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Constraining the depositional gradient of ancient alluvial river systems can aid in reconstructing landscapes and estimating paleodischarge by establishing boundaries on the climatic and tectonic history of continental sequences. We present three methods for estimating ancient depositional gradients based on the interpreted mode of sediment transport for the range of particle sizes found on the beds of modern rivers or preserved in channel fills. For sandy rivers with suspension as the dominant mode of sediment transport, these methods take advantage of observations that can be directly obtained from preserved strata, including measurements of paleo flow depth and grain size. The first method relates river slope to Shields number at bankfull flow. The second method is similar to the first but allows for variation in Shields number at bankfull flow with grain diameter. The final method relies on criteria required for both suspended-and bed-material load sediment transport in the same system and is the most comprehensive and physically justified method. We present each method and test them with modern river datasets to verify accuracy and help constrain uncertainty-the first step to adapting them to ancient systems. Results indicate that all methods estimate slope within a factor of two. These methods are potentially very powerful for interpreting sandy fluvial deposits because they can provide reasonably accurate quantitative estimates of paleoslope, an elusive yet important environmental variable.

Measured particle-size distributions are commonly reduced to one characteristic value (e.g., median grain diameter) that is used in sediment transport modeling and other analyses. These values are often interpolated from empirical distributions or from fitted distributions, usually assuming that observed grain-size populations are adequately represented by Gaussian or Normal distributions. In order to investigate the implications of this approach, we (1) statistically characterize grain-size distributions in samples of bed-material load, suspended load, and slackwater deposits from the sand-bedded Calamus, North Loup, and Niobrara rivers (Nebraska, USA), and (2) explore the potential impact of misfitting distributions on estimating percentile grain diameters. Although log-normal distributions are commonly used to characterize complete grainsize distributions in sedimentary systems, in this study, samples of transported sediment are best modeled with log-hyperbolic distributions, and slackwater deposits are best fitted by mixtures of distributions. Despite large overlaps in the grain sizes of bed-material-load and suspended-load samples, estimated parameters of fitted log-hyperbolic distributions show consistent differences between these samples across all rivers. Samples of bed-material load have higher modes and positive (coarsegrained) asymmetry, whereas suspended-load samples have lower modes and weaker asymmetry. Because it is has a general form, the log-hyperbolic distribution should adequately characterize unimodal grain-size samples because its parameters can yield both normal-shaped distributions as well as asymmetric distributions. In all three rivers, slackwater deposits contain the entire range of grain sizes present in suspension as well as a significant component of very fine-grained (, 0.02 mm) material that is not present in suspended-sediment samples. This suggests some degree of fractionated deposition of suspended sediment in areas of near-zero flow velocities. Ultimately, fitting parametric grain-size distributions to grain-size data can be a useful way to find effective particle-size values for use in sediment transport modeling and other studies. However, particularly with asymmetric grain-size distributions, fitting log-normal distributions to data may result in errors of estimated percentile grain sizes, which should be considered in studies relying on characteristic grain-diameter values.

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The Moxa Arch in the Greater Green River Basin, southwestern Wyoming, hosts two potential reservoirs for CO 2 sequestration. The Bighorn Dolomite and Madison limestone are interpreted to be independent reservoirs based on differing CO 2 compositions and production histories; the two reservoirs are separated by Devonian carbonates, siliciclastic rocks, and evaporites. On the Moxa Arch, the Bighorn ranges in thickness from 67 to 120 m with porosities from 3 to 15 percent. The massive buff-colored Steamboat Point Member comprises the bulk of the subsurface Bighorn in southwest Wyoming. Mottled dolostone (light-colored patches with higher porosity and dark-colored patches with lower porosity) is very common and is presumably the result of preferential early dolomitization of bioturbation. Core study suggests that this factor affects gas saturation and storage in this extensive reservoir. The lower Member of the Jefferson Formation is the most probable seal within the Devonian stratigraphy. Strata of this member are interpreted to have been deposited in a shallow basin semi-isolated from the deeper marine environment to the west. 87 Sr/ 86 Sr isotopic analyses of anhydrite sampled from Moxa Arch well cuttings support the interpretation of a depositional environment exposed to a mixture of seawater and freshwater. High-frequency relative sea level fluctuations superimposed on a gently sloping shelf produced alternating layers of marine carbonates, peritidal siliciclastic rocks, and evaporites. The evaporites are interpreted to seal CO 2 in the Bighorn Dolomite from the overlying Madison limestone. The lower Paleozoic strata on the Moxa Arch provide an effective trap-reservoir-seal combination for naturally occurring CO 2 with potential applications to future studies at analogous locations in the central Rocky Mountain Region.

Laramide deformation during the Late Cretaceous through early Eocene interrupted the east-flowing drainage systems from the Sevier hinterland and segmented the Western Interior Foreland