PALEO-STREAM COMPETENCY AS A TEST OF THE DISTRIBUTARY FLUVIAL SYSTEM MODEL: UPPER DEVONIAN CATSKILL FORMATION, CENTRAL PENNSYLVANIA

Fluvial deposition in actively aggrading basins is dominated by distributary fluvial systems (DFS). DFS have a fan morphology resulting from a decrease in channel size, increase in channel bifurcation, and less channelized flow downstream. Depositional style varies across DFS as a function of these geomorphic elements, with channel deposition dominating proximal DFS environments and overbank deposition characteristic of distal regions. A prograding DFS will therefore result in a vertical succession from small, relatively fine-grained to large, coarser-grained channels. These properties may serve as criteria for identifying DFS in the rock record. The Upper Devonian Catskill Formation has been interpreted as a DFS based on variability in paleosol macro- and micromorphology and increased channel sandstone body size and grain size up-section. The goal of this study is to quantify channel sandstone grain size throughout the section to support qualitative field observations. Channel sandstones were sampled from the top, middle and bottom of each of the four members of the Catskill Formation near Selinsgrove and Duncannon, Pennsylvania for petrographic analysis. Although median grain size varies minimally through the Catskill Formation, the 90^th percentile grain size (D₉₀) of channel sandstones increases from approximately 0.10 mm (very fine sand) at the base of the section to 0.45 mm (medium sand) at the top of the section. Critical shear stress (τ_C) was calculated using D₉₀ for each sample to assess variability in paleo-flow competency through time. We show τ_C increases from approximately 0.08-0.12 Pa at the base of the section to 0.20-0.45 Pa at the top of the section. The range in calculated values is due to fitting parameters used in these equations to account for unknown channel bed roughness. An increase in D₉₀ and calculated τ_C coincides with qualitative observations of increased channel body size and grain size vertically through the Catskill Formation. These results demonstrate the utility of paleo-flow competency analysis in identifying DFS in the fluvial sedimentary record.