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Stratigraphy and Depositional Environments of the Cretaceous-Tertiary Boundary Complex and Basal Paleocene section, Brazos River, 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 non-deposition or subsequent scour. The basal conglomerate (to 0.6 m) contains mudstone clasts and shells dispersed in mud matrix. This is locally overlain by granular sediment (with spherules) with large 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-l0 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 muddy debris flow, derived locally; the overlying conglomerate unit moved as a grain-dominated debris flow; the sands and calcareous, silty mud moved in suspension above the seafloor, from a distant source.
The basal Paleocene contains a laterally extensive lowstand sand 1-3 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 pyrite concretion horizon, all of which may mark minor local disconformities.
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