Resume of Depositional and Structural History of Western Montana

Abstract

The western part of Montana is not a depositional basin in the sense used in this symposium, but its depositional and structural history is related to events of nearby areas. The area discussed lies partly in the shelf region and partly in the marginal area of the Cordillera geosyncline. The decipherable part of its history begins with late Precambrian (Belt) sedimentation, during which the fundamental structural framework of western Montana evolved. Thick Belt strata are present in the western extremities and in an eastward-projecting embayment. Subsequent depositional patterns and present structural configuration are closely related to distribution of that thick sedimentary wedge. Coarse arkosic conglomerate was deposited along the southern fault-controlled margin of th Belt embayment. Cambrian through Mississippian formations and parts of the Cretaceous section are typically thicker in east-west zones, essentially coincident with the old Belt embayment, than they are north or south of the embayment. Along the southwestern Montana and Idaho border a positive arch existed against which Cambrian through Devonian formations thin or disappear. This positive element became strongly negative during Mississippian and later depositional intervals as geosynclinal subsidence encroached on the cratonic margin. Important facies changes take place in stratigraphic units across the northeast-trending Greenhorn fault in the Greenhorn-Snowcrest Range. These changes, which occur in short distances, suggest faulting or strong flexure along this zone during post-Ordovician to pre-Late Devonian and during Mississippian times. Pennsylvanian, Permian, and Triassic thicknesses also seem to be mildly influenced by relatively negative movements in this area. Other northeast-striking thickness trends in several stratigraphic units are apparent in the Sweetgrass arch area, where they seem to coincide with known present-day subsurface faults. Northeast-striking structural trends apparently also control the thickness of Upper Cretaceous and Paleocene strata in the Crazy Mountains basin. In general, Triassic, Permian, Pennsylvanian, and Mississippian formations successively underlie Jurassic beds from south to north, a relationship that has been explained by southward tilt and beveling during pre-Jurassic erosion. Irregularities in the truncational pattern and general thinning of each formation beneath the next younger unit indicate that much of the northward pinch-out is related to depositional thinning on which southward tilt was superimposed. During deposition of the marine Jurassic several large islands remained above for part or all of that interval. Late in Jurassic time the western seaway along which earlier seas had transgressed the region was destroyed by increasing tectonism in the area west of Montana, and a flood of debris was carried eastward to form the non-marine Morrison Formation. The basal conglomerate of the Kootenai Formation (Lower Cretaceous) indicates a particularly strong uplift in areas that could not have been far west of Montana. When the seas returned to this region, they came from the north and south and spread westward, inundating western Montana. In the eastern part of the area, Cretaceous and Paleocene rocks are generally separable into rock and time-rock units; however, on the west the corresponding sequence is almost entirely non-marine, sparsely fossiliferous, and exceedingly diverse in lithologic character. Four major westward advances of the sea punctuated Cretaceous deposition in an increasingly unstable tectonic setting. Locally, volcanic debris is very abundant in the Colorado Group, and strong increase in thickness westward attests to further encroachment of geosynclinal downwarping onto the cratonic margin. Laramide orogeny began in the Montana area coincident with deposition of the Eagle-Claggett and correlative units. Local areas of strong uplift, erosion, and volcanism, and the large influx of andesitic volcanic debris in these stratigraphic units, are evidence of the initial stages of orogeny. Accumulation of very thick volcanic sequences in at least two separate fields during Judith River time attests to increasing intensity of orogenic processes. Strong deformation and erosion, followed by deposition of coarse erosional products, and volcanism in the southwestern and central parts of the area, intrusion of granitic plutons in the west-central part of the area, and thick accumulation of coarse gravels in the Crazy Mountains basin, all during Lancian and Paleocene time, coincide with the culmination of orogenic activity. Some intense folding and thrusting post-date those events just mentioned, but it is reasonably certain that Laramide compressional deformation had ceased before middle Eocene time in western Montana. End_Page 1801------------------------------