Calcareous Nannofossil Systematics, Paleoecology, and Biostratigraphy of the Middle Eocene Weches Formation of Texas
AbstractEighty previously described species of calcareous nannofossils representing 25 genera arc figured with scanning-electron micrographs and photomicrographs. Improved criteria for identifying and distinguishing species and genera arc presented based upon their appearance in the scanning-electron microscope. Nine new combinations are formed. Based upon the stratigraphic range of key species, the Discoaster lodoensis Zone, Discoaster sublodoensis Zone, Discoaster tani s.l.-Sphenolithus radians Zone, Reticulofenestra umbilica-Sphenolithus furcatolithoides Zone. Pemma papillatum Zone, Reticulofenestra Datum and Discoaster saipanensis Datum of Gartner's 1971 Paleogene Nannofossil Zonation are recognized. The positions of two missing zones, Bramletteius serraculoides Zone and Chiphragmalithus quadratus Zone, have been inferred. Variations in the occurrence and abundance of species between outcrop and subsurface samples were found to exist. These variations may be the result of differential preservation or the result of living species being restricted to particular areas of the neritic environment. The following species were found only in the subsurface portion of the Weches and are considered indicators of the outer neritic environment: Chiasmolithus titus Pontosphaera plana Pontosphaera versa Micrantholithus procerus Sphenolithus furcatolithoides Sphenolithus obtusus Trochoaster radiatus Discoaster aster Discoaster diastypus Discoaster triangularis Discoaster woodringi The following species were found to occur in several subsurface samples but appeared in only one or two outcrop samples. These species probably could live throughout the neritic environment but preferred deeper portions or were better preserved in deeper water deposits. Chiasmolithus grandis Pontosphaera pectinata Braarudosphaera discula Micrantholithus flos Micrantholithus ornatus Helicopontosphaera lophota Rhabdolithes rudis Discoaster stella Discoaster barbadiensis Discoaster saipanensis Discoaster tani nodzfer Lophodolithus rotundus and Marthasterites furcatus occurred exclusively in outcrop samples. Chiasmolithus bidens, Blackites amplus and Trochoaster simplex occurred several times in outcrop samples but each is found only once in the subsurface. These five species are considered indicative of the middle to inner neritic environment. Cyclococcolithina formosa, Cruciplacolithus staurion, and Zygrhablithus bijugatus occurred in both subsurface and surface samples but were generally present in greater abundance in subsurface samples. Ericsonia muiri, Reticulofenestra coenura, Koczyia wechesensis, Traversopontis pulchriporus, Blackites tenuis, and Discoaster elegans were present in greater abundance in outcrop samples.