Ranie Lynds

Conceptual model for predicting mudstone dimensions in sandy braided-river reservoirs

Sandy braided-river deposits with high net-to-gross sand ratios are commonly attractive reservoirs, yet internal lithologic heterogeneities, particularly the presence of low-permeability mudstone deposits, significantly complicate the development of such units. Previous work has focused on measuring the scale and distribution of mudstone deposits in outcrop analogs; however, because of extreme differences in scale, discharge, sediment load, and geologic history, the results of these studies are difficult to apply with confidence to a wide range of sandy braided-river reservoirs. Based on work in modern braided rivers (Niobrara and North Loup rivers, Nebraska) and ancient braided-river deposits (Kayenta Formation, Jurassic and lower Castlegate Sandstone, Cretaceous, Colorado and Utah), we propose a process-based conceptual model for understanding and predicting the distribution and geometries of fine-grained (mudstone) intervals in sandy braided-river deposits. This model is an idealized channel-fill unit composed of five fine-grained lithofacies (mud plugs, channel-lining muds, interbar muds, inclined heterolithic strata, and flood-plain and overbank material) that scale proportional to channel-thread dimensions, including depth, cross-stream width, and downstream length. Each lithofacies is found in a different region in an individual channel fill, and lithofacies found low in a fill may be preferentially preserved. Within braided-river deposits, extrinsic depositional factors, such as aggradation rate, available accommodation, and avulsion-return time, produce different channel-fill stacking arrangements, preserving fine-grained lithofacies in different, relative proportions. This conceptual model provides an approach to reservoir characterization that deductively constrains the dimensions and distribution of fine-grained barriers to flow and may help account for the inherent variability in sandy braided-river deposits.

Paradox of downstream fining and weathering-rind formation in the lower Hoh River, Olympic Peninsula, Washington

Although downstream fining of clasts is typical in modern and ancient river systems, the Hoh River, a cobble-and-boulder-bed river in Washington state, contains surprisingly little downstream fining of the coarsest tail of the grain-size distribution along its lower 63 km. Mean rate of fining of the coarsest size fraction is only 0.24 mm/km, barely significant given uncertainties in measurement. In addition, these same clasts have weathering-rind thicknesses that change very little along the length of the river (they decrease by 0.01 mm/km). Because weathering rinds are well developed in this setting and both tumbler studies and field observations show that abrasion of weathering rinds greatly accelerates clast diminution, there seems to be a paradox between predicted and observed downstream fining rates. Field study shows that detritus is constantly being resupplied to the river by erosion of late glacial materials along the river9s cutbanks and tributaries. The clasts supplied range in size but include abundant coarse grains with very thick weathering rinds. The continuous resupply of grains strongly attenuates the rate of downstream fining, despite the fact that these weathered grains abrade relatively rapidly. Thus, the dominance of reworked glacial debris overwhelms any processes by which grains are reduced in size in the modern river. These results suggest that relatively infrequent glaciation can have a long-lived effect on river sedimentation.

Cenozoic collapse of the eastern Uinta Mountains and drainage evolution of the Uinta Mountains region

Coupled detrital sanidine and zircon data, combined with sedimentological and stratigraphic observations, provide temporal constraints on the post-Laramide paleogeographic and structural evolution of the eastern Uinta Mountains region from the late Eocene to late Miocene (ca. 36–8 Ma). Maximum depositional ages (MDAs) calculated from detrital zircon U-Pb and detrital sanidine 40Ar/39Ar ages indicate that the most significant Paleogene fluvial system in the region, represented by the Bishop Conglomerate, existed from 36 to 27 Ma. The abundance of red sandstone and gray limestone clasts, paleocurrent directions, and the large number of Grenville-age detrital zircons suggest that the Uinta Mountain Group (UMG) facies of the Bishop Conglomerate are tributaries that flowed radially away from the crest of the Uinta Mountains. To the north of the Uinta Mountains, these rivers joined a mainstem river in southwestern Wyoming represented by the Bishop Conglomerate Firehole Canyon (FC) facies. This facies consists of rounded cobbleto pebble-sized quartzite clasts with minor quantities of volcanic rocks, has westward paleocurrent directions, and abundant young (younger than 40 Ma) detrital zircon and sanidine grains. Detrital sanidine age and geochemical data suggest that these young detrital grains are tephra that originated from the Basin and Range volcanic field, which was subsequently reworked into Bishop Conglomerate sediments. The more regional headwaters of the mainstem river could have been located east of the Uinta Mountains, or in the Challis and Absaroka volcanic fields and the Wind River Mountains located to the northwest of the region. The question of whether the FC facies of the Bishop Conglomerate represents part of an integrated river system that was a precursor to the Platte River remains unresolved. Extensional collapse of the eastern Uinta Mountains was marked by the cessation of Bishop Conglomerate fluvial deposition and the onset of Browns Park Formation sedimentation within the Browns Park graben beginning ca. 25 Ma. Tuffaceous sandstone and siltstone and minor quantities of carbonate accumulated in a mosaic of fluvial and lacustrine environments representing an internally drained basin. Detrital sanidine age and geochemical data for young (younger than 40 Ma) grains also support a volcanic ash-fall origin. Some of the grains originated from the Southern Rocky Mountain volcanic field, and were reworked into Browns Park Formation deposits. New MDAs of Browns Park Formation sediments that unconformably overlie Neoproterozoic UMG rocks in westernmost Browns Park provide evidence for a younger (12–8 Ma) phase of extensional collapse of the eastern Uinta Mountains that was associated with 10–20 km of northwestward-directed lengthening of the Browns Park graben. These data are compatible with models for two stages of post-Laramide epeirogenic uplift of the Uinta Mountains region, including post–12 Ma tectonism that set the stage for subsequent integration of the Green and Colorado Rivers after 8 Ma.

CENOZOIC COLLAPSE OF THE EASTERN UINTA MOUNTAINS AND DRAINAGE EVOLUTION OF THE UINTA MOUNTAIN REGION

81501 2 Dept. of Geosciences, Colorado State University, Ft. Collins, CO 80523 3 Utah Geological Survey, Salt Lake City, UT 84114-6100 4 ExxonMobil Exploration Company, 22777 Springwoods Village Parkway, Spring, TX 77389 Wyoming Geological Survey, Laramie, WY 82072 6 Dept. of Earth and Planetary Sciences, University of New Mexico, Albuquerque, NM 87131 7 New Mexico Bureau of Geology and Mineral Resources, New Mexico Tech, Socorro, NM