The architectural variability of small-scale cycles in shelf and ramp clastic systems: the controlling factors

Many sedimentary successions are composed of metre- to decametre-scale cycles exhibiting various stratal architectures that depend on the complex interplay among several syndepositional controlling factors. These controlling factors may produce extremely different stratal and facies architectures along relatively brief distances in the same basin, but in contrast they also may form similar architectures in basins that experience a radically different tectonic setting. The recognition of factors that controlled the architecture of small-scale cycles, therefore, is very important to reconstruct the depositional history of sedimentary successions. These factors include: 1) the interplay between the rate of accommodation development and the rate of sediment supply (A/S); 2) the amplitude and shape of the relative sea-level curve; 3) the position in the shoreface-shelf system; 4) the basin physiography; 5) the shoreline trajectory; 6) the climate-driven environmental energy; 7) the siliciclastic input vs. carbonate production. Shoreface to shelf small-scale cycles can be grouped into three main categories: R cycles, showing a dominant regressive interval; T-R cycles, with transgressive and regressive intervals of similar thickness; T cycles, showing a dominant transgressive interval. Although some depositional settings may favour the deposition of a particular type of cycle, these cycle motifs appear to be mainly controlled by the interaction of local and wider-scale controlling factors. For example, T cycles may be present in both subsiding and uplifting settings, depending on the interplay between the A/S and the other factors cited above. As a result, R, T-R and T cycles may be recognized within the same depositional system. These evidences suggest that care must be taken to apply general models that predict the stratal architecture of cycles mainly in function of the long-term A/S. The correct prediction of the cycle architecture is important also for hydrocarbon researches, as the thickness of potential reservoir deposits is the result of the interplay among factors discussed here.