The proposed study will integrate diverse data sets in order to gain a better understanding of basinal and topographic response to complex faulting from early Pleistocene time to the present. This is best accomplished by combining "traditional" methods of stratigraphic and structural analysis with newer techniques in geomorphology, image processing, and age dating.
The Total Station (TS) is a combined theodolite and EDM (electronic distance measurer), with a built-in computer that allows the angles, distances, and coded information describing the feature being measured to be stored automatically. The TS efficiently and precisely locates a point in space and stores that information with sufficient labels to identify that point so that it can later be plotted. The Wild DIOR 3002S Distomat, the instrument that actually measures the distance to the point, is sufficiently powerful that no reflector is needed for distances out to ~160 m. That ability greatly simplifies surveying in terrain with high topographic relief. All of these features permit detailed analysis of microgeomorphic features, topographic mapping, terrace profiles, and height correlations to be made simply, accurately, and quickly.
In order to tie a local survey made by the TS into the local coordinate system, we use the Wild Global Positioning Satellite (GPS) System 200. The GPS system can set up a temporary network in the field to which the TS can reference. This temporary network of points is itself referenced to the local coordinate system via a Differential GPS survey (DGPS). The DGPS is done using one GPS receiver as the base and everything recorded at the second roving GPS receiver is referenced to it. The system we have in the Geological Sciences Department at Oregon can survey in a variety of modes (static, rapid-static, kinematic, and kinematic on-the-fly), depending the precision required, time limitations, and the general requirements of the job. The GPS system is best used in conjunction with the TS, to set up the local network to which the much faster TS survey is tied.
Using both the TS and GPS, we will analyze the data in the lab on standard IBM-compatible computers. Using a laptop IBM-compatible, we will download, view, and analyze the data after each day of collecting. Once the data have been downloaded, they are stored as Easting, Northing, Height (E, N, H) or X, Y, Z points that can be imported and analyzed in a variety of programs, depending on the type of survey and the features we wish to look at. Features to be analyzed in this way include fault scarps, erosion surfaces, alluvial fans, landslides, and fluvial terraces.
Using LANDSAT and SPOT satellite imagery and DEMs, we have the ability to conduct reconnaissance and interpretive mapping of tectonically active regions (e.g. Fig. 3). To aid in geologic mapping, LANDSAT images can be manipulated and analyzed in the lab using a variety of available programs that presently are available at the University of Oregon. LANDSAT images can be draped over digital elevation models (DEMs) in order to analyze structural and geomorphic complexity and relationships to bedrock variations. Programs that we currently have for the LANDSAT and DEM analysis and manipulation include: Adobe Photoshop™, ArcView GIS™, MicroDem™, Matlab™ and it's Image Analysis Toolbox, NIH Image™, and Explorer™. All of the programs run on either a Macintosh or IBM-compatible platform, except Explorer™ which runs on the SGI Indigo-2s lab that was recently created in the Department of Geological Sciences. We also have the option to obtain Mini Image Processing System (MIPS), which runs on UNIX systems and is public domain software made by the USGS.
Detailed field mapping is an essential component of any investigation of this nature. Mapping will be carried out at scales from 1:5,000 to 1:20,000 on topographic and air-photo bases, and will focus on areas relevant to the problems and questions described above. Features to be mapped include: (1) distribution of crystalline bedrock to look for distinctive compositions that might be correlated with sedimentary deposits; (2) distribution of Pleistocene sedimentary rocks, with emphasis on sedimentary lithofacies, clast composition, lateral facies changes, and interbedded tuffs and vertebrate fossil remains; (3) older faults that cut Pleistocene stratigraphy and crystalline basement rock, with the aim of understanding stratigraphic correlations and fault offsets; (4) geomorphic surfaces, terraces, and late Pleistocene to modern sedimentary deposits including lake muds, alluvial fans, debris cones, and rock avalanche deposits (Fig. 3); and (5) scarps of young and active faults.
Stratigraphic analysis will focus on lower to middle Pleistocene sedimentary rocks (Bautista Beds of Frick, 1921; Sharp, 1967) that are exposed in deeply eroded canyons and gullies in the Coyote badlands (Figs. 2, 3). This will include detailed mapping to determine vertical and lateral changes in sedimentary lithofacies measuring of sections to determine thickness trends, and descriptions of lithofacies to determine depositional processes and environments. Provenance data will be obtained from clast counts, and transport directions will be determined from paleocurrent data such as clast imbrications in conglomerate and cross bedding in sandstone. Reconnaissance field work has shown that Bautista beds are well exposed in the Coyote badlands area between Clark Lake and Coyote Creek (Fig. 3), they are preserved with sufficient continuity to permit mapping of important lateral facies changes, and the 730-Ka Bishop ash is interbedded in the sandstone part of that facies belt. Additional exposures of Bautista beds are reported by Sharp (1967) north of Clark Lake, in close proximity to the possible large rock-avalanche deposits, and north of there along the Buck Ridge fault. Pleistocene stratigraphy will be analyzed with the goal of reconstructing Pleistocene sedimentary basins, depositional systems, sediment-dispersal patterns, paleotopography, and fault-controlled subsidence.
Ar-Ar Geochronology. A distinctive white tuff interbedded with sandstone in Coyote badlands (Fig. 3a) is reported to be the 0.62-Ma Fryant Ash member of the Bishop series, based on a tentative chemical correlation performed by A. Sarna-Wojcicki (1984 unpub data, reported in Remeika and Jefferson, 1995). It is overlain by several other ash beds higher in the same section, none of which have been dated. In the Borrego badlands south of Clark Lake, the Pleistocene Ocotillo and Borrego Formations contain at least 4 interbedded tuffs which have not been dated. Their ages are tentatively constrained by a recent paleomagnetic study to be between ~ 1.0 and 0.5 Ma, and thus are roughly correlative to Bautista beds exposed in the Coyote badlands (Remeika and Beske-Diehl, 1996). Deformed strata in the Borrego badlands represent uplifted and exhumed deposits of a previously subsiding basin whose equivalents are presently subsiding beneath the modern Clark Valley (Dibble, 1984). Ash beds at both localities (Coyote and Borrego badlands) will be dated using high-precision 40Ar-39Ar age dating, to be done by Paul Renne at the Berkeley Geochronology Center (BGC) (see letter of support). Other ash beds identified in mapping will be dated at the BGC. Dating of Pleistocene stratigraphy will resolve questions about absolute ages and may permit calculation of sediment-accumulation and fault-slip rates.
Soil Chronology. Quaternary soils commonly are used to determine relative ages of alluvial surfaces. This is done by measuring soil profiles, describing various soil parameters, and considering additional factors such as annual mean temperature and rainfall parameters to determine the soil development index (Harden, 1982; Reheis et al., 1989; Rockwell et al., 1990; Kendrick et al., 1994). Relative ages are calibrated using 14C ages on charcoal or wood, or by comparison with nearby climatically similar soils that have absolute age controls. Relative soil chronologies will be developed where possible in the study area, and will be used to estimate rates of fault slip and basin exhumation. Dorsey will learn techniques in soil analysis from Greg Retallack (UO Geological Sciences) by auditing graduate classes, reading, and through individual instruction.
Paleomagnetism. Paleomagnetic study of sedimentary rocks is another useful method of age dating. Sandstone and associated fine mudstones exposed in the Coyote badlands are well suited for a study of magnetostratigraphy, to be carried out by John Stimac (see letter of interest and Budget Justification). We will be sampling to find the Matuyama-Brunhes boundary, which is known to occur ~ 50,000 yrs before deposition of the 730 Ka Bishop ash. If this boundary is identified, then it can be used to establish stratigraphic correlations where ash beds are not preserved. In addition, paleomagnetic study will be used to evaluate the possible role of vertical-axis block rotations in post-depositional uplift and basin exhumation.
Vertebrate Paleontology. Vertebrate paleontology can help define stratigraphic ranges in areas where dateable tuffs are absent. George Jefferson of the Anza-Borrego Desert State Park, and Jim Mead of Northern Arizona University, plan to conduct paleontological studies of Pleistocene mammals in the Coyote badlands in the near future. Both parties are interested in working with me on this part of the project, and we anticipate fruitful collaborations over the next couple of years. Recent work has shown that Irvingtonian (early to middle Pleistocene) mammalian faunas can be correlated locally between the Coyote and Borrego badlands (Remeika and Jefferson, 1995; Jefferson and Remeika, 1995). These faunas record open savanna-like conditions with permanent standing water and marshy lacustrine environments, in sharp contrast to modern desert conditions. Collaboration with paleontologists in this study will be important for constraining sediment ages where tuffs are not preserved, and for reconstructing Pleistocene paleoenvironments and paleoclimate which were dramatically different from the present-day regime.
Cosmogenic Isotopes. 10Be is produced in quartz by exposure to cosmogenic rays and radioactive decay of 26Al (Phillips et al., 1986; Lal, 1991; Cerling and Craig, 1994). Cosmogenic age dating is quickly becoming an important tool for determining exposure ages and erosion rates in a variety of tectonically active and stable settings (e.g. Burbank et al., 1996; Siame et al., 1997; Phillips et al., 1997). The 10Be/26Al isotopic ratio in rocks is controlled by the rate of production of 10Be and the rate of surface erosion. It may be possible to date a large granitic landslide deposit at the north end of Clark Lake using cosmogenic isotopes. The existence of the landslide first needs to be tested. If this is the correct interpretation, the age of this rock avalanche will be helpful for determining the geometry and timing of fault movements that formed and cut Clark basin. Assuming that fresh unweathered material was derived from a crystalline source, and that erosion has been slow in this arid setting since emplacement, the 10Be/26Al ratio should yield an age of emplacement. Bob Finkel at the Lawrence Livermore national Laboratory has expressed an interest in pursuing this problem in collaboration with me (see letter of support). We have agreed that a reconnaissance study is first needed to determine the feasibility of dating the landslide. The feasibility study will consist of collecting and dating 3 to 5 samples at the LLNL facility, which Finkel will do free of charge. Processing of samples for cosmogenic dating is intensive and difficult. I will have samples prepared at Paul Bierman's lab at the University of Vermont, and will pay for those costs with funds available from other sources. If the initial dates are promising, Finkel and I will design a more complete research strategy (and a new proposal) to carry out a comprehensive dating study of this and other landslide deposits around the margins of Clark Lake.
Return to San Jacinto Proposal, or Dorsey's Homepage