LOWER CRETACEOUS DEPOSITS CALIFORNIA AND OREGON

42 LOWER CBETACEOOfl DEPOSITS IN CALIFORNIA AND OREGON
aentative part of the Knoxville series. Later investigation has shown this
to be an error; the Knoxville series is distinctly Jurassic, whereas the
Paakenta group has a chronological range strictly within the Cretaceous
system—Berriasian-Valanginiaii—and is quite distinct from the Knoxville
series.
Type District.—In its type district (Elder Creek to Thomes Creek and
southward), the lithological composition of the Paskenta group begins at
the south with massive lenses of conglomerate, above which it is prevailingly
sandy, with alternating beds of sandstone and sandy shales. South
of the delta area its fauna, is largely Aucellan, containing many forms of
rugose, heavy-shelled types not found in the Knoxville series. With
these are found various species of cephalopods that show its chronological
position to be below the Hauterivian stage, including species of Berria&eUa
and Neoc-omiies. Some of these species are new, others have been described
by Stanton (1895, p. S0-S2) and others. In the Cottonwood
district few, if any, Auceiiae occtir in this group, although some of its
cephalopoda and other types of its peleeypods do occur there.
Tbe Paskenta group has, therefore, been identified stratigraphieally and
faunally on both sides of the delta area—that is, in its type area and in
tbe Cottonwood district, although there are definite faunal differences.
In the delta area also it has been identified directly by a small molluscan
fauna, including Neacomitea, and indirectly by its stratigraphic relations
with the Horsetown group above and by the absence of Knoxville beds
beneath it. Its strata can also be traced across the delta area. The
Paakenta group, like the Horsetown, is thickest in the axial areas of the
delta and thins from there toward the north and toward the south.
In the axial area of the delta the group has a thickness of 11,000 feet;
northward, on the Middle fork of the Cottonwood, its thickness is 8000
feet; on Roaring River, its thickness is about 4000 feet. Toward tho
south the thinning is more gradual. But in all sections of the series
traversed the thickness of the Paskenta is less than that of the Horsetown
group, and from this fact it may be supposed that the time interval of its
deposition was shorter and in the same proportion .
In most sections of the Shasta series the line of division between these
groups has been determined faunally, although in some places the separation
is aided by lithological changes, and the line ia marked by lenses of
conglomerate at the base of the Horsetown group. In the Cottonwood
district such conglomerates in the lower part of the Horsetown group rest
directly upon fine shales in the upper part of the Paskenta group. These
conglomerate beds begin at the north near Ono, in a single bed about
60 feet in thickness, but farther south other similar beds appear above
and beneath, until on Roaring River as many as four such beds enter the
section in a stratigraphic thickness of 1000 feet. Farther south these