UNIVERSITY

ANTICLINES IN A PART OF THE MUSSELSHELL VALLEY, MUSSELSHELL, MEAGHER, AND SWEETGRASS COUNTIES, MONTANA.

By C. F. Bowen.

INTRODUCTION.

Previous investigations had shown that the Musselshell Valley, Mont., is an area in which the rocks have undergone considerable folding. On the basis of this information work was begun in June, 1916, to determine the nature and extent of the folds and to make examination as to the possible occurrence of accumulations of oil and gas in them. The work has demonstrated the existence within the area studied of several well-developed anticlines and domes, which seem to offer structurally favorable places for the accumulation of oil and gas. The demonstration of the presence or absence of commercial accumulations of these fluids in the folds has been less conclusive, owing largely to the undeveloped condition of the area.

No direct surface indications of oil were observed, but hydrogen sulphide gas escapes from several water seeps in one part of the area. It is also reported that gas was encountered in several wells dug for water. None of these reports could be verified. At the time of the examination drilling operations within the area were confined to two wells. One of these wells, on the Big Elk dome, had reached the Kootenai formation; the other, in the Woman's Pocket anticline, was probably somewhat more than halfway through the Colorado shale. In neither place had oil been discovered. Two other wells, one about 15 miles east of the area and the other about 12 miles south of the central portion of it, had previously been drilled through the Colorado shale without any discovery of oil. Although these results are not altogether encouraging, it is believed that the field has not yet been thoroughly tested.

Sandstones that would serve as suitable reservoirs for the accumulation of oil occur at several horizons. (1) Near the top of the

21544°-19-Bull. 691-13

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Colorado shale there is a transition zone of thin sandstones and sandy shale beds. (2) About 1,200 feet below the top of the Colorado a thick porous sandstone, slightly conglomeratic at the top, is well developed in the western part of the field but seems to be nearly or quite absent in the eastern part. This sandstone has approximately the same stratigraphic position as some of the sandstones in the Frontier formation. (3) About 250 to 300 feet lower in the section is another sandstone of similar character but much thinner and more distinctly conglomeratic. (4) Associated with and underlying the Mowry shale member, in the eastern part of the field, are several thin, finely conglomeratic sandstones. (5) At the top of the Kootenai there is 40 to 50 feet of platy, rather fine grained sandstone in approximately the same position as the Greybull sand of the Big Horn Basin, Wyo. (6) Near the base of the Kootenai there is another coarse, porous sandstone of undetermined thickness. The positions of these sandstones and their approximate relations to the productive sandstones of the Elk and Big Horn basins are shown in the columnar sections A, B, C, Plate XXV (in pocket).

The Eagle sandstone is not a likely source of oil or gas in most of this area, for it has been removed from the crests of most of the anticlines, and thus any oil or gas that may have originally been stored in it has had an opportunity to escape.

It is the purpose of this report to discuss the geology of the region as a whole, and also to suggest the most favorable places for future development.

LOCATION AND SURFACE FEATURES.

The area herein described includes about 1,200 square miles lying chiefly in Tps. 7 to 9 N., Rs. 12 to 23 E. Montana principal meridian. It extends from the town of Martinsdale on the west to a point within about 10 miles of Roundup on the east, and lies for the most part north of Musselshell River but in the western portion also extends south of that stream.

Being situated on the western border of the Great Plains, the area exhibits characteristics common to both plains and mountains. To the north are the Big Snowy and Little Belt mountains, and on the west and southwest are the rugged, snow-clad Crazy Mountains. Within the area described, however, the surface features are chiefly those of a plain dissected by erosion and further modified by folding and upheaval. For the most part the highest surfaces of the plain are covered by terrace gravels which rise gradually and likewise become more continuous toward the mountains.

This absence of abrupt and extreme surface irregularity renders the establishment of routes of travel comparatively easy. The Chi

cago, Milwaukee & St. Paul Railway traverses the area from east to west along the valley of Musselshell River, and the Great Northern line between Billings and Great Falls crosses the area from southeast to northwest. These two railways, together with the numerous wagon roads and trails, bring every part of the area within easy

access.

PREVIOUS WORK.

1

2

The geology and coal resources of the Musselshell Valley were briefly described by Lindgren in 1886. The western part of the area, lying west of Twodot, was described by Weed in 1899. Later Stone, Woolsey, and Richards, who were engaged in a more detailed survey of the coal fields of this general region, mapped and described that portion of the area lying south of Musselshell River. Several other papers that deal with the stratigraphy of the western part of the area have been published, of which the most important are by Douglass, Stanton and Hatcher,' and Stone and Calvert."

FIELD METHODS.

The field work on which this report is based was done by the planetable method. Horizontal control was obtained by a system of triangulation expanded from base lines 1 mile or more long. Partly for convenience and partly for the purpose of eliminating errors that may have crept in, three such base lines were measured during the progress of the work. Land corners were located wherever practicable, and these furnished a basis for adjusting the triangulation system to the land net compiled from the plats of the General Land Office. Vertical control was carried throughout the field by means of vertical angles read to all the triangulation stations and to numerous other prominent points. The initial elevation was taken from a point on the Chicago, Milwaukee & St. Paul Railway near Roundup.

1 Lindgren, Waldemar, Montana coal fields: Tenth Census U. S., vol. 15, Mining industries, pp. 743-746, 1886.

2 Weed, W. H., U. S. Geol. Survey Geol. Atlas, Little Belt Mountains folio (No. 56), 1899.

Stone, R. W., Coal near the Crazy Mountains, Mont.: U. S. Geol. Survey Bull. 341, pp. 78-91, 1909.

1909.

Woolsey, L. H., The Bull Mountain coal field: U. S. Geol. Survey Bull. 341, pp. 62-77,

Richards, R. W., The central part of the Bull Mountain coal field, Mont.: U. S. Geol. Survey Bull. 381, pp. 60-81, 1910.

Douglass, Earl, A Cretaceous and lower Tertiary section in south-central Montana : Am. Philos. Soc. Proc., vol. 1, pp. 207-224, 1902.

'Stanton, T. W., and Hatcher, J. B., Geology and paleontology of the Judith River beds: U. S. Geol. Survey Bull. 257, pp. 59-60, 1905. Stanton, T. W., The age and stratigraphic relations of the "Ceratops beds " of Wyoming and Montana: Washington Acad. Sci. Proc., vol. 11, pp. 251-265, 1909.

Stone, R. W., and Calvert, W. R., Stratigraphic relations of the Livingston formation of Montana: Econ. Geology, vol. 5, pp. 551-557, 652-669, 741-764, 1910,

This control was checked by "tying in" to points on the railway twice during the season. From the field sheets thus constructed and the township plats obtained from the General Land Office the map accompanying this report was compiled with such adjustments as were necessary to bring the two sets of data into harmony.

Scientific assistance in the field was rendered by W. P. Woodring and George E. Dorsey, whose efficiency and general interest in the work have contributed largely to the results obtained.

GEOLOGY.

SEDIMENTARY ROCKS.

GENERAL SECTION.

With the exception of the alluvium and terrace gravel the sedimentary rocks within the area described range from the Kootenai to the Lance. Igneous rocks are also present in small amounts in the vicinity of the Big Elk and Little Elk domes. The sedimentary formations recognized are, in ascending order, the Kootenai, Colorado, Eagle, Claggett, Judith River, Bearpaw, and Lance. In the eastern part of the area these formations are well defined and easily recognized. In the western part, as a result of a marked development of andesitic material and an increasing amount of sandstone in the Claggett, the Eagle, Claggett, and Judith River merge into one another, and so far as lithologic character is concerned are difficult to separate. The areal distribution of the formations is shown on Plate XXV. Their sequence and character are set forth in the following generalized table:

Generalized section of geologic formations in a part of the Musselshell Valley, Mont.

Generalized section of geologic formations in a part of the Musselshell Valley, Mont.-Con.

Characteristics.

Dark-gray to black marine clay shale containing large calcareous concretions. An especially heavy band of sandy concretions near the base seems to develop in places into a rather well defined sandstone.

As here mapped consists of (1) an upper part composed chiefly of alternate beds of sandstone and shale forming a more or less well-defined ridge or series of ridges; (2) a middle part consisting chiefly of light-buff to gray or white clay shale with some interbedded sandstone which in places of low dip weathers into badlands; (3) a lower part, here included in the Judith River for convenience in mapping and because of lithologic similarity, which seems to be the approximate equivalent of the sandstone assigned to the upper part of the Claggett in its type area and also in the eastern part of the Big Horn Basin. This lower part consists chiefly of sandstone with a thick bed of massive ledge-making sandstone at the base. All the sandstones, especially those in the middle and upper divisions, become markedly andesitic in the western part of the field.

In the eastern part of the field the Claggett is a darkdrab concretionary shale similar to the Bearpaw. It becomes increasingly more sandy toward the west until it consists entirely of sandstone and sandy shale.

Consists of three members. The upper member is composed of moderately thick sandstones interbedded with shale, forming a ridge generally marked by several parallel ribs. The middle member consists of shale and thin-bedded sandstones that erode easily and form an area of depression. The lower member (Virgelle sandstone) is a thick massive white sandstone that forms a pronounced scarp or hogback. In the western part of the field, however, this member becomes more thinly bedded and loses its ridgemaking character. The sandstones of this formation also become more or less andesitic toward the west.

In the eastern part of the field the lower 400 to 500 feet contains at least three slightly conglomeratic sandstones 5 to 20 feet thick, above which is 10 to 30 feet of sandy, fissile indurated shale which weathers white and contains numerous scales and other fragmentary fish remains. This represents the Mowry shale. The probable representative of the Frontier formation is a zone of thin shaly sandstones about 30 feet thick, with a calcareous sandstone at the top. Farther west, in the Shawmut anticline, the Mowry shale can not be recognized. The lower 700 or 800 feet of the formation consists of alternating black, fissile, somewhat sandy shale and thin quartzitic sandstones, one of which, about 500 feet from the base, is conglomeratic. Above this zone is the Big Elk sandstone member, 200 feet or more thick, consisting chiefly of sandstone slightly conglomeratic at the top. The remainder of the formation is chiefly shale.

Lower Cretaceous.

Kootenai forma-
tion.

250 +

Consists mainly of alternating beds of maroon and white sandy shale, containing some beds of more or less concretionary sandstone in the upper part. At the top is about 50 feet of thin-bedded, platy, rusty sandstone in which are numerous markings resembling worm trails: at the base of the exposed section is a coarse gray sandstone of unknown thickness.