Three-Dimensional Numerical Modeling of Deltaic Depositional Sequences 1: Influence of the Rate and Magnitude of Sea-Level Change

A three-dimensional numerical model of deltaic deposition is used to investigate the influence of sea-level changes on delta development and sequence variability. Results illustrate the three-dimensional morphology of key stratal surfaces and architecture of stratal units (systems tracts) and highlight the importance of the rate and magnitude of sea-level change in controlling the evolution of deltaic depositional sequences. High rates of sea-level fall lead to the development of a limited number of major incised channels that focus sediment supply to a few elongate, finger-like forced regressive lobes separated by large areas of nondeposition. In contrast, low rates of sea-level fall cause only minor channel incision, which occurs late during sea-level fall. As a result, sediment is supplied more uniformly to the delta front, leading to an attached, laterally continuous forced regressive apron. During lowstand and subsequent sea-level rise, the delta morphology and internal geometry are strongly controlled by the rate of rise. High rates lead to: i) poorly developed lowstand wedges that are drowned early, ii) high-magnitude transgressions, and iii) the late development of maximum flooding surfaces. The stratigraphy developed during sea-level rise is also strongly influenced by the incised-valley system created during the preceding sea-level fall. If deep, major valleys developed that captured most of the sediment supply, the resultant stratigraphy has well developed lowstand wedges that are flooded relatively late during sea-level rise. Even within a single delta, systems tracts and key stratal surfaces show three-dimensional variability and two-dimensional sections often lack significant elements of the stratigraphy. As a result, analysis of two-dimensional sections can often lead to miscorrelation and erroneous interpretations of the controlling processes.