LOWER CRETACEOUS DEPOSITS CALIFORNIA AND OREGON

0 LOWER CRETACEOUS DEPOSITS IN CALIFORNIA. AND OREGON
each, seem to be recorded in the atratigraphical sequence and in the faunas
found in the Sacramento embayment, the results of which can be only
briefly sketched here. One of these events was initiated at the beginning
of Chico (late Albian) time; its second phase covered the subsidence that
followed, during which the earliest deposits of this series began to accumulate.
A second disturbance, differential in effects, was initiated in
early Senonian (Coniacian) time, and this stage was followed by a longcontinued
subsidence and the deposition of a sequence of Upper Cretaceous
strata which was locally interrupted in middle Senonian time, This sequence
of strata is of small arcal extent on the western bordar of the Sacramento
embayment but is of greater spread on its eastern border and
of still greater extent in the Joaquin embayment in which partly parallel
events are recorded, but continued to a later epoch.
In the Joaquin embayment, extending south from Mount Diablo,
Lower Cretaceous deposits are known in a number of districts on the east
flank of the Diablo Range (Mount Diablo, Quinto Creek, Waltham Creek,
and Devils Den), but they are not continuous, and their areas are not
large; neither do the strata attain in all places a constant thickness.
Near Mount Diablo the thickness of the Lower Cretaceous does not
exceed 5000 feet; near Quinto Creek it is much less; in the Devils Den
district it may be 12,000 feet. Near Coalinga and in Waltham Creek
Valley it is thought to attain to 15,000 feet, but this figure represents the
maximum thickness in the Diablo Bange. One may suppose that the
normal, or original, thickness of the Lower Cretaceous in the Joaquin
embayment had been from 12,000 to 15,000 feet and explain the discrepancies
in the sections in port by faulting and in part as a result of overlap
of the Chico series upon the older groups of the Cretaceous succession.
In the southern, as in the northern, embayment three diastrophic events
are recorded in the upper Cretaceous sections. One is at the base of the
Chico series; one is at the beginning of Senonian (Coniacian) time; and
another is later, probably in Late Senonian time (late CampanJaa).
The Cretaceous sections in the Joaquin embayment differ from those
in the Sacramento embayment in the almost complete absence of Horsetown
beds and in the much greater development of Upper Cretaceous,
especially during the closing epochs of the period.
In the southern coastal areas of California, including Lower California,
and also in southwestern Oregon, scattered occurrences and incomplete
data indicate that the major diastrophic and depositional events, at least,
described in the preceding paragraphs were not wholly local but are
traceable to the north and south far beyond the limits of the Great Valley
trough and its embayments.