CHARACTER AND ORIGIN OF LARGE CRUSTACEAN BURROW SYSTEMS IN PALEOCENE MARINE SHELF DEPOSITS, CLAYTON FORMATION, WESTERN ALABAMA

The Paleocene (Danian) Clayton Formation exposed at Moscow Landing on the Tombigbee River, eastern Sumter County, Alabama, includes an ~4-m interval characterized by relatively thick (18-120 cm) dark olive gray marls (50-60% CaCO₃) and thin (9-32 cm), locally glauconitic, light gray, marly limestone beds (75-85% CaCO₃) deposited in a marine shelf setting. Both marls and limestone beds are thoroughly bioturbated; background fabrics in the marls are characterized by suites of trace fossils that reflect softground conditions (e.g., small Thalassinoides, Planolites, and Chondrites). However, the most prominent ichnologic feature of these beds are unusually large, areally extensive and irregular branching burrow systems provisionally attributed to crustaceans. These burrow systems, which emanate from and are passively filled with marly limestone, occur within the marls where they weather out in positive relief. To better evaluate the character and implications of these structures, several burrow systems were carefully excavated using hand tools. Excavations reveal predominately horizontal branching networks that typically formed 50 to 60 cm below their source horizons and extend over areas of at least 3.5 m². Within individual systems, burrow segments are typically straight to slightly curved, although locally tight meanders are observed. Segments are up to 50 cm long, range widely from 1 to 7 cm in diameter, and exhibit subcircular to ovate cross sections. Angles of burrow branching vary significantly, and burrow segments locally anastomose. Burrow walls are sharp and at least locally ornamented by longitudinal scratch marks. The character of these structures suggests long-term (and possibly simultaneous) occupation of open burrow networks by a community of crustaceans (crabs and/or shrimp) of variable sizes. These burrow systems, provisionally assigned to the ichnotaxa Thalassinoides and Spongeliomorpha, represent the firmground Glossifungites ichnofacies. Marl-limestone couplets are interpreted as parasequences, and firmgrounds conditions may have developed in response to short phases of relative sea-level rise and/or condensation.