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The western part of Montana is not a depositional basin in the sense of this symposium, but its depositional and structural history are related to events of nearby areas. The decipherable part of this 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 of the state and in an eastward-projecting embayment. Subsequent depositional patterns and present structural configuration are intimately related to distribution of that thick sedimentary wedge. Course arkose conglomerates were deposited along the southern fault-controlled margin of the Belt embayment. Cambrian through Mississippian formations and parts of the Cretaceous section are t pically thicker in east-west zones essentially coincident with the old Belt embayment than they are to the north or south of the embayment.
A positive arch existed along the southwestern Montana and Idaho border against which Cambrian through Devonian formations thin and/or disappear. This positive element became strongly negative during Mississippian and later depositional intervals as geosynclinal subsidence encroached on the cratonic margin.
Abrupt changes in stratigraphic units across the northeast trending Greenhorn fault in the Greenhorn-Snowcrest Range suggest faulting or strong flexure along this zone during post-Ordovician to pre-Late Devonian and during Mississippian time. Pennsylvanian, Permian, and Triassic thicknesses also seem to be mildly influenced by relatively negative movements in this area.
Other northeast 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 structural trends apparently also control the thickness of Upper Cretaceous and Paleocene strata in the Crazy Mountains basin.
In general, successively older (Triassic, Permian, Pennsylvanian, Mississippian) formations underlie Jurassic beds from south to north, a relationship that has been explained as a result of southward tilt and beveling by 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 pinchout is related to depositional thinning on which southward tilt was superimposed. During deposition of the marine Jurassic several large "islands" remained above the sea for part of 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 nonmarine Morrison Formation. The basal conglomerate of the Kootenai Formation (Lower Cretaceous) marks 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 northeast and east.
In the eastern part of the area, Cretaceous, and Paleocene rocks are generally separable into rock and time-rock units; however, to the west the corresponding sequence is almost entirely nonmarine, sparsely fossiliferous, exceedingly diverse in lithologic character, and subdivision is very difficult.
Four major westward advances of the sea punctuate Cretaceous deposition in an increasingly unstable tectonic setting. Locally, volcanic debris is very abundant in the Colorado Group and rapid 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 strong influx of andesitic volcanic debris in these stratigraphic units is 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 th culmination
of orogenic activity. Some intense folding and thrusting post-dates those events just mentioned but is reasonably certain that Laramide compressional deformation had ceased before middle Eocene time in western Montana.
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