UPPER ORDOVICIAN MUD SLIDE: A CRINOID BIOCENOSE BECOMES A THANATOCENOSE

Some 60 years ago, at Dent , Ohio, excavations and index fossils established that a > 50 m long mudslide shallowly (~0.5m) cross-cut thin, coeval upper Ordovician (Arnheim Fm.) strata. The cross-cut strata were fossiliferous , but the cross-cutting channel filling material had few fossils. In the toe-end of this mud slide were found hundreds of beautifully preserved crinoid calyxes (Glyptocrinus, now Pynocrinus) with ~10% having symbiotic gastropods (Cyclonema) in an anal feeding position. A range of ~2 cm - ~4 cm for the calyx sizes (measured from calyx base to first ambulacral branching) is present in the preserved population. The cone axis orientations of these calyxes occur in both vertical- and parallel-positions to the subtle ~horizontal bedding of the channel’s infilling, thus indicating some turbulence of motion did occur , but conversely, the symbiotic gastropods and preservation of even fragile pinnae indicate rapid burial without excessive turbulence (Baumiller, 2008). The theorized model is that a thick mud slurry (up to 500 centipoise) caused the dislocation of a living colony of crinoids, causing relocation by deposition as the velocity of the slide diminished. All pertinent factors were applied in a Stokes Law modeling that indicates maximum mud slide velocities could have been in the range of 50cm/sec to 330 cm/sec (~1 mph to ~7.4 mph). XRD mineralogy and microscopy of the unusual white to grayish channel infill (cross-cut beds are darker gray to light brown) indicate sub-micron to micron-sized particles of calcite and quartz, no dolomite, and minor to trace amounts of hydro-grossular garnet, zeolite (gismondine), and illite. This mineralogy and the outcrop distance from the Ordovician basin rim ( >70 km) likely rule out a continental fluvial sediment source., but would support an Aeolian sediment such as volcanic ash or loess. The lack of bentonite strongly supports the idea of loess which is known in the Ordovician literature. This sample resides @ www.museumoftheearth.org