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Southern California Geology

Geologic Setting of the Transverse Ranges Province

San Bernardino Mountains

San Gorgonio Pass

View looking east through San Gorgonio Pass. View looking east through San Gorgonio Pass. To the south (right) are Peninsular Ranges-type basement rock of the San Jacinto Mountains; to the north (left) are San Gabriel Mountains-type basement rocks of the San Bernardino Mountains that rise above foothills of Tertiary and Quaternary sedimentary materials. Young Quaternary alluvial deposits occupy the center of San Gorgonio Pass. Photo by J.C. Matti, USGS. Click here for more info and enlarged image .

Geology of the San Gorgonio Pass Region

San Gorgonio Pass is an east-trending lowland that intervenes between the two high-standing masses of the San Bernardino and San Jacinto Mountains. The surface of this lowland is covered by multiple generations of alluvial-fan deposits that mainly are derived from the San Bernardino Mountains.

The northern foothills are underlain by upper Cenozoic nonmarine and marine sedimentary deposits that from oldest to youngest include sandstone and conglomerate of the Hathaway Formation (of Allen, 1957), marine mudstone and sandstone of the Imperial Formation, sandstone and conglomerate of the Painted Hill Formation (of Allen, 1957), and conglomeratic sandstone and conglomerate of the Cabazon Fanglomerate (of Vaughan, 1922, as used by Allen, 1957). Basalt flows and small intrusive bodies occur locally in the sedimentary sequence. The sedimentary section is folded, and is cut by north-dipping low-angle faults and wrench faults of the Quaternary San Gorgonio Pass Fault zone (Matti and others, 1992a). The east-trending right-lateral Banning Fault separates the sedimentary section from crystalline rocks of the San Bernardino Mountains to the north. To the east, crystalline rocks of the San Bernardino Mountains block descend beneath the Coachella Valley and there, north of the Banning Fault, are nonconformably overlain by a second occurrence of nonmarine and marine sedimentary rocks that includes from oldest to youngest the Coachella Fanglomerate (of Vaughan, 1922, as used by Allen, 1957, and Peterson, 1975), the Imperial Formation, and the Painted Hill Formation.

View looking west toward San Gorgonio Pass. View looking west toward San Gorgonio Pass. To the south (left) are Peninsular Ranges-type basement rock of the San Jacinto Mountains; to the north (right) are San Gabriel Mountains-type basement rocks of the San Bernardino Mountains that rise above foothills of Tertiary and Quaternary sedimentary materials. Photo by J.C. Matti, USGS.

Selected References

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This site last updated September 3, 2004 (ps)

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