Grain-Flow Deposits and their Implications, Santa Ynez Mountains, California

Abstract

ABSTRACT A conformable 4000 meter thick marine Lower Tertiary section, well exposed in the Santa Barbara to Ojai segment of the Santa Ynez Mountains, reveals an upward progression from deep-water turbidites with flysch features to shallow-water deposits, and is succeeded by nonmarine beds. Sandstones and conglomerates which occur generally between the flysch and the shallow-water deposits are characterized by: thick, ungraded beds with sharp borders; outsize clasts; diffuse, flat lamination; dish structure (defined here); peculiar sole markings; and direct evidence of mass flow. These primary features indicate deposition by pseudo-laminar flow of granular material, here called grain flow. Such flow may be explained on the basis of the 'Bagnold effect' of grain interaction. Depositional directions within these grain-flow deposits and associated slide deposits are generally transverse to the basin axis, in contrast to the longitudinal directions in both the older turbidites and the younger shelf deposits. These different trends are interpreted as typical of a shelf-slope-bottom configuration in a linear, fault-controlled basin, and in this instance, their succession records progressive filling of the basin. Deposition by grain flow is probably important in many steep-sided basins, particularly linear troughs along continental margins, and many massive sandstones and conglomerates probably represent grain-flow deposits. Some examples for California are cited.