PALEOPEDOLOGICAL ANALYSIS WITHIN A FLUVIAL SEQUENCE: IMPLICATIONS FOR UNDERSTANDING CHANGES IN BASE LEVEL; LATE MIDDLE DEVONIAN, CATSKILL STATE PARK, NY

Change in the degree of pedogenesis reflecting changes in the rate of sediment accumulation in an overbank floodplain environment was evaluated for a 33 meter section of the Middle Devonian, Givetian Plattekill Fm. near Haines Falls, NY. Changes in the rate of sediment accumulation record change in allo- and autocyclic controls on fluvial deposition and architecture. Pedogenic development associated with changes in accumulation rates may be indicative of the stratigraphic position of a sequence bounding unconformity. Allocyclic controls such as a change in the position of relative base level result in changes in the rate of sedimentation and can be recognized by changes in the degree of pedogenesis. A transgressive system is characterized by weakly developed paleosols and high rates of sediment accumulation; whereas a regressive system is characterized by more mature paleosols and low accumulation rates. If preserved, a sequence boundary will be marked by the development of a highly developed paleosol on an unconformity. Vertical trends identified in this section include a fining upward trend in grain size within the lower 26 m and a coarsening upward trend within the upper 7 m, a decrease in the thickness of fluvial aggradational cycles (FACs), and an increase in paleosol maturity and thickness. These trends are supported by outcrop, micromorphological, chemical, and mineralogical analysis. The increasing trend in pedogenesis is represented by increasingly more complex soil orders, including entisols, vertisols, inceptisols, and alfisols. Pedogenic features including cutan development, hydromorphic features, and precipitation of both iron oxide and carbonate glaebules were quantified allowing for a micromorphological analysis of pedogenic development. The results of this analysis indicate the presence of an increase in pedogenesis at two positions in the vertical succession. The lower is interpreted as the upper surface of a FACset and represents a paleosol at a distal position on a floodplain. The upper paleosol is bounded by meter scale sandstone beds, with an upper boundary marked by an abrupt erosion surface. The position of this paleosol interpreted as the upper boundary of the Plattekill Fm. is consistent with the conclusions of previous stratigraphic studies.