Abstract

A late Paleocene -early Eocene submarine canyon and fan complex, the Meganos Formation (or “Channel”) is exposed in the homoclinal sequence of Mesozoic to late Eocene age sediments that form the northern flank of Mount Diablo. Along this outcrop, the Meganos Formation comprises a narrow belt, 16 km long and one km wide. Exposures of the canyon-fill mudstones are largely masked by the alluvium of Deer Valley, but the resistant sandstones and conglomerates of the fan are well exposed along the southwest ridge flanking the valley.

Canyon fed sediments spilled onto the late Paleocene -early Eocene basin floor(?) and subsequently filled the lower canyon. These sediments may be divided into two facies: (1) a lower submarine fan facies of coarse-grained clastic material; and (2) an upper mudstone canyon-fill sequence. The submarine fan facies is particularly well exposed from its basal contact, which unconformably overlies the lower Paleocene Martinez Formation, to the base of the overlying mudstone or canyon-fill facies. Within this basal sequence, fan channels are filled with disorganized conglomerates and pebbly sandstones. Boulders over 2 m in diameter are the largest clasts found in the basal channel-fills near Oil Creek. The canyon fan sequence fines vertically upwards in general aspect from basal inner fan channels to the thin sandstones of the outer fan facies and finally the mudstone of the basin plain(?) and fill.

Based upon palinspastic reconstructions a minimum wall height of the submarine canyon in the vicinity of the inner fan is estimated to have been 800 m. The submarine fan was 450 m thick and the overlying mudstone of the canyon-fill facies was at least 675 m thick.

The series of north-trending high angle normal faults that cut the outcrop belt are dated as Maastrichtian-earliest Paleocene to middle Eocenein age. Palinspastic restoration of Paleocene and lower Eocene units demonstrates uplift to the east, west and possibly south during this time. Uplift related to the normal faulting controlled the position of the submarine fan system and possibly the canyon. The location of the early Paleogene Meganos and underlying Martinez canyon-fan complexes and younger (Eocene) canyons in the deepest portion of southwest tilted graben that forms the Sacramento basin suggests regional and long term tectonic control. Perhaps this southern basin edge was bounded by an ancestral Stockton Arch-Mt. Diablo uplift. This major tectonic feature may be the expression of a change of subduction rate or direction of the Early Tertiary trench system.