Laserfiche WebLink
r~ <br />U <br />YOAST <br />GENERAL GEOLOGIC DESCRIPTION <br />Regional Structure <br />The Yoast area lies in the southwest synclinal portion of the Washakie structural basin of <br />south-central Wyoming. In northern Colorado, the syncline is bordered on the east by the <br />Park Range, a large faulted anticline, and on the southwest by the Axial Basin anticline. <br />The regional synclinal structure has been modified by numerous folds, the largest of these <br />being the Tow Creek anticline, an asymmetrical fold trending towards the northeast. Other <br />prominent structural features include the Wolf Creek Dome, Chimney Creek Dome, Pagoda <br />Dome, Williams Park, and Beaver Creek anticlines, and Hat and Twenty-mile Park syncline. <br />The present structural fabric was initiated during the Laramide Orogeny, the second major <br />recorded episode of mountain building in the Rocky Mountain region, during the Late <br />Cretaceous and Early Tertiary. The forces responsible for the Laramide Orogeny continued, <br />although at a diminished rate, and during the Miocene epoch were responsible for the <br />• increased inland stress and strain which eventually elevated all of Colorado and parts of <br />Utah, Arizona, and New Mexico an additional 5,000 feet. <br />• <br />Regional Stra ti gra phy <br />The exposed rock sequences present in the region consist of approximately 13,500 of Late <br />Cretaceous, Tertiary, and Oua ternary age deposits. This sedimentary sequence is composed <br />of primarily marine, near shore, palludal, and fluvial sediments with occasional influxes <br />of volcanic ash, subsequently altered to bentonite (Figure 1). <br />The Late Cretaceous sediments were deposited along a transgressive, regressive <br />epicontinental seaway. The oldest of these formations exposed in the region is the Ma ncos <br />shale. This formation is approximately 4,900 feet thick and consists chiefly of <br />homogeneous dark gray marine shale and highly fractured, thin-bedded, fine-grained <br />fossilifero us limy sandstones and fossiliferous limestones. These limy beds, of Niobrara <br />age, have yielded shows of natural gas and oil in wells drilled along areas of structural <br />deformations. <br />The Mesaverde Groups of Late Cretaceous age conformably overlies the Ma ncos shale. The <br />1 <br />