Abstract This study addresses the stratigraphic architecture and connectivity of fluvial sandstones of the Williams Fork Formation through outcrop analysis, and static and dynamic modelling of equivalent reservoirs in the Piceance Basin, Colorado. The Williams Fork Formation is a succession of fluvial channel sandstones, crevasse splays, floodplain mudstones and paludal coals that were deposited by meandering- and braided-river systems within coastal- and alluvial-plain settings. Three-dimensional (3D) static and dynamic reservoir models that are constrained to both outcrop-derived and subsurface data show how static connectivity is sensitive to sandstone-body type and width, and varies with net to gross ratio. Connectivity analyses of 3D outcrop-based architectural-element models show how relatively wide sandstone bodies enhance connectivity. At Mamm Creek Field, connectivity of sandstones that are pay within the middle Williams Fork Formation is 12–18% higher than for the lower Williams Fork Formation. For highly constrained 3D object-based models of architectural elements, connectivity is only 4% higher when crevasse splays are included as reservoir-quality sandstones. Dynamic simulation results also suggest that the best history match is possible by considering only point bars and channel bars (reservoir-quality sandstones) as pay. Additional research is necessary to determine the impact of crevasse splays on reservoir connectivity.

Abstract The Black Canyon of the Gunnison and Unaweep Canyon in western Colorado have long been viewed as classic examples of post-Laramide Plio-Pleistocene uplift, which in the case of Unaweep, is thought to have forced the Gunnison River to abandon the canyon. Ongoing field studies of the incision histories of these canyons and their surrounding regions, however, suggest that post-Laramide rock uplift has been regional, rather than local in nature. River incision rates calculated using ca. 10 Ma basaltic lava flows as a late Miocene datum suggest that long-term incision rates range from 61 to 142 m/m.y. with rates decreasing eastward towards the central Rocky Mountains. Incision rates calculated using the ca. 640 ka Lava Creek B ash range from 95 to 162 m/m.y., decrease eastward towards the mountains, and are broadly similar in magnitude to the longer-term incision rates. Locally, incision rates are as high as 500–600 m/m.y. along the lower reaches of the Black Canyon of the Gunnison, and these anomalously high values reflect transient knickpoint migration upvalley. Knickpoint migration was controlled, in part, by downvalley base-level changes related to stream piracy. For example, abandonment of Unaweep Canyon by the Gunnison River could have led to rapid incision through erodible Mancos Shale as the Gunnison River joined the Colorado River on its course around the northern end of the Uncompahgre Plateau. Geophysical data show that abandonment of Unaweep Canyon was not caused by differential uplift of the crest of Unaweep Canyon relative to the surrounding basins. Instead, the ancestral (Plio-Pleistocene?) Gunnison River flowed through Cactus Park, a major paleovalley that feeds into Unaweep Canyon, and continued downvalley to its juncture with the Dolores River near present-day Gateway, Colorado. The average gradient of the ancestral Gunnison River through the canyon prior to abandonment was ~7.5–7.6 m/km. Lithological and mineralogical considerations suggest that the Colorado River also flowed through and helped to carve Unaweep Canyon, although the Colorado River probably exited Unaweep Canyon prior to abandonment by the Gunnison River. The ancestral Gunnison River remained in its course and incised through bedrock for a long enough period of time to produce terrace remnants in the Cactus Park region that range in elevation from 2000 to 1880 m. Abandonment of the canyon by the Gunnison River was followed by formation of a natural dam that probably led to deposition upvalley of ~50 m of lacustrine sediments in Cactus Park. Recent mapping in the lower reaches of Unaweep Canyon indicate that a landslide could have led to damming of Unaweep Canyon, perhaps while it was occupied by underfit streams.

Abstract The Mancos B interval of the Upper Cretaceous Mancos Shale is represented by up to 372 m of thinly interstratified clays tone, siltstone, and very fine- to fine-grained sandstone deposited offshore, below storm-wave base, in the Western Interior Seaway. The Mancos B is best developed along Douglas Creek Arch in western Colorado and eastern Utah, where it is a major producer of natural gas and a minor producer of oil. Combined stratigraphic and sedimentologic data suggest that the Mancos B is a regressive prodelta-plume complex genetically related to deltaic systems active along the western shoreline of the seaway. In outcrop and subsurface cores, the Mancos B is characterized by lenticular, cyclic, upward-coarsening parasequences, ranging in thickness from 2 to 32 m, and composed of five main lithofacies: silty claystone, sandstone-claystone, sandy siltstone, bioturbated muddy sandstone, and sandy dolomite (as beds and concretions). Sedimentary structures include horizontal lamination, wavy lamination, lenticular bedding, flaser bedding, ripple lamination, and horizontal lamination. Paleocurrent measurements indicate an average sediment-transport direction to the southeast (111°). Trace fossils are characteristic of the Cruziana ichnofacies. Lateral lithofacies variations in Mancos B parasequences are characterized by transitions from bioturbated muddy sandstone lithofacies into sandstone-claystone lithofacies then into sandy siltstone or silty claystone lithofacies. Horizons of sandy dolomite lithofacies cap most parasequences and represent periods of low sedimentation.

Abstract The Parachute Creek Member of the Green River Formation in the southern Piceance Creek basin, Colorado, and the eastern Uinta basin, Utah, displays four major lacustrine depositional facies: sandstone (bottom), stromatolite, marlstone, and oil shale (top). In a vertical sequence these facies record the transgressive history of Lake Uinta during its most expansive stage. The sandstone facies and the stromatolite facies represent deposition in a marginal-lacustrine environment, while the marlstone facies and oil-shale facies were deposited in an open-lacustrine setting. Sulfur-isotope values (δ 34 S) were determined for iron-sulfide minerals (pyrite, marcasite, and pyrrhotite) from all facies except the marlstone facies. Values were also determined for pyrite from the Uinta Formation in the eastern Piceance Creek basin. The sulfide minerals demonstrate a total range of about 72 permil and show strong enrichment in 34 S when compared with sulfides from marine rocks. Maximum 34 S enrichment was found in sulfides from oil shale and marlstone beds of the oil-shale facies, averaging about 35 permil (CD). Sulfides from the stromatolite facies and the sandstone facies show less enrichment in 34 S, and average about 17 permil. The Parachute Creek Member shows progressive and uniform upward enrichment in 34 S which culminated during deposition of the rich oil shale beds of the Mahogany interval of the oil-shale facies.