Upper Tertiary marginal-marine sedimentary rocks of California are important reservoirs and have been studied extensively. However, they have been difficult to analyze in detail in terms of reservoir-scale genetic rock units because of the lack of a conceptual and fine stratigraphic framework for paleoecologic analysis and because sedimentation along the tectonically active Pacific Coast was dissimilar to that along the Gulf Coast where our modern depositional models have been largely generated.

Detailed study of Pliocene and Miocene strata of the Coalinga region, integrating both macrofossil and lithologic evidence, indicates that basin-edge sediments were deposited during well-defined transgressive-regressive cycles. The thickness of strata deposited during the cycles and the proportion that was deposited during the transgressive phase of each cycle are proportional to the slope of the surface that was being transgressed. The Pliocene depositional slope in the Kettleman Hills area was very low (many cycles are represented by less than 100 ft of strata), and sedimentation occurred primarily during regression. In the Miocene examples, the depositional slope was greater, the cyclic units are thicker and probably represent longer periods, and much of the deposition occurred dur ng transgression.

Cyclicity in the marginal marine strata provides finer subdivision and more precise correlation than has been possible by use of either fossils or lithology. It also provides the conceptual framework within which genetic rock units can be distinguished at reservoir scale. Positions of shoreline, loci of deltaic deposition, and environmental gradients within the shallow-marine deposits can be discriminated in the Kettleman Hills on layer maps representing subdivisions within individual cycles.

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