Stratigraphic and Petrographic Analysis of Upper Jurassic-Lower Cretaceous Morrison and Kootenai Formations, Southwest Montana

Abstract Regression of the Middle and Late Jurassic sea and onset of regionally extensive nonmarine deposition of the Morrison and Kootenai Formations reflect a significant change in the tectonic and sedimentologic framework of the northern Rocky Mountains during Late Jurassic-Early Cretaceous time. Correlation of this change with Late Jurassic (Nevadan) orogenesis farther west suggests a process-response relation between the nonmarine deposition and orogeny. Details of this relation are documented from isopach and sand-shale ratio maps, and by petrographic and X-ray data from the Morrison and Kootenai Formations of southwest Montana. The absence of Morrison in southwesternmost Montana and a thickening of the unit eastward indicate local control of the Morrison geometry by the Belt arch, a pronounced positive feature in Middle Jurassic time, but of only limited extent during the Late Jurassic. The arch had subsided totally by the Early Cretaceous with development of true clastic-wedge configuration of the Kootenai Formation. This is also the first positive evidence of Nevadan orogenic effects on sedimentation in Montana. Morrison sandstone contains 40–50 percent quartz and 40-50 percent interstitial carbonate. Feldspar, microcrystalline quartz, and siltstone clasts are accessories. Increase of feldspar east and also south into Wyoming implies either a cratonic source or more possibly a southern source in Colorado where Precambrian crystalline rocks were exposed in Late Jurassic time. In contrast, lower Kootenai sandstone contains little interstitial carbonate and an average of 40–50 percent microcrystalline quartz, including spicule varieties and black and red cherts primarily derived from the Permo-Pennsylvanian rocks of Idaho. Upper Kootenai sandstone is similar to the Morrison in composition and texture. On the basis of the foregoing evidence and results of other similar studies in the Pacific Northwest, a history involving temporal and spatial continuity of late Mesozoic orogeny seems indicated for the northern Cordillera. Middle and Late Jurassic unconformities, intrusion, and volcanism in Oregon signal initial orogenesis in the far west. The Kootenai clastic wedge with conglomerate derived from source rocks in Idaho implies Early Cretaceous uplift in that area. Increased development of clastic-wedge geometry and pyroclastic deposition in the younger Colorado Group reflect migration of the mobile zone farther east, near the border of Montana. Climax of this migration occurred in Late Cretaceous and early Tertiary time with the Laramide orogenic pulse in central-western Montana.