PALEOENVIRONMENTAL FACTORS CONTROLLING FOSSIL PRESERVATION IN AN UPPER CRETACEOUS KONSERVAT-LAGERSTÄTTE (INGERSOLL SHALE, EUTAW FORMATION, EASTERN ALABAMA)

The Ingersoll shale is a thin (<1 m), discontinuous, clay-dominated lens within the Upper Cretaceous Eutaw Formation exposed in Russell County, Alabama. It contains an exceptionally well-preserved, primarily terrestrial biota. Sedimentologic and geochemical investigations were undertaken in an attempt to determine the conditions under which this lagerstätte formed. The Ingersoll shale is bounded below by cross-stratified, Ophiomorpha-bearing tidal sands deposited in a bayhead delta setting, and above by muddy sands and sandy muds deposited in a proximal estuarine central bay. Both upper and lower boundaries of the Ingersoll shale are erosional: the basal contact truncates cross-stratification within underlying sands, whereas the upper contact with the superjacent sandy mud is marked by a Rhizocorallium-dominated firmground ichnofossil assemblage. Sand and silt contents in the Ingersoll shale generally decrease upward. Clays in the upper part of the unit contain only narrow (1-3 mm), flattened, horizontal to subhorizontal, pyritized burrows, which reflect highly fluid substrates. The lower part of the unit is characterized by unbioturbated to weakly bioturbated, graded sand-mud couplets, the bundling of which is indicative of tidal cyclicity. Organic carbon (1.2-3.6%) and pyrite contents in the Ingersoll shale are high, indicating general oxygen-deficiency. High pyrite contents indicate normal or near-normal marine salinities. S/C ratios are high, reflecting terrestrial Fe input. Analyses of tidal rhythmites and textural data indicate that the Ingersoll shale accumulated rapidly (>50 cm/year) under progressively decreasing energy levels. Taken together, observations indicate that the Ingersoll shale was deposited during transgression in a restricted estuarine environment formed by channel abandonment. Factors that contributed to unusual fossil preservation in this setting include extremely high sedimentation rates, oxygen-deficient pore waters, and early pyrite mineralization associated with microbial sulfate-reduction.