Charles D. Harrington

Scanning electron microscope method for rock-varnish dating

Rock-varnish coatings on cobbles from geomorphic surfaces and exposed deposits in arid environments are an effective medium for dating over a time range of several thousand to a few million years. A new analytical method for dating of rock varnish is presented wherein the varnish cation ratio (VCR) is determined by a scanning electron microscope (SEM) equipped with an energy dispersive X-ray analyzer (EDAX). The experimental SEM method is a nondestructive technique that has several potential advantages over the original method of analysis, described by R. I. Dorn, that uses particle-induced X-ray emission (PIXE) of varnish scraped from rock surfaces. The SEM method can potentially eliminate analytical errors due to contamination from rock substrate because variations in varnish thickness and irregularities on the substrate surface are examined before cation ratios are determined. Because varnish surfaces remain intact, varnish sites that yield anomalous results may be reanalyzed or verified. In addition, the general accessibility of scanning electron microscopes will make rock-varnish dating more widely available for use in Quaternary studies. Cation ratios were calculated for rock varnish from Espanola Basin, New Mexico, and the Yucca Mountain region, Nevada, and were used to construct rock-varnish dating curves for these areas.

Relict colluvial boulder deposits as paleoclimatic indicators in the Yucca Mountain region, southern Nevada

Early to middle Pleistocene boulder deposits are common features on southern Nevada hillslopes. These darkly varnished, ancient colluvial deposits stand but in stark contrast to the underlying light-colored bedrock of volcanic tuffs, and they serve as minor divides between drainage channels on modern hillslopes. To demonstrate the antiquity of these stable hillslope features, six colluvial boulder deposits from Yucca Mountain, Nye County, Nevada, were dated by cation- ratio dating of rock varnish accreted on boulder surfaces. Estimated minimum ages of these boulder deposits range from 760 to 170 ka. Five additional older deposits on nearby Skull and Little Skull Mountains and Buckboard Mesa yielded cation-ratio minimum-age estimates of 1.38 Ma to 800 ka. An independent cosmogenic chlorine-36 surface exposure date was obtained on one deposit, which confirms an estimated early to middle Quaternary age. These deposits have provided the oldest age estimates for unconsolidated hillslope deposits in the southwestern United States. We suggest that the colluvial boulder deposits were produced during early and middle Pleistocene glacial/pluvial episodes and were stabilized during the transition to drier interglacial climates. By comparison to modern periglacial environments, winter minimum monthly temperatures of -3 to -5 °C were necessary to initiate freeze-thaw conditions of such vigor to physically weather relatively large volumes of large boulders from the upper hillslopes of the Yucca Mountain area. These conditions imply that early and middle Pleistocene glacial winter temperatures were at least 1 to 3 °C colder than existed during the last Pleistocene glacial episode and 7 to 9 °C colder than present. We 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 necessary to form these colluvial boulder deposits are not active on modern hillslopes in the southern Great Basin. In addition, the lack of young, relatively unvarnished colluvial boulder deposits on these hillslopes suggests that boulder-forming conditions did not exist during the late Pleistocene in this region. Modern semiarid hillslope processes primarily erode colluvium during infrequent high-intensity storms. The preservation of old, thin hillslope deposits and the less-than-2-m incision by hillslope runoff adjacent to these deposits, however, indicate that extremely low denudation rates have occurred on resistant volcanic hillslopes in the southern Great Basin during Quaternary time.

Late Cenozoic rates of erosion in the western Española basin, New Mexico: Evidence from geologic dating of erosion surfaces

Erosion surfaces in the Espanola basin formed before 350 ka and between 350 and 240, 240 and 130, and 130 and 80 ka, probably in response to climatic change and regional uplift. The surfaces are cut on Miocene, Pliocene, and Pleistocene deposits and range from about 200 m to 15 m above the present Rio Chama/Rio Grande system. Periods when the surfaces formed were dated using varnish-cation ratios from exposed clasts, the mass of soil carbonate, and amino-acid ratios in Pleistocene gastropods from underlying deposits. Thorium/uranium ages from soil carbonate were used to calibrate a local curve for varnish-cation ratios. The range in age determined for a given surface, although derived from different dating techniques, implies that parts of the surface were sites of erosion or aggradation after the surface formed. From 1.1 Ma to present, denudation rates averaged 10 cm/1,000 yr from weakly lithified sandstone, less than 7 cm/1,000 yr from indurated tuff and boulder gravel, and about 4 cm/1,000 yr from tuff and basalt. Erosion surfaces were preserved as upland benches and terraces by stream incision during periods of pluvial climate and regional uplift, but our data do not permit clear separation of the two causes.

Clast size controls and longevity of Pleistocene desert pavements at Lathrop Wells and Red Cone volcanoes, southern Nevada

Formation of desert pavement and accretionary soils are intimately linked in arid environments. Well-sorted fallout scoria lapilli at Lathrop Wells (75‐80 ka) and Red Cone (ca. 1 Ma) volcanoes (southern Nevada) formed an excellent parent material for pavement, allowing infiltration of eolian silt and fine sand that first clogged the pore space of underlying tephra and then aggraded and developed vesicular A (Av) horizons. Variations in original pyroclast sizes provide insight into minimum and maximum clast sizes that promote pavement and soil formation: pavement becomes ineffective when clasts can saltate under the strongest winds, while clasts larger than coarse lapilli are unable to form an interlocking pavement that promotes silt accumulation (necessary for Av development). Contrary to predictions that all pavements above altitudes of !400 m would have been reset in their development after late Pleistocene vegetation advances, the soils and pavements show clear differences in maturity between the two volcanoes. This indicates that either the pavements and/or soils develop slowly over many tens of thousands of years and then are very stable, or, if they are disrupted by vegetation advances, subsequent pavements are re-established with successively more mature characteristics.