Phosphatic Concretions and Ichnofossil Preservation in a Marine Lagerstätte, Ripley Formation, Central Alabama

Abstract

Phosphate concretions are common in the Upper Cretaceous Ripley Formation exposed along Dry Cedar Creek, Lowndes County, Alabama. Sedimentologic and ichnologic studies of concretions and host sediments were undertaken to investigate the origins of concretions and the role of phosphatization in trace fossil preservation.

The concretion-bearing interval is composed of bioturbated, fossiliferous, calcareous, carbonaceous mud and sandy mud. Deposition occurred on a shallow shelf between fair-weather and storm wave bases and beneath moderately oxygenated waters of relative high organic productivity. Preservation state of body fossil nuclei and textural and geochemical evidence indicate that concretions formed during very early diagenesis at shallow substrate depths prior to significant compaction. Phosphate likely was derived from biodegradation of organic matter and precipitated near the oxic/anoxic boundary, perhaps during periods of lower sedimentation rates and/or hypoxia.

Trace fossils are poorly preserved in unmineralized Ripley sediments owing to limited textural and compositional contrast, lack of three-dimensional expression, and compactional deformation. However, trace fossils are commonly preferentially preserved on or within phosphate concretions. The preserved assemblage, reflecting the work of both deposit- and suspension-feeding organisms, represents an impoverished Cruziana ichnofacies. Phosphatization enhanced ichnofossil visibility by limiting compaction and by providing partial three-dimensional views of ichnofossils. Ichnofossils locally formed microenvironments favorable for phosphatization, serving as the nuclei for concretion growth. More commonly, trace fossils were preserved by collateral mineralization on the interiors and exteriors of concretions with body-fossil (e.g., crabs, ammonites, vertebrate remains) and enigmatic nuclei. Preservation is best on concretion exteriors where, owing to differential migration of mineralization fronts, trace fossils were preserved in partial relief. Because concretions provide ichnologic information that is otherwise not available from unmineralized host sediments, they represent a form of conservation-lagerstätte.

Based on observations made in this study and by previous workers, a set of terms is proposed for describing the relationships between diagenetic mineralization and trace fossil preservation. These terms refer to the relative timing of concretion growth and trace fossil production, type of mineralization, spatial relationships between concretion nuclei and preserved trace fossils, and modes of trace fossil expression relative to concretion interiors and exteriors.