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Southern California Areal Mapping Project

 

Geologic Framework of the Twentynine Palms Marine Corps Air Ground Combat Center

| Selected References |

Geologic-mapping and Geologic-framework Investigations

In 1993-1995 SCAMP conducted geologic-mapping investigations on the Twentynine Palms Marine Corps Air/Ground Combat Center (MCAGCC). The mapping supported ongoing land-use management analyses conducted by MCAGCC staff in preparation for expanded base operations, and was integrated with hydrogeologic investigations conducted at MCAGCC by the USGS Water Resources Division Desert/Eastern Sierra Area Program (San Diego office) and with soil-survey investigations conducted by the Natural Resources and Conservation Service.

 

The primary goal of the MCAGCC geologic-mapping studies was to provide a three-dimensional geologic framework for evaluating geologic hazards and ground-water resources. Such evaluations require an accurate portrayal of how various geologic materials are distributed geographically and in the subsurface, and of how geologic structures such as faults and folds disturb the geologic materials. When combined with geophysical and hydrogeologic information, the MCAGCC geologic-map data can be used to interpret such factors as earthquake ground-rupture patterns, ground-shaking intensity patterns, the susceptibility of various geologic units to shaking-induced ground deformations, the shape and volume of ground-water-bearing sedimentary basins, and basin-recharge and basin-contamination pathways.

Findings

SCAMP investigations were presented to the MCAGCC in two administrative reports (Matti and Morton, 1994, 1995) that included geologic maps of parts of the Twentynine Palms, Deadman Lake SE, and Deadman Lake SW 7.5' quadrangles. These reports can be examined at the Natural Resources and Environmental Affairs Division of the Installations and Logistics Directorate at MCAGCC.

The mapping provided a new look at rocks and structures that otherwise form a large void in detailed geologic-map coverage of the northeastern Big Bear Lake 30' x 60' quadrangle. The studies provided information about the distribution and history of Jurassic plutonic terranes, Late Oligocene and Miocene volcanogenic rocks, late Cenozoic valley-filling sedimentary units, and Quaternary deformation patterns among right-lateral strike-slip faults of the Mojave Desert shear zone that farther west in the Big Bear Lake quadrangle generated the June 1992 M=7.3 Landers earthquake.

Specific findings include:

 

  • In the Twentynine Palms and Deadman Lake SE quads we clarified the distribution and tectonic setting of the Mesquite Lake fault, the youngest right-lateral strike-slip structure in this part of the eastern California shear zone. We extended the distribution of the fault farther northwest than previously known, and showed that surficial traces of the fault disappear in the vicinity of a tightly folded and complexly faulted structural dome in young Quaternary surficial materials. Relations suggest that right-slip on the Mesquite Lake fault steps left across the dome and is transferred to northwest-oriented faults a few km to the west in the Deadman Lake SW quad;
  • We identified previously unreported tectono-stratigraphic evidence for youthful late Quaternary displacements on the right-lateral Bullion fault, and developed a model where right slip on this structure has been succeeded by right slip on the more youthful Mesquite Lake fault to the west.
  • We found surficial evidence for several new faults and for several faults previously suspected on the basis of geophysical and (or) ground-water data. These structures include extensional dip-slip faults whose tectonic role must be factored into neotectonic models for the eastern California shear zone.
  • We successfully subdivided the Quaternary alluvial succession for a part of the southwestern Mojave Desert Province where Quaternary surficial materials previously had been undivided.

Recent SCAMP Products

Matti, J.C., and Morton, D.M., 1994, Preliminary geologic map of part of the U.S. Marine Corps Twentynine Palms Air/Ground Combat Center, California: U. S. Geological Survey Administrative Report, 45 p., scale 1:24,000. This product responds to funding provided by the U.S. Marine Corps in support of ground-water investigations led by USGS Water Resources Division in the Marine Corps Air/Ground Combat Center.

Matti, J.C., and Morton, D.M., 1995, Preliminary geologic map of the Deadman Lake SW 7.5' quadrangle, U.S. Marine Corps Air/Ground Combat Center, Twentynine Palms, California: U.S. Geological Survey Administrative Report, scale 1:24,000. This product responds to funding provided by the U.S. Marine Corps in support of ground-water investigations led by USGS Water Resources Division in the Marine Corps Air/Ground Combat Center.


Subsurface Investigations

In FY 2000, SCAMP is working cooperatively with the USGS Water Resources Division Desert/Eastern Sierra Program (San Diego Office) to examine the subsurface stratigraphy and physical properties of sedimentary materials in the Surprise Spring and Mainside Headquarters areas of the MCAGCC. These studies will be integrated with geologic-map information to yield a three-dimensional framework of the subsurface geology as it relates to ground-water flow.

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