BENTHIC FORAMINIFERA IDENTIFY SOURCE AREA OF EARTHQUAKE- AND STORM-INDUCED TURBIDITE EVENTS, MONTEREY SUBMARINE CANYON, CALIFORNIA

In Monterey Submarine Canyon off the central California coast, earthquakes and intense storm wave disturbances may cause full submarine canyon and fan channel flushing events. Evidence of these turbidite events has been documented by the movement of bottom-deployed acoustic transponders and instrument frames. While it is often assumed that these turbidity currents originate in the upper canyons, this study demonstrates that the source area of these flows ranges from the inner shelf to the slope, as identified by the benthic foraminiferal assemblages entrained within the displaced sediments.

Off central California, the foraminiferal fauna can be assigned to six biofacies: inner shelf (0-50 m), outer shelf (50-150 m), upper bathyal (150-500 m), upper middle bathyal (500-1500 m), lower middle bathyal (1500-2000 m), and lower bathyal (>2000 m). Benthic species present in surface grab or sediment trap samples can be used to identify the source areas of recent tubidites, whereas samples from cores yield historic information. Core S3-15G from the western levee of Monterey fan valley (3491 m depth) contains a 19,000-year record of hemipelagic and submarine fan overbank deposits. The sediments are deposited in 45 turbidites of sand and silt separated by hemipelagic mud. A minimum of 65 displaced species were recovered from all of the samples and the relative abundance of allochthonous foraminifers correlated positively with grain size. One of the two cross-bedded turbiditic sands (Tc) recovered originated on the inner shelf and included faunal elements from all of the shelf and slope biofacies; the source of the other was the upper slope and included only the bathyal biofacies. In both of these sands, 75% of the fauna was displaced. Of the 29 laminated turbiditic sand (Td) units, ten had faunal elements from all six biofacies, seven had only those from the slope, the remaining had faunal elements from non-adjacent biofacies, and an average of 39% of the fauna was displaced. Most of the 35 turbiditic mud (Tet) units included constituents of only three biofacies with 15% displaced specimens, whereas the 18 hemipelagic muds (Tep) had only rare (3%) allochthonous foraminifers. Turbidites originating on the shelf could be either earthquake- or storm wave-induced; those originating on the slope are unlikely storm-generated.