Fauna of the Newark Canyon Formation (Lower Cretaceous), East-Central Nevada

摘要

The Newark Canyon Formation represents the deposits of a terrestrial piggy-back basin. Lithofacies represent lacustrine mudrocks to limestones, fluvial sandstones and overbank mudrocks, and sediment-gravity flows. This unit was originally interpreted as Lower Cretaceous based upon biostratigraphy. The unit has long been known to be fossiliferous and produced the first identifiable dinosaur remains from the state of Nevada. In this study we report an updated faunal list for the Newark Canyon Formation from exposures of the Pancake, Diamond, Fish Creek, and Cortez Ranges located in east-central Nevada. Previous studies identified the fish Leptolepis nevadensis, unidentified crocodilians, and carnosaur teeth. Based upon our work we can now expand this list to include a member of the freshwater shark family Hybodontidae, gar fish (Lepisosteidae), the turtle genus Glyptops, at least two types of crocodilians, the small plant-eating dinosaur family Euornithopoda, the big plant-eating dinosaur family Iguanodontidae, the armored dinosaur family Ankylosauridae, material conferred to sauropod dinosaurs, theropod dinosaurs, and eggshell fragments attributed to the oogenus Elongatoolithus belonging to the theropod oviraptor dinosaurs and in one instance a dromaeosaur. A previous study was able to obtain a radiometric U/Pb zircon date of Aptian age for the top of the type section in the Diamond Range. As part of this study we sent off sediment samples from the Fish Creek Range to better constrain the age of the Newark Canyon Formation exposures in this area. Pollen data suggests exposures in the Fish Creek Range are Albian and perhaps Cenomanian in age. Using the previous biostratigraphic ages with the new pollen data and the radiometric date helps to temporally correlate the Newark Canyon Formation to contemporaneous units from the Sevier Foreland of east-central Utah, in particular the Ruby Ranch Member of the Cedar Mountain Formation. When the fauna of the Ruby Ranch Member of the Cedar Mountain Formation is compared to the fauna of the Newark Canyon Formation they are grossly similar to the level of taxonomic certainty we are able to interpret from the Nevada material. In particular the Cedar Mountain Formation has the same genus of turtle, hybodont sharks, and crocodiles. Small-bodied vertebrates are sensitive to subtle biogeographic boundaries. This implies that there must have been significant ecological continuity between the foreland of Utah and the Sevier hinterland of Nevada during latest Early Cretaceous time. Ecological continuity also implies that there must not have been a significant difference in elevation between foreland and hinterland depocenters during this time. That is not to say that there was not complex topography within the hinterland, as the Central Nevada Fold and Thrust belt is active during this interval and there are abundant debris flow deposits found within the Newark Canyon Formation. The broader geographic implication is that broad regional uplift of the Sevier hinterland must have been a Late Cretaceous event.