Geology of Monterey Canyon, California

Abstract

New data from marine dredgings off Monterey, California, correlated with wells and outcrops on land indicate that Monterey, Carmel, and Soquel submarine canyons are related closely to the geological history of the continent. Upper Pliocene siltstone and sandstone bodies in Monterey Canyon correlate with the Pomponio Member of the Purisima Formation in the Santa Cruz Mountains. Granodiorites exposed along the south slope of Monterey Canyon and the east slope of Carmel Canyon correlate with lower Upper Cretaceous Santa Lucia granodiorite on land. Siliceous siltstone, highly-indurated micritic limestone, and metamorphic rocks comprise the west slope of Carmel Canyon. These siltstone and limestone beds contain silicoflagellates, radiolarians, and middle Miocene diatoms, but no Foraminifera. Quartzite from Carmel Canyon probably correlates with the pre-Cretaceous Sur series. A seaward extension of Sur (Nacimiento) and Palo Colorado faults separates sedimentary and metamorphic rocks from igneous ones in Carmel Canyon; it probably continues northward to connect with the San Gregorio fault in the Santa Cruz Mountains. Monterey fault in Monterey Canyon separates upper Pliocene rocks on one side from granodiorites on the other. The sedimentary rocks in Monterey Canyon contain bathyal foraminifers stratigraphically 400 ft higher than shallow-water Pliocene rocks in the lower Salinas Valley. These relations suggest vertical movement of more than 1,000 ft on a northward extension of the Reliz fault which separates the two areas. A seaward extension of the Carmel Valley fault controls the nearshore east-west trend of Carmel Canyon and the Carmel Canyon fault (seaw rd extension of the Sur and Palo Colorado faults) controls the northwest trend of Carmel Canyon. Canyon erosion occurred in zones of poor induration and/or weakness along faults and contacts between sedimentary and igneous rocks having diverse induration. Erosion was effected by turbidity currents, by mass movement of sediment down canyon axes, and perhaps was aided by alteration of granodiorite to clay. The middle Miocene Salinian deformation resulted in more than 15,000 ft of subsidence in Monterey graben, which underlies the shelf at the north side of Monterey Bay. Additional structural features such as Monterey, Gabilan, Carmel Canyon, and Carmel Valley faults either formed at this time or were reactivated. Geomorphic features termed the erosion surface and were formed also at this time. Late Miocene, Pliocene, and early Pleistocene drainage from the Great Valley of California debouched at Monterey Bay via Elkhorn Slough at the head of the present Monterey Canyon. All canyon heads were cut or modified subaerially to an approximate depth of 300 ft below present sea level. Monterey and Soquel Canyons were cut during late Pliocene or early Pleistocene time. Carmel Canyon and Ascension Canyon initially may have been eroded earlier; they may have funneled sediments seaward before Pleistocene time. Monterey Canyon and Elkhorn Slough lie directly above the buried middle Miocene Pajaro gorge. The older canyon is not ancestral to Monterey Canyon, but differential compaction in the gorge fill may have directed rivers to reach the sea above the gorge. A sediment trap may have been eroded at the mouth of the slough whereby both littoral-drifted and river-transported sediments were deflected seaward to aid in the erosion of Monterey Canyon.