Austin Chalk
Encyclopedia
The Austin Chalk is an upper Cretaceous
geologic formation in the Gulf Coast region of the United States. It is named after type section outcrops near Austin, Texas
. The formation is made up of chalk
and marl
.
Dinosaur
remains are among the fossils that have been recovered from the formation, although none have yet been referred to a specific genus
.
The Austin Chalk consists of recrystalized, fossiliferous, interbedded chalks and marls. Exposures of Austin Chalk are mainly seen in quarries, roadcuts, and stream beds where the water eroded the soil. The Austin Chalk outcrops and can be seen throughout Dallas, and extends south underneath I-35 down into Austin and San Antonio. Volcanic ash
layers are present in the Austin chalk, and were deposited by wind from distant erupting volcanoes around 86 mya. These eruptions occurred along a 250-mile long by 50 mile wide belt of submarine volcanoes, which are located in present day south-central Texas. This belt of volcanoes coincides with the trend of the Balcones Fault zone
and is known as the Balcones volcanic province. Evidence of these ancient volcanoes is only visible in a few places since most were buried by the Austin and Taylor Group, and now are in the subsurface. The presence of this volcanism during deposition of the Austin Chalk is correlated with the Laramide orogeny
. Sea level rose for conditions to be right for the deposition of the Austin Chalk, which also coincides with the maximum extent of the Cretaceous Interior Seaway. The depths of the deposition of the Austin Chalk occurred in ~250 m or 820 ft of water. The Austin Chalk is filled with micro-organism fossils known as coccoliths.
Cretaceous
The Cretaceous , derived from the Latin "creta" , usually abbreviated K for its German translation Kreide , is a geologic period and system from circa to million years ago. In the geologic timescale, the Cretaceous follows the Jurassic period and is followed by the Paleogene period of the...
geologic formation in the Gulf Coast region of the United States. It is named after type section outcrops near Austin, Texas
Austin, Texas
Austin is the capital city of the U.S. state of :Texas and the seat of Travis County. Located in Central Texas on the eastern edge of the American Southwest, it is the fourth-largest city in Texas and the 14th most populous city in the United States. It was the third-fastest-growing large city in...
. The formation is made up of chalk
Chalk
Chalk is a soft, white, porous sedimentary rock, a form of limestone composed of the mineral calcite. Calcite is calcium carbonate or CaCO3. It forms under reasonably deep marine conditions from the gradual accumulation of minute calcite plates shed from micro-organisms called coccolithophores....
and marl
Marl
Marl or marlstone is a calcium carbonate or lime-rich mud or mudstone which contains variable amounts of clays and aragonite. Marl was originally an old term loosely applied to a variety of materials, most of which occur as loose, earthy deposits consisting chiefly of an intimate mixture of clay...
.
Dinosaur
Dinosaur
Dinosaurs are a diverse group of animals of the clade and superorder Dinosauria. They were the dominant terrestrial vertebrates for over 160 million years, from the late Triassic period until the end of the Cretaceous , when the Cretaceous–Paleogene extinction event led to the extinction of...
remains are among the fossils that have been recovered from the formation, although none have yet been referred to a specific genus
Genus
In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...
.
The Austin Chalk consists of recrystalized, fossiliferous, interbedded chalks and marls. Exposures of Austin Chalk are mainly seen in quarries, roadcuts, and stream beds where the water eroded the soil. The Austin Chalk outcrops and can be seen throughout Dallas, and extends south underneath I-35 down into Austin and San Antonio. Volcanic ash
Volcanic ash
Volcanic ash consists of small tephra, which are bits of pulverized rock and glass created by volcanic eruptions, less than in diameter. There are three mechanisms of volcanic ash formation: gas release under decompression causing magmatic eruptions; thermal contraction from chilling on contact...
layers are present in the Austin chalk, and were deposited by wind from distant erupting volcanoes around 86 mya. These eruptions occurred along a 250-mile long by 50 mile wide belt of submarine volcanoes, which are located in present day south-central Texas. This belt of volcanoes coincides with the trend of the Balcones Fault zone
Balcones Fault
The Balcones Fault Zone is a tensional structural system in Texas that runs approximately from the southwest part of the state near Del Rio to the north central region near Waco along Interstate 35. The Balcones Fault zone is made up of many smaller features, including normal faults, grabens, and...
and is known as the Balcones volcanic province. Evidence of these ancient volcanoes is only visible in a few places since most were buried by the Austin and Taylor Group, and now are in the subsurface. The presence of this volcanism during deposition of the Austin Chalk is correlated with the Laramide orogeny
Laramide orogeny
The Laramide orogeny was a period of mountain building in western North America, which started in the Late Cretaceous, 70 to 80 million years ago, and ended 35 to 55 million years ago. The exact duration and ages of beginning and end of the orogeny are in dispute, as is the cause. The Laramide...
. Sea level rose for conditions to be right for the deposition of the Austin Chalk, which also coincides with the maximum extent of the Cretaceous Interior Seaway. The depths of the deposition of the Austin Chalk occurred in ~250 m or 820 ft of water. The Austin Chalk is filled with micro-organism fossils known as coccoliths.
See also
- Geology of the Dallas-Fort Worth MetroplexGeology of the Dallas-Fort Worth MetroplexDallas–Fort Worth sits above Cretaceous-aged strata, dates ranging from ~145-65 Ma . These Cretaceous-aged sediments lie above the eroded Ouachita Mountains and the Fort Worth Basin, which was formed by the Ouachita Orogeny. Going from west to east in the DFW Metroplex and down towards the Gulf...
- List of dinosaur-bearing rock formations