CHARACTER AND SEQUENCE STRATIGRAPHIC IMPLICATIONS OF LARGE FIRMGROUND BURROW SYSTEMS IN PALEOCENE MARINE SHELF DEPOSITS, CLAYTON FORMATION, WESTERN ALABAMA

The Paleocene (Danian) Clayton Formation exposed along the Tombigbee River, Sumter County, western Alabama, features an ~4-m interval characterized by relatively thick (18-120 cm), dark olive-gray, poorly indurated marls (50-60% CaCO₃) and thin (9-32 cm), light gray, indurated, locally glauconitic interbeds of marly limestone (75-85% CaCO₃) deposited in a marine shelf setting during sea-level highstand. Both marls and limestones are thoroughly bioturbated and exhibit softground ichnofabrics dominated by small Thalassinoides, Planolites, and Chondrites. The most prominent ichnologic features, however, are large, highly irregular, branching burrow systems that are preserved in full relief in marls but emanate from and were passively filled with sediment derived from overlying limestone. Excavations of these burrow systems reveal predominately horizontal branching networks that formed 50 to 60 cm below their source horizons and extend over areas in excess of 3.5 m². Individual burrow segments are typically straight to gently curved, although rare, openly to tightly sinuous meanders are locally observed. Segments are up to 87 cm in length, range from 0.5 to 6 cm in diameter, and exhibit elliptical cross sections with width/height ratios varying from 1.2 to 1.6. Bifurcation angles vary considerably (30-90°), and burrow segments locally anastomose. Burrow walls are sharp and generally ornamented with scratch traces. Bioglyphs most commonly form rhombohedral or longitudinal patterns, but transverse patterns are also seen locally, particularly on burrow floors. 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 Spongeliomorpha and Thalassinoides, represent the firmground Glossifungites ichnofacies. Firmground conditions likely developed in response to short-term base-level falls and seafloor scour. Accordingly, marl tops are interpreted as parasequence-bounding marine flooding surfaces. Similar limestone-marl couplets with large firmground burrow systems define parasequences in more vertically extensive highstand intervals of the Clayton Formation elsewhere in the Alabama coastal plain.