LOWER CRETACEOUS DEPOSITS CALIFORNIA AND OREGON

44 LOWER CRETACEOUS DEPOSITS INT CALIFORNIA AND OREGON
or broader, and have yielded a less satisfactory sequence of faunas in
any single section, although a general order is easily seen. In this group
tbe contrasts of faunas found in corresponding beds north and south of
the delta axis are at first sight somewhat surprising but, as will be shown
later, are explainable upon the basis of ecological conditions in tbe various
districts and the different faunal fades which have been developed. . In
the districts in which the faunas of the Horsetown group are more complete
the successive zones are more numerous, more continuous, and more representative,
than in the older group. For these reasons the Horsetown group
offers better opportunity for faunal zoning, especially in the district north
of the delta areas.
There are few lithological units of small thickness in either group that
seem to have continuity or utility as smaller subdivisions, except in
limited districts, and these have not greatly affected the faunal sequence.
The broader lithological zones already described seem to have had some
selective influence upon the enclosed invertebrate assemblages, due no
doubt to conditions of deposition and to their food supplies and other
factors.
The following stratigraphic column is representative of the Lower
Cretaceous groups occurring in tbe Cottonwood district of the Sacramento
embayment and of the sequence of characteristic fauna! assemblages.
In the districts south of the delta areas the faunal order appears
to be the same, but the faunas are dominated by other types, notably
Aucellae, which, in the lower group, occur in vast numbera. The order in
which some of the cephalopoda occur does not wholly accord with that of
England, or of other countries. Only a distortion of the facts could render
their agreement more perfect.
FAUNAS 07 THE VASKENTA GfiOUP
General Statement,—The type district of the Paskenta group is immediately
about the village of this name in western Tehama County,
extending northward beyond Elder Creek, and southward toward Newville.
In thiB area the group has a thickness of 5300 to 5500 feet, the
strata of which have already been partly described. In this district the
lower half of the group is made up of conglomerates, followed by thinbedded
sandstones and sandy shales, whereas the upper portion is shaly,
with few beds of sandstone. The lower portion of the group contrasts
litbologically with the underlying Knoxville shales and also with the shaly
upper part of the group itself.
It is not believed thot these lithological features maintain in all its
sections, although as far as known the Paskenta group is prevailingly
sandy near its base, and frequently contains thick beds of basal, or nearbasal,
conglomerates. The fauna of the lower sandy beds is characterized