PHOSPHATIC CONCRETIONS AND ICHNOFOSSIL PRESERVATION IN CRETACEOUS MARINE SHELF DEPOSITS, RIPLEY FORMATION, CENTRAL ALABAMA

The Upper Cretaceous Ripley Formation in central Alabama includes concretion-bearing, fossiliferous sandy muds and marls deposited on a marine shelf. The concretions, which are composed primarily of phosphate, commonly entomb body fossils, mainly articulated crabs but also ammonites and rare vertebrates (fish). However, early diagenetic phosphate mineralization also has impacted in various ways the preservation and expression of ichnofossils. In some cases, trace fossil preservation is associated with body fossil nuclei. Both pre- and syn-mineralization traces are preserved on the exteriors of mummy concretions that nucleated on crabs carapaces, and traces (e.g., Chondrites, Thalassinoides) produced by organisms that exploited the sediment fills of ammonites are now well-manifest as semi-reliefs on exteriors of phosphatic steinkerns. In other cases, burrows of various morphologies and orientations, including large, horizontal, commonly shell-filled crustacean(?) burrows, served as phosphate nucleation sites; mineralization was either confined to burrow fills or continued a short distance beyond burrow walls into ambient sediments. Ichnofossils also are manifest on and within common, irregular phosphate concretions for which discrete nuclei are not obvious. Slabbing and polishing of the latter concretions reveal relatively uncompacted bioturbated fabrics. More important, owing to texture- or composition-controlled differences in susceptibility to mineralization, burrows or burrow segments are commonly preserved as concave semireliefs on nodule exteriors, sometimes revealing fine details of wall structure (e.g., pelleted linings). Because the Ripley concretions provide ichnologic information that is otherwise not readily obtained from unmineralized sediment, they embody both a body and ichnofossil lagerstätte.