TIDAL AND BRACKISH-WATER INDICATORS IN THE UPPER CRETACEOUS JUDITH RIVER AND MEETEETSE FORMATIONS, NORTHERN BIGHORN BASIN, WYOMING

Fine- to medium-grained fluvial channel deposits of the lower and upper parts of the Judith River Formation (JRF) in the Elk Basin anticline of the northern Bighorn Basin contain a variety of lithofacies characteristic of tidal deposition. Double mud-draped foresets and single and double mud-draped foresets arranged into discrete neap-spring bundles are preserved in 3-6 m-thick channel deposits with an overall northward and northwestward flow. Fluvial channel fills preserved between the upper and lower zones of tidally-influenced channels are of the same scale as the tidally-influenced channels, but exhibit a southward paleoflow. Tidally-influenced channel deposits in the lower JRF represent early- to middle-phases of a normal regression while those in the upper part of the formation are associated with the transgressive systems tract. Fluvial channel fills in the middle of the JRF represent the lowstand to transgressive systems tract, and the turnaround is marked by a zone of amalgamated channel sandstone. Low-diversity marine ichnofossil assemblages (Skolithos, Ophiomorpha, and Thalassinoides) are preserved in thin-bedded, ripple- and planar-laminated, fine-grained sandstone beds overlying the upper tidally-influenced fluvial succession of the JRF and underlying the basal Meeteetse Formation (MF). These assemblages, together with possibly hummocky cross-stratified, very fine-grained sandstone bodies indicate that a marine flooding surface in present in the uppermost JRF. Bentonitic shales of the lower MF in the Elk Basin anticline contain lenticular, massive- to cross-stratified sandstone that entomb fossil logs and wood fragments that are heavily bored by the marine ichnogenus Teredolites. Recognition of these zones of tidal influence and brackish to marine salinities in the JRF-MF succession improves the resolution of depositional and paleogeographic reconstructions of the northern Bighorn Basin area during the Late Campanian through earliest Maastrichtian.