Shannon A. Mahan

Late Quaternary loess in northeastern Colorado: Part I—Age and paleoclimatic significance

Loess in eastern Colorado covers an estimated 14 000 km 2 , and is the westernmost part of the North American midcontinent loess province. Stratigraphic studies indicate there were two periods of loess deposition in eastern Colorado during late Quaternary time. The first period spanned ca. 20 000 to 12 000 14 C yr B.P. (ca. 20‐14 ka) and correlates reasonably well with the culmination and retreat of Pinedale glaciers in the Colorado Front Range during the last glacial maximum. The second period of loess deposition occurred between ca. 11000 and 9000 14 C yr B.P. This interval may be Holocene or may correlate with a hypothesized Younger Dryas glacial advance in the Colorado Front Range. Sedimentologic, mineralogic, and geochemical data indicate that as many as three sources could have supplied loess in eastern Colorado. These sources include glaciogenic silt (derived from the Colorado Front Range) and two bedrock sources, volcaniclastic silt from the White River Group, and clays from the Pierre Shale. The sediment sources imply a generally westerly paleowind during the last glacial maximum. New carbon isotope data, combined with published faunal data, indicate that the loess was probably deposited on a cool steppe, implying a last glacial maximum July temperature depression, relative to the present, of at least 5‐6 °C. Overall, loess deposition in eastern Colorado occurred mostly toward the end of the last glacial maximum, under cooler and drier conditions, with generally westerly winds from more than one source.

Origin of the late Quaternary dune fields of northeastern Colorado

Stabilized eolian deposits, mostly parabolic dunes and sand sheets, cover much of the landscape of northeastern Colorado and adjacent parts of southwestern Nebraska in four geographically distinct dune fields. Stratigraphic and soil-geomorphic relations and accelerator radiocarbon dating indicate that at least three episodes of eolian sand movement occurred between 27 ka and 11 ka, possibly between 11 ka and 4 ka, and within the past 1.5 ka. Thus, eolian sand deposition took place under both glacial and interglacial climatic conditions. In the youngest episodes of eolian sand movement, Holocene parabolic dunes partially buried Pleistocene sand sheet deposits. Late Holocene sands in the Fort Morgan and Wray dune fields, to the south of the South Platte River, have trace element ratios that are indistinguishable from modern South Platte River sands, but different from Ogallala Formation bedrock, which has previously been cited as the main source of dune sand on the Great Plains. Sands in the Greeley dune field, to the north of the South Platte River, have trace element concentrations that indicate a probable Laramie Formation source. Measurements of parabolic dunes indicate paleowinds from the northwest in all dune fields, in good agreement with resultant drift directions calculated for nearby weather stations. Thus, paleowinds were probably not significantly different from present-day winds, and are consistent with a South Platte River source for the Fort Morgan and Wray dune fields, and a Laramie Formation source for the Greeley dune field. Sand accumulated downwind of the South Platte River to form the Fort Morgan duen field. In addition, sand was also transported farther downwind over the upland formed by the calcrete caprock of the Ogallala Formation, and deposited in the lee of the upland on the southeast side. Because of high wind energy, the upland itself served as a zone of sand transport, but little or no sand accumulation took place on this surface. These studies, which demonstrate the importance of fluvial-source sediments for dune fields in Colorado, may be applicable to other dune fields in North America. Because modern drift potentials in northeastern Colorado are among the highest in the world, the present stability of dunes in the region may be in part a function of the dunes being supply-limited rather than solely transport-limited. Extensive (∼ 7700 km2) late Holocene dunes document that eolian sand in northeastern Colorado is very sensitive to small changes in climate or fluvial source conditions.

Geomorphic and geochemical evidence for the source of sand in the Algodones dunes, Colorado Desert, southeastern California

The Algodones dunes of southeastern California comprise one of the largest active dune fields in the United States. The source of sand of the Algodones dunes is controversial, and the source of stabilized aeolian sand in the adjacent East Mesa area has not been investigated at all. We used mineralogical compositions and trace element concentrations to ascertain the most likely source of sand for these active and stabilized dunes. Results indicate that alluvium derived from the San Bernardino Mountains, which enters the Salton trough to the northwest of the dune fields, and alluvium derived from the Chocolate Mountains, which is deposited immediately to the northeast of the dunes, do not appear to be significant sources of sediment for the Algodones and East Mesa dunes. Both active aeolian sand from the Algodones dunes and stabilized aeolian sand on East Mesa are probably derived from sediments of ancient Lake Cahuilla, which formerly occupied part of the Salton Trough and left sandy shoreline sediments to the west and northwest of where the dune fields are now found. Lake Cahuilla sediments, in turn, were apparently derived from the Colorado River, when the river shifted its course and emptied into the Salton Trough, rather than the Gulf of California.

Chronology, sedimentology, and microfauna of groundwater discharge deposits in the central Mojave Desert, Valley Wells, California

During the late Pleistocene, emergent groundwater supported persistent and long-lived desert wetlands in many broad valleys and basins in the American Southwest. When active, these systems provided important food and water sources for local fauna, supported hydrophilic and phreatophytic vegetation, and acted as catchments for eolian and alluvial sediments. Desert wetlands are represented in the geologic record by groundwater discharge deposits, which are also called spring or wetland deposits. Groundwater discharge deposits contain information on the timing and magnitude of past changes in water-table levels and, thus, are a source of paleohydrologic and paleoclimatic information. Here, we present the results of an investigation of extensive groundwater discharge deposits in the central Mojave Desert at Valley Wells, California. We used geologic mapping and stratigraphic relations to identify two distinct wetland sequences at Valley Wells, which we dated using radiocarbon, luminescence, and uranium-series techniques. We also analyzed the sediments and microfauna (ostracodes and gastropods) to reconstruct the specifi c environments in which they formed. Our results suggest that the earliest episode of high water-table conditions at Valley Wells began ca. 60 ka (thousands of calendar yr B.P.), and culminated in peak discharge between ca. 40 and 35 ka. During this time, cold (4‐12 °C) emergent groundwater supported extensive wetlands that likely were composed of a wet, sedge-rush-tussock meadow mixed with mesic riparian forest. After ca. 35 ka, the water table dropped below the ground surface but was still shallow enough to support dense stands of phreatophytes through the Last Glacial Maximum (LGM). The water table dropped further after the LGM, and xeric conditions prevailed until modest wetlands returned briefl y during the Younger Dryas cold event (13.0‐11.6 ka). We did not observe any evidence of wet conditions during the Holocene at Valley Wells. The timing of these fl uctuations is consistent with changes in other paleowetland systems in the Mojave Desert, the nearby Great Basin Desert, and in southeastern Arizona, near the border of the Sonoran and Chihuahuan Deserts. The similarities in hydrologic conditions between these disparate locations suggest that changes in groundwater levels during the late Pleistocene in desert wetlands scattered throughout the American Southwest were likely driven by synopticscale climate processes.

Paleoecology reconstruction from trapped gases in a fulgurite from the late Pleistocene of the Libyan Desert

When lightning strikes the ground, it heats, melts, and fuses the sand in soils to form glass tubes known as fulgurites. We report here the composition of CO 2 , CO, and NO contained within the glassy bubbles of a fulgurite from the Libyan Desert. The results show that the fulgurite formed when the ground contained 0.1 wt% organic carbon with a C/N ratio of 10–15 and a δ 13 C of –13.96‰, compositions similar to those found in the present-day semiarid region of the Sahel, where the vegetation is dominated by C 4 plants. Thermoluminescence dating indicates that this fulgurite formed ~15 k.y. ago. These results imply that the semiarid Sahel (at 17°N) reached at least to 24°N at this time, and demonstrate that fulgurite gases and luminescence geochronology can be used in quantitative paleoecology.

Unusually Low Rates of Slip on the Santa Rosa Range Fault Zone, Northern Nevada

The Santa Rosa Range fault zone (srrfz) is one of the most topographically prominent normal fault systems in the northern Basin and Range province of the western United States. It has been assigned high rates of vertical slip by others and has been identified as a possible site of the future extension of the central Nevada seismic belt (cnsb). We use detailed trench mapping and luminescence dating to estimate displacements and timing of the last several large-magnitude paleoearthquakes on the southern part of the srrfz at a trench site near Orovada, Nevada. Coseismic vertical displacements ranged from 1 to 2.8 m for each of the last four events. Luminescence ages provide time limits for the last three events of 125–155 ka, 90–108 ka, and 11–16 ka. These data yield recurrence intervals of 17–65 k.y. and 74–97 k.y. and an elapsed time of 11–16 k.y. since the youngest event. Slip-rate determinations at the Orovada site are complicated by multiple fault strands, but rates calculated from a variety of data are surprisingly low (0.01–0.16 mm/yr), given the topographic prominence of the Santa Rosa Range. A lack of compelling patterns in a comparison of paleoseismic parameters indicate that the srrfz is no more likely a location for a large-magnitude earthquake than previously identified seismic gaps or along faults that lie directly north of the cnsb.

Late Pleistocene and Holocene uplift history of Cyprus: implications for active tectonics along the southern margin of the Anatolian microplate

Abstract The nature of the southern margin of the Anatolian microplate during the Neogene is complex, controversial and fundamental in understanding active plate-margin tectonics and natural hazards in the Eastern Mediterranean region. Our investigation provides new insights into the Late Pleistocene uplift history of Cyprus and the Troodos Ophiolite. We provide isotopic (14C) and radiogenic (luminescence) dates of outcropping marine sediments in eastern Cyprus that identify periods of deposition during marine isotope stages (MIS) 3, 4, 5 and 6. Past sea-levels indicated by these deposits are c. 95±25 m higher in elevation than estimates of worldwide eustatic sea-level. An uplift rate of c. 1.8 mm/year and possibly as much as c. 4.1 mm/year in the past c. 26–40 ka is indicated. Holocene marine deposits also occur at elevations higher than those expected for past SL and suggest uplift rates of c. 1.2–2.1 mm/year. MIS-3 marine deposits that crop out in southern and western Cyprus indicate uniform island-wide uplift. We propose a model of tectonic wedging at a plate-bounding restraining bend as a mechanism for Late Pleistocene to Holocene uplift of Cyprus; uplift is accommodated by deformation and seismicity along the margins of the Troodos Ophiolite and re-activation of its low-angle, basal shear zone. Supplementary material: An expanded description of the procedures used in determining OSL ages for samples given in Table 2 is available at www.geolsoc.org.uk/SUP18533.

Holocene stratigraphy and chronology of the Casper Dune Field, Casper, Wyoming, USA

Activation chronologies of dune fields within the North American Great Plains are significant sources of paleoclimate information. Although many regional chronologies exist, several dune fields have been understudied, including the Casper Dune Field of central Wyoming. This study investigated aeolian dune sediment and buried soils of the Casper Dune Field. Complex parabolic and hairpin parabolic dunes dominate the eastern dune field, while simple parabolic and linear dunes dominate the western dune field. Buried soils are found throughout the dune field, though their distribution and degree of development varies. Buried soils in the eastern dune field are weakly developed with typical A-C profiles, whereas soils in the western dune field typically exhibit A-Bt-C profiles. Optically stimulated luminescence (OSL) and radiocarbon ages were used to provide a chronology of dune field activation that spans most of the Holocene. At the onset of the Holocene, alluvium was deposited first, followed by widespread dune activity ~ 10.0—6.2 ka. Following activity, the dune field stabilized until about 4.1 ka. During this stabilization period, however, reactivation occurred in at least one locality within the dune field at 5.1 ka. Subsequent aeolian activity occurred at 4.1 ka and between 1.0 ka and 0.4 ka. The resulting activation chronology is compared with those obtained from elsewhere in Wyoming and from other west-central Great Plains dune fields.

Paleoearthquake Recurrence on the East Paradise Fault Zone, Metropolitan Albuquerque, New Mexico

A fortuitous exposure of the East Paradise fault zone near Arroyo de las Calabacillas has helped us determine a post-middle Pleistocene history for a long-forgotten Quaternary fault in the City of Albuquerque, New Mexico. Mapping of two exposures of the fault zone allowed us to measure a total vertical offset of 2.75 m across middle Pleistocene fluvial and eolian deposits and to estimate individual surface-faulting events of about 1, 0.5, and 1.25 m. These measurements and several thermoluminescence ages allow us to calculate a long-term average slip rate of 0.01 ± 0.001 mm/yr and date two surface-faulting events to 208 ± 25 ka and 75 ± 7 ka. The youngest event probably occurred in the late Pleistocene, sometime after 75 ± 7 ka. These data yield a single recurrence interval of 133 ± 26 ka and an average recurrence interval of 90 ± 10 ka. However, recurrence intervals are highly variable because the two youngest events occurred in less than 75 ka. Offsets of 0.5-1.25 m and a fault length of 13-20 km indicate that surface-rupturing paleoearthquakes on the East Paradise fault zone had probable M s or M w magnitudes of 6.8-7.0. Although recurrence intervals are long on the East Paradise fault zone, these data are significant because they represent some of the first published slip rate, paleoearthquake magnitude, and recurrence information for any of the numerous Quaternary faults in the rapidly growing Albuquerque-Rio Rancho metropolitan area. Manuscript received 17 June 1999.

User guide for luminescence sampling in archaeological and geological contexts

Abstract Luminescence dating provides a direct age estimate of the time of last exposure of quartz or feldspar minerals to light or heat and has been successfully applied to deposits, rock surfaces, and fired materials in a number of archaeological and geological settings. Sampling strategies are diverse and can be customized depending on local circumstances, although all sediment samples need to include a light-safe sample and material for dose-rate determination. The accuracy and precision of luminescence dating results are directly related to the type and quality of the material sampled and sample collection methods in the field. Selection of target material for dating should include considerations of adequacy of resetting of the luminescence signal (optical and thermal bleaching), the ability to characterize the radioactive environment surrounding the sample (dose rate), and the lack of evidence for post-depositional mixing (bioturbation in soils and sediment). Sample strategies for collection of samples from sedimentary settings and fired materials are discussed. This paper should be used as a guide for luminescence sampling and is meant to provide essential background information on how to properly collect samples and on the types of materials suitable for luminescence dating.