Texas Almanac, 1988-1989 Page: 96
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96 TEXAS ALMANAC 1988-1989
Pennsylvanian and Mississipplan
Devonian, Silurian, Ordovicion,
> Pre-Cambrlon (schist and gneiss)
Geology of Texas
This article on the geology of Texas was prepared by.the Bureau of Economic Geology at The University of Texas at
History in the Rocks
The fascinating geologic history of Texas is record-
ed in rocks - both those exposed at the surface and
those penetrated by holes drilled in search of oil and
natural gas. The rocks reveal a dynamic, ever-chang-
ing earth - ancient mountains, seas, volcanoes, earth-
quake belts, rivers, hurricanes and winds. Today, the
volcanoes and great earthquake belts are no.longer ac-
tive, but rivers and streams, wind and rain,, and the
slow, inexorable alterations of rocks at or near the sur-
face continue-to change the face of Texas. The geologic
history of Texas, as documented by the rocks, began
more than a billion years-ago; its legacy is the mineral
wealth and varied land forms of modern Texas.
Geologic Time Travel
The story preserved in the rocks requires an under-
standing of the origin of the strata and how they have
been deformed. Stratigraphy'is the study of the compo-
sition, sequence and origin of the rocks: of. what the
rocks are made, how they were-formed and the order in
which the layers were formed. Structural geology re-
veals the architecture of the rocks: the locations of the
mountains, volcanoes, sedimentary basins and earth-
quake belts. Figure 1 is a map of rocks of-various geo-
logic ages at the surface of Texas today. Figure 2 shows
the major structural features'of the state.
History concerns events through time, but geologic
time is such a grandiose concept that most of us find it
difficult to comprehend. So, geologists have named the
various chapters of earth history. Figure 3, a general-
ized geologic time scale, shows the ages of the principal
geologic units in Texas.
Precambrian rocks, more than 600 million years old,
are exposed at the surface in the Llano Uplift of Central
Texas and in scattered outcrops in West Texas, around
and north of Van Horn and near El Paso (Fig. 1). These
rocks, some more than a billion years old, include com-
plexly deformed rocks that were originally formed by
cooling from a liquid state as well as rocks that were
altered from pre-existing rocks.
Precambrian rocks, often called the "basement
complex;" are thought to form the foundation of conti-
nental masses. Precambrian rocks underlie all of Tex-
as, and the outcrop in Central Texas is only the exposed
part of the Texas Craton (Fig. 2), which is primarily
buried by younger rocks.
During the early part of the Paleozoic~ ra (approxi-
mately 600'million to 350 million years ago), broad, rela-
tively shallow seas repeatedly-inundated the Texas Cra-
ton and much of North and West Texas. The evidence
for these events is the sandstones, shales and lime-
0 100 me
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Texas Almanac, 1988-1989, book, 1987; Dallas, Texas. (texashistory.unt.edu/ark:/67531/metapth113819/m1/99/: accessed January 18, 2019), University of North Texas Libraries, The Portal to Texas History, texashistory.unt.edu; crediting Texas State Historical Association.