OF-24-06 Geologic Map of the Highline Lake Quadrangle, Mesa and Garfield Counties, Colorado

The purpose of this publication is to describe the geology, mineral and groundwater resources, and geologic hazards of the Highline Lake 7.5-minute quadrangle. This map is part of a STATEMAP geologic mapping program on Colorado’s Western Slope, near Grand Junction. Nearby completed CGS maps are the Badger Wash, Mack, Fruita, and Corcoran Point quadrangles. CGS geologists Jon White and Andrew Schmidt completed the field work on this project during the mapping seasons of 2023-24. Digital ZIP download. OF-24-06D

From the abstract:

The Highline Lake quadrangle lies in the Grand Valley in Mesa and Garfield counties of West-central Colorado, approximately 20 km northwest of the town of Fruita and 15 km east of the Utah state line. The Grand Valley is underlain by the Cretaceous Mancos Shale and a topographically low area, which consists of a landscape of subdued hills and adobe-type badlands that lie between the Uncompahgre Plateau and the Book Cliffs. Quadrangle elevations range from 1,382.3 to 1,848.3 m. The site is arid (annual precipitation is 20-30 cm) and the creeks are intermittent or ephemeral. The main creeks are the East Salt Creek and Mack Wash. The quadrangle lies at the northwestern edge of the Grand Valley where the southern portion of the map area is irrigated by the Government Highline Canal. The Colorado Highline Lake State Park lies within the map area. This reservoir, where Mack Wash is dammed, is fed by water from the nearby Highline Canal. The topography in the irrigated southern part of the map area consists of wide and subdued flats, and low hills, underlain by the Smoky Hill Member of the Mancos Shale. Irrigation and agricultural practices have heavily altered the area. The exception is the topographically inverted Mack Mesa, a gravel-capped mesa (≤30-m higher in elevation) deposited as an alluvial fan at the end of the Bull Lake glaciation. North of the flat irrigated lands and the canal, the underlying Mancos Shale bedrock is the Prairie Canyon Member that is more sandy and more resistant to weathering and erosion. Shallow dendritic drainage patterns are developed in low hills and subtle ridgelines that rise from 70 to 90 m above the irrigated lands. In the northern map area underlain by the upper part of the Mancos Shale, the shale surface is variably mantled with episodic Late to Middle Pleistocene alluvial-fan gravel deposits from erosion and transportation of rocky sediments from the Mesaverde Group sandstones of the Book Cliffs. The differing periods of gravel deposition and greater resistance to erosion than the adjacent shale slopes have formed several topographically inverted surface elevations of gently sloping mesas and mesa remnants surfaces, up to 60 m above stream base level. Structurally, formations dip gently (3-5°) to the north to northeast. At the northeast corner of the quadrangle, elevations steepen where the quadrangle boundary includes lower cliffs of the Book Cliffs where resistant sandstone units of the Sego Sandstone Member of the Mancos and the Cozzette and Corcoran members of the Upper Cretaceous Iles Formation are exposed. Formation dips also rise to about 10° in the Book Cliffs portion of the map area.