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Three-Dimensional Numerical Modeling of Deltaic Depositional Sequences 2: Influence of Local Controls
We investigate the influence of three local controls (sediment supply, subsidence, and physiography) on delta development and sequence variability, using the three-dimensional numerical model of deltaic deposition presented in a companion paper (Ritchie et al. 2004). Independently varying a single local control within geologically constrained limits produces marked differences in three-dimensional morphology, cross-sectional stratal geometry, and delta evolution during a cycle of sea-level change.
Sediment supply strongly influences not only the timing of transgressive and maximum flooding surface development during sea-level rise, and hence the diachroneity of lowstand, transgressive, and highstand systems tracts, but also the timing of onset of fluvial incision and the characteristics of incised valleys and forced regressive wedges. Low sediment supply leads to early drowning of forced regressive and lowstand prograding wedges, high-magnitude transgressions, and the late development of maximum flooding surfaces. During relative sea-level fall, low sediment supply results in early initiation of fluvial incision and development of few, but major incised valleys that route sediment to elongate forced regressive lobes at their mouths. In contrast, high sediment supply leads to early onset of normal regression and late transgression during relative sea-level rise, and numerous, poorly developed incised channels and a broad, laterally continuous forced regressive apron during relative sea-level fall.
On basin margins with a shelf-slope morphology, base-of-slope deepwater deposits may occur at times of sea-level highstand due to sediment bypass across the slope. During relative sea-level fall fluvial incision is enhanced by the relatively steep gradient of the slope, leading to the early onset of incision and rapid headward propagation. As a result, major, deep incised valleys develop that capture sediment and feed it directly to thick shelf-edge forced regressive and lowstand prograding wedges. The greater water depth ahead of the shelf edge and the steep slope gradients result in limited forced and normal regression compared to a ramp setting.
In contrast to other controls, tectonic subsidence (or uplift) leads to modification of accommodation, so that relative sea-level change may vary significantly along a basin margin. In high-subsidence settings, relative sea level may continue to rise even at times of high rates of background sea-level fall. In such settings, deltas lack incised valleys, forced regressive delta lobes, and prominent sequence boundaries. Furthermore, normal regression and even transgression in high-subsidence locations can be contemporaneous with forced regression and incised-valley development in adjacent lower-subsidence locations.
The results of this sensitivity analysis suggest that systems tracts and their bounding surfaces are likely diachronous along many basin margins and may locally be absent. More than one control can produce a similar stratigraphic response, making interpretation of controlling processes from the stratigraphic product equivocal. The models provide a framework for understanding the stratigraphy of natural basin fills, but further research on the interplay between the controls is required in order to understand the climatic, tectonic, and sea-level signals concealed in the stratigraphic record.
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