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ABSTRACT

The Great Valley sequence of the eastern Diablo Range and northern San Joaquin Valley consists of a thick accumulation of marine and nonmarine clastic rocks of Jurassic to early Paleocene age deposited in a forearc basin that was situated between the Sierran magmatic arc to the east and the Franciscan subduction complex to the west. In the western part of the basin, the sequence rests conformably on the Jurassic Coast Range Ophiolite or is faulted against the structurally underlying Franciscan Complex. Beneath the eastern San Joaquin Valley, the sequence unconformably onlaps igneous and metamorphic rocks of the Sierran magmatic arc. The sequence generally thickens westward to as much as 8-9 km in the Diablo Range, where it is unconformably overlain by late Paleocene and younger strata.

The stratigraphy of the Great Valley sequence has been the subject of much work, but problems, particularly nomenclatural, remain. Lithostratigraphic subdivisions of the sequence have not gained widespread acceptance because of the lenticularity of most sandstone bodies, abrupt facies changes in subsurface and outcrops, and the lack of detailed subsurface information from closely spaced or deep wells. Although microfossils provide the best means of regional correlation, their locally sparse distribution, together with post-deposiiional dissolution in outcrop sections, limits their usefulness. Sequence stratigraphy, in conjunction with well-log correlations, has great potential for stratigraphic analysis.

Five outcrop sections in the Diablo Range between Del Puerto Creek on the north and the Avenal Ridge-Reef Ridge area on the south, and a representative subsurface columnar section from the northern San Joaquin Valley, illustrate the diversity of stratigraphic nomenclature, depositional facies, and thickness variations of the Great Valley sequence. Although rocks as old as Tithonian (Late Jurassic) and as young as Danian (early Paleocene) are present locally, the greatest outcrop thickness of Great Valley sequence strata is of Cenomanian to Maestrichtian (Late Cretaceous) age. Although there is, in a general sense, some correlation between plate-tectonic events and changes in forearc basin sedimentation, specific causes for unconformities in the sequence remain unclear.

Outcrops of the Great Valley sequence in the Diablo Range consist mostly of submarine-fan, basin-plain, and slope deposits; the upper part of the sequence contains significant shelf and deltaic deposits. More extensive slope, shelf, and deltaic deposits underlie the San Joaquin Valley to the east. Paleocurrent data from outcrops, facies distribution patterns, and provenance information suggest predominantly westward or southwestward transport of sediments from the Sierran magmatic arc source area. The petrofacies generally reflect the petrologic evolution of the arc terranee and unroofing of Sierran batholiths in the late Mesozoic.

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