The Late Jurassic (157–150 Ma) Morrison Formation of the Western Interior of the United States contains abundant altered volcanic ash. On the Colorado Plateau, this formation accumulated behind and downwind of a subduction-related volcanic arc along the western margin of North America. The ash in these distal fallout tuffs probably drifted eastward from coignimbrite ash clouds related to collapse calderas. Altered volcanic ash is particularly abundant in the Brushy Basin Member of the upper part of the Morrison Formation. In one 110-m-thick section in eastern Utah, 35 separate beds were deposited in a 2.2 m.y. period. Alteration occurred when glassy volcanic ash fell into fluvial and lacustrine environments, where it was diagenetically altered to various mineral assemblages but most commonly to smectitic clay. Periodically, ash fell into saline, alkaline lakes, and diagenetic alteration of the glassy ash produced a crudely zoned deposit on the Colorado Plateau. Altered volcanic ash beds in the outermost part of the lacustrine deposits are argillic (with smectitic clay), whereas zeolitic (clinoptilolite, analcime) and feldspathic (K-feldspar and albite) alteration dominates the interior zones. Feldspathic ash layers contain secondary silica, and consequently immobile element (e.g., Al, Ti, and high field strength elements) abundances were strongly diluted in these rocks. In contrast, the argillic ash beds experienced strong SiO 2 depletion, and, as a result, they are enriched in the relatively immobile elements. The compositions of the zeolitic ash beds are intermediate between these two extremes and experienced the least alteration. As a result of these changes, immobile element concentrations are less reliable than ratios for determining the original magmatic composition of the ash. Most of the altered ash (regardless of type) was also depleted in water-soluble elements like the alkalies, U, and V. The latter two elements were oxidized during diagenesis of the ash, became soluble, and were partially leached away by groundwater. Locally, U and V in groundwater were reduced upon contact with organic materials and formed important ore deposits. Several aspects of the mineralogy and geochemistry of the altered volcanic ash beds yield information about their original magmatic compositions. The volcanic ash beds typically have small phenoclasts of quartz, sanidine, plagioclase, biotite, zircon, apatite, and Fe-Ti oxides. Titanite is present in ∼40% of the ash beds; pyroxene and amphibole were found in less than 5%. Phenocryst assemblages, mineral compositions, inferred high f O 2 , rare earth element patterns, and immobile element ratios all suggest the parent magmas for the altered tuffs were subduction-related dacites and rhyolites. Small numbers of tuffs have Fe-rich biotite, amphibole, and/or clinopyroxene; both pyroxene and amphibole are alkali rich. These tuffs lack titanite, but some contain anorthoclase and F-rich apatite. Combined with enrichments in Nb and Y, these features show some tuffs had an A-type character and were related to some type of within-arc extension. Paleowind directions, and distribution, radiometric ages, and compositions of the volcanic ash beds and of plutons in the western United States suggest that the most likely eruption sites were in the subduction-related Jurassic magmatic arc, which extended across western Utah and central Nevada and southward into the Mojave of California and southern Arizona (present-day coordinates). Pb isotopic compositions show that at least some of the ash was erupted from magma systems (now exposed as plutons) in the Mojave Desert. We conclude that a brief ignimbrite flare-up from 157 to 150 Ma, but focused on the time period from 152 to 150 Ma, in this region may have been driven by slab steepening and conversion to a strike-slip boundary after a preceding phase of folding and thrusting. The presence of ash beds with A-type characteristics mixed with those that have more typical subduction signatures confirms that the Late Jurassic was geologically a transitional time in North America when subduction was changing to transtensional movement along the western plate boundary.

Multiple cemented channel-fill deposits from the Late Jurassic and Early Cretaceous, once buried beneath 2400 m of sediment, are now exposed at the surface in arid east-central Utah due to erosion of the less resistant surrounding material. This field guide focuses on examples near the town of Green River where there is public access to several different types of exhumed paleochannels within a small geographic region. We describe the geologic setting of these landforms based on previous work, discuss the relevance to analogous sinuous ridges that are interpreted to be inverted paleochannels on Mars, and present a detailed road log with descriptive stops in Emery County, Utah.

Abstract Large quantities of natural gas have been produced from underpressured Cretaceous reservoirs of the San Juan Basin since 1951, yet the reasons for the under-pressuring and the containment mechanisms remain a subject of inquiry. In this investigation, compilations of reservoir pressures from the 1950s and early 1960s are used to minimize the perturbations caused by later gas production. The pressures are projected to two basin-scale cross sections showing the structural configuration and stratigraphy of Cretaceous and younger rock units. Gas pressures in the Dakota Sandstone vary according to location, with pressure/depth ratios of 0.36 psi/ft (8.16 kPa/m) in the west and 0.41 psi/ft (9.27 kPa/m) in the east, where pressures approach hydrostatic values. Gas pressures in the sandstones of the Mesaverde Group are remarkably consistent, with pressure/depth ratios of 0.24 psi/ft (5.42 kPa/m), except in the southeast corner of the gas accumulation where the pressure/depth ratio is 0.35 psi/ft (7.91 kPa/m). Pressure-elevation plots, in conjunction with cross sections and measurements of hydraulic head in water wells, show that the gas system is not buoyant in the way that a conventional gas accumulation is buoyant. Underpressuring in this basin reflects the absence of bottom water and the presence of top water. The pressure reference for the gas is at the edge of the gas accumulation instead of at the bottom, and the preproduction gas pressure is determined by the elevation of the lateral transition from downdip gas to updip water on the southwestern limb and other margins of this asymmetric basin. No pressure discontinuity between gas and water exists at the updip edge of the gas accumulation; hence, no seal in the usual sense exists, and there is no need for one. The hard seal of a shale or an evaporite formation is replaced by a capillary soft seal caused by a transition from low-permeability downdip rocks to high-permeability updip rocks. Hydrodynamic trapping, an explanation that has been cited for many years, is not required. Instead, the gas is just sitting in a pancake-shaped volume bounded by a low-permeability base, a gentle stratigraphic rise on one side, and more steeply dipping monoclines on the other three sides. The gas does not escape from the edges of the basin because no excess gas pressure can exist in the absence of an underlying aquifer.

Abstract The Upper Triassic Chinle Formation and the Upper Jurassic Morrison Formation preserve a record of lacustrine deposition along the western margin of tropical Pangaea and post-Pangaean North America. The lake deposits in these formations contain archives of sedimentary and geochemical paleoclimatic indicators, paleoeco-logical data, and characteristic stratal architecture that provide glimpses into the evolution of basins linked to global- and continental-scale tectonic events and processes, and the establishment of a mosaic of continental paleoecosystems. This field trip highlights the lacustrine and associated fluvial deposits of the Monitor Butte Member of the Chinle Formation and the Tidwell and Brushy Basin Members of the Morrison Formation in the southern part of the Colorado Plateau region, with emphases on: (1) sedimentary facies analysis and paleogeography of the paleolakes; (2) stratal architecture and high-frequency sequence stratigraphy; (3) recognition of lake basin-fill types; and (4) paleontology and ichnology of lake strata and their paleoecologic, paleohydrological, and paleoclimatic interpretation .