Cornel Olariu

Delta-front hyperpycnal bed geometry and implications for reservoir modeling: Cretaceous Panther Tongue delta, Book Cliffs, Utah

The Cretaceous Panther Tongue has an upward-coarsening and -thickening pattern and is well exposed in extensive large outcrops in the Book Cliffs area, west-central Utah. The deposits have been interpreted as having formed in a fluvial-dominated river delta environment that generated highly sediment-concentrated sustained (turbidity) flows during flooding, producing hyperpycnal-flow deposits on the delta front despite some resemblance to deep-water turbidites. The facies associations indicate terminal distributary channel, channel mouth, and proximal delta-front and distal delta-front depositional environments. The measured paleocurrents indicate a south-southwest transport of the sediments. The thickness of the hyperpycnal sandstone beds ranges from centimeters to meters. Sandstones are characteristically parallel laminated, sometimes structureless or rarely display inclined strata of cut-and-fill type. The sandstone hyperpycnal beds dominate the delta-front clinoforms and dip southward, consistent with the other paleocurrent indicators. Individual sandstone beds in the clinoforms have dips that range from 0.1 on the distal delta front (lower part of the outcrops) to 3 in the proximal parts (upper part of the outcrops). The hyperpycnal beds can be traced from a proximal mouth-bar environment to the distal delta front over a distance of hundreds of meters. As individual beds extend from mouth bar to distal delta-front environments, they become systematically finer grained and thinner. Over short distances (hundreds of meters), the beds thin with rates ranging between 0.0001 (i.e., dm/km) to 0.02 (i.e., tens of meters per kilometer). The sandstone beds thin to a greater degree in a dip direction than along strike, indicating a relatively strike-elongate (flow-normal) geometry of the hyperpycnal flows and of the delta lobes. The wider than longer geometry of the delta-front beds requires that reservoir development be more focused upon the downdip facies changes (heterogeneities) than the lateral (along strike) heterogeneities.

Deltaic process and architectural evolution during cross-shelf transits, Maastrichtian Fox Hills Formation, Washakie Basin, Wyoming

The topset compartments of two Maastrichtian basin-scale clinothems are characterized, with focus on the function they played in constructing the Lance–Fox Hills–Lewis shelf-margin sedimentary prism in the Laramide Washakie Basin, south Wyoming. Approximately 1000 well logs were used to map the delta lobes and complexes on the Fox Hills shelf and to detail their depositional character, dimensions, and orientation as they autogenically shifted during transit from an inner-shelf to shelf-edge position. The regressive transits of the deltas initiated up to 40 km (25 mi) landward from the preexisting shelf-edge and preserved river and wave-dominated deltaic deposits that thicken and concentrate sand on the outer shelf. Tidally influenced deltas (now outcropping) also occur in localized areas along the paleoshelf edge, probably where wave influence was reduced along invaginated coastal segments. Net sandstone maps of the individual clinothem topsets show that (1) coeval delta lobes exist within each clinothem, suggesting multiple rivers; (2) delta lobes have a likely autogenic compensational stacking pattern; and (3) deltas thicken and storm-wave influence become dominant closer to the shelf edge. Our results support the ideas of (1) predictable increased wave influence and (2) change to strike-elongate architecture as deltas transit the shelf. In addition, along-strike changes in process dominance cause deltaic reservoirs to be highly variable in their orientation, external shape, and internal character. Some process changes are interpreted to be autogenic responses during overall shoreline progradation. The study also provides new data on delta-lobe and delta-complex thicknesses as well as on deltaic coastline versus shelf-edge progradation rates.

Impact of tidal currents on delta-channel deepening, stratigraphic architecture, and sediment bypass beyond the shoreline

Deltas are sensitive indicators of coastal processes (e.g., waves and tides) and show dynamic changes in shoreline morphology, distributary channel network, and stratigraphic architecture in response to coastal forcing. Numerical modeling has long been used to show delta evolution associated with a single dominant coastal process, but rarely to examine the sensitivity of deltas to mixed processes. Physics-based morphodynamic simulations (Delft3D) are used to investigate the influence of tidal currents on deltas. Tidal amplitude and the sand:mud ratio of subsurface sediment have been varied in the model. The results show that increasing tidal amplitude causes deeper and more stable distributary channels and more rugose planform shoreline patterns. A new metric for channel geometry quantifies tidal influence on the distributary channel network. Stable distributary channels act as an efficient mechanism for ebb-enhanced currents to (1) bypass sediment across the delta plain, and (2) extend channel tips seaward through mouth bar erosion. The basinward channel extension leads to sandier deposits in the tide-influenced deltas than in their river-dominated counterparts. The delta-front bathymetry also reflects sediment redistribution, changing the delta-front profile from concave to convex with compound geometries as tidal amplitude increases. These results suggest that channel overdeepening is a possible tidal signature that should be considered when interpreting ancient systems, and that sand may be bypassed much farther basinward in tide-influenced than in purely river-dominated deltas.

Growth of the paleo-Orinoco shelf-margin prism: Process regimes, delta evolution, and sediment budget beyond the shelf edge

The first sedimentological characterization and correlation of onshore outcrop and offshore subsurface data (southern Columbus Basin) are presented for the paleo-Orinoco (Upper Miocene–Pliocene) shelf-margin prism, Trinidad Island, Trinidad and Tobago. The paleo–Orinoco River delta system and associated continental slope, which generated the 10-km-thick sedimentary prism, was a mixed river, tide, wave, and sediment-gravity flow system that tracked down to deep-water submarine fans. The analysis here includes: (1) an evaluation of delta-plain to deep-water turbidite sedimentary facies, which are seen in spectacular outcrops along the southwest, south, and southeast coasts of Trinidad Island, (2) well-log correlation of the same Upper Miocene–Pliocene strata across southern Trinidad and out to the southern part of the offshore Columbus Basin along an off-axis transect, because the main fairway into the most rapidly subsiding part of Columbus Basin is structurally complex, and few detailed data have been released for publication; and (3) use of published seismic data for reconstruction of clinoform morphology across the relatively undeformed segment of the margin, with quantitative sediment flux calculations and predictions. Paleo-Orinoco shelf-margin growth was generated by repeated (~<100 k.y. time scale) cross-shelf, regressive-transgressive transits (>100 km) of the Orinoco delta system, with internal variability in clinoform architecture and process-regime changes dur-

What conditions are required for deltas to reach the shelf edge during rising sea level

It is clear from multiple modeling and field studies that deltas can attain a shelf-edge position under conditions of rising sea level, and this is accepted as an alternative way to form submarine fans without relative sea-level lowstand conditions. However, the relative importance and the range of controls that generate this type of shelf-edge deltas are less well known. To investigate these controls, we input the natural range of seven accommodation and morphological variables to a geometric model that estimates the required sediment load to form the highstand shelf-edge delta. We conduct a one-at-a-time sensitivity analysis on each variable to determine their relative importance. Our results suggest that, other than sediment supply, shelf width is the most important control in forcing deltas to the shelf edge during rising sea level. Two of the traditional stratigraphic trinity, i.e., eustatic sea-level change and shelf subsidence, most likely play less important roles than implied in the literature. This result is consistent with a recent trend in sequence stratigraphic studies encouraging consideration of multiple controls in all interpretations, rather than assuming dominant control by relative sea level.

Geochemistry of a large impoundment, Part I: solute sources, mixing dynamics, and seasonal anoxia

Lake Texoma is a large impoundment on the border of Texas and Oklahoma, formed from the confluence of two river systems with different salinities, the Red River (total dissolved salt, TDS, of 2700–11 900 mg/l, average of 4862 mg/l) and the Washita River (TDS of 420–915 mg/l, average of 701.4 mg/l). Systematic analyses for major and trace elements were conducted of water samples collected spatially and with depth in the lake in different seasons. Overall, Lake Texoma waters are characterized by Na-Ca-Cl-SO4-type waters with spatial distribution shifting from Na-Cl type to Ca-SO4 type from the Red River arm to the main lake and to the Washita River arm. In addition, vertical and seasonal variations in major and trace elements concentrations indicate major elements in the lake are mainly controlled by different bedrock weathering from the two river systems. Trace elements that exhibit different distribution patterns to the major species are associated with variable sources such as river inflow, summer stratification effects, biological effects and anthropogenic activities. In Lake Texoma, differential inflow volumes and summer stratification are principal factors controlling the variation and geochemistry of lake waters and mixing dynamics. Supplementary material: Summary of major and trace element chemistry in the five zones of Lake Texoma for summer, fall and spring seasons (2003–2004) is available at http://www.geolsoc.org.uk/SUP18771

Introduction to special section: Source-to-sink system analysis of petroliferous and other sedimentary basins

The source-to-sink system comprises all areas that contribute to erosion, transportation, and deposition of sediments within an erosional-depositional system. The source-to-sink system is often described in terms of its dynamic drivers producing unsteady (allogenic) and steady (autogenic) responses