Devonian System in Central and Northwestern Montana

ABSTRACT The Devonian strata of central and northwestern Montana are confined to an Upper Devonian age. In central Montana these rocks are divided, in order of increasing age, into the Three Forks formation, predominantly shale; the Jefferson formation, composed of an upper dolomite member, including some anhydrite or evaporite-solution breccia, and a lower dense limestone member; and an unnamed basal unit of shale and shaly dolomite which bears a transgressive relationship to the underlying Ordovician and Cambrian. In northwestern Montana these terms are not applicable and the Devonian is divided, in descending order, into arbitrary units A, B, and C. Unit A is dolomite and anhydrite, or evaporite-solution breccia, unit B is dense limestone, and unit C is a red shale and shaly dolomite which bears a transgressive relationship to the underlying Ordovician and Cambrian. In northwestern Montana these terms are not applicable and the Devonian is divided, in descending order, into arbitrary units A, B, and C. Unit A is dolomite and anhydrite, or evaporite-solution breccia, unit B is dense limestone, and unit C is a red shale and shaly dolomite sequence resting on the channeled surface of the Upper Cambrian. Oil is produced from the Devonian of south-central Alberta and gas has been encountered in it in Montana. Petroleum possibilities appear to be confined to the dolomites of the Jefferson formation and unit A, and it is suggested that these possibilities may have been enhanced by favorable depositional environments and post-depositional effects.