Character and Genesis of the Ingersoll Shale, A Compact Continental Fossil-Lagerstatte, Upper Cretaceous Eutaw Formation, Eastern Alabama

Abstract

The Ingersoll shale, a thin (<1 m) clay-dominated lens within the Upper Cretaceous Eutaw Formation in eastern Alabama, contains a well-preserved, primarily continental biota that includes a diverse, carbonized, and variably pyritized flora, abundant amber with fossil inclusions, and common feathers. Geometry of the Ingersoll shale lens and its position between high-energy tidal deposits below and estuarine central bay deposits above indicate deposition in a shallow, narrow channel in the lower reaches of a bayhead delta in response to estuarine transgression. Tidal rhythmites and textural trends indicate that the channel filled very rapidly (55–80 cm per yr), under progressively waning energy regimes. Ichnofabrics, high organic carbon contents, and abundant pyrite indicate highly fluid and reducing sediments. Marine palynomorphs, sulfide contents, and δ34S values of pyrite sulfur indicate normal to near-normal marine salinities. Environmental factors and sedimentary processes contributing to preservation of the Ingersoll shale biota include (1) rapid deposition and burial (obrution); (2) reducing pore waters (stagnation), which limited bioturbation and scavenging, promoted pyritization of some fossils (diagenetic mineralization), and facilitated bacterial replacement of others (i.e., feathers); and (3) concentration of allochthonous or para-autochthonous amber clasts (preservation traps) by tidal currents. Lessons from the Ingersoll shale may help prospect for similar, isolated yet fossil-rich marginal marine deposits.