Seismically triggered turbidites in small margin basins: Alfonso Basin, Western Gulf of California and Santa Monica Basin, California Borderland

Abstract

Box cores and gravity cores collected in Santa Monica Basin, California Continental Borderland, and in Alfonso Basin, western Gulf of California, contain turbidites, some of which can be traced over much of the respective basin floors. In Santa Monica Basin, at least six of these basin-wide flows have been deposited over the past four to five centuries. In Alfonso Basin, 9–10 basin-wide turbidites have been deposited over the past few millenia. Both Santa Monica and Alfonso Basins have anoxic conditions over the deep basin floors, which inhibit bioturbation and preserve primary laminations. These deposits have been dated using the 210 Pb and AMS 14 C methods, and varve counting so it is therefore possible to date the turbidites. Those that were deposited during historic time can be matched with major floods or earthquakes in the region. In Santa Monica Basin, turbidity currents can be generated directly from decadal major flood discharges, or by centennial slope failures triggered by major earthquake shocks. The flood-generated turbidites are typically a fifth or less of the volume of the earthquake-generated turbidites. In Alfonso Basin, the tributary coastal canyon discharges are small and have a high proportion of coarse-grained sand, which is trapped on the shelves, and cannot directly supply turbidite volumes of basin-wide magnitude. Thus the turbidites seen in that basin floor are probably produced by slope failures of silty clay deposits which were seismically generated. The distribution of the dated turbidites, and a slip face in one box core from the landward slope, indicates a source on the landward depositional slope of the fault-bounded basin. Similar discontinuities of the same age have been reported on the eastern side of the Gulf in the Guaymas area. In ancient basins, the criteria that may distinguish seismo-turbidites are areal extent and volume where those factors can be estimated. Basin-wide turbidites are probably seismically triggered. If paleogeographic data are sufficient to delineate basin form, then major turbidite volumes can also be estimated. These can be compared to average annual sediment-accumulation rates based on basin volume divided by basin age, as a means of determining potential minimum magnitudes of flood-generated volumes.