Regional disconformities in Turonian and Coniacian (Upper Cretaceous) strata in Colorado, Wyoming, and adjoining states biochronological evidence

Abstract

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.