UTAH'S SOUTHERNMOST EOCENE VOLCANIC ROCKS IN THE LITTLE DRUM MOUNTAINS IN MILLARD COUNTY

The Little Drum Mountains is a small northwest-trending basin-range about six miles at its widest and about 20 miles long. Its southeastern end lies about 20 miles west of Delta, Utah. Geologic mapping of two 1:24,000 quadrangles, Smelter Knolls West and Little Drum Pass, has shown the bedrock here to be layered Eocene volcanic rocks, broken by a few normal faults of small displacement, and tilted mostly westward at low angles. The rocks are about 6,400 feet (1,900m) thick and about evenly divided into three types: lava flows, volcanic conglomeratic debris flows, and ash-flow tuffs. ⁴⁰Ar/³⁹Ar dating reveals that these rocks were deposited between 39 and 35 million years ago. They represent distal deposits from the Drum and Keg Mountains volcanic centers to the north where associated ore deposits have been much studied. The Little Drums show no overt evidence of economic mineralization and so have been little prospected. The lava flows are mostly high K₂O andesites or pyroxene-rich latites. The ash-flow tuffs are generally rich in crystals and pumice. These deposits are the southernmost record of Eocene volcanic activity in Utah. They rest unconformably on folded Cambrian strata in the adjacent Drum Mountains north of the Little Drums.