Bayhead delta interpretation of an Upper Pennsylvanian sheetlike sandbody and the broader understanding of transgressive deposits in cyclothems

The Rakes Creek Shale is noteworthy as a non-incised sheetlike sandstone body in the Shawnee Group of the Midcontinent, USA, succession of Upper Pennsylvanian cyclothems. Relative to contemporaneous settings in North America, there are comparatively few major sandy lithosomes on the low-accommodation northern high shelf of the Midcontinent, and those that have been described in detail are incised valley fills. In our study area, the Rakes Creek Shale is characterized by a consistent internal succession of eight dominant lithofacies ranging from largely nonrhythmic heterolithic strata at the base, to large-scale cross-stratified and massive sandstones, to carbonaceous and heterolithic sandstones at the top. Eight other lithofacies of minor volumetric importance also appear in the Rakes Creek Shale in our study area. At the base of the succession is a widespread well-drained paleosol that shows no evidence for incision across a transect of 30 km. Sandstones in the upper part of the Rakes Creek Shale contain abundant root casts and carbonaceous root traces, indicating colonization by land plants after accommodation space had been filled. The Rakes Creek Shale is comparatively unique in the enclosing regional Pennsylvanian succession because it provides evidence for: (1) the filling of accommodation space in the early stages of a marine transgression by comparatively coarse clastic sediments (with minimal influence by tides and waves), and (2) higher rates of sediment supply and/or slower rates of relative sea level rise compared to most other transgressive intervals in cyclothems of the Midcontinent.The regional lithofacies succession of the Rakes Creek Shale, as well as changes in that succession that appear at the eastern edge of our study area, lead us to interpret it as a series of landward-stepping, bayhead delta parasequences deposited rapidly in an embayment with minimal wave and tide energy. The Rakes Creek Shale represents a rare, and perhaps previously undocumented, variation in the expression of transgressive deposits. Ancient bayhead deltas described from the literature are typically associated with the deposits of comparatively narrow incised valleys. We also interpret the ~ 10 m thickness of the Rakes Creek Shale in the Weeping Water Valley of southeastern Nebraska as the minimum rise in relative sea level during its associated transgression. We interpret the discrepancy between this value and others calculated for Midcontinent Pennsylvanian cyclothems as direct evidence for glacioeustatic sea-level rises of varying magnitudes during the Late Paleozoic Gondwanan ice ages.