SEQUENCE STRATIGRAPHIC CONTEXT OF THE UPPER PENNSYLVANIAN INDIAN CAVE SANDSTONE, SE NEBRASKA AND ENVIRONS

Conspicuous, large sandstone bodies within Upper Pennsylvanian cyclothems of Midcontinent USA frequently lack clear stratigraphic context, yet their presence within the regional succession indicates significant shifts in sea level and facies tracts. Such bodies represent major sediment pathways across a broad, low-gradient shelf that drained southward toward the Marathon-Ouachita orogenic foreland during sea-level lowstands. These sandstones have previously been interpreted as deltaic deposits, but more recently have been reinterpreted as incised valley fills (IVF). The Indian Cave Sandstone (ICS) of southeast Nebraska and surrounding areas is here reinterpreted as the product of incision and filling of IVFs over at least two separate episodes. In a sequence stratigraphic context, these isolated linear sandstone bodies (<2000 m wide, ~30 m thick) are incised into pre-existing cyclothems, have relatively steep sides with flat bases, and rest on a well- defined sequence boundary that juxtaposes terrestrial strata against marine mudrocks and carbonates. The fills comprise crudely fining-upwards successions of trough cross-bedded sandstones overlain by heterolithic assemblages that are interpreted as fluvial-to-estuarine and upper estuarine in origin, respectively. Vertical facies successions within the bodies suggests sediment accumulation in a regime of gradually increasing accommodation, i.e., during rising relative sea level. Underlying erosion surfaces marking the position of the sequence boundary can be traced onto adjacent interfluves where they correspond to well-developed paleosols. Thus, continuity of the sequence boundaries in those areas where the IVFs are absent is established by the presence of these coeval paleosols. Within the IVF, fluvial-to-estuarine facies are interpreted as the LST to early TST; upper estuarine facies are the TST; and overlying platform carbonates represent the HST. Transgressive surfaces and maximum flooding surfaces are also recognized. Furthermore, the sequences defined herein can be correlated closely with recently published sequence stratigraphic frameworks from the region, suggesting that the ICS bodies record repeated, fifth order (Milankovitch band), relative sea-level fluctuations of a minimum of 30 m during latest Pennsylvanian time.