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Show Kenneth S. Edgett, Department of Geology, Arizona State University, Tempe, AZ 85287- 1404. POST- LACUSTRINE GEOMORPHOLOGY OF THE NORTHWESTERN GREAT BASIN: THE VOLCANICLASTIC SHIFTING SAND DUNES, CHRISTMAS LAKE VALLEY, OREGON Introduction: Aeolian dunes are tools of scientific inquiry; they preserve clues about climate variations, geologic events, and human activities. The " Shifting Sand Dunes" dune field is located in eastern Christmas Lake Valley, Oregon; the name of the dune field is from the local usage. The Shifting Sand Dunes were recently the subject of thermal infrared remote sensing studies centered on development of methods to examine and interpret similar data planned to be obtained for dune fields on the planet Mars by Mars Global Surveyor [ 1]. The purpose of the present paper, however, is to summarize what is known about this dune field and its context within the broader Holocene ( post- lacustrine) geomorphology of the northwestern Great Basin. Setting: Christmas Lake Valley is the eastern part of the Pleistocene Fort Rock Lake basin. The basin is surrounded by Pliocene and Holocene basalts and mafic hyaloclastic tuffs. The highest still stand of Fort Rock Lake was probably around 1367 m, about 53 m above the present basin floor [ 2, 3]. The ancient lake bed consists mainly of volcaniclastic fluvio- lacustrine and airfall deposits, plus diatomaceous earth [ 4]. Some parts of the Christmas Lake Valley sub- basin, particularly at Fossil Lake, have yielded important Cenozoic vertebrate fossils, including large and small mammals, birds, fish, and shellfish [ 5]. During the Holocene, the lakebed has been reworked and shaped by wind, fluctuations in levels of small lakes and ponds, and the deposition of sand- sized tephra during the terminal eruptions of Mt. Mazama 6,800 years ago [ 5- 11]. Regional Post- lacustrine History: The date of the last high- stand of Fort Rock Lake is not known for certain; Freidel [ 3] summarized the sparse literature on radiocarbon dates and suggested that it occurred about 18,000 years ago. Christmas Lake Valley was probably completely dry ( except for ponds) by 9,100 years ago, as indicated by a radiocarbon date from charcoal from the Buffalo Flat area south of the Shifting Sand Dunes [ 9], Work by Bedwell [ 12] at Fort Rock archeological sites and by Willig [ 13] in nearby Alkali Lake basin suggests that the climate during the Early Holocene ( 10,000 - 7,500 years ago; Grayson's [ 14] terminology) in this region probably ranged between being about like it is today, or slightly wetter. In the Middle Holocene ( 7,500 - 4,500 years ago), Mount Mazama experienced its terminal eruptions and the Fort Rock Basin area was covered by 15 to 100 cm of sandy tephra [ 7]. Greenspan [ 8] and Grayson [ 15] found that the Paulina Marsh, in the southwestern portion of the pluvial Fort Rock Lake basin, was largely dry during the Middle Holocene, indicating a climate that was considerably drier than today. Allison [ 6] noted that this dry climate was conducive to the aeolian reworking of Mt. Mazama tephra into dunes. The Late Holocene, less arid than the Middle Holocene, may have included a " neopluvial" episode around 2,000 - 3,000 years ago, when a good portion of Fort Rock Lake basin floor was underwater to depths of 2- 4 m [ 6]. More recent climate information has been summarized by Berry [ 16] and Hatton [ 10]. |