REDEFINING THE JURASSIC-CRETACEOUS CONTACT IN EAST-CENTRAL UTAH

Research over the past decade on the Lower Cretaceous Cedar Mountain Formation (CMF) has resulted in an appreciation of the 30 million year geological and paleontological record preserved in this relatively thin but complex rock sequence. This research has resulted in various definitions of its lower contact on a regional unconformity spanning about 20 million year at the top of the Morrison Formation (MF). The basal contact has been placed at the base of a conglomeratic sandstone (Poison Strip Ss.), at the top of a regionally extensive calcrete, and at the base of this same calcrete (currently most widely accepted). However, the calcrete in this relative stratigraphic position is not present everywhere, and major calcrete horizons occur at different stratigraphic levels in CMF regionally.

Across its northern outcrop belt west of Green River and south of I-70, the basal Yellow Cat Member (YCM) of the CMF to the north of Arches National Park preserves a distinct dinosaur fauna suggestive of a Barremian Age (polacanthine ankylosaur Gastonia, iguanodontids, brachiosaurid sauropods, and giant dromaeosaur Utahraptor). The YCM has a regionally correlative calcrete in its lower part (previous base of CMF); however, an Early Cretaceous dinosaur assemblage (including ankylosaurs, iguanodontids, and a new dromaeosaurid theropod) is now recognized from several meters lower, above a laterally persistent silcrete bed. A chert pebble lag 1-2 m below the silcrete suggests the basal CMF contact is even lower. Southeast of Green River, an Early Cretaceous dinosaur fauna apparently distinct from that recovered elsewhere in the YCM (a new giant polacanthine ankylosaur and the basal therizinosauroid Falcarius) underlies a dark-brown gravelly “caprock,” 20-50 cm thick with extensive carbonate development at its top. A chert pebble-cobble lag below the dinosaur-bearing levels marks the base of CMF.

In both areas, a chert pebble lag marks the MF/CMF unconformity well below where it has been recognized in the past. Research is underway to test for potential points of correlation between the lower YCM in the two areas (Is “caprock” equivalent to calcrete?), establish if these strata are laterally contiguous (they may well not be), and further document the earliest records of Cretaceous deposition on the Colorado Plateau.