ESTIMATING MIDDLE DEVONIAN SOIL AGES USING POINT-COUNTING OF PEDOGENIC CLAYS IN PALEOSOLS

Determining the duration of soil formation of paleosols in deep time rock records is largely constrained to qualitative assessment of pedogenic development and likewise qualitative descriptions of relative lengths of pedogenesis. We estimate soil ages (duration of pedogenesis) using point-counts of pedogenic clays and other pedogenic constituents in paleosols identified within the Manorkill Formation (Middle Devonian) of the Catskill succession. The Manorkill Fm., which crops out in the Catskill Mountains of south-central New York state, consists of fluvial aggradational cycles (FAC) that were driven by pulses of sediment delivered to the foreland basin from highlands created by the Acadian orogeny and associated eustatic sea-level fluctuations. Establishing the relative ages of the paleosols in the Manorkill Formation can be useful in defining a sequence-stratigraphic framework for this fluvial-deltaic succession as well as providing valuable paleoclimatic information for a pivotal time in Earth's history. A method based upon relative dating of Holocene soil development (Ufnar, 2007) that uses the percentages of illuviated clay as a function of time will be applied to point-count estimates from thin sections prepared from the Middle Devonian paleosols. The hypothesis to be tested is that duration of pedogenesis for stratigraphic intervals was influenced by cyclic changes in accommodation space creation and that this estimate can help define system-tract equivalents in fluvial successions. Many of the paleosols are analogous to modern USDA Vertisols, which are characterized by high clay content and extensive shrink-swell features; shrink-swell processes inhibit preservation of illuviated clay features due to movement of soil materials that breaks apart illuviated clay accumulations, nevertheless they are present in most sampled paleosols. Preliminary observations indicate a range of illuviated clay contents suggesting minimal to extensive pedogenesis for the paleosols, in some cases in association with precipitation of pedogenic calcite.