Abstract

Penecontemporaneous folds, lineations, foliation, and fold interference patterns are found in part of the Pigeon Point Formation, a turbidite sequence occurring along the central California coast. These structures are associated with slumped sheets, and they formed while the rocks were largely unlithified. The foliation, which is commonly roughly axial planar to folds, is clearly shown in the hinges of slump folds and is defined by preferred alignment of inequant grains and silt-sand domains, the latter having formed from disrupted bedding laminae. Models that attempt to explain the genesis of penecontemporaneous foliation involve simple load compaction superimposed on pre-existing slump folds (model 1) or rotation of grains facilitated by slump folding (model 2). Evidence from this study indicates that a modified version of model 2 is commonly active. It is suggested that a critical content of interstitial water is necessary, whereby sufficient water pressure can build to decrease grain cohesion and induce liquefaction of layers. Further compaction in the hinge areas of slump folds then expels water during slumping such that preferred alignment of silt-sand domains and inequant grains is facilitated.

All three of the basic fold interference patterns normally attributed to “hard rock” folding occur in these slumped sheets and, along with foliation and related structures, strongly resemble features generated by tectonic movement. These structures can persist into the higher metamorphic grades, where they may be misidentified as evidence for early repeated folding. In this case, the structural history of that part of the orogen may be reported as more complicated than it actually is.

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