ABSTRACT

At the Littig Quarry in Travis County, Texas, the Navarro claystones yield sparse ostracode assemblages with low species diversity and equitability, dominated by Haplocytheridea, that suggest stressful coastal environments with rapid sedimentation. The ostracode assemblages in the overlying Midway glauconitic claystones are fully marine, moderately diverse, and characteristically Paleocene but sparse, except in the three condensed zones, the lowest of which marks the disconformable contact.

At Walkers Creek in Milam County, the Navarro assemblages are richer and of normal-marine, nearshore aspect, with moderate diversity but low equitability, dominated by Cytherella. They belong to the "Cythereis" lixula interval-zone of Hazel and Brouwers (1982) but perhaps not to the very youngest part. The condensed zone at the disconformable base of the Midway yields a fully Paleocene fauna with moderately high diversity, either younger or further offshore than at Littig, with a few reworked specimens of Cretaceous species.

On the Brazos River in Falls County, the Navarro claystones yield assemblages of offshore aspect with moderate species diversity and equitability; they belong to the upper part of the "Cythereis" lixula zone. A barren sandstone ledge marks a turbidite deposit (Kocurek and Hansen, 1982) at which a few Cretaceous species disappear. The claystones above this ledge have very sparse, fragmentary assemblages, which gradually become more abundant, more diverse, and better preserved but less equitable upward, reflecting offshore but somewhat stressful conditions with intense naticid predation. Brachycythere plena, Bairdoppilata suborbiculata, and other characteristic Paleocene species appear one by one through this 3-m transitional interval, as holdover Cretaceous species gradually disappear, until a fully Paleocene fauna is established.