Abstract

The San Francisco Bay region between the San Andreas fault and the Great Valley is a terrane of late Mesozoic Franciscan rocks structurally overlain by five major sedimentary units, which, in ascending order, are: (1) upper Jurassic(?)-Cretaceous (Great Valley) marine sequence, (2) lower Tertiary marine sequence, (3) middle Tertiary marine sequence, (4) upper Tertiary terrestrial sequence, and (5) late Cenozoic Valley deposits.

The upper Jurassic and Cretaceous sequence was derived from an eastern granitic terrane and was deposited in an open sea under which a rapidly subsiding trough formed along the site of the west side of the Great Valley, where as much as 8300 m (27,000 ft) of sediments accumulated. The lower Tertiary sequence was also deposited in an open sea but was preceded near the site of the San Andreas fault by emergence and erosion that in places exposed the Franciscan Complex.

Deposition of the middle Tertiary sequence was preceded in Oligocene time by widespread diastrophism that caused marine withdrawal and exposed the Franciscan Complex from Santa Clara Valley northwestward and also in the Diablo Range, leaving an intervening strip of synclinally folded overlying strata in the Berkeley Hills area and southeastward. This sequence started with deposition of siliceous sediments and sands in Miocene time as the sea transgressed eastward, nearly as far as the western margin of the Great Valley, then regressed westward.

Deposition of the upper Tertiary terrestrial sequence in the Livermore Valley-Berkeley Hills area and eastward was preceded by renewed uplift of the Franciscan terrane northwestward from Santa Clara Valley and that of the Diablo range, which supplied detritus to this sequence. In late Cenozoic time valley deposits accumulated between strips of terrane squeezed up along the San Andreas and related faults.

Small gas fields and one oil field on the north flank of the Mt. Diablo uplift and the discovery of a small oil field in the adjacent Livermore basin suggest that other hydrocarbon accumulations may occur in this basin.