BUMPED, BRUISED AND BROKEN: MISSISSIPPIAN CRINOIDS AS INDICATORS OF PALEOFLOW CONDITIONS

The Fort Payne Formation is a Middle Mississippian unit that outcrops in central Kentucky and parts of northern Tennessee. The dominant lithologies of the Fort Payne are mixed carbonates and siliciclastic shale with abundant shallow marine fossils including disaggregated crinoids, bryozoans, brachiopods and sponges. This study focuses on using crinoid stalk fragments, or columnals, to try to determine paleoflow conditions during deposition.

We use a two-fold approach combining field observations with subsequent flume analysis. First, we measured the trend and plunge of 128 crinoid columnals in two separate beds: one within a channel feature and one that appeared to be outside the channel. We used stereoplot analysis to identify trends in preferred orientations of crinoids for two populations, large diameter crinoids (mean of 15 mm) and small diameter crinoids (mean of 6 mm). We found that, in general, crinoid stalk fragments within the channel showed a weak preferred orientation, while fragments located outside of the channel showed little to no preference towards a specific orientation.

Using observations from the field, we then designed a series of flume experiments to test how crinoid fragments orient in a channel with respect to flow. We generated a set of 165 synthetic crinoid columnals using oven-fired clay molds and published values for crinoid densities (~1.2 g/cm3 ) and performed a series of tests in a 10 foot-long recirculating flume. We documented the following key observations: (1) Crinoid fragments of similar densities but different diameters undergo hydraulic sorting with smaller fragments being transported greater distances than large fragments; (2) Crinoid columnals, regardless of length and diameter, orient perpendicular with respect to flow direction when water velocity is increased to sustain bedload movement; and (3) Larger crinoid fragments commonly imbricate and stack against each other in channelized flow. Our results suggest that disaggregated crinoids in the Fort Payne Formation can be used as paleoflow indicators within channel features and record hydrodynamic conditions at the time of deposition.