NEW INTERPRETATION OF MULTIPLE GLACIOEUSTATIC INCISED VALLEY AND CHANNEL FILLS OF THE “INDIAN CAVE SANDSTONE” (UPPER PENNSYLVANIAN), EASTERN NEBRASKA, USA

The Virgilian “Indian Cave Sandstone”, previously considered a single genetic unit, is herein redefined as comprising at least three incised valley fills and one channel fill, of at least two different stratigraphic intervals. These deposits represent the filling of accommodation on the high Midcontinent shelf during periods of fluctuating relative sea level. A sequence boundary delineates the base of each valley fill, with lowstand, transgressive, and highstand system tracts represented by vertical changes in lithology.

These incised bodies are encased in marine shale-limestone-dominated cyclothems. The bodies are composed of irregularly bounded, tabular to lenticular units of trough cross-bedded sandstone grading vertically into tabular units of mudstone dominated heterolith and tabular to lenticular units of sandstone dominated heterolith. Storey boundaries are delineated by bounding surfaces underlying conglomerates with heterolithic clasts. Sequence boundaries are delineated by bounding surfaces underlying conglomerates with carbonate and mudrock clasts of the cyclothem host rocks.

The incised valley fills are much wider (> 2 km) than they are deep (> 30 m), and are composed of multiple storeys grading upward from fluvial and fluvial-estuarine-dominated sedimentation to estuarine, marine-embayment and carbonate-bank sedimentation in the uppermost sections. The incised channel (> 0.5 km wide and > 25 m deep) appears to be composed of a single-storey fill, dominated by fluvial and fluvial to estuarine sedimentation at the base and grading vertically into marine-embayment and carbonate-bank deposits.

Two of the incised valley fills (Peru and Shubert, NE), previously interpreted as deltaic deposits, are younger than previously believed, and contain a complete record of at least 30 m of relative sea level fluctuation. The top of the third incised valley fill (Brownville, NE) has been extensively modified by Quaternary erosion and the base is not exposed, thus the complete relative sea level fluctuation record and exact stratigraphic position cannot be determined for this body. The incised channel (Honey Creek, NE) is the oldest and smallest lithosome, and also is the only unit with an upper bounding surface that meets the original stratigraphic definition of the “Indian Cave Sandstone”.