Variability of the sequence stratigraphic model: effects of local basin factors

Abstract

The basic parameters that control stratal architecture in depositional sequences are sediment flux, rate of change of sediment accommodation (i.e., eustasy and sea-floor subsidence/uplift) and physiography. Of these parameters, only the eustatic component associated with the generation of sediment accommodation is globally significant. Consequently, local factors within a particular basin can play a relatively dominant role, vis-à-vis eustasy, in determining the internal organization of depositional sequences. In general, eustasy and sea-floor subsidence/ uplift determine the timing of sequence bounding surfaces, whereas sediment flux and physiography are most effective in determining the stratal architecture between those bounding surfaces. Basin physiography exerts an important control on the internal organization of sequences by determining, for example, whether deep-water turbidite systems will be a major part of the lowstand systems tract and whether or not fluvial incision will occur. On ramp-type basin margins with low shelf gradients, little or no fluvial incision and no deep-water turbidite deposition will occur. Lowstand deposits in this setting will consist of shoreline sands scattered at many locations within the basin. If, however, relative sea-level fall exposes a shelf/slope break, deeper fluvial incision as well as deep-water turbidite deposition will be likely. Basin physiography also plays an important role in the stratal architecture of the transgressive systems tract. The relatively deeper water setting that lies just seaward of the last clinoform of the underlying progradational wedge is the site of a depositional unit referred to herein as the “healing phase”. These early transgressive systems tract deposits commonly are relatively sand-poor and are derived primarily from eroded delta plain/coastal plain sediments. The role of sediment supply varying as an indirect response to relative sea-level change can be significant in deep water environments. Within lowstand systems tracts that form in physiographic settings characterized by discrete shelf/slope breaks, the stratigraphic expression of depositional units that accumulate in deep-water settings primarily will be a function of sand/mud ratio of sediments supplied, as well as the physiography of the slope and basin. Commonly, the highest sand/mud ratio delivered to the deep water occurs early within the lowstand systems tract. This results in stratigraphic successions that typically grade from relatively widespread tabular-bedded sand-rich turbidite deposits early within the lowstand systems tract, to confined flow leveed-channel turbidite deposits late within the lowstand systems tract when the incoming sand/mud ratio is lower.