Abstract: Carbonate Lakes as Sensitive Indicators of Paleoclimate and Tectonics: the Triassic Gettysburg Basin, South-central Pennsylvania

Carol B. De Wet, Daniel A. Yocum

Marine sediments, although widely studied, may contain cryptic and conflicting information concerning the origin of depositional patterns. Eustacy, climate and tectonics all influence sediment deposition. By studying a closed basin, where eustatic effects are minimal, we have eliminated one variable from the equation. Interpreting the stratigraphy and sedimentology of such a basin provides insight into the role of climate versus tectonics in defining stratal patterns. This information is then added to our evaluation of marine strata of the same age.

The Triassic Gettysburg basin, located in south-central Pennsylvania, USA, is part of a roughly north-south trend of continental rift basins exposed along eastern North America. The basin fill is predominantly fluvial red sandstone and lacustrine silty claystone, but there are at least three carbonate intervals. The carbonate beds are up to 16 meters thick and formed through largely abiotic precipitation in shallow lakes. The limestones and marls contain ostracods, Charophyte fragments and gastropod molds. Paleozoic limestones and dolomites surround the rift basin, providing a source for ions through weathering of adjacent highlands.

There are well developed paleosols (vertisols) in overbank deposits associated with the fluvial sandstones. Paleosols containing calcite nodules and rhizoliths are associated with lake-margin facies. These features suggest a seasonal climate with significant precipitation. During periods of lake expansion, clastic input was reduced while water input increased to that part of the basin. Stratigraphic relationships between the underlying and overlying sandstones suggest that tectonic factors dictated water accumulation. Water ponding, due to basin sag on border faults, coupled with changing drainage patterns allowed water to collect and become supersaturated with respect to calcium carbonate. Periodic tectonically induced drainage re-organization was the principal factor in lacustrine d velopment.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994