Abstract Study of the distribution of the age-populations of detrital zircons in the Snake River system suggest that specific stream systems can be identified based on the detrital-zircon age-population signature (“barcode”) of ancient and Holocene sand deposits. Detrital zircon studies can be used on regional and local scales to determine changes in drainage patterns using both surface and subsurface data. Regional study of drainage patterns using detrital zircons found in Neogene strata of Idaho and southwest Montana suggest northeastward late Miocene to Holocene migration of the Continental Divide from the western side of the Pioneer Mountains to the current position in southwest Montana. Specifically, mixed populations of recycled Proterozoic detrital zircons that define the Wood River drainage are not found in the western Snake River Plain until after 7 Ma. Late Miocene eastward drainage from the central Snake River Plain to southwest Montana is suggested by 9–12 Ma detrital zircons found in fluvial strata less than 6 million years old, of the Sixmile Creek Formation Basalt eruptions of the Eastern Snake River Plain during the Pliocene and Pleistocene also caused drainage diversion. Detrital zircons in Pliocene sands from coreholes at Wendell and Mountain Home Air Force Base contain Big Lost River zircon provenance, suggesting that during the Pliocene, the Big Lost River flowed west along the central Snake River Plain. Late Pliocene and early Pleistocene basaltic volcanoes and rhyolite dome eruptions resulted in volcanic highlands, the Axial Volcanic Zone of the eastern Snake River Plain and the northwest-trending Arco Volcanic Rift Zone (which includes Craters of the Moon volcanic center). The development of these volcanic highlands disrupted the ancestral drainage of the Pliocene Big Lost River system, confining it to the Big Lost Trough, a volcanically dammed basin of internal drainage on the Idaho National Laboratory. After the Big Lost Trough was cut off from the main Snake River, basalt eruptions, local subsidence, and climate controlled the courses of the rivers that flowed into it. Detrital-zircon populations in core samples reveal the provenance of specific sand beds from the Big or Little Lost River systems.

Abstract Exotic Miocene and Pliocene river gravel lies on top of the Continental Divide along the Idaho-Montana border near Monida Pass. The gravel is interlayered with tuffs and basalt flows of the Heise volcanic field, which erupted from the site of the Yellowstone hotspot between 6.62 and 4.45 Ma. The gravel includes pebbles that may have been derived from bedrock outcrops in Nevada and Utah, implying a paleo-river with headwaters to the south of the modern Continental Divide and Snake River Plain. The river may have been a tributary of the pre–ice age Bell River of Canada. The field trip examines evidence for the tectonic evolution of the Monida Pass area. The course of the Miocene river appears to have been diverted around growing mountain ranges, and then pinched off at Monida Pass on the northern shoulder of the Yellowstone hotspot track.

Abstract “Precambrian rocks reach the surface in the Llano region of Central Texas in the highest part of a broad domal arch, the Llano uplift, which appears on a regional geologic map as an island of igneous and metamorphic rocks surrounded by Paleozoic and Cretaceus sedimentary rocks. The widest expanse of Precambrian rocks is about 65 mi ( 105 km), extending westward from the valley of the Colorado River through a subdued topographic basin drained by the Llano River. The broad, gentle basin carved into the Precambrian rocks is bordered by a discontinuous rim of flat-topped limestone hills which are the dissected edge of the Edwards Plateau. Within the basin and at its margins are erosional remnants and down-faulted blocks of Paleozoic rocks which form prominent hills, locally referred to as mountains.” Clabaugh and McGehee (1972, p. 9). The main Precambrian units present in the Llano region are listed in Table 1, and the distribution of some of these units is shown in Figure 1. This figure is derived from the Llano Sheet of the Geologic Atlas of Texas, scale 1:250,000 (Barnes, 1981). On that map (as well as Fig. 1), the Oatman Creek and Sixmile Granites (Table 1) are lumped as younger granitic intrusive rocky locally, in the southeast part of the Llano Sheet, the Packsaddle Schist is subdivided into four formations (McGehee, 1979). Detailed 7½-minute quadrangle maps include most of the area in which the Packsaddle Schist has been subdivided (Barnes and McGehee, 1976; 1977a, b, c).

Abstract The Lewis and Clark line (LCL) is a major transverse structure that crosses the North American Cordillera from northeastern Washington to central Montana. It initiated as a rift structure within the Mesoproterozoic Belt basin and reactivated several times during the Phanerozoic. This field trip examines the internal structure of the LCL along a transect in central-western Montana. Structural plunge permits examination of a 25-km-thick crustal section of a flower structure that formed along the LCL during Late Cretaceous-late Paleocene sinistral transpression. We will observe changes in structural style from the deepest parts of the Belt Supergroup upward to the syntectonic depositional surface.