LOWER CRETACEOUS DEPOSITS CALIFORNIA AND OREGON

CRETACEOUS SEDIMENTARY DIVISIONS 01
complete, although there are many references to the occurrence of AuceUa
crassicollis Keyserling and to a few other forms that occur in the Shasta
series of California. Wherever AuceUa crassicoUis occurs in these several
accounts it may be taken as evidence of the early Cretaceous age of tbe
beds, but perhaps not always of tbe Paskenta (Valanginian) age.
FEATURES OF THE HORSETOWN GROUP
Type District.—The Horsstown group* is typically developed in tbe
Cottonwood district north of the delta axis. Its stratigraphical relations
at both top and bottom are best shown here, and it is best supplied with
diagnostic invertebrate fossils and with faunal zones which range throughout
its Btmtigraphical sequence in such a way as to make it of unusual
value for faunal and chronological study. When taken together with the
faunas of the Paskenta group they give to the district an interest and importance
not yet found in any other West Coast area for showing the
faunal character and the faunal succession throughout its Lower Cretaceous
sequence. Indeed, this district could well supply a standard reference
column for the Lower Cretaceous faunas in all parts of the Pacific
Coast and aid in correlations of strata much farther.
Like the immediately older group, the Horsetown group in this district
becomes progressively thicker when followed southward from the North
fork of tbe Cottonwood Creek toward the delta axis, increasing from a
thickness of 6430 feet near Ono to more than 12,500 feet in its maximum
section. In all sections north of the delta the group begins at the bottom
with conglomerate beds which in some places make up a large percentage
of its lower portion, but which for the most part are lenticular. Near
Ono there is only a single bed of conglomerate, about 60 feet thick, but a
few miles to the south three beds appear, and on Roaring River there is a
succession of four such beds within a stratigraphical thickness of 10QQ
feet. When traced farther south these beds pass into the sandstones and
sandy shales of the delta axis and disappear. South of the delta similar
conglomerate beds appear near the base of the group and become conspicuous
on the Cold fork of Cottonwood Creek. The single bed found at
Ono is probably not the lowest one of the series. Immediately beneath
it there is a richly fcssiliferous zone of thin-bedded sandstones, 200 feet
thick, which can be traced eastward to Eagle Creek, and which may be
known as the Ono zone. This zone forms a near-basal part of the Horse-
< It. may be sated bwo tbmt tb* LMtfl Honwtcsm, u a. fDnnnlkilHil nam* fa; lbs nuctefition a! suit* U>
which It but boon »ppti«4, u Das a happy cbai«, Tho artn about tbe old mining camp of ttia au» In
tiebnf from Use iisif 1st in which the uroup in moat typically and eoin'pLetciy Ts-jtrosantrci and it txui oipoaa
net juara thill 600 feet ol the uppermost b«U of tbe Shrutu aerUs. which bed* ire ecutnety ifprawotative frf
the meat body ol older iiiaia indudod onicr lbs oatno by Whit*, fitaniqa, and DUlet, and fay intnr wrltera.
Although the name ii trtalntd (or the STDUP ID tlis pfreoni rotmoir. it is only Ita u» in the literature of tbe
put IbaliiutiEci lticontinunn«niaiorai»(io[llii name. The «rm Cottonwood group would have beta to
more (Lppreprint* far it.