Intrastratal Microfractured Zones in Laminated Diatomaceous Sediments from the Monterey Formation, California: A Record of Miocene Paleoseismicity?

GRIMM, KURT A., University of California, Santa Cruz, CA

Finely laminated diatomaceous sediments from the Micoene Formation possess intrastratal microfractured zones (IMZ), which are nonuniformly distributed through vertical section. These stratally discrete zones of deformation record brittle fractures with minor fault offset and complex, yet consistent, patterns of sediment and fluid redistribution in associated vein arrays. Outcrop, microscopic, and scanning electron microscope observations suggest that IMZ are attributable to a discrete episode of fluid expulsion within shallowly buried sediments. Analogy to comparable structures in diatomaceous sediments from the Peru margin supports the interpretation that organic sheaths strengthened the grain framework of Monterey diatomites and reduced their effective permeability, thus permitting he development of elevated pore fluid pressure at shallow burial depths. I credit seismically triggered loss of sediment strength for the formation of IMZ in a single episode of brittle fracture and vein propagation by fluid dilation. The consistent co-occurrence of microfaults with mud-filled veins implies synergistic coupling between the processes of fluid advection and brittle fracture.

Constituent faults and veins within IMZ are characteristically sigmoidal and occur in curviplanar-parallel arrays. Their consistent orientation is attributed to a bedding-parallel shear couple, suggesting that these sigmoidal flexures record the spatial orientation of Miocene paleoslopes. Furthermore, the association of IMZ with slump breccias, disharmonically folded strata, and extensively fluidized sediments indicates that IMZ formation may be an important mechanism for initiating sedimentary mass failure in subaqueous slope settings. As a seismically triggered(?) sedimentary structure, the presence or absence of IMZ in varved diatomites may be a useful tool for evaluating seismic history.

AAPG Search and Discovery Article #91009©1991 AAPG-SEPM-SEG-SPWLA Pacific Section Annual Meeting, Bakersfield, California, March 6-8, 1991 (2009)