uplifted. This allowed subaerial erosion to begin, of surficial Eocene sediments as well as the mantle of lower Paleocene and
Copyright © 2016. Gulf Coast Association of Geological Societies. All rights reserved.
Upper Cretaceous soft mudrocks and marls. Gradual
entrenchment of incised streams around the bank
Manuscript received October 19, 2015; revised manuscript received April 28, 2016;
margins also occurred.
manuscript accepted May 13, 2016.
(d) Beginning in late Oligocene time, the combination of
accelerating gulfward downwarping and uplift of the
GCAGS Journal, v. 5 (2016), p. 141–179.
Peter R. Rose
interior resulted in increased exposure and erosion of
the buried Central Texas Platform, until Georgetown
and Edwards rocks began to be exposed and eroded,
their detritus deposited in alluvial aprons on the adjacent coastal plain. Balcones faulting during the late
Oligocene and Miocene (~23 million years) marked the
culmination of uplift along the west and north side of
the Balcones Fault Zone, and accelerated incision of
existing streams, especially around the margins.
(e) Continued regional uplift of the Colorado Plateau during late Miocene and Pliocene (~8 million years) elevated the western margins of the exposed Edwards
carbonate bank, tilting the Plateau surface gently toward the southeast. Headward erosion from east and
south began to cut into the high-standing carbonate
mass. Streams feeding outward from the Plateau constructed sloping gravel aprons composed of carbonate
and chert debris onto the coastal plain. So far, approximately 9300 cubic miles of rock has been eroded
from the Edwards Plateau, Llano Uplift, Hill Country,
and upper Gulf Coastal Plain as the result of Tertiary
uplift and Balcones faulting, with such erosion continuing today.
I began my investigations of the Edwards Group of Texas in
1962, when I was a young geologist working for the Shell Oil
Company in South Texas, sitting wells on the so-called Edwards
Reef Trend (= Stuart City Reef), and the backreef Person-Fashing
Fault Trend a dozen miles updip to the northwest. In 1966, I
returned to the University of Texas (Austin), where my Ph.D.
dissertation was a regional monograph integrating what I had
learned about the Edwards in the subsurface (released courtesy of
Shell), with results of my 1967 surface mapping of Edwards
rocks in the eastern Edwards Plateau (Rose, 1972). During the
same period Shell geologists C. I. Smith, Jr. and Johnnie B.
Brown, under the leadership of the late Frank Lozo, were carrying out extensive stratigraphic investigations of outcropping Edwards and equivalent formations farther west and north, but this
superb work, which facilitated the stratigraphic integration of
Edwards and equivalent formations of the entire region, remained
mostly proprietary until publication by the Texas Bureau of Economic Geology (Smith et al., 2000).
As the dissertation approached completion, I was fascinated
by new questions about the Edwards Plateau, especially the geologic events that occurred after deposition of the widespread,
pelagic Buda Limestone, at the end of the Comanche Epoch.
What was the Late Cretaceous and Tertiary history of the Plateau
area? How deeply had the Edwards been buried by younger formations in the Plateau area? Was it subaerially exposed during
the Late Cretaceous? The Early Tertiary? Or was it finally exposed and eroded only during and after uplift by Balcones faulting during the late Oligocene and early Miocene? Other questions concerned the entire immense carbonate bank complex that
formed in West, Central, and South Texas, in the lee of the Stuart
City Reef—how did it relate to extensive Lower Cretaceous terrigenous clastics of the Rocky Mountain Province? How did the
Comanche carbonate shelf relate to the Cretaceous Western Interior Seaway?
Addressing these geological questions was hampered by
three basic problems: (a) most of the pertinent research was proprietary or still to be carried out; (b) Post-Edwards rocks were
mostly absent in the region, either by non-deposition or later
erosion, so there was very little direct evidence bearing on the
problem1; and (c) I was deeply involved in a professional career,
with little time to spend on personal investigations. So I put further research on the Edwards Plateau on the shelf for nearly 40
I finally returned to these long-deferred questions in 2012,
50 years since I first began to study the Edwards, and 40 years
after publication of Rose (1972). There is still much that remains
unknown about these topics, but we do have more relevant data
now, and they allow us to make reasonable inferences about what
may have happened in Central and West Texas during the ~90
my from the Late Cretaceous through the Pliocene, after Balcones faulting elevated the southeastern margins of the Edwards
Plateau during the late Oligocene and early Miocene. The present report thus addresses research questions that I have puzzled
over for many years.
The Edwards Plateau is an immense tableland that dominates the geography of West-Central Texas, covering more than
45,000 square miles, in parts or all of 29 counties (Fig. 1). Along
its northern margin, the Plateau rises 100 to 300 feet above the
adjacent rolling prairies; along its southern margin, it stands 500
to 1500 feet higher than the adjacent coastal plains of the Rio
Grande Embayment. To the east, where the Plateau is dissected
by east-flowing rivers, high-standing divides rise 100 to 400 feet
above valleys cut in older formations. Erosional remnants of
thin, deeply weathered Buda Limestone overlie the Edwards in
flat, high divides in the heart of the Plateau. The Edwards Plateau extends westward across the Pecos River, where it is sometimes called the Stockton Plateau, and to the southwest, across
the Rio Grande, where it is known as the Serrania del Burro,
which owes its much higher elevation to Laramide uplift. To the
northwest, the upper surface of the Edwards Plateau merges almost imperceptibly with the younger High Plains (or Llano Estacado) of West Texas and the Texas Panhandle (Rose, 2012).
The Plateau is the topographic and geomorphic expression
of a thick, widespread, flat-lying sequence of Lower Cretaceous
(mostly middle and upper Albian) limestones and dolostones
assigned to the Edwards Group, which thickens southwestward,
from about 400 feet on the north to more than 800 feet along the
southern edge of the Plateau. Edwards carbonate strata are generally harder and more resistant to weathering and erosion than
the underlying softer, Trinity-age sandstones and marls, which is
why the Edwards Plateau is a high-standing topographic feature,
dissected and rough-edged around its margins (Rose, 2004).
Neogene erosion of the eastern Edwards Plateau region,
related to Balcones faulting and uplift, has stripped away much
of the Edwards Group from alluvial valleys cutting eastward and
southward across Trinity-age Glen Rose and Hensel formations,
leaving only Edwards remnants in high-standing interfluvial divides. This distinctive landscape is known as the Texas Hill
Country. In the valleys of the Colorado River and its eastflowing tributaries, the Pedernales, Llano, and San Saba rivers,
erosion has cut down into Paleozoic and Precambrian rocks of
the Llano Uplift. But prior to Balcones faulting, a continuous
blanket of Edwards (and Buda) strata covered the entire Central
Texas Province, including that part which is now in the subsurface (Woodruff, 2002).
Edwards carbonate rocks record deposition on a vast offshore bank, far south and west of any substantial input of terrigenous sands, muds and clays. The climate was subtropical,
temperate to arid. Depositional environments ranged from re-
Actually, only a very thin (<40 ft) veneer of Buda strata overlies the Edwards over much of the Edwards Plateau, with even thinner remnants of underlying Del Rio pinching out on the southern flank, and overlying Boquillas (= Eagle Ford) present in the central and southwestern sectors. Southward-thickening erosional wedges of Austin Chalk are present on the far southern flank of the Edwards Plateau, where it impinges on the Chihuahua
Trough. Otherwise, there are no Upper Cretaceous or Tertiary formations present in the region, except for scattered Pliocene/Pleistocene high gravels
around the Plateau margins, and Quaternary alluvial deposits in some river valleys.
Late Cretaceous and Tertiary Burial History, Central Texas
Figure 1. Edwards Plateau, Llano Uplift, Hill Country, Balcones Fault Zone, and Coastal Plain, Central Texas.
stricted shelf interior, to low-energy shallow open shelf, to highenergy bioclastic shelf-margin. Where such environments of
deposition survived into latest Albian time (typically high on the
Central Texas Platform), the sedimentary rocks that formed there
are also called “Edwards,” even though they are coeval with uppermost Georgetown strata to the south and northeast (Rose,
1972; Young, 1974, 1986).
A large body of well-documented geologic research has
been carried out on the Edwards Group in the Edwards Plateau
region over the past 50 years. But little has been written about
the subsequent geologic history of the region after Edwards deposition ended—i.e., what younger formations may have covered
the Edwards, their thickness and areal extent. We understand
that the eastern and southern margins of the Edwards Plateau
were elevated above the Gulf Coastal Plain beginning about 25
million years ago, during late Oligocene and early Miocene time,
by Balcones faulting (Weeks, 1945a, 1945b). This event left
unmistakable sedimentary evidence—a widespread carbonate
and chert gravel-and-sand outwash plain—in Oligocene Catahoula and Miocene Oakville outcrops on the Gulf Coastal Plain
to the east and southeast, representing alluvial and coastal plain
deposits derived from recently uplifted, rapidly eroding carbonate
uplands to the west and northwest. It is generally accepted
that the gentle gulfward tilt of the Plateau is post-Miocene,
related to the regional rise of the Colorado Plateau to the
northwest (Galloway et al., 2011). Otherwise, the geologic
timing of Edwards Plateau uplift remained unknown—was the
Plateau subaerially exposed, weathered, and eroded beginning
sometime in the late Cretaceous, or during the Eocene, or only
incidental to Balcones faulting and uplift in the Oligocene and
The purpose of this paper is to summarize the geologic
events that may have transpired between the emergence of the
immense Edwards carbonate bank in the late Albian/early Cenomanian (100–98 million years ago), and the Pleistocene, especially how those events influenced the burial history of Edwards and
associated formations. Because of the general absence of conventional geologic evidence in the subject area, such an undertaking has required consideration and synthesis of many geologic
subspecialties—stratigraphy, tectonics, burial history, paleogeography, organic geochemistry, petrology, and porosity/
permeability analysis—to piece together various lines and items
of evidence to construct a credible, though admittedly speculative, geologic history of this large region over the last ~100 million years (since the end of Edwards deposition).
Peter R. Rose
Pertinent Previous Work
Cretaceous rocks of the Edwards Plateau region are now
well understood, thanks to careful, well-documented geologic
mapping and stratigraphic syntheses, mostly by geologists involved with sustained efforts by the late Frank Lozo of Shell
Development Co.: Lozo and Smith (1964), Moore (1967), Smith
(1970), Rose (1972, 1986a), Halley and Rose (1977), Smith and
Brown (1983), Miller (1984), and Smith et al. (2000). Such work
provided the necessary stratigraphic framework for many subsequent diverse and detailed research projects.
Research projects on equivalent formations in the subsurface
of central and south Texas generated counterpart mapping, stratigraphic correlation and geologic synthesis: Winter (1961), Tucker (1962), Bebout and Loucks (1974), and Rose (1986b).
Beginning in 1961, Lozo and Smith (1964) and Smith et al.
(2000) mapped, correlated and synthesized the Fredericksburg
and Washita stratigraphic succession from the central Edwards
Plateau westward across Trans-Pecos Texas, and southward, into
the Maverick Basin. Smith (1970) carried this sequence across
the Big Bend region and into the Serrania del Burro of northern
Coahuila, Mexico. Rose (1972) mapped and correlated Edwards
and associated formations of the subsurface with their outcrop
counterparts of the Balcones Fault Zone and eastern Edwards
Plateau, connecting with the findings of Lozo and Smith, thus
completing a complete stratigraphic synthesis of these formations
across Central and Southwest Texas. Young (1974, 1986) reported on ammonite zonations that confirmed the physical stratigraphic correlations of Tucker (1962) and Rose (1972). Surface
mapping of the entire Edwards Plateau region was provided by
the Texas Bureau of Economic Geology’s mammoth 1:250,000
Geologic Atlas of Texas, including the Austin (1974), Del Rio
(1977), Fort Stockton (1982), Llano (1981), Pecos (1975), San
Angelo (1976), San Antonio (1983), Seguin (1974), Sonora
(1981), and Waco (1970) sheets.
Analogous regional mapping and stratigraphic research were
also being carried out at about the same time to the north, in the
broad area of the middle Cretaceous North American Interior
Seaway, by many different geologists. Two especially pertinent
papers were a regional synthesis by Kauffmann (1977), and a
synthesis of middle Cretaceous stratigraphy in southeastern Colorado, southwestern Kansas, northeastern New Mexico, and the
Oklahoma Panhandle by Scott (1977). In 2003, Scott et al. published an integrated Albian-lower Cenomanian stratigraphic synthesis of sedimentary formations of the North Texas–Tyler Basin,
the basinal area northeast of the Edwards Plateau. Phelps et al.
(2014) published a comprehensive paper synthesizing the Cretaceous stratigraphy of the Texas Gulf Coast, focusing on sequence
In 1975, Princeton University press published the Stratigraphic Atlas of North America, an extraordinarily comprehensive series of isopach, subcrop, and lithofacies maps, with accompanying cross-sections, prepared by the Exploration Department of Shell Oil Company, and edited by T. D. Cook and A. W.
Bally. I have used this as a source for most of the isopach mapping of Upper Cretaceous and Tertiary formations included herein.
Papers addressing the structural geology of Central Texas,
including the Edwards Plateau, the subsurface Ouachita Fold
Belt, the Balcones Fault Zone, and the subsurface of the inner
Gulf Coastal Plain, include: Weeks (1945a, 1945b), Flawn et al.
(1961, 1967), Murray (1961), Grimshaw and Woodruff (1986),
and Ewing (1991, 2003, 2005).
The Cenozoic history of the Gulf of Mexico Basin was published by Galloway et al. (2000), and a comprehensive synthesis
of Cenozoic stream drainage systems feeding into the Gulf basin
was published by Galloway et al. (2011). A recent series of publications by Jackson et al. (2011), Hudec et al. (2013), and
Dooley et al. (2013) related offshore salt movements in the deep
Gulf of Mexico to Tertiary tectonics in the northern margins of
the onshore Gulf.
ELEMENTS AND HISTORY
The structural-geologic history of Central Texas is long and
complex. Fig. 2 shows structural features that are important to
the geologic history of Central Texas in general, and the deposition of Cretaceous and Tertiary formations, in particular.
Ouachita Structural Belt
In North and Central Texas the Ouachita Structural Belt,
comprehensively described by Flawn et al. (1961), lies entirely in
the subsurface. It passes from near Dallas southwesterly to the
Austin area, then begins its westward swing, under San Antonio
and Uvalde. It is interrupted by the late Paleozoic Devils River
Uplift near Del Rio, then bears northwesterly and finally westerly
into the area of the Marathon Dome (Laramide), West Texas,
where it comes to the surface. The Ouachita Structural Belt is
generally thought to ...
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