Abstract
Many of the thick-bedded sandstones of the Pennsylvanian Jackfork Group at DeGray spillway, the deposits of deep-sea sediment-gravity flows, exhibit no apparent vertical grain size trends on outcrop. This is due to the overall fine grain size and narrow range of sizes that comprise the beds. Although no previous detailed grain size data have been collected for these deposits, grading trends have been called upon to classify these units as the deposits of turbidity currents, sandy debris flows, bottom currents, and slumps. 243 thin sections were analyzed for grain size and composition. The results of these analyses show statistically significant vertical variations that illuminate the sedimentation mechanics of these units.Clean sandstones (less than 10% mud) are normal, inverse-to-normal, inverse- and ungraded; normal- and inverse-to-normal graded beds are most common. The muddiest sandstones, slurry beds (more than 20% mud), show a similar range in grain size trends, and also are mostly normally-graded. Based on these detailed grain size trends, bedding style, and primary sedimentary structures as seen in outcrop and on slabbed specimens, we believe that it is untenable to suggest that these units were uniformly the product of "sandy debris flows" or "slump/mass flows" (Shanmugam and Moiola, 1995). Instead, we view the clean, thick-bedded sandstones to be the product of turbulent sediment-gravity flows that were either continuously-fed density currents and/or relatively short-lived, collapsing sediment clouds. We envision the slurry beds deposited by flows that exhibited both non-Newtonian (cohesive) and Newtonian (non-cohesive) rheologies in a near-bed layer.