Deformation of partly dewatered and consolidated Franciscan sediments near Piedras Blancas Point, California

Summary Bedded sequences of turbidite sandstone and mudstone and associated chaotic, polymict pebbly mudstone record a post-depositional, pre-metamorphic deformation. Sandstone layers were extended and locally disrupted and fragmented and most clasts in pebbly mudstone were deformed into oblate ellipsoids. On the scale of outcrops and hand-specimens, extension in sandstone was accomplished by: (1) mesoscopically ductile pinch-and-swell behaviour and boudinage; (2) extreme necking, which locally resulted in extension fractures or brecciation; and (3) sets of parallel shear fractures inclined to bedding. On smaller scales, ductile changes in shape were accommodated by both intergranular flow of sand-rich sediment and slip along zones typically 0.1 mm or less in thickness. Microfracturing, granulation, and cataclasis of clastic grains are less commonly observed, even adjacent to microfaults. Mudstone was also mesoscopically ductile but generally it flowed by displacements on penetrative, anastomosing surfaces of slip and was transformed locally into ‘scaly clay’. These Franciscan sediments probably contained appreciable pore fluids when they were deformed, since the deformation of unconsolidated sands by homogeneous, intergranular flow rather than widespread cataclasis is fostered by high interstitial fluid pressures. More localized discontinuities, such as extension fractures, shear fractures, and microfaults, formed where sand had more completely dewatered and consolidated. Thus, material response may partly reflect water content and consolidation history. The axially symmetric extension recorded by small-scale structures suggests that sediments were deformed in a shallow-level, laterally unconfined environment on the trench slope, perhaps in response to vertical shortening and horizontal elongation that accompanied gravitational collapse and spreading.