A CEDAR MOUNTAIN TYPE FAUNA FROM THE LOWER CRETACEOUS OF SOUTHERN NEVADA

The Albian Willow Tank Formation of southern Nevada rests unconformably on the Jurassic Aztec Sandstone. The approximately 150 meter thick formation consists of two members, a basal conglomerate member, interpreted as a Donjek type braid plain, and an upper mudrock member, the focus of this study. Deposits of the upper member of the Willow Tank Formation in the Valley of Fire represent a semi-arid anastomosed fluvial environment. An abrupt transition from the braided fluvial system to an anastomosed fluvial system is recorded at the contact between the basal conglomerate member and the upper mudrock member of the formation. Sedimentary units of the upper member are consistent with models of anastomosed fluvial deposition in showing isolated channel sandstone bodies, crevasse splay deposits, and a preponderance of overbank mudrocks. These sediments were deposited in a semi-arid environment, as supported by the presence of limestone clasts in the basal member of the formation and by the presence of calcisols in the upper member.

Taphonomic modes of the Willow Tank Formation are consistent with overbank depositional environments. The majority of the fossil material is fragmentary and isolated with few exceptions. Preservation ranges from partially articulated and not weathered to isolated and unidentifiable. Taphonomic modes include microsites on a well-drained flood plain, a bone bed, a channel fill, and volcanogenic.

Due to the fragmentary nature of materials from the formation only gross taxonomic assignments can be made. The Willow Tank Formation contains a turtle taxon, Naomichelys sp., found in most other Lower Cretaceous deposits, as well as dromaeosaurid, tyrannosauroid, theropod, sauropod, iguanodontid, ornithopod, thyreoporan, and crocodylomorph remains. Willow Tank Formation fauna are similar to material recovered from the Cedar Mountain Formation and the Cloverly Formation.

Overall, the upper member of the Willow Tank Formation records deposition within a semi-arid, anastomosed fluvial system. In addition, the geographic ranges of the Early Cretaceous, North American fauna can be expanded to the Sevier Fold and Thrust front.