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The AAPG/Datapages Combined Publications Database
AAPG Bulletin
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Eight samples from the Upper Cretaceous of the northwestern Gulf Coast were studied intensively by electron and light microscopy. The samples, ranging in age from Eaglefordian to Navarroan, yielded 93 species, of which 35 are new. All are assignable to 31 genera, of which four are new. Most of the species and genera fit readily and naturally into a small number of higher taxa. Forms are distinguished primarily on the basis of gross morphology and secondarily on variations visible only by using crossed nicols on a light microscope or an electron microscope. Reworking of specimens is common with the result that a reliable zonation must be based on the entire assemblage including relative abundance of all species.
The Eaglefordian sample is characterized by two elliptical, placolith-like species related to the genus Coccolithus, which is rare or absent in younger deposits. Arkhangelskiella, Cribrosphaerella, Marthasterites, and Micula, four typical Upper Cretaceous genera, are lacking. The middle part of the Austin Chalk is marked by the first occurrence of Lucianorhabdus cayeuxi Deflandre, Micula decussata Vekshina, and a new species of Zygodiscus. The first occurrence of Cretarhabdus? decorus (Deflandre) and abundant specimens of Microrhabdulus mark the upper part of the Taylor Marl. The late Navarroan is distinguished by the appearance of Lithraphidites quadratus Bramlette and Martini and a new genus apparently related to Arkhangelskiella. Lucianorhabdus cayeuxi Deflandre and several new spe ies common to both the Austin Chalk and Taylor Marl are absent.
Many of the species appear to have notably restricted stratigraphic ranges and wide geographic distribution. Nannofossils, therefore, are a good criterion for refined zonation and intercontinental correlation.
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