Quaternary Turbidite Deposits off Central California Provide Evidence of Source of Downslope-Displaced Benthic Foraminifera and Hydraulic Sorting of Palynomorphs

M. McGann
U.S. Geological Survey, Menlo Park, CA, [email protected]

Core S3-15G from the western levee of Monterey submarine fan valley (3,491 m depth) contains a 19,000-year record of hemipelagic and submarine fan overbank deposits. The sediments were deposited in 45 turbiditic sequences of sand and silt separated by hemipelagic mud. The associated benthic foraminifers and palynomorphs yield information on the depositional processes involved.

Six modern foraminiferal biofacies are recognized off central California from inner shelf to lower bathyal depths. Based on the assemblages entrained within the displaced sediments, the depth at which the turbidites originated may be identified. Nineteen displaced species were recovered from 65 turbiditic samples investigated. The relative abundance of allochthonous foraminifers was found to correlate positively with grain size. Sands of one of the two cross-bedded turbiditic (Tc) units recovered originated on the inner shelf and included faunal elements from all of the shelf and slope biofacies between presumed origin and deposition; the source of the other was the upper slope as no shelf foraminifers were included. In both of these sands, 75% of the fauna was displaced. Of the 29 laminated turbiditic sands (Td), ten had faunal elements from all six biofacies, seven had only those from the slope, and the remaining had faunal elements from non-adjacent biofacies. On average, 9% of the fauna was displaced. Most of the 35 turbiditic mud (Tet) units contained constituents of only three biofacies with 15% displaced specimens, whereas the 18 hemipelagic muds (Tep) had only rare (3%) allochthonous foraminifers.

Pollen abundance correlates negatively with grain size; the sand deposits contain significantly fewer palynomorphs than do the muds. Higher proportions of large and hydrodynamically efficient pine pollen occur in the turbiditic sands with a reduction in the frequency of smaller-sized pollen (oak, redwood, alder), suggesting that these sands contain a pollen assemblage biased by hydraulic sorting during downslope transport.

AAPG Search and Discovery Article #90088©2009 Pacific Section Meeting, Ventura, California, May 3-5, 2009