LOWER CRETACEOUS DEPOSITS CALIFORNIA AND OREGON

34 LOWER CRETACEOUS DEPOSITS INT CALIFORNIA AND OREGON
This area is not only delta-like in form, but ft closer study of its structure,
character, volume of sediments, and other features show that it
represents the delta of a large river entering the embayment here during
Cretaceous time. A study of the several sections across its area and its
contained faunas, and their relationships, leads to the same conclusion,
namely, that this area contains a typical marine delta developed about
the debouchure of a large river entering here from the mountainous region
to tbe west. The axis of the delta lies near the position of the Shasta-
Tehama County line, and in part it is traversed by the Humboldt road
between Red Bluff and Beegum.
Although the exact position of the outflowing stream is now obscure,
various aspects of the topography, basement rocks, and the sediments
themselves, indicate that it lay near, or slightly north of, Beegum Creek,
or Beegum Peak. The sections show a decreasing thickness of Cretaceous
deposits to the north and south of this line, thinning more rapidly toward
the north and less rapidly toward the south.
Tbe oldest Cretaceous beds in this area include coarse, pebbly conglomerates
and sandstones about this apex, overlaid by sandstones and coarse,
sandy shales washed free of clay and finer silt. At higher levels in the
sequence, 4000 to 6000 feet above the base, there is a larger percentage
of shales and mudstones, in conformity with the normal order of sedimentation.
Marine fossils, elsewhere plentiful in this area, are rare in all
parts of the sequence near the axis of the delta. No molluscan fossils
have been found in tbe lower beds, but, 3000 to 4000 feet above the base,
a small form of Protocardium and a small neocomitid have been found.
At a still higher level, 11,000 feet above the base, were found many examples
of Prolocardium, a small specimen of PhyUoceras, and fragments of
ammonoids, all of which were rare. Still higher in the section fossils are
more frequent, but not plentiful. In the lower half of the sequence only
such species have been found as seem capable of a short existence in
brackish water, or such as may have drifted here from other places. These
forms include none of the larger ammonoids or other marine species that
are abundant in contemporary beds a little to the north or to the south.
In this axial area, along with a scanty moBuscan fauna, arc much vegetable
debris, finely divided leaf remains', specks of carbonised wood, and,
less frequently, pieces of poorly fossilized wood. Toward the south, along
the direction of drift, plant remains are more abundant, especially in the
lower parts of the sections, near the base of the Shasta series. From
some of these lower beds many plant species have been obtained as identified
by Knowlton (Diller, 1908), of which Diller gives a considerable list.
These may be regarded as drift materials brought into the embayment
by tbe river, carried southward by its normal currents, and dropped at
points where the currents were checked by inflowing tides or other causes.