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The Virgilian “Indian Cave Sandstone” (ICS) is herein redefined as at least four incised valley fills (IVFs) of at least two different ages. These IVFs are composed of irregularly bounded, tabular-lenticular units of trough cross-bedded sandstone, grading vertically into tabular-lenticular units of mudstone-and-sandstone-dominated heterolith. Three IVFs, wider (< 2 km) than they are deep (> 30 m), are composed of multiple storeys grading upward from fluvial-to-estuarine facies to upper-estuarine facies. Storey boundaries are delineated by bounding surfaces underlying conglomerates with heterolithic clasts. One IVF (Honey Creek, NE) is smaller than the others, (> 0.5 km wide and > 25 m deep) and appears to be composed of a single-storey fill dominated by fluvial-toestuarine facies.
A sequence boundary delineates the base of each IVF, with lowstand, transgressive, and highstand system tracts represented by vertical changes in lithology. Sequence boundaries are delineated by bounding surfaces underlying conglomerates with carbonate and mudrock clasts of the cyclothem host rocks. Outside of the confines of the IVFs, sequence boundaries are correlated to the position of interfluve paleosols interpreted to be contemporaneous with the IVFs.
Two IVFs (Peru and Shubert, NE), are younger than previously believed,and contain a record of at least 30 m of relative sea-level change. The top of the third IVF (Brownville, NE) has been modified by Quaternary erosion and the base is not exposed, thus the exact stratigraphic position and associated relative sea-level change cannot be determined. The Honey Creek IVF 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 ICS. These deposits represent the filling of accommodation on the high Midcontinent shelf during periods of fluctuating relative sea-level.
The IVFs and associated paleosols correlate to regional paleosols identified as fifth order sequence boundaries by other investigators. Thus, relative sea-level changes of at least (or >) 30 m can be correlated to fifth order cycles.
Advisor: Christopher R. Fielding