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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.〔US Geological Survey, (Austin Chalk )〕 Dinosaur remains are among the fossils that have been recovered from the formation, although none have yet been referred to a specific genus.〔Weishampel, et al. (2004). "Dinosaur distribution." Pp. 517-607.〕 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. ==See also== * Geology of the Dallas-Fort Worth Metroplex * List of dinosaur-bearing rock formations * * List of stratigraphic units with indeterminate dinosaur fossils 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Austin Chalk」の詳細全文を読む スポンサード リンク
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