The late Miocene to Early Pliocene Sisquoc Formation is a part of a Neogene lithologic succession which records basin-subsidence and infilling in the Santa Maria Basin. Core from four coreholes taken by Union Oil Company (Unocal) in the Casmalia Hills cumulatively penetrate approximately 1300 feet (400 m) of upper Sisquoc section. The base of the cored section consists of irregularly laminated to banded, and bioturbated to massive porcellaneous mudstone and minor sandstone. The opal-CT lithologies pass upward into diagenetically unaltered banded and bioturbated to structureless diatomaceous mudstone, and culminate with chert-sandy diatomaceous mudstone with minor interbedded sandstone. Sisquoc lithologies reflect the rapid accumulation of biosiliceous and continentally-derived fine-grained sediment in a borderland basin. Relative to the underlying finely laminated porcellaneous Monterey Fm, the Sisquoc Fm is enriched in terrigeneous detritus and depleted in silica. Stratiform nodular dolomite and disseminated dolomite rhombs occur locally. Isotopic analysis suggests that the nodular dolomite developed from carbon dioxide that was a by-product of anaeobic bacterial methanogenesis.
Sedimentary structures, (e.g. slump folds and angular discontinuities), paleontological data and the presence of laminated to massive units suggest that the Sisquoc Fm was deposited in an upper slope to shelf-edge environment under waters that fluctuated from being poorly- to well- oxygenated.
Pebbly units up to 46 feet (12 m) thick are encountered in the cores. Outcrop studies reveal that similar breccia and pebbly units occur in distinct zones up to 20 feet (6 m) wide, which sharply truncate bedding at high angle, suggesting that the pebbly units are not primary depositional features. Features occurring in the cores that also suggest a secondary origin for the pebbly units include: small dikes; high angle contacts that sharply truncate banded units; apparent incorporation of wallrock into the pebbly units; apparent internal flow structures; pebbles with dolomite diagenesis indicative of similar or slightly deeper burial than adjacent in-situ strata; the diagenetic maturity of pebbles decreases with decreasing depth; and transitions from pebbly units to brecciated to fractured to unfractured mudstone. The pebbly units are clast-supported and matrix-poor in lower stratigraphic occurrences, and are matrix-rich with finer clasts at higher stratigraphic levels. Pebbles are apparently derived from the Sisquoc Fm itself, mobilized in response to overpressuring of pore fluids during burial, possibly in conjunction with tectonic stresses exerted during growth of nearby anticlines and movement on the Pezzoni Fault. Release of overpressured fluids fractured and brecciated the overlying rocks, and transported clasts upward along fracture systems, rounding the clasts during transport. As energy dissipated during the upward passage of the escaping fluids, the fluids became less competent to transport clasts, and only fine clasts in a mud-rich matrix were transported upward to shallower depths.
The pebbly units commonly are highly saturated with oil. The diatomaceous mudstone and porcellaneous mudstone commonly have light to moderate oil saturation, while clay-rich diatomaceous or porcellaneous mudstone generally lacks significant oil-saturation. Oil is thought to have leaked up from the underlying Monterey Fm along fracture systems. If the pebbly units were generated from the Sisquoc Fm and emplaced in a dike-like fashion as interpreted here, then the greatest oil-saturation should occur in tabular deposits oriented at high angle to the bedding plane.
