On the perimeter of the Santa Maria basin three tectono-stratigraphic terranes of pre-Tertiary rocks are recognized. In ascending stratigraphical and structural order, these are: the Franciscan assemblage, the Coast Range Ophiolite, and the Great Valley sequence. Where exposed in the western Santa Ynez and San Rafael Mountains, the Franciscan assemblage is a melange of chiefly graywacke, argillite, chert, and blueschist rocks. The Coast Range Ophiolite forms a discontinuous tectonized belt of outcrops along the southwest edge of the San Rafael Mountains; parts of the dismembered ophiolite are exposed at Cuyama Gorge, Tepusquet-Colson Canyon, north side of Figueroa Mountain, Little Pine Mountain, and Santa Ynez River west of Gibraltar Dam. Rock types include harzburgite, p roxenite, gabbro, diorite, pillow basalt, tuff, and serpentinite. Throughout the ophiolite belt, serpentine-rimmed cold intrusions invade sedimentary strata as young as middle Miocene. The Great Valley sequence includes Tithonian through Maestrichtian submarine-fan strata as well as fluviodeltaic strata of probable Campanian age. The latter crop out in the Sisquoc River area directly west of the Sur-Nacimiento fault zone. Along the north flank of the Santa Ynez Mountain, structural superposition resulting from truncation along thrust planes has juxtaposed beds as young as Valanginian above the Franciscan assemblage.

The structural and stratigraphic relations of the three terranes support the concept of a regionally persistent late Mesozoic forearc basin and accretionary subduction complex that may have extended from the Klamath Mountains of Oregon to as far south as the Vizcaino Peninsula of Baja California, Mexico.

End_of_Article - Last_Page 450------------