An Upper Cretaceous paleodrainage system on the Coastal Plain unconformity of Alabama-Georgia

ABSTRACT Rocks of the Upper Cretaceous Tuscaloosa Formation (Cenomanian) and Eutaw Formation (Santonian) in southwestern Georgia and southeastern Alabama record an interval of fluvial and nearshore marine deposition. In the vicinity of Columbus, Georgia, basal units of the Tuscaloosa Formation consist of a residual paleosol built on crystalline rocks of the Appalachian Piedmont covered by conglomeratic sandstones deposited in braided stream systems flowing across the mid-Cenomanian Coastal Plain unconformity. The unconformity, which separates Cretaceous detrital rocks from underlying metamorphic rocks and residual paleosols built on those metamorphic rocks, lies primarily within the Tuscaloosa Formation in this region and is marked at the modern surface by the geomorphic Fall Line. Mapping of the unconformity across the region reveals areas of significant paleorelief associated with a number of distinct paleovalleys incised into the mid-Cenomanian surface. The most distinct of these lie immediately east of the Alabama-Georgia state line, within 15 km of the modern Lower Chattahoochee River Valley. Spatially, these distinct paleovalleys lie immediately north of a Santonian estuarine environment recorded in the Eutaw Formation, disconformably above the Tuscaloosa Formation. Collectively, paleo-valleys in the mid-Cenomanian surface, the fluvial nature of the Tuscaloosa Formation in southwestern Georgia and southeastern Alabama, and the estuarine environment in the younger Eutaw Formation suggest a persistent (~10 m.y.) paleodrainage system that may be a forerunner to the modern Chattahoochee River.

Large scale dinoturbation in braided stream deposits: evidence from the Cretaceous Tugulu Group of the Hami area, Eastern Xinjiang, China

Large dinosaur tracks were recently reported from locations in the Pterosaur-Yadan National Geological Park situated about 100 km south of Hami in Xinjiang Province, China. The park comprises a substantial area in a much larger arid region comprising and extensive spectrum of Cretaceous, siliciclastic, Tugulu Group, lithofacies representing proximal, basin margin, alluvial fan and braided stream deposits, grading into alluvial plain, deltaic and lacustrine facies near the depocenter. Due to the difficulties of conducting detailed geological surveys in such a vast and inhospitable area, definitive resolution of the litho-, bio- and chrono-stratigraphy is challenging in some areas, and yet to be published in detail. Nevertheless, the occurrence of large dinosaur tracks and dinoturbated units, here interpreted as sauropodan, in association with root casts, dinosaur bone and fossil wood, points to the potential of this frontier area to yield valuable paleontological information, and show that flora and fauna were found in arid braided stream systems away from the lacustrine depocenters where body fossils are more abundant and better known.

Bathymetric mapping in turbid braided mountain streams using SfM-MVS photogrammetry and statistical approaches

<p>River bathymetric investigation has a long tradition as river-bed morphology is a crucial geomorphological variable that also has implications for river ecology and sediment management. In one sense, this is becoming more straightforward with the development of UAV platforms and SfM-MVS photogrammetry. Mapping inundated and exposed areas simultaneously has proved possible either by adopting two media refraction correction or by using some form of the Beer-Lambert Law. However, both of these approaches rely upon the bed being visible which becomes restricted to progressively shallower zones as stream turbidity increases. Traditional survey techniques to collect bathymetric data for inundated zones (e.g. total station or differential GPS systems) are time consuming and require a trade-off between point density and the spatial extent of survey. In this study we test a simple hypothesis: it is possible to generalize the likely depth of water in a shallow braided stream from basic planimetric information and use such statistical relationships to reconstruct the bathymetry of inundated zones. This is based upon the principle that a suite of planimetric variables (e.g. distance from stream banks, river channel width, local curvature magnitude and direction, streamline convergence and divergence) can be used to model the spatial distribution of water depths. We attempt to do this for a shallow braided river with high suspended sediment concentrations using orthoimages and DEMs derived from application of SfM-MVS photogrammetry to UAV-based imagery. We develop separate calibration and validation relationships to train and to assess the statistical models developed. These are then applied to the stream to produce bathymetric maps of flow depth for integration with SfM-MVS derived data from exposed areas. The method produces a point specific measure of uncertainty and tests suggest that the associated uncertainties are sufficiently low that after propagation into DEMs of difference reliable data on braided river dynamics and erosion and deposition volumes can be obtained.</p>

Uniform grain-size distribution in the active layer of a shallow, gravel-bed, braided river (the Urumqi River, China) and implications for paleo-hydrology

The grain-size distribution of ancient alluvial systems is commonly determined from surface samples of vertically exposed sections of gravel deposits. This method relies on the hypothesis that the grain-size distribution obtained from a vertical cross-section is equivalent to that of the river bed. We report a field test of this hypothesis on samples collected on an active, gravel-bed, braided stream: the Urumqi River in China. We compare data from volumetric samples of a trench excavated in an active thread and surface counts performed on the trench vertical faces. We show that the grain-size distributions obtained from all samples are similar and that the deposit is uniform at the scale of the river active layer, a layer extending from the surface to a depth of approximately ten times the size of the largest clasts.

Virtual and classical Precipice Sandstone outcrops mapping for reservoir modelling

The Lower Jurassic Precipice Sandstone is an important hydrocarbon and water reservoir in the Surat Basin. It is the basal infill of the Surat Basin, commonly considered an intracratonic sag basin, although the triggering mechanism for subsidence remains unresolved. Its interpreted origin is a fluviatile system that formed a thick belt of sandstone that corresponds to the Mimosa Syncline structural axis. The Precipice Sandstone outcrops along the northern margin of the basin forming laterally continuous cliffs. This provides good conditions for 2D and 3D photogrammetry and classical analysis of sedimentary architectures, bedding and facies. Photogrammetry is a measurement technique that builds 3D photorealistic virtual models in which every pixel on the image corresponds to a real 3D point in georeferenced space. This was used to measure surfaces, correlate stratigraphy, and to measure bed and body geometries for export to a reservoir modelling system, providing a bridge between the subsurface drilling data and the outcrop analogue. The field survey mapped the lower Precipice, defined by the predominance of southeast-flowing planar and trough cross stratified sandstone (the braided stream facies), and upper Precipice, defined by a predominance of heterolithic, ripple and plane parallel stratification and slumps that transition upward into the Evergreen Formation mud-dominated unit. Sedimentary structures in outcrop suggest a northward flow on the eastern margin of the outcrop for the upper Precipice. The basin asymmetry, coincident with a major, meridional-trending fault system—the Goondiwindi-Moonie-Burunga system—and changes in upper and lower unit thickness suggest some syn-depositional control on the sedimentary architecture.