The Corona South Quadrangle lies in the northern part of the Peninsular Ranges. It contains the northwest end of the Elsinore trough which marks the trace of the Elsinore fault zone and trends northwestward across the center of the quadrangle. The Chino fault diverges north-northwestward from its apparent junction with the Elsinore fault near the eastern margin of the quadrangle.

The oldest rocks, those of the Triassic Bedford Canyon formation, are meta-sedimentary in character, and occur mainly in the Santa Ana Mountains southwest of the Elsinore fault. Here they have been intruded by late Mesozoic plutonic rocks of the Southern California batholith and in a few places are unconformably overlain by Jurassic(?) volcanic rocks. Northeast of the Elsinore fault is a succession of sedimentary rocks, several thousand feet thick, which includes rocks of Upper Cretaceous through Quaternary age. These units are of types found on the west in the Los Angeles Basin.

The dominant structural features are the Elsinore and Chino zones of high-angle reverse dip-separation faults and a synclinal trough which extends from the Puente-Chino Hills southeastward beyond Corona and lies nearly parallel with and northeast of the Elsinore fault. The surface distribution of Cretaceous sedimentary rocks indicates a probable minimum vertical displacement of 1,500 feet along the Elsinore fault southwest of Corona, whereas on the southeast a displacement of more than 5,000 feet is suggested by the difference in elevation of the basement complex on opposite sides of the fault. In post-Pliocene time, and perhaps throughout their history, the Elsinore and Chino fault zones have had an apparent high-angle reverse sense of movement. Large lateral displacement is not demo strated in the mapped area. In the Corona area the Elsinore trough is probably a faulted syncline.

Mining operations, especially for clay, crushed and broken stone, and glass sand, have been

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carried on since the late nineteenth century. Repeated though unsuccessful attempts have been made to find petroleum.

Records indicate 15 wildcat oil wells have been drilled within the quadrangle since 1925. Of these wells, 12 were put down in the period 1949-56. Several wells may have encountered noncommercial gas and oil shows, but there has been no sustained production.

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