--> Upper Cretaceous Nannofossil Assemblages in a Basin-Margin Transect Across the Western Interior Seaway, by C. E. Burns and T. J. Bralower; #90986 (1994).

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Abstract: Upper Cretaceous Nannofossil Assemblages in a Basin-Margin Transect Across the Western Interior Seaway

Celeste E. Burns, Timothy J. Bralower

Upper Cretaceous nannofossil assemblages have been investigated as a part of the Cretaceous Western Interior Seaway Continental Scientific Drilling Program. Our initial studies center on the Bridge Creek and Fort Hays Limestones in a transect of four cores across the basin. Nannofossil assemblage counts are being combined with planktonic foraminiferal assemblages and geochemical data to determine possible origins for lithologic cycles.

Relative abundances of Watznaueria barnesae, a dissolution resistant species, indicate that the material is well preserved and has not undergone extensive dissolution (average W. barnesae relative abundances are 21%). This suggests that lithologic cycles are not an artifact of variable preservation and indicate paleoceanographic changes. Particular intervals contain cyclic fluctuations in certain nannofossil taxa where carbonate measurements show no cyclic trends. In the Fort Hays Limestone, shales as opposed to limestones tend to have higher Zygodiscus spp. and Biscutum constans. These taxa are considered to be indices of high fertility, therefore we interpret these shales to represent conditions of increased productivity. We are also studying patterns in other taxa (e.g. Rhagodiscus spp., Cretarhabdus spp.) across the transect to determine onshore-offshore gradients and their relationships to sea level fluctuations.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994