Tectonic Analysis of Belt Terrane of Northwest Montana-Northern Idaho and Its Implications for Oil and Gas Potential West of Disturbed Belt: ABSTRACT

Abstract

From a recently completed comprehensive regional geologic map and cross sections across the Cordillera of the northwestern United States, the following conclusions have been made. (1) Paleozoic as well as probable late Precambrian Belt miogeoclinal sedimentation developed west of northern Idaho off the edge of the continental crust. Present position of the thick sediment wedge thereby suggests 100-200 km (62-124 mi) of eastward displacement on a passively involved cratonic basement. (2) Mesozoic orogeny started west of the continent margin, shifted east, and did not involve the Disturbed belt until earliest Tertiary time. Thus, a significant time span separated mobility and uplift in the core region of the orogene from thrusting of the foreland. Continental subduction (un erthrusting) beneath the uplifted and cold core is a probable explanation for the great shortening during the Laramide orogeny. (3) Immediately following thrusting, the entire northwest was swept by a 40-50 m.y. extensional orogeny involving, from place to place, widespread calc-alkaline magmatism, listric faulting on a grand scale, subhorizontal detachment faults, and basement mylonitization. This tectonism influenced the Belt region by fragmentation and eastward tilting of strata and thrusts, and creation of structural relief on the pre-Belt basement surface by normal faulting. A less obvious and speculative effect may be ductile mylonitization in the basement, as is exposed in the metamorphic core End_Page 946------------------------------ complexes of northeastern Washington and northern Idaho. Regional cross sections show no serious obstacles to the presence of an autochthonous or detached slab of Paleozoic rocks beneath the overthrust Belt section, although subhorizontal mylonitic rocks are not outside the realm of geologic possibility. Nevertheless, oil and gas favorability beneath Belt rocks is affected by such factors as ratio of source rock to reservoir rock, thermal and/or pressure conditions after overthrusting, and the postmigrational effects of Eocene extensional tectonism (i.e., dip reversals, magmatism, and fragmentation). End_of_Article - Last_Page 947------------