Austin Chalk stratigraphic column in Texas

The Austin Chalk is an upper

Cretaceous The Cretaceous ( ) is a geological period that lasted from about 143.1 to 66 mya (unit), million years ago (Mya). It is the third and final period of the Mesozoic Era (geology), Era, as well as the longest. At around 77.1 million years, it is the ...

geologic formation in the Gulf Coast region of the United States. It is named after type section outcrops near

Austin, Texas Austin ( ) is the List of capitals in the United States, capital city of the U.S. state of Texas. It is the county seat and most populous city of Travis County, Texas, Travis County, with portions extending into Hays County, Texas, Hays and W ...

. The formation is made up of

chalk Chalk is a soft, white, porous, sedimentary carbonate rock. It is a form of limestone composed of the mineral calcite and originally formed deep under the sea by the compression of microscopic plankton that had settled to the sea floor. Ch ...

and

marl Marl is an earthy material rich in carbonate minerals, Clay minerals, clays, and silt. When Lithification, hardened into rock, this becomes marlstone. It is formed in marine or freshwater environments, often through the activities of algae. M ...

Fossils

The putative galloanseran bird '' Austinornis lentus'' has been found in the Austin Chalk.Weishampel, et al. (2004). "Dinosaur distribution." Pp. 517-607. The general absence of dinosaurs is a reflection of the Austin limestone being marine in origin, primarily composed of microscopic shell fragments from floating sea organisms known as "

coccolithophore Coccolithophores, or coccolithophorids, are single-celled organisms which are part of the phytoplankton, the autotrophic (self-feeding) component of the plankton community. They form a group of about 200 species, and belong either to the kingdom ...

s" (the same organisms that contributed to the

White Cliffs of Dover The White Cliffs of Dover are the region of English coastline facing the Strait of Dover and France. The cliff face, which reaches a height of , owes its striking appearance to its composition of chalk accented by streaks of black flint, depo ...

, on the south coast of England). Nevertheless, the Austin Chalk will occasionally produce fossils of larger creatures, such as '' Inoceramus'' clams, ammonite cephalopods, and large marine vertebrates such as ''

Xiphactinus ''Xiphactinus'' (from Latin and Greek for "sword-ray") is an extinct genus of large predatory marine ray-finned fish that lived during the late Albian to the late Maastrichtian. The genus grew up to in length, and superficially resembled a garga ...

'', a predaceous fish.

Geology

The rocks of the Austin Chalk consist of recrystallized,

fossiliferous A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved i ...

interbedded In geology, interbedding occurs when beds (layers of rock) of a particular lithology lie between or alternate with beds of a different lithology. For example, sedimentary rocks may be interbedded if there were sea level variations in their sedimen ...

s and marls. Exposures of Austin Chalk are mainly seen in quarries, roadcuts, and stream beds where water eroded the soil. Austin Chalk outcrops can be seen throughout

Dallas Dallas () is a city in the U.S. state of Texas and the most populous city in the Dallas–Fort Worth metroplex, the List of Texas metropolitan areas, most populous metropolitan area in Texas and the Metropolitan statistical area, fourth-most ...

, and extend south underneath I-35 down into Austin and

San Antonio San Antonio ( ; Spanish for " Saint Anthony") is a city in the U.S. state of Texas and the most populous city in Greater San Antonio. San Antonio is the third-largest metropolitan area in Texas and the 24th-largest metropolitan area in the ...

Volcanic ash Volcanic ash consists of fragments of rock, mineral crystals, and volcanic glass, produced during volcanic eruptions and measuring less than 2 mm (0.079 inches) in diameter. The term volcanic ash is also often loosely used to r ...

layers are present in the Austin chalk, and dates obtained from radioactive minerals in these ash layers indicate they 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 as 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 The Laramide orogeny was a time period of mountain building in western North America, which started in the Late Cretaceous, 80 to 70 million years ago, and ended 55 to 35 million years ago. The exact duration and ages of beginning and end of the o ...

. 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, well-suited to the deposition of coccoliths.

Footnotes

References

* Weishampel, David B.; Dodson, Peter; and Osmólska, Halszka (eds.): The Dinosauria, 2nd, Berkeley: University of California Press. 861 pp. .

External links

Cretaceous Arkansas Cretaceous geology of Texas Chalk formations Austin, Texas Limestone formations of the United States {{Texas-geologic-formation-stub

Fossils

Geology

See also

Footnotes

References

External links