ABSTRACT

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 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 Tse, 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.

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