TRACE FOSSILS IN THE GREEN ZONE: ICHNOLOGIC, SEDIMENTOLOGIC AND SEQUENCE STRATIGRAPHIC IMPLICATIONS OF GLAUCONY-RICH GREENSANDS (EOCENE, TEXAS)

Glaucony-rich sedimentary units often are linked with marine transgressions, particularly in association with condensed sections in terrigenous depositional environments. The presence of glaucony can be a powerful interpretative tool when characterized as to maturity, spatial and temporal distribution, and origin. In addition, trace fossils, ichnofacies and ichnofabrics can augment our understanding of glaucony-rich zones. As part of a larger study of glaucony associated with trace fossil occurrences throughout the geologic record, the Eocene Main Glauconite Bed (MGB) of the Stone City Member (Crockett Formation) in eastern Texas was selected for evaluation of the ichnological and sequence stratigraphic implications. Previous studies of the MGB by other workers address the depositional environment, faunal communities and sedimentology. In this study, glaucony was characterized in detail using Quantitative Evaluation of Materials with Scanning Electron Microscopy (QEMSCAN) and thin section analyses. Glaucony within the MGB is predominately pelletal and is indicative of an in situ reworked origin rather than an allochthonous origin accompanied by appreciable transport. Trace fossils observed within the MGB include the crustacean burrows, Thalassinoides and Gyrolithes, while Spongeliomorpha was identified in association with a concretionary zone. Fish (stingray?) trace fossils, Piscichnus, occur in strata above the MGB. This sequence exhibits a moderate ichnodiversity, and it probably represents the Cruziana ichnofacies. Ichnofabric index range is from ii1 to ii5. The Stone City Member also contains a concretionary zone of ferruginized crustacean burrows. This may represent a firmground or marine flooding surface. Initial indications suggest a transgressive-regressive systems tract interpretation in which the glaucony grains, body fossils and trace fossils represent a time-averaged association produced during a shoreline migration.