Geological Survey Professional Paper 296 Geology of Glacier National Park And the Flathead Region, Northwestern Montana

GEOLOGY OF GLACIER NATIONAL PARK AND THE FLATHEAD REGION NORTHWESTERN MONTANA

By CLYDE P. ROSS

This report summarizes available data on two adjacent and partly overlapping regions in northwestern Montana. The first of these is Glacier National Park plus small areas east and west of the park. The second is here called, for convenience, the Flathead region; it embraces the mountains from the southern tip of Glacier Park to latitude 48° north and between the Great Plains on the east and Flathead Valley on the west. The fieldwork under the direction of the writer was done in 1948, 1949, 1950, and 1951, with some work in 1952 and 1953.

The two regions together include parts of the Swan, Flathead, Livingstone, and Lewis Ranges. They are drained largely by branches of the Flathead River. On the east and north, however, they are penetrated by tributaries of the Missouri River and in addition by streams that flow into Canada. Roads and highways reach the borders of the regions; but there are few roads in the regions and only two highways cross them. The principal economic value of the assemblage of mountains described in the present report is as a collecting ground for snow to furnish the water used in the surrounding lowlands and as a scenic and wildlife recreation area. A few metallic deposits and lignitic coal beds are known, but these have not proved to be important and cannot, as far as can now be judged, be expected to become so. No oil except minor seeps has yet been found, and most parts of the two regions covered do not appear geologically favorable to the presence of oil in commercial quantities. The high, Hungry Horse Dam on which construction was in progress during the fieldwork now floods part of the Flathead region and will greatly influence the future of that region.

The rocks range in age from Precambrian to Recent. The thickest units belong to the Belt series of Precambrian age, and special attention was paid to them. As a result, it is clear that at least the upper part of the series shows marked lateral changes within short distances. This fact introduces complexities into stratigraphic correlation and should be remembered wherever the series is studied. The stromatolites, or fossil algae, in the Belt series, although still imperfectly understood, give clues with respect to problems of ecology and stratigraphy.

The subdivisions of the Belt series within the areas covered by the present report are, in ascending order, Altyn limestone, Appekunny argillite, Grinnell argillite, Siyeh limestone, and Missoula group. Local subdivisions of the Missoula group are possible in certain areas, and all the units just named are expected to be subdivided when detailed studies are undertaken.

In the Glacier National Park and Flathead regions together, it is probable that between 25,000 and 30,000 feet of beds belonging to the Belt series, possibly more, are present. These consist largely of quartzitic argillite, quartzite, and carbonate rocks, mostly dolomitic. Small gabbroic and diabasic intrusive bodies and, at one horizon, basaltic lava are associated with the Belt series. Above the Belt series is a thick sequence of Cambrian, Devonian, and Carboniferous strata, in which limestone is dominant, followed by strata of Jurassic and Cretaceous age, largely limestone and shale and partly of terrestrial origin. Slightly consolidated gravel, sand, and silt of Tertiary age are preserved in some valleys and as erosional remnants on the plains close to the mountain border. Pleistocene and Recent glacial and fluviatile deposits are plentiful in mountain valleys and on the plains east of the mountains.

Sufficient crustal movements took place during the latter part of Belt time to produce tension cracks that permitted some intrusion and related extrusion to occur. Broad crustal warping probably took place at intervals during the Paleozoic era, but these successive movements left little record other than the absence of sedimentary rock units that might otherwise have been deposited. The same can be said of much of the Mesozoic era, but the uplift that was to give rise to the mountains may have begun during the Cretaceous period.

In or shortly after the late Paleocene, conditions changed drastically. Thrust and normal faults of major magnitude, preceded and accompanied by folds and minor fractures, resulted from a series of violent crustal movements that have not yet entirely ceased. The master structural feature produced during this deformation is known as the Lewis overthrust. Much remains obscure, but the concepts resulting from the present study are that the Lewis thrust originated fairly deep in the earth's crust and that the overthrust block moved many miles in a direction somewhat north of east over a mass of relatively incompetent rocks that were intricately folded, overturned, and overthrust to depths thousands of feet below present sea level before and during the advance of the main thrust. If, as seems quite possible, the thrust plane ever emerged at the surface, it was at some place so far to the east that erosion has since removed the evidence. The concept of a sole with which the exposed thrusts merge at depth and which is underlain by undisturbed rocks is not supported by evidence at hand. The block above the Lewis overthrust was itself deformed but, in the part now remaining, much less intensively than the rocks below. Adjustments that may have begun during the overthrusting and continued to the present have fractured the overlying block.

A series of geomorphic events not clearly recorded in the present topography reached a culmination in a mature surface (the Blackfoot surface) near the end of the Tertiary period. Several stages of glaciation with an intermediate stage in which rejuvenated stream erosion cut deep gorges have modified and, over large areas, obliterated the Blackfoot surface. Nevertheless, the present topography still reflects the structure of the underlying rocks. Since the close of the Pleistocene epoch, renewed erosion has caused the streams to cut small inner gorges in the valley floors. Also small glaciers reappeared in the uplands carved by the far larger Pleistocene ice streams, and some of these minor glaciers persist to the present day.