Paleoclimatic and Sequence Stratigraphic Significance of Mineralogy and Ichnology in an Eocene Greensand, Texas

The overarching purpose of this research project is to examine selected sites throughout the geologic record that represent trace fossils in association with “glaucony” greensands in a sequence stratigraphic context with paleoclimatic implications. An important and informative site for these studies is Stone City Bluff in Burleson County, Texas. There the Main Glauconite Bed (MGB), a trace fossiliferous greensand of the Middle Eocene Stone City Member, Crockett Formation, crops out on the south bank of the Brazos River on the Texas Gulf Coastal Plain. The MGB is a 1.7 m thick greensand, which represents a fully marine unit within the Stone City Bluff section. It has long been studied by paleontologists and stratigraphers to understand its paleoecology and its place within the regional stratigraphic framework. Our research focuses on the physical and biological processes that occurred during accumulation of the MGB, as evidenced in its ichnologic and mineralogic character in a paleoclimatic context. We examined the trace fossils, ichnofabric and ichnofacies as well as the mineralogy of bulk samples and the pelleted component of the unit.

Results demonstrate a composite ichnofabric for the MGB, which indicates a dynamic history of the deposit from a softground substrate in the central bed to a firmground substrate at the top of bed, as evidenced by the trace fossil assemblage. Initial x-ray diffraction analyses of clay minerals show iron-rich smectite in a clay mixture with kaolinite and illite. Microprobe analyses of selected pellets show that most pellets have negligible K2O. Both types of analyses indicate a lack of glaucony in the MGB. Further mineralogic analyses are needed, since glaucony typically forms in an offshore setting in modern marine environments, and for that reason its presence or absence has paleoenvironmental significance. The Stone City Bluff section exhibits parasequence level stratigraphy, demonstrates dynamic sea level fluctuation, and shows evidence of intense biological activity along the Gulf Coast 40 mya. At this time the paleoclimate was in transition during the Middle Eocene Climatic Optimum. This study of trace fossils and associated greensands provides useful information for discerning the behavior and paleoecology of benthic organisms during dynamic sea level and paleoclimatic conditions.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California