SYNSEDIMENTARY FRACTURING, FLUID MIGRATION, AND SUBAQUEOUS MASS-WASTING - INTRASTRATAL MICROFRACTURED ZONES IN LAMINATED DIATOMACEOUS SEDIMENTS, MIOCENE MONTEREY FORMATION, CALIFORNIA, USA

Thinly laminated diatomaceous sediments from the Miocene Monterey Form ation contain intrastratal microfractured zones (IMZs) with a layer-sp ecific stratigraphic distribution, IMZs are a hybrid fault/vein struct ure that are stratally bounded by undeformed sediments. IMZs record sy nsedimentary extension, brittle fracture, and en echelon normal faulti ng, and are intimately associated with a well-ordered record of sedime nt and fluid redistribution in sigmoidal, mud-filled vein arrays, Outc rop, microscope, and SEM observations suggest that IMZs are attributab le to intrastratal faulting and vein propagation within shallowly buri ed diatomaceous sediments. We interpret that IMZs formed in a setting of bedding parallel shear, on a submarine slope proximal to a region o f active tectonism. Measuring the spatial orientation of IMZs and corr ectly interpreting their vergence direction permits accurate estimatio n of paleoslope direction.The unique physical properties of organic-ri ch diatomaceous sediments contributed substantially to the formation o f IMZs, Associated sediments record contemporaneous fluidization, slum ping, disharmonic folding, complex fracturing, and brecciation, sugges ting that IMZs are part of a rheological continuum of synsedimentary s lope failure. The presence of cobbles and boulders bearing IMZs and re lated structures within and directly subjacent to soft sediment conglo merates and breccias further substantiates the synsedimentary origin o f IMZs and suggests that stratigraphic horizons bearing IMZs may have acted as detachment surfaces for some slope failure and subsequent mas s-movement events, Although the trigger for initiation of failure rema ins uncertain, our kinematic model and the apparent restriction of IMZ s (and comparable structures) to transpressive and convergent tectonic margins suggests that IMZs and some associated rudite deposits may re cord the signature of seismic events upon shallowly buried hemipelagic sediments.