SNAPSHOT OF PHOSPHATE NODULE FORMATION IN THE MISSISSIPPIAN BORDEN FORMATION, KENTUCKY: A CRINOID OBRUTION EVENT AS A SOURCE OF PHOSPHORUS

Dozens of articulated crinoids in life position are unusually preserved along a bedding plane in a shale unit within the Mississippian Borden Formation of Casey County, Kentucky. This study investigates their paleoecology, the conditions under which the specimens were deposited, as well as the meaning of their unusual preservation.

Analysis of fossil abundance and specimen proximity indicates a small, apparently mono-specific crinoid garden. The substrate on which they rested is very fine-grained, indicating that the crinoids were adapted to living on a muddy sea floor. The shales are blue-grey in color, indicating deposition in low-oxygen conditions. No other fossils were found associated with the crinoids, indicating that they were adapted to an environment that excluded other benthic species.

The crinoid garden was buried rapidly and uniformly. The depositional event that led to the obrution was not, however, a violent one; only 1% of the specimens in this study were in an orientation other than vertical. They are encased in fine-grained mud, probably indicating deposition in the distal areas of a turbidity flow. The crinoid skeleton is preserved as coarsely crystalline calcite that may represent the recrystallization of the original calcium carbonate skeleton. All specimens are associated with phosphate nodules that in some cases partially envelope the recrystallized calyxes. Because the crinoids were buried alive, phosphorus present in the living tissue dispersed into the surrounding sediments and triggered the formation of the nodule around the decaying crinoid. The enclosure of the coarsely recrystallized calyxes within the phosphate nodules implies that the recrystallization happened very quickly after death.

The Borden Formation includes abundant phosphate nodules in some units. In modern environments, phosphate nodules form slowly on the seafloor in deep water outer shelf environments. In these environments the source of the phosphorus is upwelling from deeper water environments. The association of the recrystallized crinoids within the phosphate nodules in this study area implies a different model of formation for these Mississippian nodules as well as a differing source for the phosphorus.