Ichnology, sedimentology, and sequence stratigraphy of outer-estuarine and coastal-plain deposits: Implications for the distinction between allogenic and autogenic expressions of the Glossifungites Ichnofacies

Abstract

Integration of sedimentologic, ichnologic and sequence-stratigraphic data supports that the upper member of the Río Turbio Formation records sediment accumulation in tide-dominated, outer-estuarine and coastal-plain environments. Outer-estuarine assemblages comprise facies that were deposited in channel-thalweg, subtidal-sandbar complex, oyster-reef and reef-margin environments. Coastal-plain facies assemblages include distributary-channel, floodplain, salt-marsh, swamp, and swamp margin deposits. Twelve ichnogenera assigned to soft-, stiff- and firmground suites were identified. Surfaces demarcated by autogenic and allogenic expressions of the Glossifungites Ichnofacies were recognized. The trace fossils Thalassinoides, Psilonichnus, and Bergaueria were documented within the autogenic stiffground suites, whereas Gastrochaenolites was found in both autogenic and allogenic firmground suites. Allogenically produced surfaces occur on a regional scale, creating major discontinuities, while minor autogenically produced surfaces extended laterally only over a few meters. Integrating the data derived from the substrate-controlled ichnofacies with the vertical and lateral distribution of the paleoecological data provided a more thorough understanding of the importance and scale of the erosional discontinuities. Estuarine and coastal-plain deposits contain low-diversity trace fossil assemblages inferred to have been produced by opportunistic, impoverished marine infauna, indicative of brackish-water settings. The overall distribution of ichnocoenoses within the lower portion of the upper member of the Río Turbio Formation appears to have been controlled largely by the salinity gradient, with other factors such as energy and sedimentation rate playing a more limited role.