Sandy Facies at ODP Site 893 in the Santa Barbara Basin and Implication of Proposed Drilling by the Integrated Ocean Drilling Program
Sixteen years ago, a “site of opportunity”, tacked onto the end of an Ocean Drilling Program (ODP) leg to the Cascadia margin, was cored in Santa Barbara Basin to a depth of ~200 mbsf. Cores from ODP Site 893 were transported unopened to Texas and meticulously described by a shore-based party. They discovered a rare continuous marine record from ~160 ka to the present, with resolution for paleoclimate studies approaching that of the Greenland ice cores, but with the potential of extending much further back in time. The section, spanning two glacial-interglacial cycles, consists of hemipelagic mud, laminated during warm low-oxygen periods and bioturbated when cold and oxygenated. Thin sandy laminae/beds, interpreted as turbidites, are concentrated during intermediate sea level, perhaps when sand was pushed off a narrowing shelf. Sand composition (average QFL%Q42, %F45, %L13; QmKP%Qm47, %K24, %P29; and LmLvLs%Lm36%, %Lv26, %Ls38) is consistent with a Santa Clara River source. Stratigraphic changes in sand composition are loosely linked to eustacy-related lengthening or reorganization of the fluvial drainage system to include the Ventura River. Likewise, clay mineralogic composition of fine-grained “gray layer” flood deposits are linked to the Santa Clara River catchment, but show little compositional variation with eustacy or climate, suggesting transport to the basin is sensitive to grain size.
In preparation for a return of the Integrated ODP (IODP) to extend this remarkable record to ~1 m.y., short (<5m) piston cores were taken from uplifted seafloor outcrops of strata correlative to layers more than a kilometer below Site 893. In these cores of primarily hemipelagic mud, sand content is variable through the succession but most abundant in cores from glacial intervals. These unstudied deposits offer the opportunity to study the relationship of tectonics and climate to changes in sediment source and transport pathways over a much longer time frame.
AAPG Search and Discovery Article #90088©2009 Pacific Section Meeting, Ventura, California, May 3-5, 2009