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At least 793 records · Page 44Linked to original sources
USGS2009

Geologic Map of the Atlin Quadrangle, Southeastern Alaska

This map presents the results of U.S. Geological Survey (USGS) geologic bedrock mapping studies in the mostly glacier covered Atlin 1:250,000-scale quadrangle, northern southeastern Alaska. These studies are part of a long-term systematic effort by the USGS to provide bedrock geologic and mineral-resource information for all of southeastern Alaska, covering all of the Tongass National Forest (including Wilderness Areas) and Glacier Bay National Park and Preserve. Some contributions to this effort are those concerned with southwesternmost part of the region, the Craig and Dixon Entrance quadrangles (Brew, 1994; 1996) and with the Wrangell-Petersburg area (Brew, 1997a-m; Brew and Grybeck, 1997; Brew and Koch, 1997). As shown on the index map (fig. 1), the study area is almost entirely in the northern Coast Mountains adjacent to British Columbia, Canada. No previous geologic map has been published for the area, although Brew and Ford (1985) included a small part of it in a preliminary compilation of the adjoining Juneau quadrangle; and Brew and others (1991a) showed the geology at 1:500,000 scale. Areas mapped nearby in British Columbia and the United States are also shown on figure 1. All of the map area is in the Coast Mountains Complex as defined by Brew and others (1995a). A comprehensive bibliography is available for this and adjacent areas (Brew, 1997n).

Scientific Investigations Map↗
USGS1999

Preliminary geologic map of the San Guillermo Mountain Quadrangle, Ventura County, California

New 1:24,000-scale geologic mapping in the Cuyama 30' x 60' quadrangle, in support of the USGS Southern California Areal Mapping Project (SCAMP), is contributing to a more complete understanding of the stratigraphy, structure, and tectonic evolution of the complex junction area between the NW-striking Coast Ranges and EW-striking western Transverse Ranges. The 1:24,000-scale geologic map of the San Guillermo Mountain quadrangle is one of six contiguous 7 1/2' quadrangle geologic maps in the eastern part of the Cuyama map area being compiled for a more detailed portrayal and reevaluation of geologic structures and rock units shown on previous geologic maps of the area (e.g., Dibblee, 1979). The following observations and interpretations are based on the new San Guillermo Mountain geologic compilation: (1) The new geologic mapping in the northern part of the San Guillermo Mountain quadrangle allows for reinterpretation of fault architecture that bears on potential seismic hazards of the region. Previous mapping had depicted the eastern Big Pine fault (BPF) as a northeast-striking, sinistral strike-slip fault that extends for 30 km northeast of the Cuyama River to its intersection with the San Andreas fault (SAF). In contrast the new mapping indicates that the eastern BPF is a thrust fault that curves from a northeast strike to an east strike, where it is continuous with the San Guillermo thrust fault, and dies out further east about 15 km south of the SAF. This redefined segment of the BPF is a south-dipping, north-directed thrust, with dominantly dip slip components (rakes > 60 deg.), that places Middle Eocene marine rocks (Juncal and Matilija Formations) over Miocene through Pliocene(?) nonmarine rocks (Caliente, Quatal, and Morales Formations). Although a broad northeast-striking fault zone, exhibiting predominantly sinistral components of slip (rakes < 45 deg.), extends to the SAF as previously mapped, the fault zone does not connect to the southwest with the BPF but instead curves into a southwest-directed thrust fault system a short distance north of the BPF. Oligocene to Pliocene(?) nonmarine sedimentary and volcanic rocks of the Plush Ranch, Caliente, and Morales(?) Formations are folded on both sides of this fault zone (informally named the Lockwood Valley fault zone [LVFZ] on the map). South-southeast of the LVFZ overturned folds have southward vergence. Several moderate-displacement (< 50 m), mainly northwest-dipping thrust and reverse faults, exhibiting mostly sinistral-oblique slip, flank and strike parallel to the overturned folds. The fold vergence and thrust direction associated with the LVFZ is opposite to that of the redefined BPF, providing further evidence that the two faults are distinct structures. These revised fault interpretations bring into question earlier estimates of net sinistral strike-slip displacement of as much as 13 km along the originally defined eastern BPF, which assumed structural connection with the LVFZ. Also, despite sparse evidence for repeated Quaternary movement on the LVFZ (e.g., Dibblee, 1982), the potential for a large earthquake involving coseismic slip on both the LVFZ and the central BPF to the southwest may not be as great as once believed. (2) Several generations of Pleistocene and younger dissected alluvial terrace and fan deposits sit at various levels above modern stream channels throughout the quadrangle. These deposits give testimony to the recent uplift and related fault deformation that has occurred in the area. (3) A vast terrane of Eocene marine sedimentary rocks (Juncal and Matilija Formations and Cozy Dell Shale) exposed south of the Big Pine fault forms the southern two-thirds of the San Guillermo Mountain quadrangle. Benthic foraminifers collected from various shale intervals within the Juncal Formation indicate a Middle Eocene age (Ulatisian) for the entire formation (K. McDougall, unpub. data, 1998) and deposition at paleodepths as great as 2,000 m (i.e., lowe

California↗