Low-accommodation detrital apron alongside a basement uplift, Pennsylvanian of Midcontinent North America

Abstract

The northern end of the 650-km-long Nemaha Uplift (Nebraska and Kansas, USA) is an important example of basin-margin sedimentation in the North American Midcontinent. An apron of coarse, basal Pennsylvanian arkosic clastic sediments (BPC) was deposited on the flanks of the uplift while marine cyclothems were encroaching from the east. Small-scale fining-upward intervals, many with demonstrably erosional bases, dominate the BPC and are interpreted as overridingly fluvial in origin. Weak paleosols, desiccation cracks, and reddened intervals in the BPC record episodic subaerial exposure. Multiple, burrowed horizons and heterolithic strata of probable tidal origin and rare marine fossils also indicate episodic marine influence. The BPC appear to have been deposited as a thin apron of coalesced, alluvial fans and fan deltas. Deposition of the BPC occurred during the waning of uplift and subsequent quiescence. The comparative thinness and large-scale packaging of the BPC are compatible with the controlling effects of relict relief, regional subsidence, and eustasy, rather than ongoing, major vertical displacements along active faults. A strong autocyclic influence on sedimentation is evidenced by stacked fining-upward intervals of poorly-sorted conglomerates, sandstones, and sandy mudstones. Correlations demonstrate that the accumulation of the BPC took place over more than seven major sea-level cycles, beginning in Cherokee Group times (middle Moscovian/middle Pennsylvanian) and ending only when the eroded uplift was inundated and buried by marine cyclothems. On the basis of local correlations with marine cyclothems, and using black phosphatic shales (so-called “core shales” of Heckel, P.H., 1986. Sea-level surve for Pennsylvanian eustatic marine transgressive-regressive depositional cycles along Midcontinent outcrop belt, North America: Geology 14, 330-334., Heckel, P.H., 1994. Evaluation of evidence for glacio-eustatic control over marine Pennsylvanian cyclothems in North America and correlation of possible tectonic effects. In: Dennison, J.M., Ettensohn, F.R. (Eds.), Tectonic and Eustatic Controls on Sedimentary Cycles, SEPM Concepts in Sedimentology and Paleontology No. 4, pp. 5-87) as marker beds, we speculate that the BPC exist in backstepping sequences and/or parasequences alongside the flanks of the Nemaha Uplift. The BPC are lithologically comparable to contemporaneous deposits alongside the Ancestral Rockies, Amarillo–Wichita Uplift, and other buried basement highs in North America. Nonetheless, the BPC are an order of magnitude thinner, are dominantly retrogradational, rather than progradational, and their occurrence was not associated with major displacements along basin-bounding faults. In this manner, the BPC are a useful example of low-accommodation, eustasy-dominated, coarse-grained terrigenous clastic deposition around an uplifted basement block.