Comparative Study of Rocks from Deep Sea Drilling Project Holes 467, 468, and 469 and the Southern California Borderland

Abstract

Miocene and Pliocene sedimentary and volcanogenic rocks drilled in Holes 467, 468, and 469 of Leg 63 resemble rocks of the same age that have been sampled from other parts of the southern California borderland. The purpose of this report is to compare in general terms the composition, thickness, and age of the preQuaternary rocks drilled at these sites with rocks from two test wells, seafloor outcrops, and island areas. From these comparisons, inferences are made concerning the developmental history of the borderland. Emphasis is placed on the composition and origin of coarse clastic and volcaniclastic Miocene rocks. Because the stratigraphic sequences on the mainland shelf, in nearshore basins, and on parts of the northern Channel Islands are unlike the sections in the DSDP holes, they are omitted from most of the comparisons. From the DSDP cores, 275 samples, chiefly coarsegrained Miocene rocks, were selected and examined. Thin sections of 78 of these selected samples were studied. All available seafloor samples of volcanic and volcaniclastic rocks from the borderland north of 32 °N (exclusive of the mainland shelf and nearshore banks) were re-examined for this report (Fig. 1). Igneous rocks are classified according to petrographic determination of mineral content and not on geochemical analysis. Chronostratigraphic assignments follow the usage of Bukry (this volume). The upper Tertiary sections drilled in two deep stratigraphic test wells, one west of Point Conception (OCS-CAL 78-164 No. 1), the other at the southeast end of Cortes Bank (OCS-CAL 75-70 No. 1), are dissimilar in many respects to those in the DSDP holes. Though information on recently drilled deep exploratory wells on the southern part of Santa Rosa-Cortes Ridge and near Santa Barbara Island is proprietary, the location of these holes suggest that they spudded in middle Miocene or older rocks and penetrated stratigraphic sequences that are, for the most part, older than those in the DSDP holes. Stratigraphic sections of Miocene and Pliocene rocks on some of the islands are reviewed, as they provide possible analogues for subsea sequences that ordinarily are represented only by widely spaced, short cores.