Stratigraphic and Paleoenvironmental Context of the Ingersoll Shale, An Upper Cretaceous Conservation Lagerstatte, Eutaw Formation, Eastern Alabama

Abstract

The Ingersoll shale, a thin (<1 m) clay-dominated lens within the Upper Cretaceous Eutaw Formation exposed in Russell County, Alabama, contains an exceptionally well-preserved, primarily terrestrial biota that includes a diverse, carbonized and variably pyritized flora, abundant amber with fossil inclusions, and common feathers. Stratigraphic, sedimentologic, and geochemical studies were undertaken in an attempt to determine the conditions under which this conservation Lagerstätte formed. Geometry of the Ingersoll shale lens and its position between high-energy tidal sand facies below and estuarine central bay sediments above indicate that deposition occurred in a shallow, narrow channel in the lower reaches of a bay-head delta in response to estuarine transgression. Well-developed tidal laminites in the lower part of the shale lens and textural analysis of the entire unit indicate that the channel filled very rapidly (up to 77 cm/yr), under progressively waning energy regimes, in response to diurnal tidal rhythms. Ichnofabrics, high organic carbon, and abundant pyrite indicate that sediments in the channel were likely highly fluid and oxygen deficient. Abundance of marine palynomorphs, sulfide contents, and stable isotopic signatures of pyrite sulfur indicate that waters within the channel were characterized by normal to near-normal marine salinities. Environmental factors and sedimentary processes contributing to unusual preservation of the Ingersoll shale biota include (1) extremely rapid tidal deposition and burial of organic remains (obrution); (2) reducing conditions in pore waters (stagnation), which limited bioturbation and scavenging, resulted in pyritization of some fossils (diagenetic mineralization), and facilitated the growth of bacterial mats on fossil feathers and, possibly, other fossil elements (bacterial sealing); and (3) concentration of allochthonous or para-autochthonous amber clasts (preservation traps) by tidal currents. Documented conservation Lagerstätten differ significantly in terms of paleoenvironments, fossil biotas, and modes and causes of preservation. Nonetheless, knowledge gained from the study of the Ingersoll shale may help prospect for similar isolated marginal deposits that may contain comparable well-preserved, predominately terrestrial fossil assemblages.