Rocky Mountain Section - 72nd Annual Meeting - 2020

Paper No. 16-3
Presentation Time: 8:30 AM-4:30 PM


BLOUNT, Charles W., Geosciences, Idaho State University, 881 22 Road, 921 S. 8th Ave. 8072, Pocatello, Id 83209, Grand Junction, CO 81505

Pleistocene terrace gravels are a common feature adjacent to creeks and washes in the Grand Valley south of the Book Cliffs. The terraces associated with Westwater Creek in the Harley Dome Quadrangle, Eastern Grand County, Utah are among the largest and are spread out across the valley. The quadrangle was mapped by Grant Willis of the Utah Geological Survey in 1994. Willis recognized what he called “Alluvial Pediment and Terrace Mantle Deposits” and labeled them Qap3 to Qap7 (oldest). The Qap7 Terraces were dated by the presence of detrital 630K Lava Creek B Ash from Yellowstone.

The present study consists of detailed mapping of stream terraces in the Harley Dome and adjacent quadrangles. A result has been the division of Qap4 terraces into 6 members and Qap5 terraces into 3 members. The age of these terraces can only be estimated based on their elevation position between Westwater Creek (below them) and the 630K Qap7 terrace (topographically above). Below is a list of terraces, their elevation above Westwater Creek, their approximate age, and the age range suggested by Willis (1994).

Terrace Elevation Approximate Willis (1994)
feet K age
Qap3 30-60 55-111 30-70
Qap4a 80 148 Qap4 100-200
Qap4b 100 185
Qap4c 190
Qap4d 200
Qap4e 120 220
Qap4f 135 250
Qap5a&b 260-280 480-500 350-400
Qap5c 300 535
Qap7 340 630 550-620

In the Holocene and late Pleistocene the dominant geomorphic process in the Grand Valley has been erosion that has removed an enormous amount of material. The terraces are a tiny but significant remnant of this valley fill. The Qap4, 5 and 7 terraces have features that indicate rapid deposition by flash flooding events such as monsoonal storms. These terraces are large, thick, extend many miles out into the valley, contain a mixture of clast sizes from coarse sand to very large boulders, and lack silt and clay. The distribution of terrace elevations and ages indicates that such large monsoonal storms are rare events from interglacial periods.

The location of these terraces reflects the shifting position of Westwater Creek. Qap3, Qap4a and Qap4b are adjacent to the modern stream valley. Qap4c, d, e and f are along an older stream channel about ½ mile north. Qap5 and Qap7 are along a stream channel over a mile to the north. There is a large elevation and age difference (250 feet) between the Qap4 Terraces and the Qap5 terraces.