LOWER CRETACEOUS DEPOSITS CALIFORNIA AND OREGON

CRETACEOUS SEDIMENTARY DIVISIONS 87
part from early Mesozoic time, with only minor changes through successive
periods.
The land areas bordering these troughs on the west were in part peninsular,
and in part mountainous islands, between which waterways gave
ingress from the open sea into partially land-locked, trough-like basins
or embayments. One of the most important of the outpost land areas
was that embracing the present Klamath Mountain complex, here described
under the name Klamathonia, but others of less importance existed
to the south.
The geographical extent and physiographic aspect of these land areas
can only bo inferred from the distribution, character, thickness, and
attitude of the marine deposits now found within the enclosed troughs.
In the Great Valley trough of California these deposits lie chiefly along
its western border—that is, along the eastern flanks of older mountain
areas limiting the trough on the west. In such positions these deposits
outcrop in a somewhat discontinuous xane for a total distance of nearly
400 miles, extending to the northwest and southeast.
Within the trough of the Great Valley, two distinct embayments, hero
known as the Sacramento and the Joaquin embayments, have been recognized.
The geological records in these embayments have not been
parallel throughout the period. In the former the stratigraphical thickness
of the deposits is much greater, and deposition covered most of the
chronological stages known in other parts of the world, beginning in
Berriasian time and continuing into Senonian, with only partial interruptions.
Later stages are but little represented in this embayment, although
in places they appear in its southern part. In this embayment the Cretaceous
deposits attain an aggregate thickness of at least 36,000 feet, of
which the Shasta series alone reaches a maximum of nearly 27,000 feet.
This series is divided into two principal groups—the Paskenta and the
Horsetown—attaining a thickness of 11,300 and 15,700 feet respectively.
Evidences of disconformity are now recognized between these groups.
Unconformably upon the Shasta series rests the Chico series, with a
maximum thickness of 9000 feet, and in some places it overlaps the older
series and rests directly upon the basement rocks. This series of strata
includes the oldest part of the California Upper Cretaceous, its deposition
beginning in late Albian time and continuing in the Sacramento embayment
to near the close of Senonian time.
In the northern part of the Sacramento embayment lower Cretaceous
deposits constitute a typical delta, filling a large triangular area in which
it spreads fan-like from an apex at the west, which marks the point of
river discharge from the mountainous background within the limits of
Klamathonia. Within its areal limits this delta, including the southward
drift of its sediments north of Thomes Creek, contains approximately
4S00 cubic miles of Cretaceous sediment, not including any portion that