Stratigraphy, Depositional Environments, and Diagenesis Related to Porosity Development and Destruction in Jefferson Formation, Northwestern Montana: ABSTRACT

Abstract

The Jefferson Formation in northwestern Montana consists of a lower member (< 155 m or 508 ft) and the Birdbear Member (< 72 m or 236 ft), which are correlative to the Duperow and Birdbear Formations of the Williston basin. The lower member consists of sucrosic and nonsucrosic dolomites, limestones, and evaporite solution breccias. The Birdbear Member consists of silty nonsucrosic dolomites and limestones, and sucrosic evaporite solution breccias. Paragenesis of diagenetic events based on petrographic cross-cutting relations, cathodoluminescence, microprobe analysis, conodont alteration indices, and field relations indicate that most porosity development and destruction occurred at relatively shallow depth prior to thrusting. Devonian events include early cementation of desiccation structures and intergranular spaces, localized vuggy and moldic porosity development, and dolomitization and formation of intercrystalline porosity. Mississippian to pre-Jurassic events include burial to oil window, kerogen maturation, and loading stylolitization. Later erosion of Mississippian strata and ground-water circulation resulted in brecciation, subvertical fracture development related to collapse, and dedolomitization followed by near-complete calcite cement infilling of available porosity. Paleocene to early Eocene thrusting events include stylolitization, folding of breccias, and nonfilled fracture formation. Post-early Eocene exposure-induced events results in vuggy porosity and precipitation of speletic cements.