Scale in sequence stratigraphy

Abstract

Abstract Stratigraphic cyclicity can be observed at different scales. At each scale of observation (i.e., hierarchical level), the building blocks of the sequence stratigraphic framework are represented by sequences and their component systems tracts and depositional systems. At the smallest stratigraphic scales, systems tracts and component depositional systems consist solely of sedimentological cycles (i.e., beds and bedsets). At any larger scales, systems tracts and depositional systems consist of lower rank stratigraphic cycles (i.e., sequences). The stacking pattern of sedimentological cycles defines the lowest rank systems tracts, which are the smallest stratigraphic units of a sequence stratigraphic framework. The scale of the lowest rank systems tracts at any location defines the highest stratigraphic resolution that can be achieved, and the limit between sedimentology and stratigraphy sensu stricto. The formation of depositional systems, and implicitly of systems tracts, requires typically minimum timescales of 102 yrs, and it may be sustained for as long as the defining environments are maintained as dominant sediment fairways. Within the transit area of a shoreline, where changes in depositional environment are most frequent, only the lowest rank depositional systems consist strictly of process-related facies accumulated in specific environments; these depositional systems sensu stricto develop commonly at scales below the resolution of seismic stratigraphy. At larger scales (higher hierarchical ranks), depositional systems sensu lato reflect dominant depositional trends, but may record higher frequency changes in depositional environment. The distinction between depositional systems sensu stricto and sensu lato becomes less meaningful outside of the shoreline transit area, where stratigraphic cyclicity may develop without changes in depositional environment. High-frequency sequences are commonly observed at scales of 100–101 m and 102–105 yrs, which defines the scope of high-resolution sequence stratigraphy. The stacking pattern of high-frequency sequences defines depositional systems and systems tracts of higher hierarchical ranks in lower resolution studies.