Stratigraphic models for deep-water sedimentary systems

Abstract

Stratigraphic models predict sedimentary architecture. Prediction requires understanding systems across a sufficient range of scales. To be predictive a model must address the interaction of multiple process-response relationships. For deep-water systems these processes include (1) subaqueous flow initiation and transformation, (2) linkages between channel, levee and lobe processes, and (3) shelf-to-basin profile evolution. Thickness, lithology and the geomorphic hierarchy of sedimentary bodies are responses that can be used to define phases in deep-water episodes recording both external (allogenic) and internal (autogenic) controls. Shelf-to-basin studies of the Middle Permian Brushy Canyon Formation demonstrate that the more complete basinal record correlates to an incomplete shelf record; this incongruity impacts recognition of allogenic forcing. Preserving the signature of external controls, internal changes in local gradient and topography also impact the deep-water record requiring complete basin analysis. Independent but nested autogenic and allogenic stratigraphic models address these challenges and predict patterns of deep-water sedimentation. Tectonics and climate modulate sediment supply and sea level, which are considered the principal allogenic controls on deep-water sedimentation as described by the phases of the AIGR ( Adjustment-Initiation-Growth-Retreat ) model. The complete AIGR cycle commences with the Adjustment (A) phase, which defines the initial profile gradient and topography. The Initiation (I), Growth (G), and Retreat (R) phases describe variations in sedimentary response. Autogenic controls on deep-water sedimentation include (1) lateral offset and compensational stacking of lobes, (2) channel migration, switching and avulsion, and (3) longitudinal translation of the channel-lobe transition zone. The BCFS ( Build-Cut-Fill-Spill ) model describes autogenic controls on local gradient and confinement based on a hierarchy of channel-fill, channel-flank, and lobe sedimentary bodies, which vary in proportion and arrangement in each phase. The sedimentation phases of the AIGR and BCFS models describe the systematic increase and decrease in sedimentation energy recorded in hierarchical stratigraphy. When linked to gradient, the models form the axes of a sedimentary system energy matrix (SSEM) for sedimentary architecture. The BCFS model for submarine channels is embedded within the AIGR basin model and, together they facilitate the correlation of a hierarchy of internally and externally generated stratigraphic cycles.