Jonathan O. Davis

Correlation of late Quaternary tephra layers in a long pluvial sequence near Summer Lake, Oregon

Abstract Near Summer Lake in southern Oregon, 54 tephra beds of late Quaternary age are exposed in pluvial lake sediments of Lake Chewaucan. Seven of the tephra beds near the top can be correlated with tephra deposits younger than 117,000 yr at Mount St. Helens, Washington, at Crater Lake, Oregon, and in northwestern Nevada in the deposits of pluvial Lake Lahontan. However, most of the section at Summer Lake lies below the correlated units, and contains 39 tephra beds older than 117,000 yr. Major-element chemistry of tephra glasses was determined by electron microprobe analysis; petrography supports the correlations made from chemical evidence. Compositions correlated range from 70 to 76% SiO 2 ; the least silicic Summer Lake glass contained 57%. Extrapolation of depositional rate suggests that most of the sediments at Summer Lake are younger than about 335,000 yr, but older lake beds containing tephra layers occur at one place. The long lacustrine record suggests that Lake Chewaucan persisted through the last interpluvial stage, and that the lake may have dried up at the end of the Pleistocene due to diversion of the Chewaucan River by relict shore features.

Mono Lake geomagnetic excursion found at Summer Lake, Oregon

A 25-cm zone of anomalous paleomagnetic directions from lacustrine sediments near Summer Lake, Oregon, has the same age and the same paleomagnetic signature as the younger part of the proposed Mono Lake geomagnetic excursion. Both field observations and rock magnetic studies support the hypothesis that the anomalous directions represent actual geomagnetic field behavior, thereby confirming the existence of the Mono Lake excursion at the Oregon site. Although confirmation of its existence at this site does not make the Mono Lake excursion any more suitable as a chronostratigraphic horizon, this excursion may now be used as an additional constraint on models of geomagnetic field behavior including those that predict the morphology of transitional geomagnetic fields.

Level of Lake Lahontan during deposition of the Trego Hot Springs tephra about 23,400 years ago

Abstract The Trego Hot Springs tephra bed is a silicic tephra about 23,400 yr old, found at several localities in pluvial lake sediments in northern Nevada, southern Oregon, and northeastern California. It has been characterized petrographically, by the major and minor element chemistry of its glass, and by its stratigraphic position with respect to other tephra layers. At a newly described locality on Squaw Creek, northwest of Gerlach, Nevada, at the north end of the Smoke Creek Desert, Trego Hot Springs tephra has been found in sediments of the Sehoo and Indian Lakes formations. The depositional environments of these sediments show that when the tephra fell, pluvial Lake Lahontan stood between 1256 and 1260 m, and that immediately thereafter the lake rose to at least 1275 m. These data corroborate earlier findings by Benson ( Quaternary Research 9 , 300–318) from radiometric dating of calcareous tufa. However, the Lake Lahontan area has been affected by isostatic subsidence and rebound in response to changing water loads, so that caution is required in the use of lakeshore elevations in correlation.

The middle to late Pleistocene geomagnetic field recorded in fine-grained sediments from Summer Lake, Oregon, and Double Hot Springs, Nevada, U.S.A.

Abstract A 400,000 year record of the paleomagnetic field has been acquired from 22 meters of middle to late Pleistocene fine-grained sediments from Summer Lake in south-central Oregon and Double Hot Springs in northwestern Nevada. The stratigraphy is based on 55 tephra layers, nine of which have been correlated with tephra layers from other localities on the basis of their distinct major- and trace-element geochemistry and their distinct petrography. The paleomagnetic samples carry a strong and stable magnetization that does not appear to have been affected by the inclination error commonly associated with the magnetization of sediments. The samples have accurately recorded the declination and inclination of the geomagnetic field at or near the time of deposition except for errors arising from rotations of discrete blocks of sediment predominantly about vertical axes. Errors introduced by this type of rotation were corrected by using paleomagnetic directions associated with correlated tephra layers. The Summer Lake paleomagnetic record suggests that secular variations occurred throughout the middle and late Pleistocene often maintaining the same waveform through several oscillations. The amplitudes of these variations were similar to those of Holocene variations, and the periods ranged from 15,000 years to greater than 100,000 years.

A paleomagnetic record from Pyramid Lake, Nevada, and its implications for proposed geomagnetic excursions

Abstract A well-dated, 3.3-m section of deep-lake clays from pluvial Lake Lahontan, now exposed along the shore of Pyramid Lake, Nevada, has provided a paleomagnetic record of secular variation during the time interval 25,000–36,000 years B.P. The measured ranges of inclination and declination are 40° and 75°, respectively, and represent values which are comparable to observed secular variation at northern mid-latitudes. These results and those from a previously reported study from Clear Lake, California, together provide strong evidence for the conclusion that northern California and western Nevada were not affected by any geomagnetic excursion during the time interval 21,000–36,000 years B.P. Such a conclusion puts severe constraints on the nature and timing of the proposed Mono Lake, Laschamp and Lake Mungo geomagnetic excursions.

Dating late Pleistocene pluvial events and tephras by correlating paleomagnetic secular variation records from the western Great Basin

Abstract Paleomagnetic records are used to correlate sedimentary sequences from pluvial Lakes Chewaucan and Russell in the western Great Basin. This correlation is the basis for age control in the relatively poorly dated sequence from Lake Chewaucan. The resulting chronology supports a lack of sedimentation in Lake Chewaucan during the interval 27,400 to 23,200 yr B.P., an assertion supported by the presence of a lag deposit at the corresponding stratigraphic horizon. Because the Lake Chewaucan outcrop (near Summer Lake, Oregon) is near the bottom of the lake basin, we conclude that Lake Chewaucan was at a lowstand during this time interval. The Chewaucan lowstand is coeval with the lowstand accompanying the Wizards Beach Recession (isotope stage 3) previously seen in the geologic record from nearby pluvial Lake Lahontan. The ages of six tephra layers, including the Trego Hot Springs tephra, were also estimated using the paleomagnetic correlation. Together, the new age of the Trego Hot Springs tephra (21,800 yr B.P.) and the lake surface level prehistory of Lake Chewaucan imply a revised model for the lake surface level prehistory of Lake Lahontan. The revised model includes a longer duration for the Wizards Beach Recession and the occurrence of a younger lowstand of short duration soon after the lowstand corresponding to the Wizards Beach Recession.

Dating volcanic ash by thermoluminescence: Test and application

Abstract We have developed a new physical dating method for Late Pleistocene tephrochronometry. Tests of laboratory procedures applied to independently dated samples show that the thermoluminescence (TL), additive-dose dating method can yield accurate ages for both proximal and distal tephra from a few hundred years to at least 400 ka. The technique involves purification of 4–11 μm glass by heavy liquid centrifugation, pre-TL-readout heating at 50–75°C for eight days and careful regression analysis of TL growth curves. Five unknown-age tephra beds from Summer Lake, Oregon, were dated directly by this method for the first time. The TL ages for these tephra are: 67.3 ± 7.5 ka (ash 2), 102 ± 11 ka (ash N), 165 ± 19 ka (ash R), 200 ± 27 ka (ash KK), and 162 ± 35 ka (ash LL). These results indicate a nonuniform sedimentation rate for the enclosing pluvial lake deposits, a sedimentation hiatus in the interval 100–160 ka, and maximum ages roughly half of the previous indirectly inferred ages.

The North American Great Basin: A Sensitive Indicator of Climatic Change

Climatic change has become a major scientific and political issue during the past decade. Articles concerning global warming due to the greenhouse effect and ozone depletion from industrial chemicals are common in the news media and scientific literature. As stated by Schneider (1989), “the intense heat, forest fires, and drought of the summer of 1988 and the observation that the 1980s are the warmest decade on record have ignited an explosion of media, public, and governmental concern that the long-debated global warming has arrived”.