Geological and paleontological investigations of the Upper Jurassic Morrison Formation at the basin margins of central Montana and the western panhandle of Oklahoma

Abstract

During the past century, the research focus area of the Morrison Formation has been the Colorado Plateau. There are two main reasons for the research emphasis in the region. First, the Colorado Plateau contains world-class exposures of the formation. Second, the majority of Morrison Formation vertebrate sites (dinosaur quarries) are located within its boundaries. Due to the emphasis where the formation is well-exposed, the geology and paleontology of the Morrison Formation at the basin margins has not received much attention from researchers. The focus of this project is to investigate two different regions of the Morrison foreland basin margin and research the geology and paleontology of these areas. Exposures of the Morrison Formation are limited in central Montana. Relatively recent dinosaur discoveries in the northern portion of the basin resulted in an investigation of the formation in the region. The investigation led to the discovery, description, and first confirmed occurrences of the fossil wood Xenoxylon meisteri in the Upper Jurassic of North America. In addition to the fossil wood Xenoxylon, numerous carbonate mound springs were discovered and are described. These carbonate tufa deposits are the first recorded occurrences for the Morrison Formation and are some of the oldest tufa deposits in North America. The presence of Xenoxylon and the carbonate mound springs have paleoclimatic significance for the northern basin margin. Whereas climate models for the Morrison Formation on the Colorado Plateau suggest that it was hot and dry, the presence of Xenoxylon and the mound springs in central Montana indicate a higher precipitation rate and likely cooler temperatures than more southern regions. The second basin margin investigated was the western panhandle of Oklahoma. The Morrison Formation in the western portion of the state is divided into three newly formed members (Cimarron, Boise, and Kenton) that likely correlate with the three main members of the Colorado Plateau (Tidwell, Salt Wash, and Brushy Basin). The three Oklahoma Morrison members are the lateral distal depositional facies of each of the members of the Colorado Plateau. The depositional facies and climatic proxies (e.g. Xenoxylon) of the new members of the Morrison Formation in Oklahoma each recorded a distinct climatic interval. Overall, the climate of the Morrison Formation in Oklahoma was more temperate and wetter than the Colorado Plateau.