Field studies document an apparent eustatic control on facies patterns in isolated basins along a tectonically active margin. In the San Diego embayment and along northern Baja California, progradational-retrogradational shoreline sequences characterize Late Cretaceous and Eocene fore-arc basin-margin stratigraphy. Extensive paleontologic control helps establish the age and distribution of facies changes along these depositionally compact, steep-gradient margins. The observed depositional sequences may be stratigraphically arranged into three scales and patterns of sedimentary cycles. Timing of the two largest cycles provides relative sea level curves that correlate exceptionally well with worldwide sea level curves of Vail and others.

The major depositional cycle is asymmetric--a "hemicycle" hundreds of meters thick, characterized by a thin, basal retrogradational sequence overlain by a thick progradational sequence--and corresponds to eustatic supercycles. Depositional hemicycles are composed of smaller scale rhythmic successions controlled by sea level cycles and paracycles. Depositional pulses produced by local conditions, in turn, overprint these two larger scales of sedimentation.

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Coeval hemicycles and depositional rhythms in coastal basins from Oregon to Baja California further indicate a primary eustatic control on sedimentation.

Figure

Field-based facies analysis, having resolution far greater than that provided by seismic stratigraphy, thus supports using the "Vail curve" as a predictive tool in exploration. Deposition, distribution, and geometries of reservoir rocks can be modeled prior to drilling. Initiation and duration of sedimentary cycles defined by this study may be estimated ahead of the bit. Variations in expected facies patterns yield improved structural and stratigraphic interpretations for basin analysis. Worldwide comparison of stratal patterns in coeval basins from various tectonic settings ultimately will provide a data base for developing basin models.

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