ICHNOFOSSIL PRESERVATION ENHANCED BY EARLY DIAGENETIC MINERALIZATION: AN EXAMPLE FROM THE CRETACEOUS RIPLEY FORMATION, ALABAMA

Early diagenetic mineralization is one of the most important factors governing the preservation of body fossils and is integral to the formation of conservation lagerstätten, particularly those that preserve soft parts. Ichnofossils also are subject to taphonomic controls, including those in the diagenetic realm. The tendency for trace fossils to be selectively mineralized by pyrite, silica, carbonate, or phosphate is generally well known. However, the potential role of early diagenetic mineralization, particularly concretion growth, in preserving ichnologic information is not fully appreciated. The positive impacts that diagenetic mineralization may have on ichnofossil preservation are exemplified via comparative studies of phosphate concretions and unmineralized host sediments in marine shelf facies of the Cretaceous Ripley Formation exposed in central Alabama. Unmineralized calcareous muds are virtually completely bioturbated, yet discrete trace fossils are rarely expressed. In contrast, a low-diversity Cruziana ichnofacies assemblage of pre-mineralization traces is manifest on and in early-formed concretions. In some cases, large crustacean burrows and other structures served as concretion nuclei. This incidental preservation reflects higher permeabilities and/or organic contents of burrows or burrow fills. More commonly, trace fossils were preserved within or on exteriors of concretions that grew from body-fossil and other, enigmatic nuclei into surrounding bioturbated sediments (collateral preservation). Ichnofossils manifest on concretion exteriors commonly are preserved in relief, reflecting differential migration of mineralization fronts. Syngenetic concretion growth improved ichnofossil preservation, and hence the paleoenvironmental signal that trace fossils carry, by enhancing contrasts between biogenic structures and ambient sediments, by precluding deformation of structures during burial and compaction, and by rendering structures in three dimensions.