A primary objective in exploration for and development of fluvial reservoirs is determining the thickness and width of sandstone-conglomerate bodies (mainly channel-belt deposits). Most of the existing techniques for estimating the dimensions of fluvial reservoirs have major drawbacks. A fresh approach to the problem is made using recent theoretical, experimental, and field studies. This new approach involves (1) new models for the lateral and vertical variation of lithofacies and petrophysical-log response of river-channel deposits with explicit recognition of the different superimposed scales of strata, (2) distinction among single and superimposed channel bars, channels, and channel belts, (3) interpretation of maximum paleochannel depth from the thickness of channel bars and the thickness of sets of cross-strata formed by dunes, and (4) evaluation of various methods for estimation of widths of sandstone-conglomerate bodies that represent either single or connected channel belts (outcrop analogs; correlation of sandstone-conglomerate bodies between wells; use of empirical equations relating channel depth, channel width, and channel-belt width; theoretical models; and three-dimensional seismic data).

Interpreting the Dimensions of Ancient Fluvial Channel Bars, Channels, and Channel Belts from Wireline-Logs and Cores

Tetanuran theropods represent the majority of Mesozoic predatory dinosaur diversity and the lineage leading to extant Aves. Thus their history is relevant to understanding the evolution of dinosaur diversity, Mesozoic terrestrial ecosystems, and modern birds. Previously, the fragmentary and poorly sampled fossil record of basal (non-coelurosaur) tetanurans led to uncertainties regarding their basic interrelationships. This in turn prevented determining the relationships of many incompletely known taxa that nonetheless document a global radiation spanning more than 120 million years. We undertook an exhaustive examination of all basal tetanurans and all existing character data, taking advantage of recent discoveries and adding new morphological, temporal and geographic data. Our cladistic analysis of 61 taxa achieved significantly improved phylogenetic resolution. These results position several ‘stem’ taxa basal to a succession of monophyletic clades (Megalosauroidea, Allosauroidea and Coelurosauria). Mega...

The phylogeny of Tetanurae (Dinosauria: Theropoda)

A quantitative, three-dimensional depositional model of gravelly, braided rivers has been developed based largely on the deposits of the Sagavanirktok River in northern Alaska These deposits were described using cores, wireline logs, trenches and ground-penetrating radar profiles The origin of the deposits was inferred from observations of: (1) channel and bar formation and migration and channel filling, interpreted from aerial photographs; (2) water flow during floods; and (3) the topography and texture of the river bed at low-flow stage This depositional model quantitatively represents the geometry of the different scales of strataset, the spatial relationships among them and their sediment texture distribution Porosity and permeability in the model are related to sediment texture The geometry of a particular type and scale of strataset is related to the geometry and migration of the bedform type (eg ripples, dunes, bedload sheets, bars) associated with deposition of the strataset In particular, the length-to-thickness ratio of stratasets is similar to the wavelength-to-height ratio of associated bedforms Furthermore, the wavelength and height of bedforms such as dunes and bars are related to channel depth and width Therefore, the thickness of a particular scale of strataset (ie medium-scale cross-sets and large-scale sets of inclined strata) will vary with river dimensions These relationships between the dimensions of stratasets, bedforms and channels mean that this depositional model can be applied to other gravelly fluvial deposits The depositional model can be used to interpret the origin of ancient gravelly fluvial deposits and to aid in the characterization of gravelly fluvial aquifers and hydrocarbon reservoirs

A quantitative, three‐dimensional depositional model of gravelly braided rivers

Depositional models of braided rivers are necessary for rational interpretation of ancient deposits, and to aid in the characterization of subsurface deposits (e.g. aquifers, hydrocarbon reservoirs). A comprehensive depositional model should represent bed geometry, flow and sedimentary processes, and deposits accurately, quantitatively, and in detail. Existing depositional models of braided rivers do not meet these requirements, and there are still many misconceptions about braided rivers and their deposits that need to be expunged. Over the past decade, there have been major advances in our understanding of braided rivers, making it possible to develop new and improved depositional models. First, the use of ground- penetrating radar in combination with cores and trenches has allowed detailed description of the differ- ent scales of deposit in braided rivers that vary widely in channel size and sediment size. Second, the study of braided channel geometry and kinematics has been facilitated by the use of aerial photographs taken at short time intervals. In some cases, these photographs have been analysed using digital pho- togrammetry to produce digital elevation models. Third, water flow and sediment transport at the all- important flood stages have been studied using new equipment and methods (e.g. acoustic Doppler current profilers, positioning using differential global positioning systems). However, such high-flow studies are rare, which is one reason why there has not been much progress in development of realistic theoretical models for the interaction between bed topography, water flow, sediment transport, erosion and deposition. Laboratory experimental studies of braided rivers have continued to be useful for examining the controls on channel geometry and dynamics, but have not been able to generate all of the different types and scales of strata observed in natural braided river deposits. New depositional models for sand-bed and gravel-bed braided rivers are presented here based mainly on studies of natural rivers. They comprise: 1 maps showing idealized active and abandoned channels, compound bars and lobate unit bars; 2 cross-sections showing large-scale inclined strata and their internal structures, associated with migration of compound bars, unit bars and their superimposed bedforms; 3 vertical logs of typical sedimentary sequences through different parts of compound bar deposits and channel fills.

Depositional Models of Braided Rivers

Realistic physical models of meandering rivers have proven extremely difficult to produce, particularly in comparison to the formation of braided rivers in the laboratory. Here we address the question of why such realistic model meanders are so difficult to reproduce, through the most realistic physical modeling of meandering channels yet achieved. This paper demonstrates that cohesion is a key variable in the development and maintenance of single-thread channels. In particular, cohesion must be sufficient to force the planform away from a braided state but low enough for active migration to continue and for the avoidance of a gradual reduction and eventual cessation of planform movement (ossification). The enhanced realism of the experiments also enables the processes of meander evolution, and critically the resultant alluvial architecture, to be examined in a physical model for the first time. Planform history can be linked to deposits, and this process–product linkage enables the depositional development of the experimental deposits to be compared against, and to test, existing models of bedding geometries within point bars. Here we document three mechanisms for bend cutoff, provide new process explanations for certain modes of bend evolution in coarse-grained meandering rivers, examine the geometries and spatial distribution of alluvial architecture, and demonstrate that existing models of point-bar geometry successfully reproduce the larger-scale aspects of point-bar accretion in rivers dominated by episodic unit-bar accretion.

Meander-Bend Evolution, Alluvial Architecture, and the Role of Cohesion in Sinuous River Channels: A Flume Study

Fluvial deposits of the Cretaceous Chubut Group, San Jorge Basin, Argentina, were studied in outcrop to provide analogs for adjacent subsurface hydrocarbon-bearing strata. Outcrops were described using photomosaics and detailed sedimentological logs. Particular attention was paid to describing the geometry (e.g., width, thickness), lithofacies, and spatial distribution of sandstone bodies. Sediment accumulation rates were calculated using radiometric ages obtained from the tuffs and ignimbrites that are an important component of these strata. Interpretation of depositional environment included quantitative reconstruction of the geometry, hydraulics, and mode of migration of paleochannels. The proportion, connectedness, and spatial distribution of channel-belt sandstone bodies were interpreted using alluvial stratigraphy models. Sandstone bodies are generally meters thick and tens to hundreds of meters across (normal to paleocurrent direction). Channel-form sandstone bodies represent channel bars and fills within channel belts, whereas sandstone sheets, wedges, and lenses represent the deposits of overbank sheet floods, levees, and crevasse splays, respectively. Most of the rivers were single-channel and sinuous (sinuosity less than 1.2), but there were also braided rivers. The rivers flowed eastward and were perennial. Individual channel widths were on the order of tens of meters (mainly 35 to 65 m) and maximum channel depths were on the order of meters (mainly 2 to 6 m). The thickest and widest sandstone bodies (up to 16 m thick and in excess of 1 km wide) represent the largest channel belts or superimposed channel belts. Inasmuch as the proportion of channel-belt deposits is generally less than 0.5, most channel belts are unconnected. Channel-deposit proportion varies laterally and vertically on a 100-m scale. These variations are related to changes in the dimensions of channel belts, but they may also be related to variations in the deposition rate, floodplain width, and the timing and location of avulsions. These factors may in turn be related to intrinsic fluvial processes, tectonic tilting of the floodplain, or variations in sediment supply related to climate, tectonism, and igneous activity. Thickness and orientation of the sandstone bodies are similar to those interpreted from adjacent subsurface data. However, the width of subsurface sandstone bodies estimated from well-to-well correlation is greater than measured in outcrop. This discrepancy is because: (1) subsurface sandstone-body width less than the well spacing (typically 300 m) cannot be resolved; (2) the width of some of the subsurface sandstone bodies may be overestimated in well-to-well correlation; and (3) the full extent of the widest sandstone bodies cannot be observed in the smaller outcrops.

Geometry, Lithofacies, and Spatial Distribution of Cretaceous Fluvial Sandstone Bodies, San Jorge Basin, Argentina: Outcrop Analog for the Hydrocarbon-Bearing Chubut Group

Guillermo A. Jalfin

Papers

Geometry, Lithofacies, and Spatial Distribution of Cretaceous Fluvial Sandstone Bodies, San Jorge Basin, Argentina: Outcrop Analog for the Hydrocarbon-Bearing Chubut Group