Abstract

Coastal belt rocks are a thick flyschlike sequence of interbedded sandstone and mudstone. Between Laytonville, California, and the coast, beds strike northwest and dip moderately northeast. Pull-apart features and discontinuous bedding are common throughout the sequence.

A major unit of highly disturbed bedding averaging 2 km thick can be traced for more than 35 km along strike. The unit is a composite of several thinner units of disrupted bedding. It is characterized by pinch-and-swell, pull-apart sandstone beds, boudins with rounded terminations, sheared mudstone, argille scagliose, intricate discontinuous folds, and exotic blocks of volcanic rock and limestone. Axes of sigmoidal folds in sandstone boudins and in thin clay-rich units generally parallel regional strike but show no preferred sense of rotation. At one place, undisturbed strata lie on this unit with sharp contact; however, most of its contacts are gradational. Pebbly mudstone and a large exotic block of chert were found in other smaller disrupted units.

Several features indicate that extensional deformation occurred while sediments were unconsolidated. Pebbly mud-stone and large exotic blocks suggest that downslope mass movements were important. Deformation here probably reflects a combination of submarine sliding and intrastratal shearing; these conditions might be expected on trench slopes during active subduction.

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