Abstract This guide consists of three general sections: an introduction that includes discussions of Raton basin stratigraphy and the Cretaceous Tertiary (K-T) boundary; descriptions of the geology along the route from Denver, Colorado, to Raton, New Mexico; and descriptions of several K-T sites in the Raton basin. Much of the information is from previous articles and field guides by the authors together with R. M. Flores and from road logs co-authored with Glenn R. Scott, both of the U.S. Geological Survey.

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.

The junction between Archean and Proterozoic crust forms a zone of weakness near the north margin of the Uinta trend that extends west from Colorado to the edge of Precambrian continental crust in central Nevada. In the vicinity of the Wasatch Mountains of central Utah, areas north and south of this crustal boundary differ markedly in thicknesses of Middle and Late Proterozoic sedimentary rocks and in the degree of involvement of basement rocks in thrusting in the Sevier orogenic belt. North of the crustal boundary, where Proterozoic sedimentary rocks are absent and the total thickness of the section is less than half of that directly south of the crustal boundary, a large area of Archean crust was uplifted above and west of the Farmington ramp. Stratigraphic and sedimentological data show that rock in the thrust belt northwest of the Uinta Mountains reflects deformation in older parts of the belt to the west in Early Cretaceous time. Palynological dating and stratigraphic correlations in the upper East Canyon area, north of the Uinta trend, indicate deformation less far to the west in Cenomanian, Turanian, Coniacian, and Santonian times. Paleocurrent data from the Echo Canyon Conglomerate and the position of its proximal fades on upturned rocks of the east margin of the northern Utah highland suggest that deposition of the Echo Canyon was concurrent with formation of the Farmington ramp and the overlying fold, chiefly in late Coniacian and Santonian time. Similar conglomerates interbedded with the underlying Frontier Formation suggest that the ramp and fold may have begun to form as early as Cenomanian time. By the time the fold was completely formed, early movements on the Absaroka thrust in southwestern Wyoming had occurred. South of the Uinta uplift, an anticline that formed in Campanian time during movement associated with the Charleston thrust is unconformably overlain by the basal beds of the Currant Creek Formation that were deposited in late Campanian or early Maastrichtian time. East of the anticline the uppermost beds of the Mesaverde Formation are of Santonian age; this suggests Campanian uplift along part of the south flank of the Uinta Mountains at the same time that the Charleston thrust was being emplaced. Dips in the Currant Creek Formation are moderately steep in its basal part and decrease upsection, suggesting that it was deposited during a major pulse of uplift of the Uinta Mountains. No ages have been obtained more than 100 m above the base that would date the main body of the formation and thus the uplift. In the Neil Creek area at the northwest corner of the Uinta Mountains, the North Flank fault zone splays out and cuts the Absaroka thrust. Rocks in the footwall of that thrust have been overturned and sliced by deformation along that fault zone. The youngest rocks involved are the late Campanian-early Maastrichtian Hams Fork Member of the Evanston Formation, and the Wasatch Formation of late Paleocene age overlies several of the splays of the North Flank fault zone. Time of major movement along the Hogsback thrust 150 km north of the Uinta Mountains is between early Campanian and middle Paleocene. No information is available on time of movement on that fault near the Uintas, because it is entirely in the subsurface. Seismic data are said to show that the North Flank fault zone overrides the Hogsback thrust at the margin of the Uintas. Major movements on the Uinta uplift occurred in late Paleocene and early Eocene time and continued into Oligocene time. Structural relief on the Cottonwood uplift is mainly, but not entirely, the result of Neogene faulting. Data on tuning of structural movements are interpreted to show that the major movement along the Sevier thrusts did not overlap times of major uplift of the western Uinta Mountains, but many events are not closely dated.

Abstract Tertiary and Cretaceous nonmarine sandstones are reservoirs for large amounts of natural gas at Natural Buttes field in the eastern part of the Uinta basin, Utah. A cored interval in the Upper Cretaceous Tuscher Formation dominantly comprises fine- to medium-grained, moderately to well-sorted sandstones and less abundant carbonaceous and coaly shale beds. These rocks represent sedimentation on the lower part of an alluvial braidplain. The Paleocene and Eocene Wasatch Formation unconformably overlies Cretaceous rocks and intertongues with marginal lacustrine strata of the Green River Formation. The cored interval in the upper part of the Wasatch consists of fine-grained lenticular sandstones with small-scale cross-bedding, argillaceous siltstones, and variegated mudstones, all of which were deposited in lower delta plain settings along the margin of Lake Uinta. Cored sandstones in the Tuscher and Wasatch formations have been extensively modified by minor quartz overgrowths; by the precipitation and subsequent dissolution of a carbonate mineral assemblage comprising iron-free calcite, ferroan calcite, dolomite, and ankerite; by local occurrences of anhydrite and barite; and by the formation of authigenic illite, mixed- layer illite-smectite, kaolinite, chlorite, and corrensite. Most authigenic carbonate formed during early burial before significant compaction. During later stages of diagenesis, anhydrite and barite precipitated locally, replacing detrital grains and mineral cements such as carbonate. Porosity and permeability have been significantly reduced in the sandstones owing to clay mineral development and the formation of carbonate cement. Large amounts of natural gas are stratigraphically trapped in these lenticular, diagenetically modified low-permeability sandstones. Potential source rocks in the Tuscher Formation may have generated thermogenic gas even though they are only moderately mature with respect to liquid hydrocarbon generation.