James C. Yount

Dating precariously balanced rocks in seismically active parts of California and Nevada

Precariously balanced boulders that could be knocked down by strong earthquake ground motion are found in some seismically active areas of southern California and Nevada. In this study we used two independent surface-exposure dating techniques—rock-varnish microlamination and cosmogenic 36 Cl dating methodologies—to estimate minimum- and maximum-limiting ages, respectively, of the precarious boulders and by inference the elapsed time since the sites were shaken down. The results of the exposure dating indicate that all of the precarious rocks are >10.5 ka and that some may be significantly older. At Victorville and Jacumba, California, these results show that the precarious rocks have not been knocked down for at least 10.5 k.y., a conclusion in apparent conflict with some commonly used probabilistic seismic hazard maps. At Yucca Mountain, Nevada, the ages of the precarious rocks are >10.5 to >27.0 ka, providing an independent measure of the minimum time elapsed since faulting occurred on the Solitario Canyon fault.

Late Quaternary eolian and alluvial response to paleoclimate, Canyonlands, southeastern Utah

began at ca. 6 ka and ended by ca. 3‐2 ka, followed by a shift to drier modern conditions; localized mobilization of dune sand has persisted to the present. These interpretations are similar to those of studies at the Chaco dune fi eld, New Mexico, and the Tusayan dune fi eld, Arizona, and are consistent with paleoclimate interpretations of pollen and packrat middens in the region. A period of rapid deposition and infi ltration of eolian dust derived from distant igneous source terranes occurred between ca. 12 and 8 ka. Before ca. 17 ka, and apparently back to at least 45 ka, paleosols contain little or no such infi ltrated dust. After ca. 8 ka, either the supply of dust was reduced or the more arid climate inhibited translocation of dust into the soils.

Compositional trends in aeolian dust along a transect across the southwestern United States

[1] Aeolian dust strongly influences ecology and landscape geochemistry over large areas that span several desert ecosystems of the southwestern United States. This study evaluates spatial and temporal variations and trends of the physical and chemical properties of dust in the southwestern United States by examining dust deposited in natural depressions on high isolated surfaces along a transect from the Mojave Desert to the central Colorado Plateau. Aeolian dust is recognized in these depressions on the basis of textural, chemical, isotopic, and mineralogical characteristics and comparisons of those characteristics to the underlying bedrock units. Spatial and temporal trends suggest that although local dust sources are important to the accumulated material in these depressions, Mojave Desert dust sources may also contribute. Depth trends in the depressions suggest that Mojave sources may have contributed more dust to the Colorado Plateau recently than in the past. These interpretations point to the important roles of far-traveled aeolian dust for landscape geochemistry and imply future changes to soil geochemistry under changing conditions in far-distant dust source areas.

Regional significance of an early Holocene moraine in Enchantment Lakes basin, North Cascade Range, Washington

Abstract The upper Enchantment Lakes basin in the North Cascade Range of Washington displays two moraine belts, each recording an episode of glacier advance after the end of the last glaciation. The inner belt, the Brynhild, 0.1 to 0.5 km beyond existing glaciers, postdates Mount St. Helens Wn tephra (∼450 yr old), which lies only beyond the moraines. The morainal surface is only slightly weathered, is almost barren of lichens, and is devoid of soil, evidence suggesting that the Brynhild moraines are no more than a century old. The outer moraine, the Brisingamen, 0.3 to 0.7 km beyond existing glaciers, is weathered and is covered with large lichens. On and behind the Brisingamen moraine the Mazama ash (6900 yr old) is present beneath the Mount St. Helens Yn and Wn tephras. Despite more than 7 millennia of weathering, the rock surface behind the Brisingamen moraine is measurably less weathered than the surface beyond, which was last glaciated during the Rat Creek advance about 13,000 yr ago. The age of the Brisingamen moraine therefore is probably early Holocene. The Brisingamen moraine evidently correlates with moraines near Glacier Peak, near Mount Rainier, in northeastern and central Oregon, in the southern Canadian Rockies, and in the northern U.S. Rocky Mountains. These regional effects suggest that a climatic episode of cooling or increased snowfall affected the entire region some time during the early Holocene.

Ground Cracks Associated with the 1994 Double Spring Flat Earthquake, West-Central Nevada

The 1994 Double Spring Flat earthquake ( M W 5.8) occurred within a densely faulted step-over between the Genoa and Antelope Valley faults, two principal normal faults of the transition zone between the Basin and Range Province and the northern Sierra Nevada. The earthquake created zones of ground cracks from 0.1 to 2.8 km long along at least five northwest- to north-northwest-striking faults in the epicentral area. Individual cracks had extensional openings generally from 1 to 10 mm wide. No cracks displayed obvious vertical separation, and only one zone showed permissive evidence of right-lateral separation. Over the 8 days following the mainshock (the period over which the cracks were found), aftershocks formed a dominant northeast trend suggesting the earthquake occurred along a northeast-striking structure. However, no ground breakage was found along faults striking parallel to this northeast aftershock alignment, and subsequent aftershocks formed a conjugate northwest trend. Based on the location and character of the five zones, the observed cracks are attributed to secondary fault slip and shaking effects. The earthquake also created ground cracks along at least two faults 15-25 km from the epicenter. In both of these cases, the faults had documented histories of prior ground cracking, indicating that they are particularly susceptible to such triggered deformation. Manuscript received 21 August 2002.