--> Abstract: Depositional Units of the K-T Boundary Complex and Basal Paleocene Section, Brazos River Section, Texas, by T. E. Yancey; #90950 (1996).

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Abstract: Depositional Units of the K-T Boundary Complex and Basal Paleocene Section, Brazos River Section, Texas

Thomas E. Yancey

The Brazos River section contains 1.5 m of K-T boundary complex, deposited in several high energy events, and 4-5 m of basal Paleocene (P0 to P1c zones), deposited under changing sea levels. The boundary complex contains conglomerate beds overlain by multiple sands. Units vary laterally due to local nondeposition or subsequent scour. The basal conglomerate (to 0.5 m) contains mudstone clasts and shells dispersed in mud matrix. This is locally overlain by granular sediment (with spherules) with mudstone and carbonate clasts. Up to four truncated sand units are present. A complete sand unit has a scour base, a thin (0-5 cm) coarse calcareous sand overlain by 5-15 cm of quartzose, hummocky sand, topped by 5-10 cm of fining-upwards calcareous silty mudstone. Sands of separate events may amalgamate into a composite sand bed. Hummocky cross-bedding may change upwards to oscillation ripples and climbing ripples. Sediment transport mechanisms change: the basal conglomerate moved as a local slurry flow; the overlying conglomerate unit moved as a seafloor grain flow; the sands and calcareous, silty mud moved in suspension above the seafloor.

The basal Paleocene contains a laterally extensive lowstand sand 1-2 m above the K-T boundary complex, correlatable to Walker Creek (15 miles southwest of the Brazos River) where the lowstand sands are a basal lag on Cretaceous sediments. Deposition of the lowstand sands marks a major change in water depths. Other persistent marker horizons at Brazos include calcareous concretion horizons and a pyrtic concretion horizon, all of which may mark minor local disconformities.

AAPG Search and Discovery Article #90950©1996 AAPG GCAGS 46th Annual Meeting, San Antonio, Texas