Texas Almanac, 1992-1993 Page: 66
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66 TEXAS ALMANAC 1992-1993
complex," are thought to form the foundation of conti-
nental masses. Precambrian rocks underlie all of Texas,
and the outcrop in Central Texas is only the exposed part
of the Texas Craton, which is primarily buried by young-
er rocks.
Paleozoic Era
During the early part of the Paleozoic Era (approxi-
mately 600 million to 350 million years ago), broad, rela-
tively shallow seas repeatedly inundated the Texas
Craton and much of North and West Texas. The evidence
for these events is the sandstones, shales and limestones,
similar to sediments that form in seas today, and fossils
of animals, similar to modern crustaceans - the brachi-
opods, clams, snails and related organisms that live in
modern marine environments. Early Paleozoic rocks
are now exposed around the Llano Uplift and in far West
Texas near Van Horn and El Paso and exist in the sub-
surface over most of West and North Texas.
By late Paleozoic (approximately 350 million to 240
million years ago), the Texas Craton was bordered on
the east and south by a long, deep marine basin, called
the Ouachita Trough. Sediments slowly accumulated in
this trough until late in the Paleozoic Era. Plate-tectonic
theory postulates that the collision of the North Ameri-
can Plate (upon which the Texas Craton is located) with
the European and African-South American plates
uplifted the thick sediments that had accumulated in the
trough to form the Ouachita Mountains, which at that
time extended across Texas. Today, in Texas that old
mountain range is entirely buried by younger rocks and
all that remains at the surface of the once majestic Oua-
chita Mountain chain is exposed only in southeastern
Oklahoma and southwestern Arkansas.
During the Pennsylvanian Period, however, the Oua-
chita Mountains bordered the eastern margin of shallow
inland seas that covered most of West Texas. Rivers
flowed westward from the mountains to the sea bringing
sediment to form deltas along an ever-changing coast-
line, and the sediments were then reworked by waves
and currents of the inland sea. Today, these fluvial, delta
and shallow marine deposits compose the late Paleozoic
rocks that crop out and underlie the surface of North
Central Texas.
Broad marine shelves divided the West Texas seas
into several sub-basins, or deeper areas that received
more sediments than accumulated on the limestone
shelves. Limestone reefs rimmed the deeper basins. To-
day, these reef limestones are important oil reservoirs in
West Texas. These seas gradually withdrew from Texas,
and by the late Permian Period, all that was left in West
Texas were shallow basins and wide tidal flats in which
salt, gypsum and red muds accumulated in a hot, arid
land. Strata deposited during the Permian Period are
exposed today along the edge of the Panhandle as far
east as Wichita Falls and south to Concho County and in
the Trans-Pecos.
Mesozoic Era
Approximately 240 million years ago, the major geol-
ogic events in Texas shifted from West Texas to East and
Southeast Texas. The European and African-South
American plates, which had collided with the North
American plate to form the Ouachita Mountains, began
to separate from North America. A series of faulted ba-
sins, or rifts, extending from Mexico to Nova Scotia were
formed. These rifted basins received sediments from
adjacent uplifts, and as Europe and the southern conti-
nents continued to drift away from North America, the
Texas basins were eventually buried beneath thick de-
posits of marine salt within the newly formed East Texas
and Gulf Coast basins. Jurassic and Cretaceous rocks in
East and Southeast Texas document a sequence of broad
limestone shelves at the edge of the developing Gulf of
Mexico. From time to time, the shelves were buried be-
neath deltaic sandstones and shales, which built the
northwestern margin of the widening Gulf of Mexico to
the south and southeast. As the underlying salt was bur-
ied more deeply by dense sediments, the salt became
unstable and moved toward areas of least pressure. As
the salt moved, it arched or pierced overlying sediments
forming, in some cases, columns known as "salt domes."
In some cases, these salt domes moved to the surface;
others remain beneath a sedimentary overburden. This
mobile salt formed numerous structures that would later
serve to trap oil and natural gas.
By the early Cretaceous (approximately 140 million
years ago), the shallow Mesozoic seas covered a large
part of Texas, eventually extending west to the Trans-
Pecos area and north almost to the present-day state
boundaries. Today, the limestones deposited in thoseseas are exposed in the walls of the magnificent canyons
of the Rio Grande in the Big Bend National Park area
and in the canyons and headwaters of streams that drain
the Edwards Plateau, as well as in Central Texas from
San Antonio to Dallas.
Animals of many types lived in the shallow Mesozoic
seas, tidal pools and coastal swamps. Today these lower
Cretaceous rocks are some of the most fossiliferous in
the state. Tracks of dinosaurs occur in several localities,
and remains of terrestrial, aquatic and flying reptiles
have been collected from Cretaceous rocks in many
parts of Texas.
During most of the late Cretaceous, much of Texas
lay beneath marine waters that were deeper than those
of the early Cretaceous seas, except where rivers, deltas
and shallow marine shelves existed. River delta and
strandline sandstones are the reservoir rocks for the
most prolific oil field in Texas. When discovered in 1930,
this East Texas oil field contained recoverable reserves
estimated at 5.6 billion barrels. The chalky rock that we
now call the "Austin Chalk" was deposited when the
Texas seas became deeper. Today, the chalk (and other
Upper Cretaceous rocks) crops out in a wide band that
extends from near Eagle Pass on the Rio Grande, east to
San Antonio, north to Dallas and eastward to the Texar-
kana area. The Austin Chalk and other upper Cretaceous
rocks dip southeastward beneath the East Texas and
Gulf Coast basins. The late Cretaceous was the time of
the last major seaway across Texas, because mountains
were forming in the western United States that
influenced areas as far away as Texas.
A chain of volcanoes formed beneth the late Creta-
ceous seas in an area roughly parallel to and south and
east of the old, buried Ouachita Mountains. The
eruptions of these volcanoes were primarily on the sea
floor and great clouds of steam and ash likely accompa-
nied them. Between eruptions, invertebrate marine ani-
mals built reefs on the shallow volcanic cones. Pilot
Knob, located southeast of Austin, is one of these old vol-
canoes that is now exposed at the surface.
Cenozoic Era
At the dawn of the Cenozoic Era, approximately 65
million years ago, the northern and northwestern mar-
gins of the East Texas Basin were sites of deltas fed by
rivers flowing eastward that drained areas to the north
and west. Although there were minor incursions of the
seas, the Cenozoic rocks principally document extensive
seaward building by broad deltas, marshy lagoons,
sandy barrier islands and embayments. Thick vegeta-
tion covered the levees and areas between the streams.
Coastal plains were taking shape, under the same proc-
esses still at work today.
The Mesozoic marine salt became buried by thick
sediments in the coastal plain area, and the salt began to
form ridges and domes in the Houston and Rio Grande
areas. The heavy load of sand, silt and mud deposited by
the deltas eventually caused some areas of the coast to
subside and form large fault systems, essentially parallel
to the coast. Many of these coastal faults moved slowly
and probably generated little earthquake activity. How-
ever, movement along the Balcones and Luling-Mexia-
Talco Zones, a complex system of faults along the west-
ern and northern edge of the basins, likely generated
large earthquakes millions of years ago.
Predecessors of modern animals roamed the Texas
Cenozoic coastal plains and woodlands. Bones and teeth
of horses, camels, sloths, giant armadillos, mammoths,
mastodons, bats, rats, large cats and other modern or
extinct mammals have been dug from coastal plain de-
posits. Vegetation in the area included varieties of plants
and trees both similar and dissimilar to modern ones.
Fossil palmwood, the Texas "state stone," is found in
sediments of early Cenozoic age.
The Cenozoic Era in Trans-Pecos Texas was entirely
different. There extensive volcanic eruptions formed
great calderas and lava flows. These eruptions ejected
great clouds of volcanic ash and rock particles into the
air - many times the amount of material elected by the
Mount St. Helens eruption. Ash from the eruptions drift-
ed eastward and is found in many of the sand-and-
siltstones of the Gulf Coastal Plains. Lava flowed over
the older Paleozoic and Mesozoic rocks, and igneous
intrusions melted their way upward into the crustal
rocks. These volcanic and intrusive igneous rocks are
well exposed in the arid areas of Trans-Pecos Texas to-
day.
In the Texas Panhandle, streams originating in the
recently elevated southern Rocky Mountains brought66
TEXAS ALMANAC 1992-1993
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Kingston, Mike. Texas Almanac, 1992-1993, book, 1991; Dallas, Texas. (https://texashistory.unt.edu/ark:/67531/metapth279642/m1/70/?rotate=270: accessed April 26, 2024), University of North Texas Libraries, The Portal to Texas History, https://texashistory.unt.edu; crediting Texas State Historical Association.