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Sedimentologic Controls on Brittle Deformation of Siliceous and Dolomitic Mudstones: Pismo Fromation, California

Jon R. Schwalbach, Dalton F. Lockman

Sedimentologic parameters and diagenetic history are important factors influencing the brittle behavior of fine-grained rocks in a given structural environment. The complexity of these relationships is highlighted by the fact that different parameters influence the level of fracturing in different rock types of the same depositional sequence. This can be illustrated with data collected from 23 outcrop stations from the upper Miocene to lower Pliocene Miguelito Member of the Pismo Formation in the Pismo basin.

The samples were divided into two lithologic groups based on outcrop description, x-ray diffraction data, and thin-section analysis. Bed thickness was measured in the field, and matrix porosity determined in the laboratory. The magnitude of brittle deformation was estimated by calculating the fracture intensity (fracture surface area per unit rock volume) of individual bedding units.

For the 16 opal-CT siliceous mudstone samples, the d-spacing values determined by x-ray diffraction have the highest correlation coefficient with fracture intensity (0.65). The ratio of biogenic to detrital silica, which has been shown to influence d-spacing values, has a correlation coefficient of 0.59. Other variables that have lower correlation coefficients with fracture intensity include matrix porosity (0.49), the detrital content (0.48), and the biogenic silica content (0.40). The best two-variable model for predicting fracture intensity of these mudstones combines d-spacing and matrix porosity (R-squared equals 0.72).

For the seven dolomite samples, individual lithologic components exhibit the best correlation coefficients with fracture intensity, including detrital content (0.74), dolomite content (0.64), and biogenic silica content (0.59). Bed thickness has a reasonable correlation coefficient with fracture intensity (0.59), but matrix porosity has a very low correlation (R-squared equals 0.23).

AAPG Search and Discovery Article #91035©1988 AAPG-SEPM-SEG Pacific Sections and SPWLA Annual Convention, Santa Barbara, California, 17-19 April 1988.