THE UPPER PENNSYLVANIAN INDIAN CAVE SANDSTONE, NORTHERN MIDCONTINENT, USA: FLUVIAL SYSTEM RESPONSE TO GLACIOEUSTASY UNDER LOW ACCOMMODATION CONDITIONS

The uppermost Pennsylvanian Indian Cave Sandstone (ICS) overlies a shallow incision surface eroded into the Towle Shale Member (Onaga Shale Formation) in a succession of marine cyclothems. ICS facies in the vicinity of Peru, Nebraska include: basal, trough cross-bedded (0.25-1 m sets) fine to medium sandstones (facies St1); thinly-bedded, ripple cross-laminated and mud-draped sandstones with localized flaser and wavy bedding, which appear at the tops of some of the individual beds, or at the bases of individual trough sets; low angle trough cross-bedded sandstones (facies St2) with ripple cross-lamination and low relief basal scour surfaces, which typically appear above St1. Paleoflow in both St1 and St2 at Peru is to the SW. Intraformational conglomerates (facies Ci) truncate these facies along apparent SE-dipping storey-bounding surfaces. The ICS grades vertically through at least three storeys. The ICS grades upward into an interbedded sandstone and shale facies (IBF) that appears to be rhythmically bedded in places. Hummocky cross-stratified sandstones (HCS) appear in the uppermost ICS and thin, carbonaceous shales are dispersed throughout the unit. The shallow-marine Aspinwall Limestone overlies the ICS. The logging of 0.5-1.5 km-long continuous exposures indicates that the ICS is a multistorey, fining-upward sandstone body filling a broad paleovalley. Storeys are generally delineated by facies Ci, are laterally and vertically offset, and may include smaller-scale lateral accretion units.

Vertical facies changes in the ICS indicate a progression from a large, tidally-influenced river, to an estuary, and then to storm-influenced offshore sandy shelf and carbonate platform environments. We interpret the ICS as a complex transgressive fill that records one or more cycles of sea level oscillation on a low relief platform with little accommodation space. Widely-accepted models imply that the deposition of the ICS and other contemporaneous sandstones was controlled by glacioeustatic sea-level fluctuations, and the age of the ICS coincides with recently-documented major glaciations in the Southern Hemisphere.