CHARACTERIZATION OF UPPER CRETACEOUS FRONTIER SANDSTONES, NORTH-CENTRAL WYOMING: PROVENANCE AND TECTONIC INTERPRETATION (Invited Presentation)

The exposed Cretaceous sedimentary rocks of north-central Wyoming, particularly the Frontier Formation, contain numerous sandstone beds and have enormous stratigraphic and tectonic significance. Although they tend to occur widely, their usefulness in stratigraphy has been limited to comparatively short-range correlations due to their difficulty of distinguishing one bed from another. The basal sandstone is composed of relatively thin layers of very fine to fine grained, bioturbated sandstone interbedded with irregularly laminated dark gray, poorly bioturbated mudstone. Small-scale trough-cross-stratification and small 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. The basal, middle, and upper Frontier sandstone in north-central Wyoming consist of subequal amounts of quartz and rock fragments and subordinate amounts of feldspar. The rock fragment fraction is almost entirely composed of chert and other grains of sedimentary/volcaniclastic origin. The basal Frontier sandstone with its generally consistent patterns in QFL roughly suggests the existence of a depositional basin with very little topographic or structural irregularity and presumably a homogeneous upland source area to the northwest. On the contrary, local intrabasinal control in the form of intraforeland uplifts associated with a dissimilar source terrain to the northwest must have exerted significant control on the establishment of final mineralogy for upper Frontier sandstones.

Funded by a PSC-CUNY Research Grant (#43) awarded to the author.