Fine-grained slope and basin deposits, California continental borderland: Facies, depositional mechanisms and geotechnical properties

Abstract

Surficial sediments from slopes and slope-aprons in Santa Barbara, Santa Cruz and San Nicolas basins (California borderland) are dominantly silts and clays. Facies are defined and interpreted on the basis of sedimentary structures as follows: (1) disorganized beds — mudflow or debris flow deposits; (2) beds with deformation features, micro-faults or bowl-shaped dislocations — slump deposits; (3) beds with fluid escape structures — partly fluidized or liquefied beds; (4) graded beds with Bouma sequences — turbidities; (5) ripple cross-bedded silts with mud interlaminae — traction current deposits; (6) horizontally laminated silty clay — suspensate or nepheloid flow deposits; and (7) bioturbated beds. Clay fabrics reflect mode of deposition or later deformation of some of the facies. Facies 1 shows random or swirled fabrics, while facies 2 has horizontal domains which alternate with random zones. Where fluidization or liquefaction has occurred (facies 3) clay fabrics consist of vertically aligned domains near fluid escape tubes, or are random. Other facies (4, 6, 7) show mostly random fabric patterns. Plasticity indices and water contents vary with texture; both are higher with increasing clay content and organic carbon content. Individual facies are also an influencing factor. For example, beds which were sufficiently consolidated to develop microfaults have lower values than the other facies. Facies distributions correlate with sedimentation rates. Proximal basins (> 50 mg cm −2 yr −1) have more mudflow, debris flow and slump deposits while distal basins (<10 mg cm −2 yr −1) have more turbidites and laminated deposits. Similarly within individual basins, facies correlate to local seafloor gradients, with debris flow and slump deposits more common in steeper areas and turbidites and laminated “varves” more common in flatter areas. Mudflow deposits are more common in intermediate slope areas, especially in those areas undergoing creep and syn-sedimentary gravity-faulting in Santa Barbara Basin.