ABSTRACT This field trip will test global tectonic models against detailed geologic and paleomagnetic data from within the Belt basin, and will evaluate the significance of its Neoproterozoic to early Cambrian rift truncation. The trip will focus on the stratigraphy and paleotectonic evolution of the Mesoproterozoic Belt basin on an E-W transect from the shallow-water facies of the east margin of the basin at Rogers Pass through its deep-water interior near Sandpoint, Idaho, USA. With some variations, this trip will follow Don Winston’s classic trans-basin field guide from the east edge of the basin at Rogers Pass, Montana, to its western rift-truncation near Sandpoint, along Montana Highway 200 and Interstate 90 along the Lewis and Clark Line.

Abstract In the absence of land plants, broad pediments may have formed stable landforms that beveled Proterozoic continents. Braided streams would have transported a thin layer of clastic sediment across such Proterozoic epicontinental pediments. The Proterozoic pediment–braidplain system may be represented by extremely flat regional unconformities beneath locally preserved, supermature, braidplain sandstones. Continental rifting would have destabilized Proterozoic epicontinental pediments by funneling runoff along rift axes to create large rivers, which otherwise were not favored in the Proterozoic landscape. The sedimentological history and detrital–zircon provenance of the intracratonic Mesoproterozoic Belt–Purcell basin of western North America may be described in terms of destabilization of a late Paleoproterozoic to early Mesoproterozoic epicontinental pediment by a three–armed rift system with the Belt–Purcell basin at its center. A model using a Siberia–Laurentia–Australia paleocontinental reconstruction implies that the sedimentary veneer of the pediment washed down the western branch of the rift system to enter the Belt–Purcell basin at a point source on its western side. Capture of clastic sediment in delta fans on the western side of the basin permitted clean carbonate to precipitate on the northeast side. Reconfiguration of the basin by renewed rifting appears to have changed composition, grain size, and sedimentary provenance during deposition of the Missoula Group (upper Belt–Purcell Supergroup).

Abstract The Barrick Golden Sunlight Mine (GSM) in Whitehall, Montana, is an industry leader in safe, responsible resource extraction. With more than 3 million ounces of gold poured since 1983, and current proven and probable reserves of 318,000 ounces of gold, GSM is the largest gold producer in Montana. The gold-silver deposit is localized in a hydrothermal breccia pipe related to Late Cretaceous latite porphyry magmatism hosted by the Mesoproterozoic Belt Supergroup, and is influenced by younger cross-cutting faults and fracture systems. The deposit has been mined by both underground and open pit methods, and the current open pit operation was recently permitted for expansion. The mill and tailings operations practice efficient and environmentally responsible resource recovery by processing ore from historical tailings and dumps from around the state in addition to ore from the Golden Sunlight property. This trip will explore the complex geologic and tectonic controls on mineralization and review how GSM has addressed the technical challenges of mining, milling, and reclamation.