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Turbidite Stratigraphic Architecture – Hierarchy Development Associated with Allocyclic Factors


Many if not most turbidites originate as upper slope/outer shelf slope failures that transform from mass transport into full turbidity currents. The frequency and magnitude of these events at the shelf edge control stratigraphic patterns observed down-system from the slope to the basin floor. Commonly, a major failure event at the shelf edge is followed quickly by a succession of progressively smaller failures as slopes re-equilibrate. The result of this succession of progressively smaller flows is a gradual filling of channels (i.e. elements), producing storeys. These flows may occur over a period of days to years. Because each flow is smaller and less energetic than the preceding one, little channel migration occurs and onlap fill is the dominant architecture. Another implication of having a succession of smaller flows is that each successive flow will go less far than the immediately preceding flow, resulting in progressive backstepping as the element fills. Thus, at any given location the basal element fill may be dominated by bypass facies, overlain by deposition from progressively more of the body of the turbidity current. This backstepping facies expression also would manifest itself within the unconfined fan that lies outboard of the channel-dominated environment. Once the slope has re-equilibrated a period of quiescence occurs until eventually slope instability once again may develop and the same process occurs. Because the first event of this second series of flows is significantly more energetic than the last event of the previous series, significant erosion and channel migration can occur as a second channel element forms and then fills just as before, producing a succession of offlapping channel fills, or a channel complex. After a succession of such events, an avulsion may occur such that the next series of events is located outside the previous succession of elements. Multiple avulsions may result in multiple clusters of elements (i.e. complexes) forming a complex set. In a simple sea-level cycle, all of these events would take place within the context of relative sea level fall followed by lowstand and then accelerating sea level rise. From a process perspective, overall, flows would get progressively smaller and more mud-rich as the lowstand episode winds down. This forcing factor would act in concert with the aforementioned factors to produce a predictable stratigraphic and geomorphic succession.