ICHNOLOGY OF ESTUARINE STORM BEDS, UPPER CRETACEOUS EUTAW FORMATION, WESTERN GEORGIA

The Upper Cretaceous Eutaw Formation of western Georgia is characterized by a variety of estuarine facies. Mud-dominated, distal and proximal central bay deposits exposed in the Columbus area contain numerous thin (< 30 cm), carbonaceous, hummocky cross-stratified, fine-grained sand beds that likely reflect deposition during major storm events. Storm beds are moderately bioturbated (ichnofabric index=2 to 3) and contain a moderately diverse assemblage of trace fossils. These assemblages include, in order of decreasing abundance: robust, thickly-lined Ophiomorpha; funnel- and dish-shaped structures; Teichichnus; Rosselia; Thalassinoides; Palaeophycus(?); Asterosoma; Planolites; backfilled cylindrical burrows (Taenidium?); and escape structures. Most of these ichnofossils also occur in background muds and can be attributed to the work of deposit feeders that colonized the sands upon re-establishment of fair-weather conditions. Ophiomorpha and funnel- and dish-shaped structures only are found in event-bed sands and likely are related in some way to opportunistic colonization by shrimp. Funnel- and dish-shaped structures, both of which originate from sand-bed tops and are filled with highly bioturbated muds, are intimately associated with Ophiomorpha. Funnel-shaped structures, measuring 7 to 12 cm in diameter and 5 to 8 cm in depth, locally are connected at their apices to vertical to oblique Ophiomorpha segments. Dish-shaped structures, typically 18 to 27 cm wide and 8 to 10 cm deep, commonly truncate Ophiomorpha shafts or tunnels. Some funnel-shaped structures may be related to the construction, abandonment, and/or collapse of these pellet-lined burrow networks. Dish-shaped structures closely resemble previously described ray-feeding structures. Their close link to Ophiomorpha suggests a possible predator-prey relationship.