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The composition of sandstone In the upper Miocene and Pliocene Etchegoln Formation In the vicinity of the southern Dlablo Range is highly variable. Results of assays of framework grains and heavy minerals in Etchegoin sandstone from four stratigraphic sections, three in the Coalinga region and one in the San Benito region, suggest that many 11thologically distinct types of detritus are found in Etchegoin strata. The detritus is identified on the basis of the relative abundance of quartz (Q), volcanic rock fragments (VRF), and metamorphic rock fragments (MRF) in the framework composition, and on the relation of these framework components to heavy mineral associations, which are Inferred primarily from factor analysis. Three distinct types of plutonic detritus are found in sandstone of the Etchegoin and they are characterized by (1) Q found in association with green and brown hornblendes, (2) Q found in association with green and brown hornblendes, zircon, sphene, epldote, and garnet, and (3) 0 associated with green and brown hornblendes, epidote, and zircon. Three types of volcanic detritus are found in the sandstone, and they are characterized by (1) VRF associated with augite and brown hornblende, (2) VRF associated with hypersthene and brown hornblende, and (3) severely altered VRF not associated with volcanic accessory minerals. Metamorphic detritus is found in moderate amounts in sandstone of the Etchegoin, and it is characterized by MRF associated with actinolitetremolite and glaucophane. Plutonic detritus characterized by Q associated with hornblende, zircon, sphene, epidote, and garnet and severely altered volcanic detritus are most abundant in sandstone in the Sulphur Creek region, and they are rare, in the Etchegoin Formation in the Coalinga region. Provenance of the Etchegoin Formation is inferred on the basis of the lithology and spatial distribution of detritus found within the sampled strata. The suite of detrital grains characterized by Q associated with green and brown hornblendes is Interpreted to have been derived from hornblende-quartz-gabbroic rocks adjacent to the VergalesZayante fault, most of which are now covered by post-Pliocene rocks. The detritus characterized by Q associated with green and brown hornblendes, zircon, sphene, epidote, and garnet contains many grains with morphologic characteristics suggestive of a reworked origin. This detritus is similar in heavy mineral content to that found in sandstone in the Santa Margarita Formation along Anticline Ridge, from which it probably was derived. A Santa Margarita Formation source for Etchegoin detritus is supported by the presence of coarse-grained Santa Margarita detritus, Crassostrea fragments and felsic volcanic clasts, which are found as clasts in Etchegoin conglomerate. Detritus characterized by 0 associated with green and brown hornblendes, epidote, and zircon is found in some sandstone of the Etchegoin in the Coalinga region. This detritus may have been derived from the Sierra Nevada. A Sierra Nevada source is suspected because green and brown hornblende, epidote, and zircon are common in the mafic and intermediate plutonic rocks of the Sierra Nevada, and Sierran plutonic detritus is expected to be found in Etchegoin strata because east of the Coalinga region Etchegoin strata grade laterally into the plutonlc-rlch rocks of the Kern River Formation. Kern River strata are thought to have been deposited as alluvial fans built westward into the San Joaquin basin from the plutonic basement of the Sierra Nevada. The volcanic detritus characterized by VRF associated with auglte and brown hornblende or hypersthene and brown hornblende was derived from the Mehrten Formation and from coeval volcanic rocks at the crest of the Sierra Nevada in central California. A Sierra Nevada source is likely, because (1) the mineralogy of volcanic detritus in Etchegoin strata is similar to that found in the Mehrten Formation, and (2) the vertical succession of augite-rich to hypersthene-rlch volcanic detritus found in Etchegoin strata in the Coalinga region occurs also in the upper Miocene volcanic rocks at the crest of the Sierra Nevada. The severely altered VRF found in Sulphur Creek sandstone is attributed to detritus recycled from volcanic-rich Franciscan rocks. A Franciscan source is inferred because altered mafic volcanic detritus is abundant in the nearby weakly metamorphosed to blueschlst facies volcanic-rich strata in the Franciscan in the Dlablo Range. Altered or metamorphosed mafic volcanic and intrusive rocks are found locally in the Franciscan, and they may have also.contributed detritus to sandstone of the Etchegoin. The metamorphlc detritus characterized by MRF associated with actinolitetremolite and glaucophane probably was derived from blueschist facies Franciscan rocks in the Diablo Range. Provenance of the Etchegoin Formation reflect major paleogeographic changes that occurred during the late Miocene and Pliocene in central California. Prior to the deposition of Etchegoin strata, the late Miocene of central California was characterized by two marine embayments: one in the San Francisco Bay region, the other in the central and southern parts of the San Joaquin Valley. At this time, the Gabilan Range, Santa Cruz Mountains region, and the central part of the Diablo Range were emergent. The paleogeographic changes that accompanied onset of deposition of Etchegoin strata include uplift in the northeastern and southern parts of the Diablo Range, and submergence of the Gabilan Range and most of the Santa Cruz Mountains region. Uplift in the northeastern part of the Diablo Range was responsible for changing the locus of deposition of Sierran volcanic detritus from the San Francisco Bay region to the San Joaquin Valley. Emergence of the southern Diablo Range is reflected by the presence of Santa Margarita Formation detritus as framework components and clasts in Etchegoin strata. Submergence of the Gabilan Range and most of the Santa Cruz Mountains region is inferred, because detritus characteristic of these regions is lacking in Etchegoin strata. However, the Gabilan Range probably remained a subsea high that prevented Santa Lucia Range detritus from entering the San Joaquin basin. The latter stage of deposition of Etchegoin strata was characterized primarily by continued deposition of Sierran and Diablo Range detritus in a restricted San Joaquin basin. The San Joaquin basin became restricted as a result of a closure of the southern seaway at about 6 Ma, and from a partial blockage of the northern seaway, outboard of the Coalinga region, by the Gabilan Range subsea high. The Gabilan Range subsea high was positioned outboard of Coalinga as a result of movement along the San Andreas

Provenance of the upper Miocene and Pliocene Etchegoin Formation; implications for paleogeography of the late Miocene of Central California