--> Paleosol Variability Within the Upper Devonian Catskill Formation (Central Pennsylvania): Evidence for a Prograding Distributary Fluvial System

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Paleosol Variability Within the Upper Devonian Catskill Formation (Central Pennsylvania): Evidence for a Prograding Distributary Fluvial System

Abstract

The Catskill Formation consists of approximately 2 kilometers of marginal marine and alluvial strata deposited into the Appalachian basin during the Acadian Orogeny. Previous studies have interpreted the Catskill Formation as a prograding deltaic complex. Others have interpreted this system as a prograding low-gradient alluvial plain grading into muddy shoreline depositional environments. Recent observations in actively aggrading basins show fluvial deposition dominated by distributary fluvial systems (DFS). A DFS is characterized by a fan morphology, increasing channel bifurcation downstream and less channelized flow downstream. The purpose of this study is to assess the applicability of the DFS depositional model to the Catskill Formation using paleopedological analysis and to clarify Late Devonian paleoclimatic trends. Pedogenesis on modern DFS is largely controlled by variation in hydrologic regime as a function of geomorphic position. In source proximal DFS environments, flow is confined to channels and overbank environments are well-drained, as depth to the water table is greatest in these settings. Channel confinement and water table depth decrease downstream on DFS, resulting in poorly-drained, poorly-developed, hydromorphic soils forming in source-distal DFS environments. Paleosols associated with a prograding DFS will therefore be manifested in the rock record as a vertical succession from poorly-developed, hydromorphic paleosols to well-drained, well-developed paleosols. Pedofacies associated with the lower-most member of the Catskill Formation show hydromorophic features and are generally poorly developed. An increase in paleosol drainage up-section in the Catskill Formation is inferred based on a transition from hydromorphic pedofacies to calcareous and vertic pedofacies. These results are consistent with deposition of the Catskill Formation by DFS processes. Identifying DFS in the sedimentary record has implications for paleoclimatic interpretations based on paleosols. Paleosols associated with DFS show a drying trend up-section as a result of changing hydrologic conditions, which could potentially be misinterpreted as a change from prevailing humid to arid paleoclimatic conditions. Recognizing DFS in the rock record also has implications for basin reconstruction and characterizing fluvial hydrocarbon reservoirs, as the geometry and scale of lithofacies distributions associated with DFS are fundamentally different from tributary systems.