Chronostratigraphy of the Quaternary Santa Barbara Basin: an integrated geophysical, sedimentologic and paleoceanographic approach

Abstract

The tectonically active Santa Barbara basin on the California continental margin contains an ultra-high-resolution sedimentary record of climate, oceanographic and continental environmental change that can be extended through much of the Quaternary. The stratigraphic record in this setting also provides detailed information on the location, rates and evolution of structural deformation in the Santa Barbara fold and fault belt, and consequently on modern seismic hazards in the region. In the absence of deep, continuous scientific drilling, an integrated method was employed to date and characterize strata older than the ~165 kyr record recovered by a 200-meter core at Ocean Drilling Program Site 893. We conducted a multi-year integrated seismic acquisition and piston coring campaign in which more than forty 2—11 meter-long piston cores were acquired that provide ~2—9 kyr windows into past climate behavior and sediment age. We identified and mapped distinctive seismic stratigraphic horizons across the basin to seafloor outcrop in pre-existing multichannel seismic (MCS) reflection data and in high-resolution MCS and towed chirp data acquired during our research cruises. Horizons to ~735 ka were dated by interpolation between the well-studied ODP Site 893 near the basin center and chronological datums of recovered tephra layers, microfossil ranges and the climate record (Marine Isotopic Stages) identified in the new cores taken at or near the horizons' sea-floor outcrop. Ages of older strata were interpolated back to a previously published ~1 Ma horizon. Sedimentation rates are high enough (70-130 cm/kyr) over broad areas of the basin to preserve ultra-high-resolution data on the rate and character of climate/ocean change. Within depositional troughs or the basin center, sedimentation maintained an overall relatively consistent sedimentation rate (varying by less than a factor of 2) during most stratigraphic intervals of the past 1 Myr. Sediment accumulation within the primary depositional trough was unevenly distributed only during a few intervals when changes in accomodation space confined most sediment to the northeast portion of the offshore basin. Unsurprisingly, multiple seismically-resolvable lowstand and transgressive system tract stratigraphic packages pinch-out or are absent on the structurally growing paleobathymetric high of the Mid-Channel trend anticline. Yet, quite surprisingly, once sedimentation resumed near the top of the submarine structure in each cycle, the short-term accumulation rate of bathyal depth, low-oxygen sediment resumed at rates remarkably consistent with the overall long-term 100-kyr to 1-Myr sedimentation rates near the basin center. This relationship suggests that most hemipelagic sediment that accumulated in the basin — even on bathymetric highs — was deposited from mid- to bottom-water-column suspension (nepheloid plumes) and was switched on or off depending on the relative paleobathymetric depth of the seafloor as modulated by sea level and tectonics.

AAPG Datapages/Search and Discovery Article #90266 © 2016 AAPG Pacific Section and Rocky Mountain Section Joint Meeting, Las Vegas, Nevada, October 2-5, 2016