The Upper Cretaceous Frontier Formation exposed along the western margin of the western interior foreland basin is composed of clastic sediments that were deposited during the initial late Cretaceous transgressive-regressive phases of the Western Interior Seaway across Wyoming (Figure 1). Current study is aimed at providing attendees with the best practices associated with tectonic, lithofacies and sedimentologic interpretation of foreland basin sequences and emphasizing the need for utilizing field, structural, geochemical, and compositional data for reconstructing a viable scenario connected with foreland basin sedimentary packages. An attempt will be made to compare documented sedimentological findings with foreland basins located in the northeast of Qinghai-Tibet Plateau. The lithofacies belonging to the Frontier Formation are subdivided into three distinct units which are, in ascending order: the lower Frontier unit, characterized by bioturbated sandstone, mudstone, bentonitic mudstone, and chert-pebble conglomerate; the middle Frontier unit, a thick sequence of sandstone, carbonaceous mudstone, bentonites, and chert-andesite-granite-quartzite pebble conglomerate; and the upper Frontier unit which incorporates thick beds of sandstone, siltstone, and bioturbated mudstone. Lateral as well as vertical variation in lithofacies is pronounced and sediment-fill is asymmetric. The basal sandstone (Peay sandstone) is composed of relatively thin layers of very fine to fine grained, bioturbated sandstone interbedded with irregularly laminated dark gray and poorly bioturbated mudstone. Small-scale troughcross-stratification and minute intraformational rip-up clasts are also present within this sandy facies. The middle Frontier sandstones are generally less well-sorted than those of the basal sandstones; although the grain size coarsens up section within this middle unit. In addition, the middle Frontier sandstones consist of low-angle, trough cross-stratification, plant debris, and thin beds of lignite. The upper Frontier unit is composed of thick beds of fine to medium-grained sandstone. Ripple bedding is common in this unit. Carbonaceous shale, sideritized mudstone, root traces, and plant remains are frequently observed within this unit. Small horizontal to inclined burrows (Ophiomorpha) are encountered in the outcrop.
Nazrul I. KHANDAKER, 2013. Tectonose Dimentologic Significance of the Upper Cretaceous Foreland Basin Siliciclastics: Western Interior, USA. Acta Geologica Sinica (English Edition), 87(supp.): 212-214.