Montana oil and mining journal. [volume] (Great Falls, Mont.) 1931-1953, 1941-01-25
FINDS DEVONIAN

lCOntinued from Pass onel
Montana by the Montana-Dakota
deep test which found Devonian
oil. Whether Devonian Is present
in central Montana was considered
doubtful. Calvert and Reeves re-
ported that Mis5issippian hmestones
rest directly upon Cambrian rocks,
and that Devonian sediments are
absent in the Big Snowy Mountains.
DR.. Deiss found $50 feet of Devon
ian dmestones and dolomites rest
ing upon an eroded surface of
Upper Cambrian shales and lime
stones near Hah Noon Pass. These
Devonian limestones in the Big
Snowy mountains were tentatively
correlated with the Upper Devon
ian Three Forks shale.
DR.. Deiss write:
Found Throughout Region
Throughout the Rocky Mountains
Of western Montana, Upper or
Middle Camlbrian sediments are
overlain by Middle and Upper De-
yonian rocks called the Jefferson
formation" by Peale (U.S.G.S).
These Devonian limestones are
oyerlain by Mdssisslppian rocks, also
named by Peale, the Madison
limestone. -
The writer measured II sections
of PaleozCic rocks in northwestern
Montana during 1882 and 1888,
and. in that connection. examined
the literature of the subject for the
entire state. That study disclosed
the fact that the only Paleozoic
section in western and central
Montana in which DevOnian rocks
were reported to be absent is In
the Big Snowy Mountains. Because
the jefferson formation is we'll de-
Yeloped and relatively uniform in
thickness throughout the state
and particularly in the Little Belt
mountains, the reported absence OF
Devonian rocks in the Big Snowies
only 80 miles west, seemed very
improbable. Similar anomalies in
the Cambrian section OF these
mountains. such as Calvert's fig
ure of $50 feet OF Wolsey shale,
Cambrian sediments appeared like
wise doubtful. because the Wolsey
averages only 240 feet in thickness
elsewhere In Montana, and the
Upper Cambran rocks thicken east
ward in all of the other sections
in central and western Montana.
In 1885, during a revision of the
Cambrian type sections and forma-
tons of Montana and Yellowstone
National park, 1t became necessary
to remeasure the Half Moon Pass
section in the Big Snowy mountains
and to examine the everything Pale
ozoic rocks in contact with the
highest Cambrian beds. The OPP
portunity was thus afforded to
study hastily the Devcnian and
Lower Mississippian rocks.
Are PetroIifcrOus Lines
The top of the Cambrian section
is composed of 80 feet of moroon,
oIive-green and gray fissile shales,
massive argsllaceous limestones and
hard, crystalline, intraformatiOnal
conglcmerates. These beds con-
tain Upper Cambrian triloIbites and
are tentatively correlated with the
Dry Greek shale instead of the
Meagher limestone OF the Little
Belt mountains. The upper part
OF the Cambrian rocks is eroded
to depths of three feet within hori-
zontal distances of so feet or less.
Overlying the Dry Creek shale dis
are $50 feet OF thick
and thin-bedded, gray, buff and |
chocolate-gray. argillaceous. gem
orally dolomite PETROLIFEROUS |
DEVONIAN LIMESTONES (caps |
by editor) which are strikingly |
different in composition and APP
pearance from the subjacent Cam
brian and superjacent Mississippian
beds. Within a zone 2O-35 feet be-
OW the ton Of these DeVonfan rocke