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Description
The Santiago Peak volcaniclastic rocks are composed of volcanically derived sedimentary rocks made up of interbedded breccias, sandstones, siltstones and mudstones that were deposited in a marine environment. The largest outcrop exposures of the Santiago Peak volcaniclastic rocks are found in three stream canyons located west of the main body of the Santiago Peak Volcanics in western San Diego County. Detailed stratigraphic section measurements and petrographic examinations were undertaken to determine the depositional environment, the petrology, and provenance of the Santiago Peak volcaniclastic rocks. The exposures in the small canyon north of La Zanja Canyon, in Los Peñasquitos Canyon, and in Lusardi Canyon were found to have similar petrologies, while their individual sedimentary sequences revealed different environments of deposition. Los Peñasquitos Canyon contains a series of massive, andesitic-dacitic, granule to boulder breccias, interbedded feldspathic andesitic litharenites, minor pyroxene andesite, and mudstone. Deposition of these rocks was in the inner (upper) fan to middle fan channeled portions of a submarine fan. In the small canyon north of La Zanja Canyon the rocks are composed predominantly of interbedded siltstones and feldspathic andesitic-dacitic litharenites, which were deposited as turbidites in a submarine fan system in the transitional regions of the middle fan fringe or depositional lobe environment and the outer fan environment. The exposures in Lusardi Canyon are almost entirely made of interbedded mudstones and siltstones. A minor sequence of andesitic-dacitic, pebbly breccias and andesitic litharenites may be the result of smallscale channeling or slumping of coarser materials into the lower slope or basin-plain environments represented by the mudstones and siltstones. The presence of minor amounts of metamorphic and plutonic rock fragments signifies the minor input of these grains from either an older, pre-volcanic source terrane or from the contemporaneous erosion into the heart of the magmatic arc. Petrologically, the Santiago Peak volcaniclastic rocks have essentially the same composition as the Santiago Peak Volcanics, and are considered to be derived from them. Paleocurrent and paleoslope data are usually ambiguous; however, based upon the physical association of the Santiago Peak Volcanics being located east of the exposures of the volcaniclastic rocks, a simplified east to west direction of transport and deposition might be indicated. If the three canyon exposures of the volcaniclastic rocks are taken to represent portions of a single fan system, then a northerly sense of direction may also be indicated, with the Los Peñasquitos Canyon exposures representing the "apex" of the fan and the exposures in Lusardi Canyon representing the "distal(?)" slope to basin-plain environments. During the Late Jurassic large quantities of volcanic detritus were shed in a westerly to northerly direction from an active volcanic island arc system, as turbidity currents and other sediment gravity flows. These turbidites, the Santiago Peak volcaniclastic rocks, were deposited in submarine fan-type environment located within the trench slope portions of the island arc system.
