NETWORK CRUSTACEAN BURROWS IN AN OFFSHORE FIRMGROUND: AN EXAMPLE FROM THE MIDDLE EOCENE LISBON FORMATION OF ALABAMA

The Lisbon Formation of Alabama records deposition on a shallow, mixed carbonate-siliciclastic shelf that flanked the eastern margin of the Mississippi embayment during the Lutetian and early Bartonian. A conspicuous, regionally mappable firmground occurs within the upper part of the Lisbon in exposures along the banks of the Tombigbee River at Coffeeville Landing (Clarke County). This surface is expressed as a low-relief disconformity developed on a homogenous, silty claystone and is overlain by a condensed shell bed. Fossiliferous sand is piped downward into Gyrolithes and networks with the characteristic triple junctions and swellings of the elite crustacean burrow Spongeliomorpha in a fabric superposed on earlier softground Thalassinoides. The tracemakers of Spongeliomorpha were ecological engineers whose irregular burrows are common in firmgrounds from marsh creek to sediment-starved offshore marine environments. Field and subsurface relations suggest that these strata occur within the upper depositional sequence of the Lisbon, between the regional unconformities associated with the Sparta and Yegua depositional episodes. Thus, it likely records the combined effects of sediment starvation and submarine erosion during the maximum flooding of the shelf. Accordingly, it also serves to delineate the shift in deposition from a transgressive siliciclastic shelf to a regressive carbonate margin. The occurrence of the Glossifungites ichnofacies in this context may reflect the dual sources of sediment characteristic of mixed-lithology systems and their differential expression through the relative sea level cycle.