STRATIGRAPHIC POSITION OF A CERATOPSID DINOSAUR SITE IN THE UPPER CRETACEOUS KAIPAROWITS FORMATION, GRAND STAIRCASE-ESCALANTE NATIONAL MONUMENT, UTAH: IMPLICATIONS FOR AGE OF THE KAIPAROWITS LOCAL FAUNA

The 800 m thick Kaiparowits Formation exposed in Grand Staircase-Escalante National Monument yields a diverse Late Cretaceous terrestrial vertebrate fauna poorly known anywhere else in the world. Age estimates for the Kaiparowits range from entirely Campanian to entirely Maastrichtian. Whether the fauna represents a new vertebrate biozone, an anomolous occurrence of endemics, or both, is still unresolved. Answering this question will require precise chronostratigraphic correlation of specifically identifiable vertebrate remains, which to date are few. Recent discovery of a potentially complete ceratopsid skull in the lower part of the formation has created the opportunity to place an identifiable taxon in a precise sequence stratigraphic context, which may in turn be correlable regionally. Near the skull site the lower contact of the Kaiparowits Fm. is placed at the base of a poorly exposed gray shale above a massive tan sandstone characteristic of the underlying Wahweap Formation. The overlying 96 m of strata are a monotonous sequence of crossbedded fine-to- coarse chert grain-rich cut and fill sandstones with minor chert pebble conglomerates arranged in fining upward cycles interpreted as stacked channel deposits. Mudstones are rare or absent. At 96 m a persistant 2m thick mudstone was encountered that proved a good marker bed for local correlation. The top of this bed is 2 m below the skull horizon. Above the skull level, mudstones become more common, even dominating the middle third of the formation. We interpret the 96 m thick stacked fluvial channel interval as a typical lowstand systems tract. The succeeding mudstone-rich portion of the Kaiparowits, characterized by meandering stream channel deposits isolated in mud-rich overbank beds, represents the transgressive and highstand systems tracts of the overlying sequence. The lowest persistant mudstone bed possibly represents terrestrial equivalents to a flooding surface. Assuming the Kaiparowits highstand correlates with known regional sea level events, it most likely represents the Bearpaw Late Campanian-Early Maastrichtian incursion, an interval from which virtually no terrestrial vertebrate record exists anywhere else in North America. Less likely are correlations with the Clagget marine cycle or strictly local tectonic forcing.