Texas Almanac, 1998-1999 Page: 66
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66 Texas Almanac 1998-1999
EXPLANATION
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a nd Oee P Im n lo min d Mbslalppl n
cretno nend
,ca.-,--Sri") - Ignom tundifferenloed)from adjacent uplifts. As Europe and the southern continents
continued to drift away from North America, the Texas basins
were eventually buried beneath thick deposits 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 beneath 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
buried 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 those seas are exposed in the walls of
the magnificent canyons of the Rio Grende 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 terres-
trial, 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 theearly 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 Texarkana area.
The Austin Chalk and other upper Cretaceous rocks dip south-
eastward 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 beneath 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 vol-
canoes were primarily on the sea floor and great clouds of
steam and ash likely accompanied them. Between eruptions,
invertebrate marine animals built reefs on the shallow volcanic
cones. Pilot Knob, located southeast of Austin, is one of these
old volcanoes 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 margins of the East
Texas Basin were sites of deltas fed by rivers. These streams
flowed eastward, draining areas to the north and west.
Although there were minor incursions of the seas, the Ceno-
zoic rocks principally document extensive seaward building by
broad deltas, marshy lagoons, sandy barrier islands and
embayments. Thick vegetation covered the levees and areas
between the streams. Coastal plains were taking shape, under
the same processes still at work today.
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Ramos, Mary G. Texas Almanac, 1998-1999, book, 1997; Dallas, Texas. (https://texashistory.unt.edu/ark:/67531/metapth162515/m1/66/: accessed March 29, 2024), University of North Texas Libraries, The Portal to Texas History, https://texashistory.unt.edu; crediting Texas State Historical Association.