ABSTRACT Shales exhibit a wide range of textures, compositions, and mechanical properties, which are interlinked by their diagenetic history. During hydraulic fracturing of shales, the matrix is subjected to shear deformation, which may create microfractures and enhance hydrocarbon transport from nanoscale, organic matter (OM)-hosted pores to the larger, induced fracture network. To study the nanoscale response to shear deformation of shale pore systems with different diagenetic histories, we deformed shale samples from a formation in the Northern Rocky Mountains and the Eagle Ford Group in Texas, using confined compressive strength tests. N 2 and CO 2 adsorption were performed to quantify fracture effects on pore morphology including pore size distribution, porosity, surface area, and surface fractal dimensions. Most samples increased their gas adsorption quantity, pore volume, and surface area after failure. The surface fractal dimensions were less sensitive to shear deformation. Results show that varying nanometer-to-micron-scale fracture patterns are in part caused by contrasting rock fabrics that are preconditioned by their distinctive diagenetic histories. For example, fractures tend to propagate along the OM laminae, whereas others cut across OM grains and access OM pores. Other possible mechanisms for porosity increase include the deformation of relatively uncemented clay aggregates and contrasting amounts of intra-OM pores between samples. Thus, the mechanisms for syn-deformational porosity changes at the micro scale are highly dependent on diagenetic history, particularly the maturation of OM, and the cementation history relative to clay content.

Abstract This field guide covers geology across north-central Idaho from the Snake River in the west across the Bitterroot Mountains to the east to near Missoula, Montana. The regional geology includes a much-modified Mesozoic accretionary boundary along the western side of Idaho across which allochthonous Permian to Cretaceous arc complexes of the Blue Mountains province to the west are juxtaposed against autochthonous Mesoproterozoic and Neoproterozoic North American metasedimentary assemblages intruded by Cretaceous and Paleogene plutons to the east. The accretionary boundary turns sharply near Orofino, Idaho, from north-trending in the south to west-trending, forming the Syringa embayment, then disappears westward under Miocene cover rocks of the Columbia River Basalt Group. The Coolwater culmination east of the Syringa embayment exposes allochthonous rocks well east of an ideal steep suture. North and east of it is the Bitterroot lobe of the Idaho batholith, which intruded Precambrian continental crust in the Cretaceous and Paleocene to form one of the classical North American Cordilleran batholiths. Eocene Challis plutons, products of the Tertiary western U.S. ignimbrite flare-up, intrude those batholith rocks. This guide describes the geology in three separate road logs: (1) The Wallowa terrane of the Blue Mountains province from White Bird, Idaho, west into Hells Canyon and faults that complicate the story; (2) the Mesozoic accretionary boundary from White Bird to the South Fork Clearwater River east of Grangeville and then north to Kooskia, Idaho; and (3) the bend in the accretionary boundary, the Coolwater culmination, and the Bitterroot lobe of the Idaho batholith along Highway 12 east from near Lewiston, Idaho, to Lolo, Montana.

Abstract The northern Rocky Mountains encompass an array of tectonic provinces representing tectonic and magmatic events spanning more than three billion years of Earth history. This field guide presents a diverse collection of trips highlighting the rich geology of the region, from the Precambrian, through the Sevier/Laramide orogeny, to the Quaternary history of Yellowstone. This volume is an essential update to the classic field-oriented literature of the northern Rocky Mountain Region of Montana and Idaho, and will be an invaluable addition to the libraries of researchers, educators, and students interested in the dynamic geology of the northern Rockies.