EVIDENCE FOR TIDAL BORES IN UPPER PENNSYLVANIAN INCISED VALLEY FILLS IN EASTERN NEBRASKA AND KANSAS, USA

Incised valley and channel fills in the Pennsylvanian succession of the Midwestern USA are widely documented. Most of these erosionally-based, internally complex lithosomes preserve at least one cycle of sandstone-dominated fluvial deposits, overlain by more heterolithic facies of interpreted estuarine origin, and locally capped by marine deposits. These cycles record drawdown and subsequent rise of relative sea-level associated with eustatic sea level changes driven by the expansion and contraction of Gondwanan ice centers. Tidal influence is possible because paleovalleys were likely flooded far landward during early transgressional stages. Analogous indented coastlines today are affected by tidal bores, wherein the leading edge of the flood tide forms a water wave or waves that migrate upstream. Herein, we present evidence for the action of tidal bores in the Virgilian Indian Cave Sandstone (ICS) of SE Nebraska and eastern Kansas and possibly in the underlying Pony Creek Shale. Putative bore deposits in the ICS comprise erosionally based, ragged sheets of mixed and unstratified, muddy fine-grained sandstone that are overlain by stratified, fine-grained sandstone in which ripple cross-lamination indicates up-valley flow. These facies are associated with erosionally-based sandstone units that are internally dominated by upstream-directed cross-bedding and ripple cross-lamination. Cross-beds are cut by multiple reactivation surfaces, contain paired mud drapes and have a rounded, preserved crestline, all features characteristic of deposition under the influence of tides. Our interpretation is important because: 1. Tidal-bore deposits have not previously been recognized in Pennsylvanian incised valley deposits of the region, and 2. The presence of such deposits many tens of km upstream from paleoshorelines indicates that estuarine (transgressive) Pennsylvanian coasts of the Midcontinent were at times macrotidal to hypertidal.