Abstract

Glacioeustasy constituted a first-order control on Pennsylvanian stratigraphic cyclicity, but inadequately explains the diverse sedimentologic expression of many Pennsylvanian cycles, particularly mixed carbonate-siliciclastic varieties. By analogy with the Quaternary record, significant glacial-interglacial climate change probably accompanied Pennsylvanian glacioeustasy. Sedimentologic features within Upper Pennsylvanian cycles of the Pedregosa and Orogrande basins include eolian siltstones and calcareous paleosols that record relatively arid conditions, as well as evidence for hyposaline bays and arborescent forests that suggest more humid conditions. Hence, within the paleoequatorial setting of these basins the climate apparently fluctuated between subhumid and semiarid conditions during interglacial and glacial extremes respectively.

Glacioeustatic and glacioclimatic influences acted within paleogeographic constraints to dictate sedimentologic signals of Pennsylvanian cycles. Tectonically controlled paleogeography of depositional basins ultimately determined the nature and volume of the siliciclastic influx on a grand scale. However, intensified aridity and wind strength during peak glacial phases led to decreased fluvio-deltaic siliciclastic sedimentation and increased eolian activity. Conversely, increased precipitation during interglacial phases reactivated or intensified fluvio-deltaic sediment yield.

Evidence from Pedregosa and Orogrande strata suggests that coupled glacioeustatic and glacioclimatic changes were sufficiently severe to re-configure depositional systems between climatic extremes, implying that each cycle potentially represents a product of discrete environmental mosaics that developed significantly separated in time. This scenario contrasts with the interpretation that cycles represent the product of a completely contemporaneous suite of lithofacies that migrated laterally to form the observed vertical succession. This is a critical difference that largely has been lost as a result of mis-application of Walther’s Law to Pennsylvanian cyclostratigraphy.