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Petrology and Provenance of Upper Cretaceous Sandstone, Southern San Rafael Mountains, Santa Barbara County, California

Cameron D. Toyne

Petrologic analysis of 24 medium to coarse-grained sandstone samples, collected from a 2,950-m submarine fan complex of late Campanian-early Maestrichtian age exposed within Mono Creek Canyon, reveal commonly calcite cemented, poorly sorted, subangular biotitic arkoses. Framework averages 86.0%. Matrix--primarily detrital quartz, feldspar, and lithic fragments finer than 0.03 mm and mechanically and chemically altered phyllosilicates and labile aphanites--averages 8.9%. Calcite cement averages 4.2%. Porosity averages 0.9%. Gazzi-Dickinson point counts of 400 framework grains per slide yield modal averages of Q37.7 F49.8 L12.5; Qm27.4 F49.8 Lt22.8; Qm35.6 P43.7 K20.7; and Qp< UB>49.4 Lv22.1 Ls28.5. P/F averages 0.68, Lv/L averages 0.45, Qp/Q averages 0.27, and detrital phyllosilicate, predominantly biotite, averages 5.7% of total framework. Neither primary nor secondary parameters vary systematically with stratigraphic position. Miscellaneous constituents average 1.3% of framework and include epidote, garnet, amphibole, pyroxene, zircon, and tourmaline as well as carbonaceous blebs, opaque minerals, and unidentifiable lithic fragments. Separate analysis of 100 medium-sized quartz grains per slide indicates a mean population of 63.0% nonundulatory monocrystalline quartz, 9.1% undulatory monocrystalline quartz, 10.1% polycrystalline quartz of 2 to 3 crystals, and 17.9% polycrystalline quartz composed of more than 3 crystals.

Modal data, plotted upon provenance discrimination diagrams, indicate a plutonic provenance transitional between a dissected magmatic arc and uplifted basement terrane. Paleocurrent data, neglecting possible clockwise rotation, indicate sediment transport from the north. While a Sierra Nevadan or Pennisular Ranges source terrane cannot be excluded with certainty, a Salinian provenance is inferred. This corroborates current hypotheses suggesting amalgamation of the Sur-Obispo and Salinian terranes by the Campanian time.

AAPG Search and Discovery Article #91038©1987 AAPG Annual Convention, Los Angeles, California, June 7-10, 1987.