Abstract

Porosity, grain density, and composition were determined for different rock types from a 1.2-km-thick Neogene section of the Monterey and Sisquoc Formations in the Point Pedernales area, California. Porosity decreases and dry bulk density increases with burial depth in these predominantly siliceous rocks. Porosity reduction is controlled largely by initial composition and silica diagenesis. There is a positive correlation between porosity and biogenic silica (diatom) content of diatomaceous shales. The porous, incompressible diatom frustule maintains a high porosity relative to detrital, organic, and calcareous components of the rock. Porosity reduction by physical compaction from increasing overburden pressure is much slower at burial depths greater than 0.5 km. Solution of the porous diatom frustule and precipitation of cryptocrystalline opal-CT resulted in a highly variable porosity reduction. Porosity reduction during the opal-A to opal-CT transformation depends on the amount, composition, and texture of the nonsilica grains. A large porosity reduction occurred locally in nodules and beds of pore-filling dolomite and silica.

The porosity-depth relation of siliceous sediments and rocks from this and previous studies is used with biostratigraphic age data and estimated paleobathymetry to reconstruct the burial history of the Point Pedernales area. The sedimentation rate, corrected for compaction, quadrupled during deposition of the Sisquoc Formation. Using the constant water depth proposed for the Monterey Formation, and assuming that basin filling occurred during rapid deposition of the Sisquoc Formation, results in a tectonic subsidence curve similar to that observed for cooling oceanic lithosphere. Upwelling of asthenosphere near the surface would have produced a significantly higher paleogeothermal gradient, and the opal-A to opal-CT transformation is predicted to have occurred 3 m.y. earlier than if a constant, presentday geothermal gradient is assumed. Basin subsidence by faulting or flexure, without upwelling of asthenosphere, does not significantly alter the thermal history.

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