Vol. 5] Louderback.—Benitoite. 335
inga.
This particular anticlinal lobe is of considerable interest in another
way, for its lower portion near the valley has produced and is
producing large quantities of petroleum, and it was along the nose of
this anticline that the first important development of the Coalinga oil
fields took place.
The
next succeeding primary anticlinal component forms that part of the
range to the west of Los Gatos Creek and Coalinga. The synclinal area
between the two is well marked topographically by the depression in
which flows Los Gatos Creek and its tributary, White Creek, the pass at
the head, and the depression occupied by San Benito Creek on the
western slope. These corresponding depressions (in part structural,
but modified by erosion) and the saddle between them naturally
determine the position of a road crossing the mountains from the
interior valley towards the coast, and this is the road followed to
reach the benitoite locality from Coalinga.
Ascending
the mountains one sees a remarkable display of formations starting with
the Recent of the valley, crossing in succession various divisions of
the Quaternary, Pliocene, Miocene, Eocene, Upper and Lower Cretaceous
and ending with the Franciscan which with its associated intrusives
occupies the highest portions of the range. It is in these last named
rocks that the minerals under discussion occur.
Owing
to the general structure just described, the exposures of the
Franciscan and their associated igneous rocks do not everywhere occupy
the summit line of the main range, but extend out along the axes of
the anticlinal components. Thus these rocks of the benitoite locality
extend southward along the spur that runs into the valley northeast of
Coalinga, while later rocks occupy the divide at the pass. The older
rocks again appear at the surface along the range line farther south
along the axis of the next anticlinal component.
All
of the rock formations of this section down to and including the
Knoxville (usually considered Lower Cretaceous), as is common in the
Coast Ranges, are unaltered or but slightly altered sediments and show
nothing in the nature of schist formation and very little in the way of
veination—and this of superficial origin. The Franciscan series is in
marked contrast.
