An oxygen isotope study of illite and calcite in three Appalachian Paleozoic vertic Paleosols

Abstract

ABSTRACT The oxygen isotope compositions of pedogenic calcite and illite in three Paleozoic vertic red bed paleosols record a complex history of pedogenic and diagenetic processes. The paleosols are preserved in the upper Mississippian (Chesterian) Mauch Chunk (Pennsylvania), Hinton (West Virginia), and Pennington (Tennessee) Formations in the Appalachian Basin and were buried to 7-8 km (230-260°C), 3-4 km (100-145°C), and 1.5-3 km (70-100°C), respectively. The submicron-size clay fractions of the paleosols are presently composed of R3-ordered illite/smectite (I/S), inferred from K-Ar dates and soil morphology to have been authigenically produced by the reaction of smectite to illite. Oxygen isotope fractionations, calculated pore-fluid compositions, and K-Ar dates were used to constrain the conditions and timing of I/S and pedogenic calcite (re)crystallization in each of the paleosols. The data were interpreted in light of three possible end-member environments for formation and recrystallization: (1) the pedogenic environment; (2) the burial diagenetic environment; and (3) a burial environment dominated by hot, tectonically driven fluid. The 18O values of pedogenic calcite and authigenic I/S in vertic red bed paleosols generally reflect recrystallization and exchange during burial diagenesis and are thus not suitable for interpretation of paleoenvironment or paleoclimate. The involvement of orogenic fluids during burial cannot be ruled out, but it is not required by any of the paleosols. In the coastal-margin Pennington Formation paleosol, I/S formation may have been initiated in the pedogenic environment in response to cyclical wetting and drying of the soil clay matrix. Continued smectite transformation and coarsening and ordering of I/S during a long period of shallow burial led to its isotopic reequilibration with pore fluids at slightly elevated temperature, but oxygen isotope compositions of pedogenic calcite in the paleosol are preserved.