CHARACTERIZING DEFORMATION OF NEOGENE ROCKS NEAR POINT ARENA, CALIFORNIA

Deformation of the Neogene, Monterey Formation equivalent, Schooner Gulch, Galloway, and Point Arena Formations represents the last of several structural events in the Gualala block. The deformation within these formations is dominated by NW-SE trending meter to the tens of meters scale folds, meter-scale thrust faults, fault propagation folds, fault bend folds, NE-SW striking veins, as well as N-S striking normal faults. We took measurements along a 4 kilometer section of coastal bluff exposures near Point Arena, CA, extending from Moat Creek beach south to Schooner Gulch beach. The folds typically plunge shallowly to the NW or SE. Fold axes have an average trend and plunge of 319:07, nearly parallel to the local trace of the San Andreas Fault (SAF). The folds range from open to closed with rounded to angular hinge zones and are inclined to the SW with some sections locally overturned. The Galloway and Point Arena Formations contain 1-3 m diameter dolomite concretions, some of which have quartz overgrowths that show evidence for rotation.

A maximum compression direction of 210° was calculated from an M-plane analysis of the thirteen meter-scale thrust faults and their associated slickenlines. This is consistent with the orientation of our fold axes and thus is nearly perpendicular to the trace of the SAF. Elder (1998) interprets this deformation to be the result of a series of largely blind or poorly exposed imbricate thrust faults formed due to transpression along the SAF system.