ESTUARINE FACIES ON A TRANSGRESSED LATE PENNSYLVANIAN INTERFLUVE, EASTERN NEBRASKA, USA

Clastic sediments of the Rakes Creek-Oskaloosa interval (RCO) in the Tecumseh and Deer Creek formations (Pennsylvanian, Virgilian) in Cass County Nebraska overlie an interfluve with a prominent, widespread paleosol. Sandstones are otherwise rare in the enclosing Shawnee Group at the northern end of the Midcontinent outcrop belt. The RCO differs from many other sandstone bodies in the regional Pennsylvanian succession because it is a tabular body showing no evidence for incision at its base and no large channel forms over a transect of at least 8 km and in scattered outcrops a few tens of kilometers away. Basal RCO facies are 0-60% burrowed, heterolithic laminites (Hst1, Hst2, Hs) with rare rhythmites. These facies grade upward into very fine sandstone, including rippled (Sr) and massive (Sm1, Sm2, and Sm3) facies, some of the massive sandstones containing carbonate nodules and/or carbonaceous rooting and large root traces. In some outcrops, subtle, large-scale (~1.5-2 m) trough cross-strata are visible in the interval equivalent to the massive sands logged in cores, and we surmise that cross-stratification is actually common in that interval. Sandstone facies are usually overlain by a ~70% bioturbated sandstone-mud heterolith (Hsbc). An irregular ravinement surface having tens of centimeters of relief typically appears at the top of Hsbc. Thin, burrowed, carbonaceous sands (Sbc) or intraformational conglomerates (Csm) appear above this surface. Burrowed carbonaceous very fine sandstone with few mud laminae and brachiopods (Hsc) always appears at the top of the RCO and grade upward into calcareous marine shales.

An estuarine origin is interpreted from heterolithic (= tidal) facies, evidence for flow reversal on reactivation surfaces in cross-stratified sandstones, low trace-fossil diversity, and a lack of marine body fossils (except in Hsc). The tabular geometry of the RCO sandstone body, the lack of incision at its base, and the persistence of a well-developed paleosol underneath it strongly suggest that deposition in the study area took place over a broad interfluve rather than within an incised valley. Estuarine facies on interfluves have received much less attention than incised valley fills have, despite their importance in interpreting paleogeography and sequence stratigraphy