PALEOENVIRONMENTAL CONTEXT OF THE INGERSOLL SHALE, AN UPPER CRETACEOUS KONSERVAT-LAGERSTÄTTE, EUTAW FORMATION, EASTERN ALABAMA

The Ingersoll shale, part of the Upper Cretaceous Eutaw Formation exposed in Russell County, Alabama, is a thin (<1 m), discontinuous, clay-dominated lens containing an exceptionally well-preserved terrestrial biota. Field and laboratory studies were undertaken in attempt to evaluate the general depositional setting and environmental conditions for the Ingersoll shale. The shale lies in sharp contact upon Ophiomorpha-bearing, cross-laminated, coarse-grained tidal sands. Detailed textural data and observations of depositional cyclicity reflected locally in sand-shale laminae indicate that the Ingersoll shale accumulated rapidly, under progressively decreasing energy levels, in a tidally influenced setting. Ichnofossils within the clay are dominated by small (2-3 mm), horizontal to subhorizontal, pyritized slime trails or burrows, which reflect soupground conditions during deposition. Organic carbon (1.2-1.8%) and pyrite contents in the Ingersoll shale are high, indicating oxygen-deficient conditions. S/C ratios are high, reminiscent of those observed in euxinic basin sediments. High pyrite contents within the clay indicate normal marine or brackish conditions. Sharp-walled Thalassinoides and Rhizocorallium pipe down into the shale from above. This indicates firmground conditions and the development of a disconformity between the shale and the immediately superjacent muddy sands, which were deposited in an estuarine central bay setting. Synthesis of results acquired thus far suggest that the Ingersoll shale was deposited in a restricted tidal creek near the head of a mixed-energy estuary during transgression.