Sand Provenance and Compositional Evolution at ODP Site 893, Santa Barbara Basin: Implications for Reservoir Characterization

Kathleen M. Marsaglia, Karl C. Rimkus, Richard J. Behl

During ocean Drilling Program Leg 146, Site 893 was drilled in the Santa Barbara Basin, off southern California. Cores recovered at Site 893 consist predominantly of late Quaternary (0-160 ka) hemipelagic mud with minor (4%) very thin to thick beds of sand. The stratigraphic sequence (196.5 m) consists of laminated, non-laminated and intermittently laminated subunits that reflect glacial/interglacial oscillations and global changes in deep-water circulation. Two main pulses (Subunits 1C and 1F) of coarse clastic sediment into the Santa Barbara Basin were associated with falling sea level.

Detrital modes of sand deposited at Site 893 suggest that its most likely source was a combination of the Santa Clara and Ventura rivers at the eastern margin of the basin. Sand from Hole 893A is quartzo-feldspathic with subequal amounts of quartz and feldspar grains and a lesser lithic component. P/F ratios range from 0.4 to 0.7, and metamorphic and sedimentary lithic fragments are more common than volcanic lithic fragments.

Gradational changes in composition at the top of Subunit 1C, such as increasing QFL%Q and decreasing P/F and LmLvLs%Ls, probably reflect evolutionary changes within the drainage basin associated with the lengthening and expansion of the Santa Clara/Ventura river system as sea level dropped and shelf areas were exposed (between ~55 and 25 ka). Lithic proportions vary considerably within and between subunits; trends in LmLvLs%Ls within Subunit 1C possibly reflect small scale eustatic (shoreline migrations) or tectonic (uplift) cycles within the source area. These compositional trends have important ramifications for the potential reservoir characteristics of such sequences.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995