A depositional sequence is defined as a relatively conformable succession of genetically related strata bounded by subaerial unconformities or their correlative conformities. The critical part of this definition is that sequences are bounded above and below by subaerial unconformities, or by correlative conformities, that is, surfaces that correlate updip to a subaerial unconformity. A subaerial unconformity is a surface formed through subaerial exposure and erosion, and includes features formed by downcutting rivers, soil processes, and karst processes.
In addition, every depositional sequence is the record of one cycle of relative sea level. As a result, depositional sequences have a predictable internal structure of surfaces and systems tracts (suites of coexisting depositional systems, such as coastal plains, continental shelves, and submarine fans). There are several models of systems tracts within depositional sequences, and we'll cover what is called the four systems tract model. In this model, all depositional sequences contain the following systems tract in this order: lowstand systems tract, transgressive systems tract, highstand systems tract, and falling-stage systems tract. In this view, a sequence begins with the slow rise following a fall in sea level, and continues through the next fall in sea level. These systems tracts are bounded by important named surfaces. The lowstand and transgressive systems tracts are separated by the transgressive surface. The transgressive and highstand systems tracts are separated by the maximum flooding surface. The highstand and falling-stage systems tract are separated by the basal surface of forced regression.