LOWER CRETACEOUS DEPOSITS CALIFORNIA AND OREGON

62 LOWEB CBETACEODB DEPOSITS IN CALIFORNIA. AND OREGON
town group and the oldest port of tbe Hauterivian stage in the Cottonwood
district.
No clearly marked stratigraphical break between the Paakenta and-
Horsetown groups has been found here, although the conglomerate beds
extending south from Ono may be taken as such. Otherwise, only an
arbitrary line has been drawn for it. The rich fauna of the Ono zone at
the bottom of the Horsetown group seems to represent a low Hauterivian
horizon on the scale of Europe, and for this reason it forms a convenient
reference plane, serviceable for correlation, and it is believed that tbe sequence
of faunas in the Cottonwood district above and beneath it fully
supports this view. In the district of Themes and Elder creeks, Tehama
County, the exact equivalent of the Ono zone has not been found, although
it is probably a little below the horizon of "Zone R" of the McCarthy
Creek section (Anderson, 1033, opp. p. 1242). This is indicated by some
of the species named by Stanton and by various moll us can forms found by
the writer.
In the Cottonwood district, above the conglomerate overlying the Ono
zone, there is a thick succession of sandstones, sandy shales, and argillaceous
shales, totaling 6500 feet of strata as exposed along tbe stream
east of Ono and on Hulen Creek, a northern tributary of it. The uppermost
bed of the Horsetown group in this section crosses the North fork
and also Hulen Creek, about half a mile above their junction. On both
streams the Horsetown group is overlaid unconformably by the basal
beds of the Chieo series. East of the east branch of Hulen Creek the
Cretaceous beds are covered by the Tehama formation, consisting of
volcanic tuffs.
Five miles northeast of Hulen Creek the canyon of Clear Creek has cut
through tbe Tehama formation, exposing the uppermost beds of the Horsetown
group at the site of the old mining camp of this name, and also at
Jackass Flat, a mile or more east of Horsetown. Tho horizon exposed at
Texas Springs may he above its upper Emit, although many of the species
found at Horsetown also occur here.
Southward from the North fork of Cottonwood Creek many of the
more characteristic fossil zones of the Horsetown group may be recognized
upon all the streams coming from the west, and the group can
be traced across the delta areas and as far south as Redbank, Elder, and
McCarthy creeks, and as far as the valley of Berryessa Creek. South of
the delta axis, east of Stephenson's on the Cold fork, conglomerate bedB
similar to those on the North fork seem to occupy the basal part of the
Horsetown, although the stratigraphic details have not been studied here.
South of McCarthy Creek the Horsetown group baa been recognized
only by its stratigraphical relations at top and bottom, although several
diagnostic fossils have been found in it.