New Biostratigraphic and Paleotectonic Interpretation of Devonian and Mississippian Rocks in Southwestern Montana Thrust Belt: ABSTRACT

Abstract

The structurally complex area of previously undifferentiated Mississippian rocks below the Meramecian Kibbey Sandstone in the southwestern Montana thrust belt of the northern Tendoy Range actually comprises an unusual combination of Devonian and Mississippian basinal, slope, platform, and nearshore deposits. The lower part of this sequence comprised two widespread western Montana formations: the Devonian part of the Three Forks Formation over 65 m (213 ft) thick, containing all three members, and the basinal Kinderhookian and Osagean Paine Limestone, 228 m (748 ft) thick. However, the Paine is succeeded by an eastern tongue of the older, Osagean part of the lower-slope Middle Canyon Formation, 266 m (872 ft) thick. This tongue consists of clinoform cherty micrite and bedd d chert, with some encrinite debris flows that increase upward in number and thickness. The overlying Osagean and Meramecian Mission Canyon Limestone comprises 102 m (335 ft) of an upper-slope encrinite lower member and 74 m (243 ft) of a mainly shelf-margin wackestone and encrinite upper member. Thus, the Mission Canyon here represents the distal part of a broad carbonate platform. Succeeding the Mission Canyon is 140 m (459 ft) of the newly named Meramecian McKenzie Canyon Limestone, which comprises a sabkha, back-mound, and lagoonal sequence of evaporite-solution breccia, micrite, dismicrite, and pelmicrite, with interbeds of crinoidal wackestone, encrinite, and bio-oopelsparite. This formation represents beds absent at an unconformity elsewhere in western Montana and western Wyoming. Thus, the sequence from the Paine through the McKenzie Canyon, constituting the newly named Tendoy Group is 810 m (2,657 ft) thick and represents one of the most complete records of Kinderhookian to Meramecian carbonate deposition in the northwestern United States, displayed in the predominantly upward-shallowing part of a eustatic megacycle. The reconstructed detailed biostratigraphy aids new structural interpretations and provides several new plays for petroleum exploration in the Montana thrust belt. End_of_Article - Last_Page 865------------