Monday, 6 October 2008: 2:45 PM
George R. Brown Convention Center, 320F
The late Eocene-Oligocene White River Group (WRG) of the northern Great Plains is composed of volcaniclastic and siliciclastic claystones and siltstones, sandstones, and limestones deposited within fluvial, lacustrine, and eolian environments. The overall climatic trend during this time is a regional change to progressively cooler and drier conditions. On a regional scale, lacustrine deposits of the White River Group define shifting depocenters between southwest South Dakota and northwest Nebraska. Lacustrine carbonates define the top of the late Eocene Chadron Formation in South Dakota, but in Nebraska temporally equivalent deposits are dominated by siliciclastics. On a local scale, lacustrine deposits in South Dakota range from siliciclastic to carbonate dominated. Siliciclastic deposits have only been identified within late Eocene paleovalleys carved into the Chadron Formation, whereas carbonate systems are seen either as parts of former stable, pedogenically modified late Eocene alluvial plains that were later incised by base level fall (Chadron Formation), or within Oligocene eolian dominated environments punctuated by periods of nondeposition (Brule Formation). In Nebraska, only carbonate and gypsum dominated lacustrine deposits have been documented, and only from the late Eocene Chadron Formation. These deposits are highly localized and interbedded with fluvial overbank, pedogenically modified clastic materials. The hydrology of these lacustrine systems is varied. Previous workers have identified structurally controlled conduits that acted as sources for some lakes, whereas others appear to represent emergence of the water table during wet climate periods, or possibly episodes of decreased sedimentation. Paleosols lateral to these emergent lakes also record increased groundwater influence and expansion of the lake system.