Mineralogy And Sedimentation History, Santa Barbara Basin, California

Abstract

ABSTRACT Holocene sediments in Santa Barbara Basin, California Continental Borderland, consist primarily of laminated olive gray lutite intercalated with abundant layers of gray lutite. On the basis of detailed sedimentologic and mineralogic analyses of representative river sediments and a 7 m core from the center of the basin, the gray layers, previously interpreted as turbidites, have been found to be suspended-load deposits derived mainly from the Santa Clara River during large floods. Average periodicity of the gray layers is estimated at 120 years. The Santa Clara River is also the major source of laminated lutite. Predominance of Santa Clara River sediments is suggested by drainage area calculations, and supported by a comparison of <62µ mineral suites and chlorite content of basin sediments and sediments of peripheral rivers. An origin as flood deposits is postulated for the gray layers on the basis of their smaller mean diameter, better sorting, smaller sand fractions, lower carbonate content, and greater mineralogic similarity to the Santa Clara River than the laminated lutite. Mineralogic variations within the core occur mainly in the silt size-range, and are caused by source variations. The modern Santa Clara River suite becomes less dominant in the lower half of the core, probably as a result of more abundant shelf sediment that was available during lowered sea level.