LOWER CRETACEOUS DEPOSITS CALIFORNIA AND OREGON

78 3WJWEH CRETACEOUS DEPOSITS IN CALIFORNIA AND OREGON
accounts that the Lower Cretaceous deposits containing AuceUa. cra&si*
collis and associated fossils rest in some places upon Upper Jurassic (?)
strata with Aueella cf. piochi, and in other places upon Triassic, or even
upon Paleozoic formations (Chitins Valley, Admiralty Island, Rampart-
Tanana district, and upper Yukon region). These facta may be taken
aa evidence that in Alaska, as in California and Oregon, the Lower Cretaceous
sequence began with a "marine transgression," following a widespread
subsidence that led the aea into many already-formed troughs,
ancestral to existing valleys. As far aa known, the faunas most characteristic
of the Lower Cretaceous in Alaska are similar to those of the
Paskanta group in the Shasta series in California, and, without evidence
to the contrary, they may be assumed to be contemporary with them.
Concerning the forms of Aucella found in the early CretaccoiiB of the
Matanuska region Stanton (Martin, 1928, p. 313) says, in part:
"This ia the same large species which was identified aa A. cravricoUit Koyaarling
in MeadenhaH's collection from Bubb Creek. While this Alaskan species may not
belong to A. crasticollis, aa restricted by the Russian paleontologists, who recognize
a very largo number of species of Aueetia, it does belong to a group of forms which
is most abundant in the Lower Cretaceous, and is therefore suggestive of Cretaceous
age,. .
Horsetown Group,—Beds that may be correlated with the upper part
of the Horsetown group seem to have been found in scattered localities
in Alaska (Alaskan Peninsula, Chitina Volley, and lower Yukon), but, if
we may judge from the summary accounts and especially the lists of
marine invertebrates given by Martin, the evidence is not abundant and
not always very convincing, An examination of the fossil lists from the
various sections of the Lower Cretaceous in Alaska has failed to reveal
any species characteristic of the lower part (Hauterivian or B&rremian)
of the Horsetown group. None of the larger ammonoids found in the
Cottonwood beds have been recorded, and in many places the absence of
species representing upper Horsetown beds leads to the suspicion that they
are absent. In this respect the Lower Cretaceous in Alaska resembles
that in California in that slow subsidence led to transgreseive overlaps
throughout Cretaceous time. It has already been shown that south of
the Great Valley of California no faunol evidence of Horsetown strata
has been recorded.
Mexico and Tezas.—The Lower Cretaceous deposits in Mexico and
Texas have little resemblance to the Shasta series in California and Oregon,
either faunally, lithologieally, or in stratigraphic thickness. For the
region of Mazapil and Concepcifin, south-central Mexico, Burckhardt
(1930) gives a general column (Valanginion to Cenomanian) totaling
3380 feet; for northern Chihuahua, Bitee (1910b) gives a maximum for
Middle and Lower Cretaceous of 870Q feet, of which less than half Is
older than Albian. Kellum (1936) gives a summary of the Cretaceous