SEDIMENTOLOGICAL RESPONSE IN SOUTHERN EURAMERICA (APPALACHIAN BASIN) TO LATE PALEOZOIC GLOBAL CLIMATE CHANGE

Late Paleozoic strata in the Appalachian basin reflect climate change in response to a northward drift of southern Euramerica from 30° S in the Late Devonian to near equatorial latitudes by the Early Permian. However, second- to fourth-order changes in lithostratigraphy are best explained by wet-dry climate cycles that were controlled by changes in ice-sheet volume.

Early Famennian terrestrial red beds contain numerous calcic paleo-Vertisols that are indicative of a warm-mode and a dry subhumid climate. In contrast, late Famennian diamictites and laminites containing striated and faceted pebble- to boulder-size clasts indicate that glacial conditions and a cool humid climate extended nearly to sea level in the vicinity of 30° S. The Late Devonian cool-mode and humid climate persisted well into the Visean as indicated by the continued influx of siliciclastic sediment containing abundant terrestrial organic matter. An abrupt return of red beds containing Aridisols, evaporites, and marine limestones in the mid-Visean is coincident with sea-level rise, the development of an arid climate, and ice melting. Limestone deposition was followed by a late Visean mixed carbonate-siliciclastic sequence. Increasing amounts of siliciclastics and terrestrial organic matter were deposited during the Serpukhovian as sea-level fell. Ice build-up coincident with a progressive increase in rainfall ultimately led to perhumid climates under which Ultisols, Spodosols, and domed peat deposits (coal) formed during fourth-order low stands in the Bashkirian. Subsequent ice melting culminated in the Kasimovian when calcic Vertisols once again developed during fourth-order low stands and dry subhumid climates. Ice build-up during the Gzhelian led to sea-level fall and the development of fourth-order humid-climate paleosols.

These trends are best explained by a paleo-atmospheric climate model in which tropical southern Euramerica were relatively humid whenever high-pressure over ice sheets fixed the position of high-latitude polar fronts and the intertropical convergence zone near the equator. Conversely, ice sheet decay resulted in land-mass heating that caused the ITCZ to move between hemispheres. During warm modes, cross-equatorial monsoonal circulation intensified, tropical rainfall became more seasonal, and the tropics became increasingly dry.