Ichnofossils and Ichnofabrics in Syngenetic Phosphatic Concretions in Siliciclastic Shelf Deposits, Ripley Formation, Cretaceous, Alabama

摘要

Studies of phosphatic concretions and associated marine-shelf sediments in the Upper Cretaceous Ripley Formation, central Alabama, demonstrate the positive impacts that early diagenetic mineralization may have on ichnofossil preservation. Distinct ichnofossils are poorly expressed in highly bioturbated, calcareous, organic-rich muds owing to lack of three-dimensional expression, limited contrast between biogenic structures and ambient sediments, and compactional deformation. In contrast, phosphate concretions that formed at shallow sediment depths during early diagenesis exhibit a Cruziana ichnofacies assemblage. This assemblage indicates both (1) incidental preservation, whereby large burrows served as concretion nuclei, and (2) collateral preservation, whereby biogenic structures were incorporated within or on exteriors of concretions as diagenetic fronts migrated outward from body-fossil and other nuclei into surrounding sediments. Although most Ripley Formation ichnofossils were emplaced prior to mineralization, some apparently reflect burrowing that was contemporaneous with and influenced by phosphatization. In either case, concretion growth enhanced ichnofossil visibility by boosting contrast with host sediment, by rendering structures or portions thereof in three dimensions, and by precluding significant compaction. Concretion-hosted ichnofossil assemblages in the Ripley Formation and comparable deposits, analogous to conservation traps in the body-fossil realm, can be regarded as ichnofossil Lagerstätten.