Deposition of resedimented sandstone beds in the Pico Formation, Ventura Basin, California, as interpreted from magnetic fabric measurements

Abstract

Settling velocity distributions and magnetic fabrics in sediments from the Pico Formation were studied in order to determine the relationships between these properties and the observed sedimentary structures, and to evaluate the processes of deposition of turbidites. Three basic types of settling velocity frequency distributions were recognized: P1 profile, a low, flat distribution pattern indicating very poor sorting; P2 profile, a distribution which shows a distinctive mode; and P3 profile, a slope-shaped pattern composed predominantly of fine materials. P1 was found in the graded and massive divisions of turbidites; P2 was found in the lower division of horizontal stratification and the ripple stratification division; P3 was found in the uppermost divisions. Comparison of these patterns with previous results from modern sediments reveals that P2 and P3 are similar to patterns found in fluvial environments, whereas P1 was quite rare in “normal” current- or wave-formed deposits. The uniqueness of the graded and massive division is also evident in the results of the magnetic analysis. Although the magnetic fabrics in the upper parts of turbidites show similarity to other current-formed fabrics, the magnetic fabrics of the graded and massive divisions are quite different. The magnetic fabrics in the graded and massive divisions are characterized by (1) the presence of the current-normal orientation, and (2) less foliated and in-homogeneous fabrics which are indicated by high imbrication and q -value as well as large standard deviations of q -value and K max directions. Comparison with results from modern sediments indicates that fabric characteristics in the sediments of the graded and massive divisions are best explained by a combination of (1) an orientation mechanism related to layer by layer grain collision in a highly concentrated flow and (2) an orientation mechanism related to the suspension of grains in a viscous flow. This evidence indicates that a highly concentrated and partly viscous basal flow in turbidity currents may be responsible for the deposition of the lower part of the graded division and the massive division, whereas a more diluted flow may be responsible for the deposition of the upper divisions.