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ABSTRACT

Unmetamorphosed strata of Turonian to middle Miocene age exposed around the margins of the Los Angeles basin (LAB) predate the formation of the basin and respond to a different structural framework. Highly organic middle and late Miocene (Luisian and Mohnian) strata also predate the present LAB framework: the Puente Hills received a thicker sequence than did the area southwest of the Whittier fault, and the Santa Monica Mountains were at the distal end of Mohnian turbidites derived from farther north. The central trough became a major depocenter during deposition of the upper Mohnian (after 8 Ma), and it achieved its present northwest-southeast trend about 4 Ma. The central trough filled with “Delmontian” and Repettian turbidites shallowing upsection to Pliocene and Pleistocene deposits. Miocene and early Pliocene source rocks were buried beneath the oil-generating thermal threshold so that oil migrated to stratigraphic and broad structural traps formed during deposition. Many oil fields contain a thick stack of reservoir turbidites; the boundary between highly productive turbidites and overlying water-bearing turbidites, also in trapping position, is abrupt. Late Quaternary deformation after basin filling distorted rather than enhanced oil traps, and some oil accumulations were breached by erosion. New oil prospects could result from a better understanding of (1) the Mohnian basin framework in contrast to the far different post- Mohnian framework and (2) the post-Mohnian fold and thrust belt tectonicaUy loaded by the southern margin of the Transverse Ranges onshore and offshore.

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