Ichnology of fair-weather and storm deposits in an Upper Cretaceous estuary (Eutaw Formation, western Georgia, USA)

Exposures of the Eutaw Formation in western Georgia, eastern Gulf coastal plain, USA, were examined to evaluate the ichnologic signature of fair-weather and storm-related strata deposited within the central bay of a Late Cretaceous estuary. Ichnofabrics of fair-weather sandy muds and muddy sands compare favorably with those of previously well-documented estuarine facies of the Cretaceous Western Interior foreland basin. Trace fossil assemblages vary with sediment texture, reflecting proximity to sources and frequency of deposition of sands, but they generally are dominated by simple structures attributed to activities of trophic generalists (Terebellina, Teichichnus, and Planolites). Although this likely reflects environmental stress associated with estuarine dynamics, the degree to which salinity fluctuations governed trace fossil assemblages cannot be confidently assessed. Trace fossil assemblages in estuarine storm sands also are variable. Distal central bay tempestites include fair-weather ichnofossil suites. However, commonly in proximal parts of the central bay, rapid event-related accumulation of suspended clays immediately followed sand emplacement and thereby precluded burrowing of sands by subsequently established fair-weather organisms. By virtue of proximity to fluvial point sources and/or salinity gradients, this phenomenon may be unique to and diagnostic of certain estuarine settings. Ophiomorpha is nearly ubiquitous in storm sands. Normally attributed to post-storm colonization by opportunistic organisms, these burrow systems instead may represent the work of storm-transported crustaceans.