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

STRUCTURE (continued)

BROAD FLEXURES OR WARPS

PRECAMBRIAN STRUCTURAL FEATURES

Throughout northwestern Montana and adjacent regions the Belt series shows little evidence of deformation prior to the deposition of Cambrian strata. The lack of clean-cut evidence of unconformity on Cruiser Mountain was noted in the description of that mountain. Similar conditions exist in many places in northwestern Montana. For example, south of Missoula the passage from beds of the Belt series into the Flathead quartzite of middle Cambrian age is made without noticeable hiatus. When that area was visited in fieldwork related to compilation of the new geologic map of Montana (Ross, Andrews and Witkind, 1955), much doubt was felt as to the existence of any stratigraphic break at the top of the Belt series there. However that may be, such conditions emphasize the fact that any deformation that occurred prior to the deposition of Paleozoic strata must have been gentle.

In the part of the State here described the igneous rocks of Precambrian age afford evidence that even before deposition of the Belt series ceased gentle deformation took place. The small intrusions of metagabbro, which are related to the Purcell lava, presumably had to take advantage of paths of easy access. That is, if the great thickness of resistant strata of the Belt series had been undisturbed and unbroken, the gabbroic magma might well have been unable to penetrate into the positions it did. Probably enough diastrophism occurred to produce long tension cracks, now filled by thin dikes of metagabbro. The sills of similar rock (figs. 3, 16) may have occupied places where warping had tended to buckle the beds and thus to produce weaknesses or even actual openings between beds. As the magma reached the surface and produced lava flows, the load of sedimentary rocks above the sills was not great. The irregular intrusive masses of metagabbro in the Flathead region and the small exposures of effusive basalt west of the Apgar Mountains are at stratigraphically higher horizons than the dikes, sills, and flows in the northern and central parts of Glacier National Park. Hence, igneous activity persisted even after the principal flows had erupted. The character of much of the Missoula group is in harmony with the idea that, through uplift or otherwise, the Belt seas were retreating.

The conclusion to be drawn from the character and distribution of the bodies of igneous rock is that gentle but widespread diastrophic disturbances began fairly early in the deposition of the Missoula group. Deiss (1935) has assembled data in favor of the concept of an unconformity between the Belt series and the lowest Paleozoic strata in and near western Montana. His evidence indicates that, whether or not sedimentation was interrupted everywhere, there was uplift and perhaps some flexure of the rocks of the Belt series. The paucity of evidence of marked angular discordance anywhere in Montana indicates that the diastrophism was not intense. At the most, it effected regional uplift and gentle warping without sharp folding or faulting. In the regions here described, no evidence of discordance at the top of the Belt series has been obtained, but the inference from the ideas expressed by Deiss (1935) and widely held among geologists is that some uplift took place. Presumably it was a continuation of that commented on above.

PALEOZOIC STRUCTURAL FEATURES

The only record available of diastrophic disturbances during the Paleozoic era in Glacier National Park and the Flathead region is the absence of sedimentary rocks that might otherwise have been deposited. The crustal fluctuations that began during the Precambrian may have continued into Early Cambrian time, but conclusive evidence on this point was not obtained during the present investigation.

At higher horizons, hiatuses can be demonstrated by means of fossils even though angular discordances are not recorded. Thus, the fact that Devonian strata rest on those of late Cambrian age implies retreat of the seas during that interval, which in turn suggests widespread relative uplift. Similarly interruptions of deposition in Late Devonian and late Carboniferous time may result from relative uplift of broad regions. The crustal movements were so broad and gentle that other results from them have not been detected. The regional flexures were not, however, of continental scope, for they did not take place during the same intervals in Alberta to the north.

MESOZOIC STRUCTURAL FEATURES

The diastrophic movements of Mesozoic time were much like those of the Paleozoic. No sedimentation took place from the end of deposition of the Hannan limestone late in Carboniferous time until some time in the Jurassic period. The presence of the Ellis group suggests that downwarp occurred in Late Jurassic time, but the Ellis is thin compared to many of the older units. Whatever warping may have permitted ingress of the Ellis sea must have been slight, especially as the beds next formed are of terrestrial origin. After deposition of the Kootenai formation (Lower Cretaceous), another downwarp may have occurred because marine deposition was resumed and continued without major interruption throughout the rest of the Cretaceous period. Presumably the region rose gently late in that period, for parts of the St. Mary formation and all of the Willow Creek formation have been interpreted as of continental origin. The uplift that was to give rise to the mountains may have begun during the Cretaceous period, but there is no evidence of violent diastrophism in the region until after the deposition of the Willow Creek formation in Paleocene time.