RELICT COLLUVIAL BOULDER DEPOSITS AS PALEOCLIMATIC INDICATORS IN THE YUCCA MOUNTAIN REGION, SOUTHERN NEVADA

Early to middle Pleistocene boulder deposits are common features on so uthern Nevada hillslopes. These darkly varnished, ancient colluvial de posits stand out in stark contrast to the underlying light-colored bed rock of volcanic tuffs, and they serve as minor divides between draina ge channels on modern hillslopes. To demonstrate the antiquity of thes e stable hillslope features, six colluvial boulder deposits from Yucca Mountain, Nye County, Nevada, were dated by cation-ratio dating of ro ck varnish accreted on boulder surfaces. Estimated minimum ages of the se boulder deposits range from 760 to 170 ka. Five additional older de posits on nearby Skull and Little Skull Mountains and Buckboard Mesa y ielded cation-ratio minimum-age estimates of 1.38 Ma to 800 ka. An ind ependent cosmogenic chlorine-36 surface exposure date was obtained on one deposit, which confirms an estimated early to middle Quaternary ag e. These deposits have provided the oldest age estimates for unconsoli dated hillslope deposits in the southwestern United States. We suggest that the colluvial boulder deposits were produced during early and mi ddle Pleistocene glacial/pluvial episodes and were stabilized during t he transition to drier interglacial climates. By comparison to modern periglacial environments, winter minimum monthly temperatures of -3 to -5-degrees-C were necessary to initiate freeze-thaw conditions of suc h vigor to physically weather relatively large volumes of large boulde rs from the upper hillslopes of the Yucca Mountain area. These conditi ons imply that early and middle Pleistocene glacial winter temperature s were at least 1 to 3-degrees-C colder than existed during the last P leistocene glacial episode and 7 to 9-degrees-C colder than present. W e conclude that at least several early and middle Pleistocene glacial episodes were colder, and perhaps wetter, than glacial episodes of the late Pleistocene in the southern Great Basin. Geomorphic processes ne cessary to form these colluvial boulder deposits are not active on mod ern hillslopes in the southern Great Basin. In addition, the lack of y oung, relatively unvarnished colluvial boulder deposits on these hills lopes suggests that boulder-forming conditions did not exist during th e late Pleistocene in this region. Modern semiarid hillslope processes primarily erode colluvium during infrequent high-intensity storms. Th e preservation of old, thin hillslope deposits and the less-than-2-m i ncision by hillslope runoff adjacent to these deposits, however, indic ate that extremely low denudation rates have occurred on resistant vol canic hillslopes in the southern Great Basin during Quaternary time.