ORBITAL- TO SUB-ORBITAL-SCALE CYCLICITY IN SEISMIC REFLECTIONS AND SEDIMENT CHARACTER IN EARLY TO MIDDLE PLEISTOCENE MUDSTONE, SANTA BARBARA, CA

High-resolution seismic reflection records and well logs from the Santa Barbara Channel suggest that much of the Pleistocene hemipelagic sedimentary sequence records climate variability on orbital to sub-orbital scales, like strata of the last glacial cycle studied at ODP Site 893. We have developed and tested a new method to extract lithologic cyclicity from high-resolution marine seismic profiles. Seismic reflection data (towed chirp) collected on the R/V Melville 2008 Cruise (MV08) penetrate 10's of meters below seafloor into a ~1 km-long sequence of south-dipping seismic reflections. Spectral analysis of seismic reflection data and gamma ray logs from stratigraphically similar Pleistocene sections identify similar cyclic character through the section. This correlation suggests that acoustic impedance and physical properties of sediment are linked by climatically-driven oscillations in lithologic composition and fabric during deposition. Furthermore, shifts in spectral character permit identification of basin-wide changes in sedimentation rate or climatic forcing.

Where it crops out along the northern shelf of the central Santa Barbara Channel, the early to middle Pleistocene succession (~1.6-0.7 Ma, middle to upper Pico Formation) is a bathyal hemipelagic mudstone with remarkably rhythmic planar bedding, finely laminated fabric, and well-preserved foraminifera. Unlike the coarser, turbiditic successions in the central Ventura and Los Angeles basins, this sequence has the potential to record Quaternary global climate change at high resolution. We suggest the following procedure for analysis of this dipping sub-sea succession: 1. Measure reflection amplitude values from several, parallel horizontal transects (constant 2-way-time) of a displayed profile, using absolute values of the SEG-Y data to test the usefulness of the data. Correlate, stack, and spectrally analyze the data series. 2. Sample several horizontal series of the digital SEG-Y data, correlate and stack the series, and analyze for cyclicity. Convert measured distance to true stratigraphic thickness. This method may be used to evaluate the sedimentation rates, age range, and paleoceanographic potential of sedimentary strata before a coring vessel is deployed.