MESO-TIDAL INTERPRETATION OF HARTSELLE SANDSTONE (LOWER CARBONIFEROUS, ALABAMA) BY ANALOGY WITH BAY OF FUNDY MACRO-TIDAL ICHNOFACIES

A large tidal flat at Lubec, Maine extends for more than a kilometer at low tide. Tide ranges from 7 to 8 meters depending on lunar phase and time of year. The elevation gradient increases monotonically towards the beach where the flat is bound by a steep gradient to the shoreline. Surface sediments are heterogeneous, ranging from coarse gravel to silt. The surface of the flat is also biologically heterogeneous with areas of the mid-flat covered with algae while others are barren. The upper few centimeters of sediment are light-colored and oxic, with dark, H₂S-rich sediment below. The keystone natural predator on the tidal flat is the herring gull, Larus argentatus, followed by smaller shorebirds. The feeding gulls especially favor crabs (Cancer borealis, C. irroratus, and Carcinus maenas), and the smaller shorebirds prey on smaller crustaceans. Surface traces (grazing, crawling, and resting) are relatively sparse but infaunal traces (feeding and dwelling) are abundant. A variety of small crustaceans, predominantly the amphipod Corophium volutator in muddy sediment and the isopod Chiridotea coeca in sandy sediment, inhabit the upper reach of the flat (more than 8-10 hours exposure per tidal cycle). Middle reach sediment (4-8 hours exposure) is inhabited primarily by pelecypod mollusks (notably Mya arenaria), with relatively dense collections of the annelid Arenicola marina. Decapod crustaceans, gastropods, and echinoderms are found in the lower reach of the flat (0-4 hours exposure) where they are often stranded by spring tides. Annelids, both polychaetes and oligochaetes, are abundant everywhere on the flat. Trace fossil zonation in the Hartselle SS (Mississippian) exposed on Fielder Ridge, Colbert Co., Alabama is compared with the modern Lubec ichnofacies and is demonstrated to be analogous by sedimentologic and ichnotaxa morphologic criteria. Pascichnia, repichnia, cubichnia, and many of the domichnia traces in the Hartselle can be attributed to crustacean arthropods rather than trilobites or worms. Many of the traces were apparently made on exposed inter-tidal sediment and these data support an interpretation of meso- to macro-tidal deposition. The resultant tidal flux into and out of the Black Warrior Basin would have resulted on a strong tidal influence on the regional depositional pattern of the Hartselle.