Lithologic and Diagenetic Sequences of the Monterey Formation, Molino Field, Offshore Santa Barbara, California

Abstract The Molino Field is located along the axis of a fault-controlled anticline trending east-west, parallel to the California coast, 30 miles west of Santa Barbara. The middle-late Miocene Monterey Formation of the Molino Field consists of thinly-bedded hemipelagic sediments deposited during a high sea-level stand at apparently low sediment-accumulation rates under suboxic conditions. The Formation has been fractured as a result of north-south compressional stresses in the Santa Barbara - Santa Ynez region. Lithostratigraphically, the sequence is equivalent to those exposed along the Santa Barbara coast described by Isaacs (1983). Cores from two wells, State PRC 2920 #7 and #8, were recovered during a 1984-85 new pool exploratory test program. Several depositional lithological features, post-depositional geochemical features, and post-depositional tectonic and compaction-related features are observed in the cores. The most distinct sedimentary features are the fine laminations to thin beds, soft-sediment structures and compacted fractures indicating deposition in a suboxic environment followed by post-depositional tectonic activity and compaction. Core and log analysis indicates that the opal-CT to quartz diagenetic transition in the siliceous rocks of the Monterey occurs in the center of the stratigraphic section. Thus silica in the upper Monterey is in the opal-CT phase, while in the lower Monterey it is in the quartz phase. However, the transition does not occur at a sharp boundary and diagenetic quartz and opal-CT co-exist over an interval several hundred feet thick. Recent studies of the Monterey, based on gamma-ray log analysis and stratigraphic inference from well logs, indicate that Molino represented a slope environment, situated midway between banktop facies exposed along Santa Barbara coastal beaches, and basinal facies occurring in the vicinity of Hondo Field. With the occurrence of nodular phosphate (a banktop indicator) in Molino cores, it is possible that Molino represents an extension of a topographically dipping banktop, onto which the oxygen-minimum zone was impinging. The location of the steeply dipping slope would therefore be south of the Molino, and would be quite narrow. The sequence of tectonic and depositional events recorded in the Molino cores is discussed along with the lithologic and diagenetic framework.