ALBERT

Description: How and when sediment moves from terrestrial sources to deep-water sinks is a significant area of research. We have used an array of seismic, borehole, and gravity core data sets to explore the timing and magnitude of sediment-routing to Pearl River slope over the last 478 k.y. As predicted by existing sequence stratigraphic models, most sediment dispersal to deep water is shown to have occurred during glacial sea-level falls; however, clastic detritus was still being transported into deep water during interglacial sea-level rises. We suggest that sediment routing to deep water during interglacial sea-level rise is caused by summer monsoon strengthening and resultant warmer and wetter climates, both of which have enhanced effective precipitation and sediment supply. Although some models for the delivery of sediment to deep-water basins stress the importance of proximity of canyon heads and coeval shorelines, we observed that sediment routing to deep water could occur regardless of the distance between channel head and coeval shorelines. In the present case, the success of delivery is related to the combined effects of (1) the short duration and high amplitude of sea-level oscillations during the past 478 k.y. and (2) the enhanced sediment supply caused by more humid climates and greater temperature difference between glacial and interglacial period. This hypothesis is supported by (1) observations that outer Pearl River deltas prograded as an apron over preexisting shelf edges for 10–15 km (6–9 mi) and (2) the occurrence of slope channels extending back to prodelta reaches of Pearl River shelf-edge deltas.

Description: A 300-km (186-mi) transect was reconstructed across the 500-m (1640-ft)-thick, 3-m.y. duration Iles clastic wedge in southern Wyoming and northern Colorado using well logs and stratigraphic columns. This wedge developed into the middle–late Campanian-aged Western Interior seaway by progradation from the active Sevier fold and thrust belt and adjacent uplifted areas. The wedge thickens basinward because late-stage, widespread uplift of the fold belt caused multiple unconformities across the proximal (former foredeep) reaches of the foreland basin. Transect analysis allows the wedge and its component sequences to be better understood and permits a two-dimensional characterization of the sand and mud distribution. The Iles wedge exhibits 11 low-gradient, regressive-transgressive, high-frequency sequences that were correlated across several hundred kilometers. Sediment partitioning analysis along the transect shows the following. (1) Within the regressive limb of the Iles wedge, the component higher order regressive compartments tend to thicken into the medial reaches of the wedge, whereas transgressive compartments thicken landward. This geometry is driven by preferential erosion in proximal areas during regression, bypassing much sediment to the marine shorelines, and transgressive backfilling into proximal areas previously eroded more deeply. (2) The greatest concentration of sands tends to be located in the proximal fluvial and estuarine facies of the transgressive compartments and within the medial (shoreline) deltaic facies of the regressive compartments. (3) As the high-frequency sequences develop, the effectiveness of basinward sand partitioning reaches a maximum value near the peak regression level of the wedge, apparently reflecting stronger erosion and sediment bypass during these times. 3rd revised manuscript received December 15, 2009 Carolina Gomez-Veroiza received an M.S. degree in geology from Universidad de Chile in 2003 and a Ph.D. from the Jackson School of Geosciences, University of Texas at Austin in 2009. Her Ph.D. research work included source-to-sink characterization of a clastic wedge at different scales, especially in the fluvial to marine transition. She currently works as an exploration geologist for the Chilean state oil and gas company Enap Sipetrol. Ron Steel is a professor and a Davis Chair at the University of Texas at Austin and a Sixth-Century Chair at the University of Aberdeen, Scotland. He was previously a professor at the universities of Wyoming and Bergen and worked as a scientist and manager at Norsk Hydro, Bergen and Oslo. His interests are in dynamic stratigraphy and sedimentology, with current focus on shelf-margin growth, sedimentation and tectonics, and tidal bars and dunes.

Description: 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.

Description: Identification of bypass at the shelf margin is critical to deep-water exploration. We examine the shelf margin of an early Eocene fourth-order sequence with an attached basin-floor fan in the Spitsbergen Central Basin. Turbidity currents were fed mainly by hyperpycnal flow emerging from shelf-edge deltas. The life span of any turbidity current was determined primarily by the sediment concentration of the flow and the duration of the river flood. High-density hyperpycnal flows created sand-filled slope-channel complexes 10–15 m (33–49 ft) thick and 100–200 m (328–656 ft) wide that served as conduits for bypass to the basin floor. Low-density hyperpycnal flows were unconfined and deposited heterolithic lobes on the slope. Shelf-margin accretion of about 1.5 km (0.9 mi) during the falling stage gave way abruptly to bypass in the early lowstand. Most of the basin-floor fan growth was achieved after shelf-edge incision and before relative sea level rise. Coastal-plain aggradation in the late lowstand sequestered sediment from the shelf-edge distributaries, effectively diminishing high-density hyperpycnal flow output. The late lowstand was therefore marked by a second phase of shelf-margin accretion with only limited bypass to the basin floor, and a heterolithic, prograding complex downlapped the early lowstand channels. Transgression ultimately led to the abandonment of the shelf-edge delta complex and the accumulation of mainly mudstone on the margin. The shelf-margin architecture exhibited by this sequence should serve as a type example of a deep-water feeder system in which hyperpycnal flow is the primary initiator of turbidity currents for sand accumulation on the slope and basin floor. Andrew L. Petter is a Ph.D. aspirant in the Jackson School of Geosciences, University of Texas at Austin. He earned an M.S. degree in geological sciences from the University of Texas at Austin in 2005, where he focused on the topic covered in this article. He is currently studying the Paleogene shelf margin of the Gulf of Mexico and its relationship to transport of reservoir-quality sediment to the deep water. A University of Texas professor, Ron Steel received his B.Sc. degree in 1967 and his Ph.D. in 1971 from the University of Glasgow. His research is aimed primarily at using clastic sedimentology to address problems in basin analysis and particularly to decipher the signatures of tectonics, sea level change, and sediment supply in stratigraphic successions.

Description: 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. Cornel Olariu is a research associate at the University of Texas. He earned a geology engineering degree in 1995 from the University of Bucharest, and an M.S. degree in 2002 and a Ph.D. in 2005 both in geological sciences from the University of Texas at Dallas. He is interested in clastic sedimentology with focus on shallow-water depositional systems and more specifically on mechanisms of sediment transfer from shallow to deep water. Ronald J. Steel is a professor and Davis Centennial Chair at the University of Texas at Austin. He received his B.Sc. degree in 1967 and his Ph.D. in 1971 from the University of Glasgow, United Kingdom. His research is aimed primarily at using clastic sedimentology to address problems in basin analysis and particularly to decipher the signatures of tectonics, sea level change, and sediment supply in stratigraphic successions. Andrew L. Petter is a Ph.D. candidate in the Jackson School of Geosciences, University of Texas at Austin. He earned a B.S. degree in geology from Rice University in 2002 and an M.S. degree in geological sciences from the University of Texas at Austin in 2005. He is currently studying the partitioning of sediment across alluvial, deltaic, and shelf-margin systems using numerical models, flume experiments, and outcrop and subsurface data.

Description: Using a seismic database from the Qiongdongnan Basin in the South China Sea, this study demonstrates that shelf-edge trajectories and stratal stacking patterns are reliable, but understated, predictors of deep-water sedimentation styles and volumes of deep-water sand deposits, assisting greatly in locating sand-rich environments and in developing a more predictive and dynamic stratigraphy. Three main types of shelf-edge trajectories and their associated stratal stacking patterns were recognized: (1) flat to slightly falling trajectories with negative trajectory angles (Tse) (-2° to 0°) and negative shelf-edge aggradation to progradation ratios (dy/dx) (-0.04 to 0) and associated progradational and downstepping stacking patterns with low clinoform relief (Rc) (150-550 m [492-1804 ft]) and negative differential sedimentation on the shelf and basin (As/Ab) (-0.6 to 0); (2) slightly rising trajectories with moderate Tse (0°-2°) and medium dy/dx (0-0.04), and associated progradational and aggradational stacking patterns with intermediate Rc (250-400 m [820-1312 ft]) and intermediate As/Ab (0-0.6); and (3) steeply rising trajectories with high Tse (2°-6°) and high dy/dx (0.04-0.10) and associated dominantly aggradational stacking patterns with high Rc (350-650 m [1148-2132 ft]) and high As/Ab (1-2). Each trajectory regime represents a specific stratal stacking patterns, providing new tools to define a model-independent methodology for sequence stratigraphy. Flat to slightly falling shelf-edge trajectories and prograda-tional and downstepping stacking patterns are empirically related to large-scale, sand-rich gravity flows and associated bigger and thicker sand-rich submarine fan systems. Slightly rising shelf-edge trajectories and progradational and aggradational stacking patterns are associated with mixed sand/mud gravity flows and moderate-scale slope-sand deposits. Steeply rising shelf-edge trajectories and dominantly aggradational stacking patterns are fronted by large-scale mass-wasting processes and associated areally extensive mass-transport systems. Therefore, given a constant sediment supply, then rse, dy/dx, Rc, and As/Ab are all proportional to intensity of mass-wasting processes and to amounts of mass-transport deposits, and are inversely proportional to the intensity of sand-rich gravity flows and to amounts of deep-water sandstone. These relationships can be employed to relate quantitative characteristics of shelf-edge trajectories and stratal stacking patterns to deep-water sedimentation styles.