ALBERT

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 ( $${T}_{\mathrm{se}}$$ ) (–2° to 0°) and negative shelf-edge aggradation to progradation ratios ( $$\mathrm{d}y/\mathrm{d}x$$ ) (–0.04 to 0) and associated progradational and downstepping stacking patterns with low clinoform relief ( $${R}_{\mathrm{c}}$$ ) (150–550 m [492–1804 ft]) and negative differential sedimentation on the shelf and basin ( $${A}_{\mathrm{s}}/{A}_{\mathrm{b}}$$ ) (–0.6 to 0); (2) slightly rising trajectories with moderate $${T}_{\mathrm{se}}$$ (0°–2°) and medium $$\mathrm{d}y/\mathrm{d}x$$ (0–0.04), and associated progradational and aggradational stacking patterns with intermediate $${R}_{\mathrm{c}}$$ (250–400 m [820–1312 ft]) and intermediate $${A}_{\mathrm{s}}/{A}_{\mathrm{b}}$$ (0–0.6); and (3) steeply rising trajectories with high $${T}_{\mathrm{se}}$$ (2°–6°) and high $$\mathrm{d}y/\mathrm{d}x$$ (0.04–0.10) and associated dominantly aggradational stacking patterns with high $${R}_{\mathrm{c}}$$ (350–650 m [1148–2132 ft]) and high $${A}_{\mathrm{s}}/{A}_{\mathrm{b}}$$ (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 progradational 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 $${T}_{\mathrm{se}}$$ , $$\mathrm{d}y/\mathrm{d}x$$ , $${R}_{\mathrm{c}}$$ , and $${A}_{\mathrm{s}}/{A}_{\mathrm{b}}$$ 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.

Description: A series of short and steep unidirectionally migrating deep-water channels, which are typically without levees and migrate progressively northeastward, are identified in the Baiyun depression, Pearl River Mouth Basin. Using three-dimensional seismic and well data, the current study documents their morphology, internal architecture, and depositional history, and discusses the distribution and depositional controls on the bottom current–reworked sands within these channels. Unidirectionally migrating deep-water channels consist of different channel-complex sets (CCSs) that are, overall, short and steep, and their northeastern walls are, overall, steeper than their southwestern counterparts. Within each CCS, bottom current–reworked sands in the lower part grade upward into muddy slumps and debris-flow deposits and, finally, into shale drapes. Three stages of CCSs development are recognized: (1) the early lowstand incision stage, during which intense gravity and/or turbidity flows versus relatively weak along-slope bottom currents of the North Pacific intermediate water (NPIW-BCs) resulted in basal erosional bounding surfaces and limited bottom current–reworked sands; (2) the late lowstand lateral-migration and active-fill stage, with gradual CCS widening and progressively northeastward migration, characterized by reworking of gravity- and/or turbidity-flow deposits by vigorous NPIW-BCs and the CCSs being mainly filled by bottom current–reworked sands and limited slumps and debris-flow deposits; and (3) the transgression abandonment stage, characterized by the termination of the gravity and/or turbidity flows and the CCSs being widely draped by marine shales. These three stages repeated through time, leading to the generation of unidirectionally migrating deep-water channels. The distribution of the bottom current–reworked sands varies both spatially and temporally. Spatially, these sands mainly accumulate along the axis of the unidirectionally migrating deep-water channels and are preferentially deposited to the side toward which the channels migrated. Temporally, these sands mainly accumulated during the late lowstand lateral-migration and active-fill stage. The bottom current–reworked sands developed under the combined action of gravity and/or turbidity flows and along-slope bottom currents of NPIW-BCs. Other factors, including relative sea level fluctuations, sediment supply, and slope configurations, also affected the formation and distribution of these sands. The proposed distribution pattern of the bottom current–reworked sands has practical implications for predicting reservoir occurrence and distribution in bottom current–related channels.