Regional Studies of Foreland Basins
Intra- and inter-basinal correlations between outcrop and subsurface over most of northern and central Wyoming indicate that chert-bearing conglomerates in the lower Cretaceous Cloverly Formation in the foreland of central Wyoming occupy three distinct stratigraphic levels. The two older conglomerates are in the lower Cloverly Formation in the western Wind River Basin and reflect northerly to northeasterly dispersal. The youngest conglomerate is in the upper Cloverly Formation in the eastern portion of the basin; gravels in this interval also were transported to the north and northeast.
The two older conglomerates are separated from the youngest conglomerate by up to 35 m of purple to gray, smectite-rich mudstones that contain distinctive 10 to 90 cm-thick layers of white to dark green devitrified tuff, as well as silica and carbonate nodular beds. Fission-track ages of 125–128 Ma have been obtained from three samples of tuff in the Wind River Basin. These tuffs can be correlated to prominent tuffs further north in the Bighom Basin where a paleomagnetic stratigraphy has been established. Fission-track ages of zircons from devitrified tuff layers and magnetostratigraphy of mudstones suggest that the older two conglomerates in the Wind River Basin were deposited between 133 and 128 Ma and the youngest conglomerate at about 118 to 115 Ma.
Three-dimensional, spatially controlled and temporally constrained reconstructions of paleodrainage systems for Cloverly conglomerates illustrate the complexity of fluvial drainage networks within the evolving Early Cretaceous foreland basin. Sand-body geometry and dispersal patterns within these fluvial networks were partially controlled by tectonic activity, which created a series of northeast-oriented horsts and grabens in the Wind River Basin. Location of trunk rivers was controlled by the positions of grabens within the basin.
Figures & Tables
Stratigraphic Evolution of Foreland Basins - A strong case can be made that foreland basins are where the casual links between sedimentation and tectonic events were first recognized, as evidenced by the interpretations of geologists working in classic foreland areas. This Special Publication was derived from a Research Symposium entitled ?Stratigraphic Sequences in Foreland Basins ?held at the AAPG-SEPM joint annual meeting on June, 1992, in Calgary, Alberta, Canada. This volume provides a well-balanced perspective of current research on foreland basin stratigraphy and also serves as another element in the evolving framework that comprises our understanding of foreland basins. Given that so many of earth?s resources are found in foreland basins and that foreland basin strata often provide the only preserved record of the tectonic events that led to basin development, the impetus for continued studies of foreland basin strata should remain for many generations of geologists to come.