Abstract Mafic intrusions within the Mesoproterozoic Belt-Purcell basin of the Northern Rocky Mountains of Montana record important Mesoproterozoic and Neoproterozoic rifting episodes. Previous studies established four major Mesoproterozoic igneous intrusive events. This project focuses on sills and dikes that represent a fifth, Neoproterozoic intrusive event and establishes their regional extent and relationship to the Gunbarrel magmatic event and late Proterozoic Windermere rift activity. The sills and dikes of this study intruded Archean basement rock and the Mesoproterozoic formations of the Belt-Purcell Supergroup and are unconformably overlain by the Cambrian Flathead Sandstone. When restored on a palinspastic map, the thickest and deepest segments of the sills, along with dike swarms in western Montana and northern Wyoming, are colinear with the Belt-Purcell basin axis. The sills and dikes fall on the eastern margin of Windermere rock exposures along the margins of the Laurentian craton. The sills and dikes are tholeiitic diabase and have locally undergone low-grade metamorphism and alteration. Granophyre is associated with some sills, and Belt-Purcell xenoliths found in the granophyre show some signs of interaction with the magma. Geochemical analysis is compatible with a single intrusive event. A U-Pb zircon date of 777.5 ± 2.5 Ma obtained from granophyre in the Holland Lake sill supports argon dates of previous studies. Discordant U-Pb data from two other sills, exposed at Turah and near Rogers Pass, are consistent with this date, although it is also possible that these sills were emplaced at different times. The 777.5 ± 2.5 Ma date from the Holland Lake sill establishes the emplacement of this sill during the Gunbarrel magmatic event (ca. 780 Ma) and may reflect earliest Windermere rift activity.

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 It has been shown that sediments are being introduced into the ocean along the Southern California coast largely by swollen streams during the winter floods. There are, however, extensive areas of the sea floor off the coast on which no appreciable sedimentation is occurring. There is reason to believe that by-passing is the general rule on continental and insular shelves, on banks, ridges, steep submarine slopes, and in submarine canyons. Also deposition is thought to be very slow on the broad saddles between banks and islands and other deep topographic highs. Evidence has been presented to show that much of the coarse debris introduced by streams is returned to the land as beaches and later in part is blown farther landward by onshore winds. On the other hand finer sediments, principally silts and clays, are believed to be accumulating along the gentler slopes and in the basins and troughs. The areas that are preserving a state of marine unconformity are characterized by abundance of rocky outcrops and by the markedly uneven texture of the sediments both laterally and vertically. Much of the coarser detrital material in these areas appears to be derived from an older generation of erosion and deposition probably related to former low stands of sea-level. Far from shore, however, other materials of sand texture, notably recent and fossil foraminifera, miscellaneous calcareous fragments, and glauconite predominate because of the removal by currents of the finer clastic sediments that would otherwise accumulate in these environments. Evidence is given to show that the by-passing on steep slopes and in the submarine canyons is in part the result of mud flows or other sliding action. The sediments of the basins and troughs are predominantly clays of uniform texture and compare in fineness of grain with the sediments of the deep ocean and with those of protected estuaries along various coasts. The exceptionally high organic matter content of the basin sediments is one of their striking characteristics and is thought to be due to the virtual stagnation of the water below sill depths. The presence of finegrained calcium carbonate below the surface in core samples from the basins may also be ascribed to the alkaline reducing conditions accompanying low oxygen supply.

Abstract It has been shown that sediments are being introduced into the ocean along the Southern California coast largely by swollen streams during the winter floods. There are, however, extensive areas of the sea floor off the coast on which no appreciable sedimentation is occurring. There is reason to believe that by-passing is the general rule on continental and insular shelves, on banks, ridges, steep submarine slopes, and in submarine canyons. Also deposition is thought to be very slow on the broad saddles between banks and islands and other deep topographic highs. Evidence has been presented to show that much of the coarse debris introduced by streams is returned to the land as beaches and later in part is blown farther landward by onshore winds. On the other hand finer sediments, principally silts and clays, are believed to be accumulating along the gentler slopes and in the basins and troughs. The areas that are preserving a state of marine unconformity are characterized by abundance of rocky outcrops and by the markedly uneven texture of the sediments both laterally and vertically. Much of the coarser detrital material in these areas appears to be derived from an older generation of erosion and deposition probably related to former low stands of sea-level. Far from shore, however, other materials of sand texture, notably recent and fossil foraminifera, miscellaneous calcareous fragments, and glauconite predominate because of the removal by currents of the finer clastic sediments that would otherwise accumulate in these environments. Evidence is given to show that the by-passing on steep slopes and in the submarine canyons is in part the result of mud flows or other sliding action. The sediments of the basins and troughs are predominantly clays of uniform texture and compare in fineness of grain with the sediments of the deep ocean and with those of protected estuaries along various coasts. The exceptionally high organic matter content of the basin sediments is one of their striking characteristics and is thought to be due to the virtual stagnation of the water below sill depths. The presence of finegrained calcium carbonate below the surface in core samples from the basins may also be ascribed to the alkaline reducing conditions accompanying low oxygen supply.

Abstract The Lewis thrust plate in Glacier National Park contains the displaced margin of the Mesoproterozoic Belt Basin, which was transported some 130 km (~80 mi) northeastward during the Late Cretaceous to late Paleocene Laramide orogeny. This two-day field trip examines the Lewis plate on an eastward transect along the Going-to-the-Sun Road, crossing the Continental Divide at Logan Pass. The trip begins in basinal facies of the Belt Supergroup, and crosses eastward into shelf facies. The shelf-to-slope transition coincides with the axis of the Akamina syncline along the Continental Divide. The trip visits classic exposures of the Lewis thrust along the Rocky Mountain front, where the lower Belt Supergroup overlies a duplex zone in the Late Cretaceous marine section.