Abstract: Distribution and Significance of Trace Fossils in Upper Devonian Glen Aubrey Formation (Turbidite to Shelf Transition), New York
Quarry exposures of basal Glen Aubrey rocks demonstrate that substitution in trace fossil assemblages occurs prior to lithologic change in sediments and reflects a sharp upward transition from slope to shelf environments. Slope sediments consist of channeled, discontinuous turbidites arranged in laterally continuous packages alternating with sets of thin continuous turbidites with higher mudstone/sandstone ratio. Most trace fossils low in the combined turbidite section are hypichnial structures predating sandstone deposition. Representatives are typically the Nereities assemblage and include varied trail networks, starlike and errant feeding structures, and arthropod walking tracks. Most resulted from deposit-feeding activity. Trace fossils higher in the turbidite section consist of nearly equal proportions of pre- and postdepositional hypichnial forms. Network structures are replaced by less systematic grazing trails, and both paired or unpaired burrows of suspension feeders. Extensive postdepositional burrow systems are hypichnial and endichnial. The appearance of varied epichnial trails on siltstones and intense bioturbation coincides with beginning of shelf-mud deposition. Systematic burrows are no longer present. Intraturbidite distribution of trace fossils suggests that preservational processes account for major differences in assemblages in channeled and nonchanneled parts of turbidites. Variation in feeding types within the turbidite section signals subtle changes in nonsedimentologic factors such as nutrient availability and dissolved oxygen. Pro esses accompanying marine regression initiated environmental changes in the deeper basin which may have changed trophic groups prior to the onset of Glen Aubrey deposition.
AAPG Search and Discovery Article #90968©1977 AAPG-SEPM Annual Convention and Exhibition, Washington, DC