Archives

A guide to the bedrock geology of Range Creek Canyon, Book Cliffs, Utah

Range Creek Canyon, located within the Book Cliffs of eastern Utah, contains some of the most abundant and well-preserved archaeological sites in North America. Its cliffs and landscapes provide a canvas for rock art panels and a foundation for granaries, ruins, and artifacts of the prehistoric Fremont Indians. In order to place these Range Creek sites within a geologic context, an illustrated geologic field guide was created for the general public. The guide focuses on the major bedrock formations that crop out in the canyon, as well as many indicators that facilitate geologic interpretation of these rocks. Outcrops of the Paleogene Flagstaff and Colton Formations (~58 to 48 million years old) in Range Creek Canyon were investigated in order to interpret their depositional environments. The lacustrine Flagstaff Limestone contains limestone beds and fossils of freshwater gastropods, oysters, and turtles indicative of lake environments. The unit coarsens upward with an increase of interbedded sandstone, which was deposited in and near ancient river channels. This trend suggests dynamic levels of the ancient lake, with overall encroachment of river systems near the contact with the Colton Formation. The fluvial Colton Formation is characterized by discontinuous, stacked beds of sandstone, representing a succession of migrating river channels and floodplain deposits. The Colton Formation exhibits a general upward trend of increased grain size and increased channel belt (continuous sandstone beds) frequency and lateral extent, implying a transition to higher energy river systems through time. These dynamic, ancient rivers may have been flowing generally northward into Eocene Lake Uinta, recorded in deposits of the Green River Formation north of Range Creek Canyon.

The major pre-Mississippian unconformity in Rock Canyon, central Wasatch Range, Utah

In central Utah, the major pre-Mississippian unconformity is fairly well understood at most of the localities where it is recognized. However, the unconformity is more enigmatic in Rock Canyon of the central Wasatch Range. At this locality, dolomitization of most pre-Mississippian rocks obscures stratigraphic identification of Devonian and older units. The absence of any identifiable angular relationship further complicates resolution. Because of this, both identification of the stratigraphic level of the unconformity and, consequently, its magnitude remain controversial. Large-size dolomite samples taken in Rock Canyon at closely spaced intervals for the 3.6-m directly below definite Upper Devonian rocks yield microfossils, including conodonts, in the uppermost 1.6-m of that interval that indicate no unconformity exists between the Cambrian Maxfield Limestone and the Upper Devonian-Lower Mississippian Fitchville Dolomite at the horizon previously identified as unconformable. Rather, an unknown thickness of dolomitized Upper Devonian Pinyon Peak Formation and probable older rock (possibly Bluebell Dolomite and Victoria Formation) occurs between the top of definite Maxfield and base of the Fitchville. The identification of the unconformity horizon remains unknown. Our preliminary work outlines a promising procedure for future understanding of the magnitude and stratigraphic level of the unconformity.