Fault-controlled Basin Research Articles

Morphology and Sediments of Redondo Submarine Fan, Southern California

Abstract The following abridged article is included because of the strong similarity between the modern San Pedro basin with the associated Redondo Canyon submarine fan system and the Plio/Pleistocene Ventura basin and sedimentation. Both basins were tectonically active during cycles of sediment influx with each being constrained by an outboard series of topographically positive seamounts. The interrelationship of tectonic activity during sediment influx and outboard resulted in juxtaposition of sedimentary depositional features not normally observed in the “Classic” unconstrained submarine fan model. Through Haner’s work in the modern fan setting, a better understanding of associated sedimentary features observed in ancient basin environments is now available (Haner, 1971). The Redondo Submarine Fan fills the northern end of the San Pedro basin, one of several inshore, fault-controlled basins of the continental borderland of southern California. Fan segmentation and channel morphology were analyzed in detailed echo soundings, sub-bottom seismic profiles, and box cores. They reveal a threefold segmentation of the submarine fan along its axis as indicated by changes in fan gradient and channel patterns unique to each segment. The upper fan is steepest, and the channel is entrenched. Intersection of channel with fan surface and meandering characterize the middle segment, whereas a slightly steeper lower fan gradient results in a braided channel in the lower segment. The recognition of these geomorphic divisions reveals the that both proximal and distal facies exist together in the middle segment. Movements along faults in the basin slope are responsible for steepening of the upper fan segment; he active channel responds to the displacement either by entrenchment or abandonment and creation of a new channel. Turbidity currents are confined solely to the channel of the upper fan segment, but below the intersection zone (mid fan) they flow longitudinally within the channel confines or spread laterally over the fan surface. Associated internal sedimentary structures reveal these different current deceleration environments. The total geometry of sand units, lateral and longitudinal changes in sedimentary facies, and channel morphology provide a model which has greater scope and flexibility than previous sedimentary longitudinal facies models which other writers postulated for unconstrained fan development. The model can be applied to the vertical stratigraphic changes and to the distribution of sediments on the submarine fan surface.

Newly Discovered Cenozoic Basins, Bering Sea Shelf, Alaska

Recent geophysical studies over the Bering shelf have outlined the basic structure of three previously unknown Cenozoic basins, Norton basin and Pribilof and Zhemchug depressions. The upper part of the crust beneath the Bering shelf consists of a stratified sequence of generally undeformed Cenozoic deposits overlying a basement of Mesozoic sedimentary, volcanic, and plutonic rocks, and older Paleozoic sedimentary and metamorphic units. The average thickness of the Cenozoic deposits is approximately 1.0 km. However, they are more than twice that thick where the basement has been downfolded or downfaulted to form the major intrashelf basins (such as Bristol, Anadyr, and Norton basins) that typically underlie major bays and gulfs, and the narrower, fault-controlled outer shelf basins (such as Pribilof and Zhemchug depressions) that trend parallel with the continental margin. Norton basin underlies the northern part of the Bering shelf north of St. Lawrence Island. It is an east-west trough filled with terrestrial and neritic deposits of Paleogene age and younger, as much as 2.0 km thick. Minor folds and offsets in this fill are present along the flanks of the basin. Pribilof and Zhemchug depressions underlie the outer shelf and the northeast-trending canyon heads of Pribilof and Zhemchug Submarine Canyons, respectively. These depressions in the basement rocks are grabens and/or half grabens, and are filled with stratified neritic deposits of Paleogene age and younger, as much as 2.0 km thick. The Cenozoic fill is deformed above faults that offset the basement and the sea floor near the Pribilof Islands.

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

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.