VOLUME 40 NUMBER 1

Evidence for marine influence on a low-gradient coastal plain: Ichnology and invertebrate paleontology of the lower Tongue River Member (Fort Union Formation, middle Paleocene), western Williston Basin, U.S.A.

Triassic paleosol catenas associated with a salt-withdrawal minibasin in southeastern Utah, U.S.A.

Shane J. Prochnow, Lee C. Nordt, Stacy C. Atchley, Michael Hudec, and Thomas E. Boucher

Catenas for lithified paleosols have been rarely described in the literature because they are seldom preserved and, where present, are difficult to trace laterally. This study focuses on paleocatenas associated with a Triassic salt-withdrawal minibasin (rim syncline). The catenary surfaces formed during an episode of pro-longed landscape stability along a margin of a salt-withdrawal minibasin and were identified by a series of continuous, high-resolution photopanoramas of canyon outcrops. The paleocatenas had paleoslopes of 1%, and were laterally extensive over distances ranging from 2.8 to 7.9 km. Paleocatenas within the Chinle are associated with lateral taxonomic differences in paleosol morphology. A paleocatena formed on the upper boundary of the lower Chinle has well-drained alfisols on its upper slopes and a poorly drained, over-thickened inceptisol at the toe of the slope. A second paleocatena documented within the upper Chinle includes inceptisols with carbonate nodules characterized by thick paleosols on upper slopes and thin paleosols at the toe slope. Solum thickness, horizon thickness, color, and profile development index (PDI) values were evaluated relative to slope position. Solum thickness, E horizon thickness, Bt horizon thickness, and PDI values have relatively robust covariance to slope position, which suggests that they were influenced by pedotopographic factors. The Bw horizon thickness, Bk horizon thickness, and BC horizon thickness have less robust correlation to slope, and may instead, reflect the influence of other soil forming factors. Both paleocatenas were probably unstable, a conclusion supported by evidence for downslope sediment movement. Upper-slope paleosols have truncated surfaces whereas toe-slope paleosols were either buried or over thickened due to cumulic aggradation. Soil development along the paleocatenas was influenced primarily by erosion, deposition and soil-moisture processes, similar to modern catenas. Thus, landscapes associated with these salt tectonic features significantly influenced paleosol morphology in the Chinle.

William Lee Stokes: Earth historian from Hiawatha

Key Words: History of geology • biography • Utah • Great Basin • Earth history • teaching

Aaron S. Otteman and Arthur W. Snoke

The western border of the Hanna Basin is defined by the Rawlins uplift, a Laramide, basement-involved, faulted arch. This north-northwest south-southeast-trending structure separates the Hanna Basin on the east from the Great Divide Basin (part of the greater Green River depositional basin) on the west. The Rawlins uplift is a west southwest-vergent, macroscale, fault-related fold. Detailed geologic mapping, construction of serial cross sections, and the incorporation of data from a seismic-reflection profile indicate that displacement along the fault zone flanking the uplift's western margin cannot account for the net structural relief between the Hanna and Great Divide Basins (37,000 vertical ft [11,285 vertical m] and 27,000 vertical ft [8,235 vertical m], respectively). The exposed frontal fault traces are interpreted as high-angle (70) splays off a shallowly dipping (25), master fault zone developed within Archean granitic rocks of the Wyoming province. A low-dipping, braided, plastic-to-brittle thrust-fault zone in the Precambrian basement is inferred to accommodate much of the fault displacement and thus account for the structural relief between the core of the uplift and the adjacent basins. Within the study area, displacement along the exposed frontal fault zone decreases from south to north. Within the map area, bedding attitudes along the southwest limb (forelimb) of the uplift range from 3090 with only local areas of overturned beds. However, southwest of the map area, Upper Cretaceous strata are sub-vertical to overturned. On the homoclinal backlimb, dips are 1015 east-northeast into the Hanna Basin. Eastward structural bends at the southern and northern margins of the uplift suggest a component of left-lateral, oblique-slip displacement along the southern margin and right-lateral, oblique-slip displacement along the northern margin. Pre-existing basement anisotropies or discontinuities are likely responsible for these important changes in structural grain.

Key Words: Laramide orogeny • fault-related arch • basement-involved uplift • brittle fault zone • Rocky Mountain foreland deformation • Rawlins uplift • Wyoming

Jason C. Ruf and Eric A. Erslev