Gary S. Weissmann

Fluvial form in modern continental sedimentary basins: Distributive fluvial systems

Analysis of more than 700 modern continental sedimentary basins that are both endorheic (internally drained) and exorheic (externally drained) and cover a wide range of climatic and tectonic settings shows that sedimentation is dominated by distributive fluvial systems (DFSs). Facies distributions on DFSs are different from those of rivers in degradational settings, yet rivers in non-aggradational settings are commonly used to develop fluvial facies models. DFS rivers typically decrease in size downstream, are not confined to valleys, and form a radial pattern from an apex. Confined rivers are present in specific locations in sedimentary basins, including basin axial positions, areas between adjacent DFSs, and valleys incised into the DFS. DFSs and adjacent axial fluvial systems develop in a predictable manner that allows interpretation and prediction of fluvial architecture at the basin scale.

Recognition and importance of amalgamated sandy meander belts in the continental rock record

Meandering fluvial channels and their meander belts are common in modern continental sedimentary basins, yet compose a minor constituent of the reported fluvial rock record. Here we document exhumed amalgamated meander belt deposits from the upper Jurassic Morrison Formation, Utah (United States). The size of the amalgamated meander belt (9000 km2) is significantly larger than any documented previously and comparable in size to those from modern sedimentary basins. We describe a representative outcrop of sandy point bar deposits that shows features considered characteristic of both braided and meandering fluvial systems. Lateral accretion sets compose <5% of the outcrop area, yet point bar morphology is clearly visible in plan view. We suggest that difficulties in the identification of sandy, amalgamated meander belt deposits indicate that they have gone largely unrecognized in the rock record. Their recognition has important implications for basin-scale reconstructions of fluvial systems and interpretation of tectonic setting.

Characterization and quantification of aquifer heterogeneity using outcrop analogs at the Canadian Forces Base Borden, Ontario, Canada

Accurate characterization of internal structures and geometries of aquifers is critical for evaluation of plume migration and dispersion of contaminants. For this reason, high-resolution transport study field sites, such as the Waterloo Groundwater Research Site at the Canadian Forces Base Borden, Ontario, Canada, have been established. However, geological characterization at this site is based primarily on cores and shallow geophysical data. Outcrop analog studies offer detailed horizontal data that can help to build quantified models of aquifer heterogeneity. We used a combination of ground-based light detection and ranging (LiDAR) and high-resolution photographs at a sand quarry cut into Borden aquifer sediment to evaluate and quantify the distribution of lithofacies and hydrofacies that control flow properties in the Borden aquifer. We exposed sixteen ∼20 m × 1.5 m outcrops, LiDAR surveyed, photographed, and field mapped lithofacies at each exposure, and segmented the lithofacies from photographs to produce maps of facies distributions. We grouped different lithofacies into hydrofacies based on overall facies geometry and estimated hydraulic properties. We used LiDAR scans and the segmented facies maps to produce a digital outcrop model (DOM) of the site. The resultant DOM combines lithofacies and hydrofacies derived from field observations with high-resolution (5 mm) LiDAR data to reconstruct hydrofacies distributions in a three-dimensional (3-D) geomodel. The DOM offers a relatively complete horizontal correlation structure that was used in transition probability geostatistical modeling to create realizations of hydrofacies distributions in the aquifer at the study site.

Scale invariance in fluvial barforms: implications for interpretation of fluvial systems in the rock record

Understanding the controls on the size and shape of sandstone bodies deposited by fluvial systems is important in the reconstruction of ancient fluvial deposits and construction of quantitative reservoir models. Measurements and analyses of sandbodies from remotely sensed imagery have allowed quantification of width and length ratios of barforms in modern fluvial systems. For bank-attached bars the width:length ratios range between 0.12 and 0.47 (arithmetic mean: 0.25), for lateral bars between 0.19 and 0.42 (arithmetic mean: 0.30), for mid-channel bars between 0.09 and 0.49 (arithmetic mean: 0.28), and for point bars between 0.14 and 0.50 (arithmetic mean: 0.30). The majority of width:length ratios for all bar types range between 0.15 and 0.35. Examination of other parameters such as basin type, planform geometry, apparent stream width, river length, gradient over the investigated area, aggradational or degradational system, tectonic setting and climate do not significantly affect the width:length ratio. Therefore, the bar planform shape, the width:length ratio, can be considered to be scale invariant. The recognition that bar planform shape in fluvial systems is scale invariant will be useful in the construction of subsurface three-dimensional models of fluvial deposits with variable dimensions. Supplementary material: Data tables with information obtained for all of the rivers studied are available at http://www.geolsoc.org.uk/SUP18745.

Controls on the apex location of large deltas

The majority of sediment transport to the worlds oceans is routed via large deltas. We examine controls on delta apex location using a database of 84 of the worlds largest deltas. Of the dataset, 94% of apices are controlled by either bedrock valleys (80%) or Pleistocene alluvial valleys (14%), suggesting that the principal control on modern apex development is valley exit and/or bedslope-mediated avulsion and not hydrodynamic backwater length. Valley exit control on large delta apex location may have been as important in the rock record as it is today, and should be considered as a key control on delta development. Supplementary material: Tabulated data on backwater length and apex type for studied deltas available at https://doi.org/10.6084/m9.figshare.c.3469770

Uranium distribution as a proxy for basin-scale fluid flow in distributive fluvial systems

We infer system-scale fluid flow in the Late Jurassic Salt Wash fluvial succession (SW USA) by plotting uranium deposit distribution against sedimentological data, using uranium distribution as a proxy for subsurface fluid flow. More than 90% of uranium deposits in the Salt Wash occur where sandstone forms 40–55% and sand-rich channel-belts form 20–50% of the succession, which coincides with changes in channel-belt connectivity and gross-scale architecture. The paucity of uranium below these cut-off values suggests that fluid flow is related directly to predictable downstream fining and facies variations in distributive fluvial systems. Supplementary material: A summary table of location data, key trends and the amalgamation ratio method is available at https://doi.org/10.6084/m9.figshare.c.2849581.

Geochemical & Physical Aquifer Property Heterogeneity: A Multiscale Sedimentologic Approach to Reactive Solute Transport

This project is testing the hypothesis that sedimentary lithofacies determine the geochemical and physical hydrologic properties that control reactive solute transport (Figure 1). We are testing that hypothesis for one site, a portion of the saturated zone at the Hanford Site (Ringold Formation), and for a model solute, carbon tetrachloride (CT). The representative geochemical and physical aquifer properties selected for quantification in the proposed project are the properties that control CT transport: hydraulic conductivity (K) and reactivity (sorption distribution coefficient, Kd, and anaerobic transformation rate constant, kn). We are combining observations at outcrop analog sites (to measure lithofacies dimensions and statistical relations) with measurements from archived and fresh core samples (for geochemical experiments and to provide additional constraint to the stratigraphic model) from the Ringold Formation to place local-scale lithofacies successions, and their distinct hydrologic property distributions, into the basinal context, thus allowing us to estimate the spatial distributions of properties that control reactive solute transport in the subsurface.