PRELIMINARY INTERPRETATION OF PARAGENESIS OF MINERAL AND SEDIMENT INFILL IN SCYPHOCRINITES LOBOLITHS FROM THE LOWER DEVONIAN ROSS FORMATION OF WESTERN TENNESSEE

Exposures of the Ross Formation (Lower Devonian) at the Horse Creek Wildlife Sanctuary near Savannah, Tennessee preserve abundant root bulbs (loboliths) of the crinoid Scyphocrinites. Loboliths (N=100) were studied in situ in stream exposures (cross-section, bedding plane surface); ten thin-sectioned for petrographic analysis. Primary goals were to determine taphonomic history, primarily paragenesis of infill, including timing of burial, compaction, and diagenesis. Internal relationships of infilling sediment and minerals reveal a composite sequence involving multiple deformation-infill phases. Loboliths are typically three to five chambered with undeformed chambers entirely infilled with sediment (matching external sediment lithology). Progressing taphonomically, specimens show varying infill and compaction histories. For example, in some specimens, some chambers are infilled with crystalline calcite and late growth chert (either exclusively, or with minor amounts of sediment), while other chambers are completely sediment filled. In these loboliths, the sediment always occurs between calcite crystals and the original lobolith wall. The general trend of the paragenesis appears to be (1) crushing prior to (2) deposition of sediment internally through breaches, (3) continued crushing contemporaneously with at least some (4) calcite crystallization. Both of these subsequently are replaced by (5) a stage of chert formation. Earlier infill sediment then (6) begins to recrystallize into sparry calcite (crystal size up to 2 cm). Minute (< 5mm) pyrite grains look to be contemporaneous with the calcite recystallization phase and are associated with fossil fragments. In many specimens, the sediment is iron-oxide stained. Massive (1-2cm) crystalline pyrite clusters (7) occur in some specimens, occasionally dominating crystal mass. Additional specimens show evidence of (8) another crystallization phase of calcite and chert. The most extreme deformation involves multiple events of crushing, with some specimens becoming only a cherty mass due to shell dissolution or recrystallization. The mechanism for compaction is dewatering of enclosing shale prior to infilling chambers. Data from infilling may provide clues to resolving the controversy over Scyphocrinites life habit.