The Spreckels quadrange is located south of Salinas, CA, and is located along the valley and mountains visible in the left half of this photograph. Toro Regional Park at top center is covered in clouds in this photograph. Fremont Peak is primarily made of Paleozoic to Jurassic metamorphic rocks intruded by Cretaceous granitic rocks. The Toro Regional Park is primarily underlain with Tertiary continental sedimentary rocks and Cretaceous granitic rocks. The photograph was taken by Oliver Bowen looking southwest towards Salinas (at right center) from Fremont Peak in 1954.
The Spreckels quadrangle lies at the north end of the Sierra de Salinas and extends from the Salinas Valley on the northeast across Los Laurelles Ridge south to Carmel Valley, an intermontane valley that separates the Santa Lucia Range from the Sierra de Salinas (fig. 1). The Toro Regional Park occupies the east-central part of the quadrangle, whereas the former Fort Ord Military Reservation covers the northwestern part of the area and is the probable locus of future development. Subdivisions largely occupy the older floodplain of Toro Creek and the adjacent foothills, with less dense development along the narrower canyons of Corral de Tierra and San Benancio Gulch to the south. The foothills southwest of the Salinas River are the site of active residential development.
Geologically, the study area has a crystalline basement of Upper Cretaceous granitic rocks of the Salinian block and older metasedimentary rocks of the schist of the Sierra de Salinas of probable Cretaceous age. Resting nonconformably upon these basement rocks is a sedimentary section that ranges in age from middle Miocene to Holocene and has a composite thickness of as much as 1,200 m. One of the purposes of the present study was to investigate the apparent lateral variation of the middle to upper Miocene sections from the typical porcelaneous and diatomaceous Monterey Formation of the Monterey and Seaside quadrangles to the west (Clark and others, 1997) to a thick marine sandstone section in the eastern part of the Spreckels quadrangle.
Liquefaction, which seriously affected the Spreckels area in the 1906 San Francisco earthquake (Lawson, 1908), and landsliding are the two major geological hazards of the area. The landslides consist mainly of older large slides in the southern and younger debris flows in the northern part of the quadrangle.
The geologic map shows the distribution of the crystalline basement rocks, the Tertiary rocks, and the Quaternary deposits together with the geologic structure of the Spreckels quadrangle. The distribution of basement rocks is primarily from Ross (1976a).
Approximately three weeks of fieldwork in March and May of 1998 by Clark, Brabb, and Rosenberg resulted in local modification of Ross' (1976a) basement rock distribution, in mapping of the Tertiary deposits and of the structure of the area, and in modification of the earlier mapping of the Quaternary deposits by Dupré (1990a) and of an unpublished map compilation by Rosenberg.
The landslide distribution was mapped mainly by Brabb in June 1998 primarily from U.S. Soil Conservation Service (SCS) aerial photos taken on May 14, 1971, mission ABG 1MM, approximate scale 1:20,000 and secondarily from partial coverage of the quadrangle by SCS photos taken on October 25, 1937 and on August 1, 1956; from U.S. Geological Survey Geologic Division color photos taken on July 18, 1974, scale 1:36,000; and from a brief field check during the week of March 16, 1998.
The first comprehensive geologic study of the area was by Herold (1935), whose geologic map (scale 1:62,500) of the Salinas 15-minute quadrangle includes the Spreckels 7.5-minute quadrangle. Cassell's (1949) study of the Monterey Formation includes a map (scale 1:31,250) that extends into the western part of this quadrangle. Bowen (1965, 1969) mapped the geology of the quadrangle at 1:24,000 and described the Tertiary stratigraphy and oil possibilities of the area. Dibblee's (1973) mapping of the Salinas 15-minute quadrangle modified Bowen's earlier mapping. Ross' (1976a) geologic map (scale 1:125,000) of the pre-Cenozoic basement rocks includes the area of this study.
Barron (1976) described the diatoms from the Monterey Formation of the western part of the quadrangle, and Govean and Garrison (1981) described and interpreted the depositional environment of the diatomite beds from this same Monterey section. Dupré (1990a) mapped the distribution and liquefaction susceptibility of the Quaternary deposits. Staal, Gardner & Dunne, Inc. (1991a, 1991b) extensively studied the hydrogeology of the Corral de Tierra region.
Files available for downloading:
mf2349a.t.gz A tarred, gzip'd package that contains two PostScript files (skmap.ps and skmf.ps) that make up the map (13.5 MB).
skmap.pdf A PDF file of the map sheet (7.3 MB).
skmf.pdf A PDF file of the accompanying pamphlet for the map (80 kb).
mf2349c.t.gz A tarred, gzip'd package that contains all the Arc/Info coverages that make up the spatial database (including import.aml), a file that describes how to use the database (as both a text-only and PDF file), and the metadata (496 kb).
mf2349d.met.txt An ASCII text file of the FGDC-compliant metadata for this publication (68 kb).
For questions about the scientific content of this report, contact Earl Brabb
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Created: January 1, 2001 (cad)
Updated: April 20, 2007 (bwr, mfd)