Abstract For over 100 years the Frontier Formation has been investigated in the Bighorn Basin in northern Wyoming and southern Montana. Recently, the U. S. Geological Survey (USGS) has had the opportunity to synthesize existing studies, analyze the latest subsurface information, and develop a stratigraphic framework for the central and southern part of the Bighorn Basin in Wyoming. The framework incorporates surface to subsurface correlations, projects biostratigraphic control from outcrop studies into the subsurface, and relates the ages to specific horizons mapped in the subsurface. As defined here, the Frontier Formation is part of an interval that is about 450 ft thick in the western part of the basin and about 1,075 ft thick in the southeastern part. This interval is mostly of Cenomanian age with a significant disconformity in the upper part, which is overlain by strata of Turonian and Coniacian age. The main subsurface correlation horizons are marine-flooding surfaces and condensed sections identified on the basis of geophysical log character and confirmed in core and on outcrop. Marine-flooding surfaces are identified by the transposition of marine shale facies over nearshore-marine sandstone facies, signifying a deepening event and relative sea-level rise. Condensed sections are interpreted for the accumulation of bentonite (altered volcanic ash deposits) deposited in sufficient quantity to register as high gamma-ray responses on geophysical logs and these responses are correlated with specific ashes found on outcrops and in cores. The subsurface mapping resulted in identifying ten surfaces that can be correlated throughout parts or all of the study area with varying degrees of certainty.
Siliciclastic and calcareous sedimentary rocks of early Late Cretaceous age in the Western Interior of the United States have been assigned to, in ascending order, the Graneros Shale, Greenhorn Formation, Carlile Shale, Niobrara Formation, and their lateral equivalents (including members of the Frontier Formation and overlying formations). This sequence of formations was deposited intermittently within and near an epicontinental seaway during the Cenomanian, Turonian, and Coniacian stages of the Cretaceous. It encloses three conspicuous and widespread disconformities that reflect regional marine regressions and transgressions as well as moderate tectonism. The disconformities and associated lacunae occupy three large areas within Wyoming, Colorado, and adjoining states. In parts of that region, as in northwestern Wyoming, a lacuna can represent more than one period of erosion and more than a single disconformity. Evidence for these disconformities was obtained from about 175 collections of molluscan fossils and from sedimentological studies of outcrops and borehole logs, supplemented by previously published data.
The oldest of the three disconformities, within the Frontier Formation and partial age-equivalents (including the Carlile Shale), separates Cenomanian or lower Turonian beds from middle Turonian beds in central and western Wyoming, northwestern Colorado, and adjoining areas of Montana and Utah. In parts of that region, the maximum duration of the associated lacuna is about 3 m.y. Erosion of the region in the late early Turonian was associated with a marine regression and transgression as well as mild local tectonism. The area where strata underlying the unconformity are oldest is partly overlain by the youngest of the succeeding transgressive beds. These youngest overlying beds presumably were deposited in an uplifted area where the eroded surface had a slightly higher elevation.
A younger disconformity, within the Frontier Formation and lateral equivalents, separates upper Cenomanian or lower or middle Turonian strata from middle or upper Turonian strata in central and eastern Wyoming, southwestern South Dakota, western Nebraska, and central and eastern Colorado. Locally in that region, the duration of the lacuna is as much as 5 m.y. The oldest beds underlying this contact are of late Cenomanian age and are distributed in north-central and southeastern Wyoming and in north-central Colorado, where the erosional surface was affected probably by slight uplifts and by fluvial drainage systems. In that region, the oldest beds are partly overlain by the youngest (late Turonian) of the transgressive strata. The areal distribution of the younger overlying beds in central Wyoming could indicate a westward migration of marine prodelta environments during the late Turonian.
At the youngest of the three disconformities, strata of middle or late Turonian ages in the Carlile Shale and lateral equivalents are overlain by upper Turonian or lower or middle Coniacian beds of the basal Niobrara Formation in Wyoming, Colorado, Nebraska, and parts of adjoining states. The maximum duration of the associated lacuna is more than 4 m.y. in northwesternmost Wyoming and northeasternmost Nebraska. Beds underlying this disconformity are oldest (early middle Turonian) in northwestern Wyoming, northeasternmost Nebraska, and possibly elsewhere in Nebraska, which apparently were areas of comparatively higher elevation and greater truncation. The underlying beds are youngest in a northeast-trending area that extends at least from eastern Utah to northeastern Wyoming. This area presumably was uplifted less than adjoining areas possibly in the late Turonian. Strata overlying this disconformity are oldest in northeastern New Mexico and much of Colorado and are youngest in northeastern Utah, northwestern and east-central Wyoming, north-central Kansas, and northeastern Nebraska, which indicates a marine transgression that progressed mainly northward.
Most of the ages used for the following calculations are estimates; consequently the resulting quantitative interpretations are speculative. The duration of the lacuna between the uppermost Carlile and the basal Niobrara increased northwestward from about 0.8 m.y. in south-central Colorado to about 4.3 m.y. in northwesternmost Wyoming. It also increased northeastward from 0.8 m.y. in Colorado to about 5.1 m.y. in northeastern Nebraska. Ages of basal beds of the Niobrara decrease northwestward from about 89.3 Ma in southeastern Colorado and northeastern New Mexico to about 88.7 Ma in northwesternmost Wyoming. Apparently, the Niobrara sea transgressed northwestward about 500 mi (805 km) from southeastern Colorado to northwesternmost Wyoming in about 0.6 m.y. Ages of the basal Niobrara also decrease toward the northeast, from 89.3 Ma in southeastern Colorado to 87.6 Ma in northeasternmost Nebraska. The Niobrara sea in that region, where chronologic data are notably sparse, possibly transgressed more than 480 mi (772 km) in about 1.7 m.y.
The Niobrara Formation in the Raton basin of northeastern New Mexico consists of the Fort Hays Limestone Member, about 20 ft thick, and the overlying Smoky Hill Shale Member, 800-900 ft thick. The Fort Hays Member is composed of seven to ten beds of light-gray-weathering, hard, dense limestone separated by soft, calcareous shale. A more varied lithology characterizes the Smoky Hill Member, which consists of four unnamed units. A 35-40 ft thick sequence of calcareous shale and shaly limestone, that forms the basal part of the Smoky Hill, is herein referred to as the shale and limestone unit. It is overlain by 95-125 ft of calcareous shale and minor shaly limestone, herein called the lower shale unit. These rocks are overlain by a sandy unit, 410 440 ft thick, most of which is moderately soft, shaly, very fine-grained sandstone that weathers yellowish brown to grayish orange. Parts of this unit contain calcareous concretions. The uppermost part of the Smoky Hill Member, herein referred to as the upper shale unit, is 265-295 ft thick and consists mostly of calcareous shale. The unit also includes at least 10 beds of orange-weathering argillaceous limestone, some beds of sandy shale, and limestone concretions. All contacts between the units of the Smoky Hill Member appear conformable, and the Smoky Hill rests conformably on the Fort Hays Member. The Niobrara Formation was deposited in a marine environment and is moderately fossiliferous. Molluscan fossils are dominant. Inoceramids prevail; ammonites are scarce in the Fort Hays Limestone Member and in the lower two units of the Smoky Hill Shale Member. The molluscan fossils in the Raton basin indicate a late Turonian age for the Fort Hays Member and possibly a very late Turonian to early Campanian age for the Smoky Hill Member. Geochemical data from pyrolysis assays of eight outcrop samples indicate that the Niobrara Formation in parts of the Raton basin contains potential source rocks for oil and gas. In some areas of the basin, the Niobrara is thermally mature and presumably has generated hydrocarbons.
In the Bighorn Basin of north-central Wyoming and south-central Montana, the Frontier Formation of early Late Cretaceous age consists of siliciclastic, bentonitic, and carbonaceous beds that were deposited in marine, brackish-water, and continental environments. Most lithologic units are laterally discontinuous. The Frontier Formation conformably overlies the Mowry Shale and is conformably overlain by the Cody Shale. Molluscan fossils collected from outcrops of these formations and listed in this report are mainly of marine origin and of Cenomanian, Turonian, and Coniacian ages. The lower and thicker part of the Frontier in the Bighorn Basin is of Cenomanian age and laterally equivalent to the Belle Fourche Member of the Frontier in central Wyoming. Near the west edge of the basin, these basal strata are disconformably overlain by middle Turonian beds that are the age equivalent of the Emigrant Gap Member of the Frontier in central Wyoming. The middle Turonian beds are disconformably overlain by lower Coniacian strata. Cenomanian strata along the south and east margins of the basin are disconformably overlain by upper Turonian beds in the upper part of the Frontier, as well as in the lower part of the Cody; these are, in turn, conformably overlain by lower Coniacian strata. Thicknesses and ages of Cenomanian strata in the Bighorn Basin and adjoining regions are evidence of regional differential erosion and the presence of an uplift during the early Turonian centered in northwestern Wyoming, west of the basin, probably associated with a eustatic event. The truncated Cenomanian strata were buried by lower middle Turonian beds during a marine transgression and possibly during regional subsidence and a eustatic rise. An uplift in the late middle Turonian, centered in north-central Wyoming and possibly associated with a eustatic fall, caused the erosion of lower middle Turonian beds in southern and eastern areas of the basin as well as in an adjoining region of north-central Wyoming. Similarly, in east-central Wyoming and an adjacent area to the south, Cenomanian strata are disconformably overlain by upper middle and lower upper Turonian strata that probably reflect uplift and erosion in that region during the interim period of middle Turonian time. During later subsidence and a marine transgression, upper Turonian deposits buried Cenomanian beds in areas along the south and east margins of the Bighorn Basin and buried lower middle Turonian beds in much of northern Wyoming. Upper Turonian and lower Coniacian strata are apparently conformable in eastern and southern areas of the basin as well as near Riverton, Kaycee, and Casper in central Wyoming. Upper Turonian strata are absent on the west flank of the Bighorn Basin and in outcrops west of the basin, where middle Turonian beds are disconformably overlain by lower Coniacian beds . The conformable upper Turonian and lower Coniacian beds apparently transgressed an eroded middle Turonian surface in the region, but only Coniacian strata overlie middle Turonian beds on the west side of the basin and areas farther west. Coniacian strata onlap the truncated lower middle Turonian surface west of the basin, indicating a region that had higher elevation possibly resulting from tectonic uplift. In east-central Wyoming and an adjoining region to the south, upper middle Turonian and lower upper Turonian strata are disconformably overlain by lower and middle Coniacian beds. That region apparently was uplifted and eroded during the latest Turonian.
