Janet L. Slate

Cosmogenic 10Be and 36Cl geochronology of offset alluvial fans along the northern Death Valley fault zone: Implications for transient strain in the eastern California shear zone

The northern Death Valley fault zone (NDVFZ) has long been recognized as a major right-lateral strike-slip fault in the eastern California shear zone (ECSZ). However, its geologic slip rate has been difficult to determine. Using high-resolution digital topographic imagery and terrestrial cosmogenic nuclide dating, we present the first geochronologically determined slip rate for the NDVFZ. Our study focuses on the Red Wall Canyon alluvial fan, which exposes clean dextral offsets of seven channels. Analysis of airborne laser swath mapping data indicates ∼297 ± 9 m of right-lateral displacement on the fault system since the late Pleistocene. In situ terrestrial cosmogenic ^(10)Be and ^(36)Cl geochronology was used to date the Red Wall Canyon fan and a second, correlative fan also cut by the fault. Beryllium 10 dates from large cobbles and boulders provide a maximum age of 70 +22/−20 ka for the offset landforms. The minimum age of the alluvial fan deposits based on ^(36)Cl depth profiles is 63 ± 8 ka. Combining the offset measurement with the cosmogenic ^(10)Be date yields a geologic fault slip rate of 4.2 +1.9/−1.1 mm yr^(−1), whereas the ^(36)Cl data indicate 4.7 +0.9/−0.6 mm yr^(−1) of slip. Summing these slip rates with known rates on the Owens Valley, Hunter Mountain, and Stateline faults at similar latitudes suggests a total geologic slip rate across the northern ECSZ of ∼8.5 to 10 mm yr^(−1). This rate is commensurate with the overall geodetic rate and implies that the apparent discrepancy between geologic and geodetic data observed in the Mojave section of the ECSZ does not extend north of the Garlock fault. Although the overall geodetic rates are similar, the best estimates based on geology predict higher strain rates in the eastern part of the ECSZ than to the west, whereas the observed geodetic strain is relatively constant.

Death Valley research revises age of last deep lake

The last deep lake in Death Valley probably existed during marine isotope stage VI, more than 100,000 years earlier than previously thought, according to a paper presented this past spring at a conference on geologic research in Death Valley. The long accepted paradigm of a deep lake, known as Lake Manley in the very late Pleistocene appears to have fallen in light of recent U-series dating of high shorelines. This and other new research were the topics of an interdisciplinary meeting on the “Status of Geologic Research and Mapping in Death Valley National Park.” As its title indicates, the conference was organized to compile up-to-date information on the status of geologic research and mapping in Death Valley National Park and surrounding areas. It also was intended to establish a network of active researchers to create synergy for cooperative, interdisciplinary research endeavors and to present recent and current research results in an informal setting, thus encouraging dialogue.

Geologic map of the Fort Morgan 7.5' quadrangle, Morgan County, Colorado

[Calibrated radiocarbon ages are expressed as “cal ka B.P.,” which stands for calibrated thousand years before present (0 yr B.P. = 1950 A.D.). Uncertainties are given at the 95 percent (2σ) confidence level. Calibrated ages are reported as the midpoint of the calibrated range. In cases where calibration produced more than one age range with a probability of 5 percent or more, ages are based on the mean of the ranges weighted by their probabilities and are presented without uncertainties. Soil-horizon designations and other descriptive soil terminology used in this report follow criteria outlined in Soil Survey Division Staff (1993), Birkeland (1999), and Schoeneberger and others (2012). Most colors are field dry colors and based on Munsell soil color charts (Munsell Color, 1975). The term “consistence” is the resistance to crushing of soil or surficial material in the hand, as described by Soil Survey Division Staff (1993). Textures are field estimates. In descriptions of clast lithology, the term “granite” refers to phaneritic igneous or meta-igneous rock types that are felsic to intermediate in composition. Geochronology sites are named by letters in their field numbers (tables 1–3, sheet 2), and correspond to letters shown in red on figure 1]

Late Neogene–Quaternary tephrochronology, stratigraphy, and paleoclimate of Death Valley, California, USA

Sedimentary deposits in midlatitude continental basins often preserve a paleoclimate record complementary to marine-based records. However, deriving that paleoclimate record depends on having well-exposed deposits and establishing a sufficiently robust geochronology. After decades of research, we have been able to correlate 77 tephra beds exposed in multiple stratigraphic sections in the Death Valley area, California, United States. These correlations identify 25 different tephra beds that erupted from at least five different volcanic centers from older than 3.58 Ma to ca. 32 ka. We have informally named and determined the ages for seven previously unrecognized beds: ca. 3.54 Ma tuff of Curry canyon, ca. 3.45 Ma tuff of Furnace Creek, ca. 3.1 Ma tuff of Kit Fox Hills, ca. 3.1 Ma tuff of Mesquite Flat, ca. 3.15 Ma tuff of Texas Spring, 3.117 ± 0.011 Ma tuff of Echo Canyon, and the ca. 1.3 Ma Amargosa ash bed. Several of these tephra beds are found as far northeast as central Utah and could be important marker beds in western North America. Our tephrochronologic data, combined with magnetic polarity data and ^(40)Ar/^(39)Ar age determinations, redefine Neogene sedimentary deposits exposed across 175 km^2 of the Death Valley area. The alluvial/lacustrine Furnace Creek Formation is a time-transgressive sedimentary sequence ranging from ca. 6.0 to 2.5 Ma in age. The ca. 2.5−1.7 Ma Funeral Formation is typically exposed as a proximal alluvial-fan facies overlying the Furnace Creek Formation. We have correlated deposits in the Kit Fox Hills, Salt Creek, Nova Basin, and southern Death Valley with the informally named ca. 1.3−0.5 Ma Mormon Point formation. In addition, our correlation of the late Pleistocene Wilson Creek ash bed 15 in the Lake Rogers deposits represents the first unambiguous sequences deposited during the Last Glacial Maximum (marine isotope stage [MIS] 2) in Death Valley. Based on this new stratigraphic framework, we show that the Pliocene and Pleistocene climate in Death Valley is consistent with the well-established marine tropical/subtropical record. Pluvial lakes in Death Valley and Searles Valley began to form ca. 3.5−3.4 Ma in the late Pliocene during MIS MG5. Initiation of lakes in these two hydrologically separated valleys at the same time at the beginning of a cooling trend in the marine climate record suggests a link to a cooler, wetter (glacial) regional climate in North America. The Death Valley lake persisted until ca. 3.30 Ma, at the peak of the M2 glaciation, after which there is no evidence of Pliocene lacustrine deposition, even at the peak of the Northern Hemisphere Glaciation (ca. 2.75 Ma). If pluvial lakes in the Pliocene are an indirect record of glacial climate conditions, as they are for the Pleistocene, then a glacial climate was present in western North America for ∼200,000 yr during the Pliocene, encompassing MIS MG5−M2. Pleistocene pluvial lakes in Death Valley that formed ca. 1.98−1.78 Ma, 1.3−1.0 Ma, and ca. 0.6 Ma (MIS 16) are consistent with other regional climate records that indicate a regional glacial climate; however, Death Valley was relatively dry at ca. 0.77 Ma (MIS 19), when large lakes existed in other basins. The limited extent of the MIS 2 marsh/shallow lake in the Lake Rogers basin of northern Death Valley reflects the well-known regional glacial climate at that time; however, Death Valley received relatively lower inflow and rainfall in comparison.