Abstract This field trip presents field evidence for Neogene evaporite tectonism, dissolution of evaporites, and related collapse in Eagle River valley and along the southwestern flank of the White River uplift. In the Eagle collapse center, Pennsylvanian evaporite flowed to form anticlinal diapirs, dissolved, and disrupted a lower Miocene basaltic plateau originally at elevations as high as 3.35 km by tilting, faulting, and sagging to elevations as low as about 2.1 km. Also in the Eagle collapse center, the 30 x 10-km, homoclinal Hardscrabble Mountain sank into evaporite during Triassic and Permian collapse followed by Neogene(?) tilting and collapse, based on seismic reflection data. Along the southwestern flank of the White River uplift in the northwestern part of the Carbondale collapse center, parts of the Grand Hogback monocline have collapsed northeastward toward a series of strike-elongate extrusive diapirs. The volume of evaporite removed from the Eagle and Carbondale collapse centers during the Neogene (about 2,250 km 3 from an area of roughly 4,500 km 2 ) was calculated by measuring the departure of collapsed basalts from an assumed original basalt plateau. Regional Neogene uplift and incision of the Rocky Mountains, which locally began about 8-10 Ma, probably triggered dissolution and collapse. Presently the Colorado River removes a dissolved-solids load of about 1.4 x 10 9 kg per year from the two collapse centers.

In Montana, Wyoming, North and South Dakota, and Minnesota, Cretaceous strata are preserved in the asymmetric Western Interior foreland basin. More than 5,200 m (17,000 ft) of Cretaceous strata are present in southwestern Montana, less than 300 m (1,000 ft) in eastern South Dakota. The asymmetry resulted from varying rates of subsidence due to tectonic and sediment loading. The strata consist primarily of sandstone, siltstone, mudstone, and shale. Conglomerate is locally abundant along the western margin, whereas carbonate is present in most areas of the eastern shelf. Sediment was deposited in both marine and nonmarine environments as the shoreline fluctuated during major tectonic and eustatic cycles. A discussion of Cretaceous strata from southwestern to east-central Montana, the Black Hills, eastern South Dakota, and southwestern Minnesota shows regional stratigraphy and facies relations, sequence, boundaries, and biostratigraphic and radiometric correlations. The thick Cretaceous strata in southwestern Montana typify nonmarine facies of the rapidly subsiding westernmost part of the basin. These strata include more than 3,000 m (10,000 ft) of synorogenic conglomerate of the Upper Cretaceous part of the Beaverhead Group. West of the Madison Range, sequence boundaries bracket the Kootenai (Aptian and Albian), the Blackleaf (Albian and Cenomanian), and the Frontier Formations (Cenomanian and Turonian); sequence boundaries are difficult to recognize because the rocks are dominantly non-marine. Cretaceous strata in east-central Montana (about 1,371 m; 4,500 ft thick) lie at the approximate depositional axis of the basin and are mostly marine terrigenous rocks. Chert-pebble zones in these rocks reflect stratigraphic breaks that may correlate with sequence boundaries to the east and west. Cretaceous rocks of the Black Hills region consist of a predominantly marine clastic sequence averaging approximately 1,524 m (5,000 ft) thick. The Cretaceous System in eastern South Dakota (457 to 610 m; 1,500 to 2,000 ft thick) consists of a marine shelf sequence dominated by shale and limestone. Major sequence boundaries in South Dakota are at the base of the Lower Cretaceous Lakota Formation, Fall River Sandstone, and Muddy Sandstone, and bracket the Upper Cretaceous Niobrara Formation.

Laramide-style deformation of the Rocky Mountain foreland began in the Lima region of southwest Montana in Coniacian to Santonian (Late Cretaceous) time with the growth of the Blacktail-Snowcrest uplift. The Lima Conglomerate of the Beaverhead Group locally onlaps its deformed source terrane, the Laramide-style (thick-skin) Snowcrest-Greenhorn thrust-fault system of the foreland, along the southeastern margin of this uplift. Associated sandstones as old as Coniacian to Santonian, also derived from this uplift, are here reinstated into the Beaverhead Group. Northeast of Lima, the Snowcrest thrust transported Archean gneiss, marble, and schist southeastward over deformed Phanerozoic rocks. These Phanerozoic rocks are locally overturned and intensely fractured, and they exhibit many cross-faults. The Archean rocks exhibit locally intense cataclasis, microfaults, and pressure solution at grain boundaries. The first incursion of Sevier-style (thin-skin) thrusting into the Lima region followed Campanian erosion of the Blacktail-Snowcrest uplift, locally to Archean basement. This thrusting shed thick quartzite-roundstone conglomerates eastward. These are placed in a new informal stratigraphic unit, the Little Sheep Creek conglomerate unit, which appears to conformably overlie the fining-upward sequence at the top of the Lima Conglomerate, palynologically dated as mid-Campanian. Quartzite clasts in the Little Sheep Creek conglomerate unit were probably recycled from proximal fans adjacent to deeply eroded hinterland thrust sheets to the west. However, this unit contains large slide blocks of Mississippian limestone from the front of the closer Four Eyes Canyon sheet, the lower bounding thrust of which reached the land surface, probably in late Campanian time. The Little Sheep Creek conglomerate unit is overridden by the Tendoy thrust. Consequently, the Tendoy is younger than the Four Eyes thrust to the west. Complex structural imbrication of upper Paleozoic and Triassic through Lower Cretaceous rocks of the Tendoy thrust sheet occurs in the Lima Peaks area, above the inferred southwestern extension of the older Snowcrest-Greenhorn thrust system. This imbricate stack, transported east-northeast on the Tendoy thrust, subsequently was folded about a N70°E axis, together with the Tendoy thrust and Lima Conglomerate of its footwall, by possible later reactivation of Snowcrest-Greenhorn thrust system. Structural relationships within the Little Water Canyon and McKnight Canyon areas northwest of Lima indicate that northeast-trending structures of probable foreland origin developed in these areas prior to emplacement of the Tendoy thrust sheet, but subsequent to emplacement of the Four Eyes Canyon thrust sheet. Therefore, these northeast-trending structures are younger than the Snowcrest-Greenhorn thrust system east of Lima and may be Maastrichtian, based on structural involvement of Beaverhead rocks palynologically dated as late Campanian to early Maastrichtian in the northern part of the McKnight Canyon area. Cessation of thrusting in the Lima region is still poorly dated. The youngest Beaverhead conglomerates, those derived in part from the Tendoy thrust sheet, underlie middle to upper Eocene basin beds northeast of Dell.

Abstract Two orthogonal systems of lineaments reflect recurrent structural movement in the basement rocks of the Rocky Mountains and adjacent plains—the area of the middle Phanerozoic Cordilleran continental shelf. The shelf lay between the Transcontinental arch and the Cordilleran continental margin (or miogeocline) from Arizona and New Mexico to Montana and North Dakota. Major tectonic features that affected middle Phanerozoic sedimentation on this shelf were the Big Snowy trough and the Williston basin, the Ancestral Rocky Mountains, and the Zuni uplift. Some of the boundaries of these and other structural elements in the shelf area seem to be related to an orthogonal system of two perpendicular sets of lineaments, one trending northeast and the other northwest. The first parallels the northeast-trending Cordilleran miogeocline in Nevada, and the other parallels the northwesterly continuation of the miogeocline north of Idaho. The two sets are roughly parallel to the Transcontinental arch and the margin of the Canadian shield, respectively. A second, less important orthogonal system is oriented north-south and east-west. Differential vertical movements of rectangular basement blocks bounded by the fracture systems were apparently propagated upward through the strata, forming a variety of structures. Movements of the blocks at different times in the Phanerozoic influenced erosion, deposition, and lithofacies of the sediments and thus influenced the distribution of sedimentary minerals and petroleum deposits and later igneous intrusions, volcanism, and hydrothermal ores. The lineaments appear to have been avenues for passage of hydrothermal solutions, so their identification should help guide exploration for mineral deposits. Tectonic movements in the Phanerozoic seem to have been mainly along the northeast and northwest system of lineaments, but the north-south and east-west system also influenced the formation of Laramide structures and the present landscape in the Rocky Mountains. Deformation associated with the two systems is probably related to events at the North American plate margin or to incipient continental fragmentation of the plate.

Abstract The Upper Silurian Tonoloway Limestone at Pinto, Maryland, is divided into three informal members on the basis of field, paleontologic, and petrographic studies. The lower member is characterized by thin bedding, stromatolites, gypsum molds, intraclasts, and mud cracks. The rocks typically are laminated micrite and pelmicrite. In the field and in thin section, this member is similar to the upper one. Faunal diversity is extremely low in both. The middle member, composed of biopelsparite and biomicrite, is more fossiliferous than either of the other members and shows an extreme variability in the development of communities. This member can be traced along the outcrop from Pinto, Maryland, to Greenbrier County, West Virginia. Under the eastern Appalachian Plateau, dolomite and anhydrite occur in the upper and lower parts of the Tonoloway, whereas the middle member is chiefly limestone. Farther west, the subsurface equivalent of the Tonoloway is the Salina Formation, which consists of light to dark-gray dolomite and anhydrite with minor green and gray shale. Several salt beds generally are developed. Most of these are in the Salina F unit (equivalent to the upper Tonoloway member), but in Marshall County, West Virginia, salts as low in the section as the D evaporite (lower Tonoloway) are also well developed. Total salt thickness exceeds 200 ft (61 m) in several wells; however, excessive thicknesses may be due to salt flowage in anticlines. The upper and lower members of the Tonoloway in the outcrop belt were deposited on inter-tidal-supratidal mud flats. In these two members beneath the eastern Plateau, salinity apparently increased toward the depocenter, and halite precipitated within the deeper central region of the evaporite basin. The evaporite basin was closed, with periodic influxes of normal sea water. The middle member of the Tonoloway was deposited in environments where water depth fluctuated from intertidal to shallow subtidal to deeper subtidal. A widespread transgression led to the more normal marine conditions during deposition of the middle member. Effects of this transgression can be traced into the basin, where the calcareous middle member supplanted evaporites.