OCR Interpretation

Montana oil and mining journal. [volume] (Great Falls, Mont.) 1931-1953, September 29, 1945, Image 5

Image and text provided by Montana Historical Society; Helena, MT

Persistent link: https://chroniclingamerica.loc.gov/lccn/sn86075103/1945-09-29/ed-1/seq-5/

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zone, the contact is arbitrarily
placed near its center.
The contact between unit MB and
the underlying unit MC is also
somewhat transitional, but may be
placed within very close limits be
tween the more heavily bedded
limestones at the base of unit MB,
and the thin-bedded, argillaceous,
dense limestones below. A charac
teristic feature of unit MB is the
presence of occasional beds of frag
mental limestone, which may be
used to distinguish unit MB from
unit MC in the subsurface.
In the outcrop areas the unit is
divisible into two subunits, desig
nated MB. and MB.. The upper sub
unit, MB., consists of white to light
gray, buff, and brown coarsely
crlnoid-fragmental massive lime
stone. A few beds of medium-brown
gray, dense to sparsely fragment
unestone are present, as are zones
hearing abundant white to light
gray chert. At the top, in the ques
tionable transition zone, are a few
tens of feet of dense to saccharoidal
dolomite interbedded with frag
mental limestone. Subunit MB. con
sists of well-bedded medium-brown
to dark-gray, dense to fragmental
limestone, In beds up to two feet
thick, separated by thin partings of
calcareous shale or shaly limestone.
The dense limestone carries an
abundance of gray chert. In the sub
surface the distinction between MB,
and MB, is drawn with difficulty,
but the break may be logged on
the basis of texture and color.
Unit MB bears a brachiopod and
coral assemblage typical of the
Osage portion of the Madison group.
In lithologie composition
graphic position it appears that MB.
represents the Mission Canyon lime
stone, whereas MB, represents the
Wood hurst limestone member of
the Lodgepole limestone. Unit MB
is approximately the same as Deiss'
Rooney chert member of the Madi
son and is correlative with the
Osage portion of the Rundle lime
stone of Alberta.
Unit MC. —Unit MC, the basal
unit of the Mississippian of this
area, rests with complete conform
ity on the Upper Devonian. In the
outcrop areas the relationship ■ is
generally obscured by slumping and
brecciation involving the upper
beds of the Devonian and in some
areas extending up into the Missis
sippian. The most obvious break is
marked by the base of a black
fissile carbonaceous shale; however,
between the black shale and the
lop of the Devonian in many local
ities there intervene a few feet of
gray or brown, dense to fragmental
limestones bearing a Kinderhook
fauna. In most places the base of
the zone of dense to fragmental tex
ture, resting on the zone of saccha
roidal texture, coincides with the
lowest appearance of Mississippian
fossils; in the Sawtooth range, how
ever, the picking of the contact is
very difficult. In the subsurface the
top of the Devonian is readily es
tablished by the top of a persistent
green shale.
Lithologically, unit MC is charac
terized by thin-bedded dense black
argillaceous limestones interbedded
with black or gray calcareous shales
and bearing an abundance of black
al 1
and strati
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chert, some of which may be bed
ded. Near the base, black to dark
brown shale or highly carbonaceous
platy limestone is often encoun
tered, but the carbonaceous mate
rial does not seem to form a persis
tent zone. Throughout the unit thin
lentils of fragmental material are
present and crinoid-fragmen tal lime
stone is often prominent in the
basal few feet On
arch the lower portion of the unit
is characterized by persistent sandy
limestone and limy sandstone.
Unit MC is -roughly equivalent to
Deiss' Dean Lake chert member of
the Madison and is probably corre
lative with the Banff shale of Al
berta and the Paine shale member
of the Lodgepole limestone of cen
tral Montana. A Kinderhook age is
indicated by preliminary interpre
tation qf the megafauna.
The black shale zone is similar
stratigraphic no
shaw shale of Albe
has shown that the type Exshaw
is Upper Devonian in age. It is
highly probable that the shale is
equivalent to a black shale zone en
countered in central and southwest
ern Montana and in the subsurface
of Alberta and Montana. *
Devonian Rocks
the Sweetgrass
sitlon to the Ex
rta, but Warren *
Unit DA. —Unit DA rests conform
ably on the Devonian Unit DB, the
contact being between saccharoidal
dolomites and dense limestones. In
the subsurface the unit is charac
terized by massive anhydrite Inter
bedded with brown, dense to saccha
roidal dolomite, the dolomite in
creasing and the anhydrite decreas
ing toward the base. At the top are
20 to 60 feet of green and gray
green noncalcareous shale; similar
shale appears in minor amounts
interbedded with anhydrite and
dolomite throughout much of the
unit. A few beds
occur. In the outcrop areas, the
anhydrite has been removed by
solution, causing slumping and brec
ciation of much of the upper three
fourths of the unit and making pos
sible a division into two subunits
DA, and DA,. Unit DAi, the breccia
zone, is remarkably persistent and
contains a mixture of all the rock
types mentioned above, except an
hydrite, as well as fragments of
Mississippian rocks collapsed from
above. The breccia is composed of
angular blocks ranging from a frac
tion of an inch to several tens of
feet across, firmly cemented by
dense to saccharoidal brown lime
stone or dolomite. There are in the
breccia relatively undisturbed se
quences of strata which represent
dolomite beds of sufficient thick
ness to be unaffected by solution
of anhydrite. Unit DA, in its outcrop
is almost without breccia and con
sists of massive brown coarsely
saccharoidal dolomite, often cross
laminated. Insoluble residues from
the cross-laminated zones* exhibit
no sand grains.
Fossils are rare and poorly pre
served in the unit; however, the
of limestone also
Toole County
Abstract Company
following corals are recognized;
Phillipsastraea sp. cf. P. ma couni
Disphylltun catenatum Smith.
Correlation is indicated with the
upper part of the Minnewanka
limestone of Alberta, and part of
the Threeforks formation of central
and southwestern Montana. The
Silvertip conglomerate member of
the Madison, and the Spotted Bear
limestone and Lone Butte limestone
members of the Jefferson described
by Deiss apparently fall within this
unit. Perry * has applied the term.
Potlatch anhydrite, to this unit in
the subsurface of the Sweetgrass
arch (type section is section 12
the chart) and the usage appe
be applicable throughout Iht
surface of the area covered by the
chart. "
Unit DB.—On the Sweelgra
arch unit DB lies disconformably on
Cambrian shales; elsewhere it is
underlain conformably by Devonian
unit DC, the contact being marked
by a fairly abrupt transition from
limestones above to shale and shaly
dolomite below. Unit DB is domin
antly brown to brownish-gray dense
limestone, becoming slightly argil
laceous toward the base. A few thin
zones of saccharoidal dolomite are
present, and some of the limestones
bear globular segreg
brown saccharoidal
dolomite. Many beds are marked by
anastomosing worm burrows or so
lution channels filled withyellow
ish argillaceous limestone. The unit
is well-bedded and tends to form
steep cliffs. Large masses of strom
are to
e sub
allons of olive
limestone or
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atoporoids are common. In the sub
surface the limestones contain a
few thin anhydrite beds, and a few
thin breccia zones appear in out
'Delss, Charles, Paleozoic forma
tions of northwestern Montana:
Montana Bur. Mines and Geology,
Mem. No. 6, 1933.
•Deiss, Charles, Stratigraphy
structure of southwest Savoo c
Bull. Vol. 54
ie, Montana: Geol. Soc.
, pp. 228-231, 1943.
•Warren, P. S., Age of the Exshaw
shale in the Canadian Rockies: Am.
Jour. Sei., 5th ser., Vol. 33, pp. 454
457 1937
^Cooper, C. L„ and Sloss, L. L.,
Conodont fauna and distribution of
a lower Mississi
Montana and Alberta: Jour. Paleont
ology, Vol. 17. pp. 168-176,1943.
•Perry, E. S„ The Kevln-Sunburst
and other oil and gas fields of the
Sweetgrass arch: Montana Bur.
Mines and Geology, Mem. No. 1 (2d
ed.), p. 5, 1929.
n black shale in
Important Note: It was found
impossible to run this article in
its entirety in this issue. Con
cluding portion will appear next
Licensed Bonded

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