dockum - G EOL 4200 I n te rp retation of the Dockum Group...

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Unformatted text preview: G EOL 4200 I n te rp retation of the Dockum Group (T r iassic) in Caprock Canyons State Par k, T exas T he Dockum Group in Caprock Canyons State Park is a fining upward sequence. I t consists of a channel-related facies and a f lood-plain facies (Lehman 2005). The lowest unit is 8-10m thick and consists of a friable sandy siltstone. This silt stone is most likely an overbank f lood-plain facies. I t contains thin dipping cross laminations. Above the fr iable sandy silt stone is a cliff forming unit consisting of conglomeritic sandstones, sandstones, and interbedded silt stones. The entire unit is 7-8m thick. The conglomerit ic sandstones and sandstones contain cross laminations, cross bedding, t rough cross bedding, and mud clasts. All of the beds are lenticular and fine upwards. The interbedded siltstones contain r ipple cross-laminations. This sequence of lenticulaer conglomerates, sandstones, and siltstones are consistent with a channel facies capped by overbank f lood-plain facies ( Lehman 2005). Paleo-currents were recorded for this unit and appear to oscillate from Northeast to Northwest in the sandstones. This could be due to changes in a meandering s tream patterns. The siltstones have a southern paleo-current direction, perpendicular to t he sandstone sequences. Because the siltstones are part of an overbank f lood-plain, the currents should be perpendicular to the sandstone currents. The upper most unit is most l ikely a overbank f lood-plain facies. I t consists of thinly laminated fining upward very fine sand. The top unit is the contact for Dockum Group and Ogallala Formation. References: Lehman, Thomas, and Chatterjee, Sankar. Depositional Setting and Vertebrate B iostratigraphy of the Triassic Dockum Group of Texas. Journal of Earth Systems Science, Vol. 114, No. 3, June 2005, pp. 325-351. ...
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dockum - G EOL 4200 I n te rp retation of the Dockum Group...

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