Abstract
Relationship between paleoceanographic changes related to the California Current, marine sedimentation, and the seismic reflection record in the California Borderland over the last 5 My has been investigated based on core and log data from Ocean Drilling Program Leg 167 and two seismic datasets. The correlation between cores and seismic data show that fluctuations of current-related productivity produce variations in the sediments. Thus, the seismic record is the direst result of paleoceanographic changes.Processed displays of 4-channel seismic data obtained with an 80 cubic inch watergun source were correlated with borehole data from Tanner Basin using synthetic seismograms. The representative frequency of these seismic data is 40 kHz, providing a vertical resolution of about 10 m. The synthetic seismograms and field seismic data match well and permit correlation and dating of several horizons. Within the analyzed interval (60--430 mbsf), density has been shown to be the primary cause of acoustic impedance variations, and the density-depth profile is ultimately controlled by the interplay between calcium carbonate and organic carbon, which derives from fluctuations in primary productivity related to upwelling and changes in water column oxygenation. The distribution of seismic reflections follows cyclic patterns, with 200 ky and 1 My cycles especially pronounced.Additional work focused with more detail on the upper 60 m of Pleistocene sediments imaged with high resolution 3.5 kHz sub-bottom profiles, which provide submeter vertical resolution. In the unconsolidated Pleistocene sediments the source of the density contrasts and seismic horizons closely and inversely correlates with organic carbon content (i.e., relatively high organics produce a low density). This is the first location we know of where large-scale variations in organic carbon content determine seismic reflections. The similarity of the density profiles in the top 60 m of the investigated basins (Tanner, East Cortes, and San Nicolas) indicates that the sedimentary regime was similar in all of these basins throughout the Pleistocene. The reflections can be traced amongst the three basins studied, and likely could be extrapolated to other Borderland basins (e.g., San Clemente), where they could be used as time markers for neotectonic studies in the region.