Stratigraphy and tectonic implications of Upper Cretaceous rocks in the Powder River basin, northeastern Wyoming and southeastern Montana

Sedimentary rocks of Late Cretaceous age (98.5-65.4Ma) in the Powder River Basin of northeastern Wyoming and southeastern Montana include siliciclastic, carbonate, pyroclastic, and coaly strata that crop out along the eastern, southern, and western margins of the basin and have been penetrated in the subsurface of the basin by many boreholes drilled for oil and gas.Siliciclastic beds predominate and are composed mainly of quartz, potassium feldspar, plagioclase, mixed-layer clay minerals, mica-illite, chlorite, kaolinite, and fragments of older rocks (sedimentary, metamorphic, and igneous).These Upper Cretaceous strata conformably overlie Lower Cretaceous beds and are conformably overlain by Paleocene beds; however, they enclose several widespread unconformities.The strata thicken southward in the basin from about 1,300 m (4,300 ft) in Powder River County, Montana, to about 3,000 m (9;800 ft) in Converse County, Wyoming.The rocks were deposited in continental, nearshore-marine, and offshore-marine environments on the west side of a north-trending epeiric sea, and they record at least six cycles of marine transgression and regression, some of which correspond to eustatic events.Most of the sediments were derived from erosion of areas in central and northwestern Wyoming, eastern Idaho, and western Montana.Paleocurrent directions determined from outcrops of sandstone in the Powder River Basin are northeast, southeast, and southwest.In the northwestern and northeastern parts of the basin the average rate of sedimentation during the Late Cretaceous was about 8.1 cm (3.2 in.)/l,000 years and 5.8 cm (2.3 in.)/l,000 years, respectively, whereas in the southern part of the basin the average rate of sedimentation was 12.1-13.6cm (4.8-5.4 in.)/l,000 years.The Powder River Basin is a large structural depression, having an area of about 57,000 km2 (22,000 mi2), that formed early in Tertiary time as a result of compression of a foreland in the Western Interior of North America.At the end of the Cretaceous, the total tectonic subsidence of the base of the Upper Cretaceous sequence was about 523 m (1,716 ft) near the northwestern margin of the basin, about 306 m (1,004 ft) near the northeastern edge, about 681 m (2,234 ft) on the southeastern flank, and about 761-762 m (2,497-2,500 ft) on the southwestern flank.The higher rates of tectonic subsidence (>9 cm [3.5 in.]/l,000 years) were determined from stratigraphic units mostly in the southern part of the basin.Subsidence in the basin region during the Late Cretaceous was occasionally interrupted by regional or local uplift, particulary in the middle and late Turonian, early Campanian, late Campanian, and late early to early late Maastrichtian.Nearby areas in central and southeastern Wyoming evidently were rising during the middle and late Turonian, early and late Campanian, and early and latest Maastrichtian.The greater thickness, higher sedimentation rate, and higher subsidence rate of several Upper Cretaceous stratigraphic units in the southern part of the basin probably are consequences of structural location.They apparently reflect the subsidence of downwarps or downthrown blocks, as well as the uplift of adjoining areas to the south in central and southeastern Wyoming. Figure 1 (facing page).Map showing outline (dashed line) and troughline (line with facing arrows) of Powder River Basin (where blanketed by Tertiary strata) and locations of major structural features, important boreholes (©) and outcrops (x), and lines of profile A-A' (fig.5) and cross section B-B' (fig.6), southeastern Montana and northeastern Wyoming.Outcrop locations are referred to by number and are described in table 1.of siliciclastic, carbonate, pyroclastic, and coaly beds that were deposited in marine and continental environments along the west side of a north-trending epeiric seaway (fig.2).These rocks thicken southward from about 1,300 m (4,300 ft) in Powder River County, Montana, to about 3,000 m (9,800 ft) in Converse County, Wyoming, (fig.3) (W.D. Grundy and C.T. Pierson, written commun., 1991).On the southeastern flank of the Powder River Basin in eastern Niobrara County, Wyoming, outcropping strata of Late Cretaceous age are, in ascending order, the Mowry Shale, Belle Fourche Shale, Greenhorn Formation, Carlile Shale, Niobrara Formation, Pierre Shale, Fox Hills Sandstone, and Lance Formation (fig.4, table 1).Laterally equivalent strata on the northwestern flank of the basin in Big Horn County, Montana, are, from oldest to youngest, the Mowry Shale, Belle Fourche Formation, Greenhorn Formation, Cody Shale, Judith River Formation, Bearpaw Shale, Fox Hills Sandstone, and Hell Creek Formation.Rocks of the same age in the southwestern part of the basin are, from oldest to youngest, the Mowry Shale, Frontier Formation, Cody Shale, Mesaverde Formation, Lewis Shale, Fox Hills Sandstone, and Lance Formation.The information shown in figure 4 for the southwestern area pertains to scattered outcrops in southeastern Natrona County.Figure 4 also displays information for comparable strata on the southwestern flank of the Bighorn Basin in northwestern Wyoming.In the Powder River Basin, Upper Cretaceous beds conformably overlie Lower Cretaceous beds, either at or slightly below the base of the Mowry Shale, and are conformably overlain by Tertiary (Paleocene) strata of the Fort Union Formation.