Stratigraphic model for glacial-eustatic Pennsylvanian cyclothems in highstand nearshore detrital regimes

Abstract

Lower (distal) positions on gently sloping shelves (ramps) affected by Pennsylvanian glacial-eustatic sea-level fluctuation display classic marine transgressive-regressive cyclothems, which are separated by exposure surfaces or by shallow marine to thin terrestrial beds deposited at lowstand. In contrast, higher shelf (proximal) positions display marginal marine units and coastal-plain coal beds deposited at marine highstand and separated by thicker terrestrial units. Both marine and terrestrial strata deposited in high-shelf positions underwent significant fluvial incision and valley formation during long stands of lower sea level, resulting in discontinuous erosional remnants and subsequent localized terrestrial deposition on irregular leached and eroded surfaces. Examples from the Appalachian, Illinois, and Sydney basins display features such as erosional disconformities with rotated slumps, buried hilltops preserving erosional remnants of marine units, colluvially reduced landscape relief, and buried paleovalleys filled with stacked channel sands or locally with marginal marine deposits from incursions of lesser extent. The basic cyclothem in highstand nearshore (proximal) detrital regimes consists of a widespread marginal marine-associated coal-bearing unit formed during early highstand, overlain by late highstand progradational, largely terrestrial siliciclastics. These grade upward in some places to thick paleosols and are truncated in other places by valley-fill deposits. The lower-stand weathering, erosional, and depositional features in lower areas are overlain by colluvial/alluvial to late transgressive estuarine deposits, and all lower-stand features are capped by the next major highstand coal-bearing to marginal marine unit. All features produced by sea-level fall reflect the primacy of global glacial-eustatic control over the cyclic nature of Pennsylvanian stratigraphy.