Late Cretaceous Paleogeography Near Wheeler Gorge, Ventura County, California

Abstract

A continuous 1,500-m. succession of Upper Cretaceous marine sedimentary rocks near Wheeler Gorge consists chiefly of interbedded siltstone and black mudstone, which contrast markedly with conglomerate-sandstone units in the succession. The siltstone beds are mostly 2 mm.-3 cm. thick, and grade upward into nearly structureless mudstone. They have considerable lateral extent, and show cross-lamination, sole marks, and disturbed bedding. These features are thought to indicate deposition by small-scale turbidity currents on a relatively deep and level sea floor, on which fine mud also settled. The conglomerate units contain abundant angular intraclasts ripped from the conformable underlying mudstone, a feature ascribed to final deposition of the conglomerate in the deep-water, black-mud environment. However, the variety and extreme rounding of the extra-formational clasts suggest diverse terrestrial sources and abrasion on a beach. It is therefore suggested that the conglomerate, lubricated by wet sand, reached deep water by mass gravitational movement. Abundant sole marks (chiefly flute casts) suggest that the movement was turbulent, and indicate consistent paleocurrent flow from ESE. to WNW. The presence of mylonite and anorthosite clasts in the conglomerate is consistent with an easterly source, for they are very similar to distinctive rock types of the San Gabriel compl x, which probably was east of Wheeler Gorge in Cretaceous time. The Late Cretaceous paleogeography of the area thus was dominated by an eastern land mass shedding coarse detritus westward to the coast. Here the clasts accumulated, were abraded, and were displaced periodically into the deep-water environment where finer sediments were being deposited continuously. Sedimentation was terminated by uplift but was resumed during the middle Eocene marine transgression which followed.