Sequence Stratigraphy: A New Look at Old Rocks

Abstract Sequence stratigraphy is the use of time-stratigraphic units to classify, correlate, and map sedimentary rocks. Time-stratigraphic mapping in the field was first done using unconformities and marker beds. It did not become popular until the late 1970s when seismic reflection was applied to basin fill on a regional basis (seismic stratigraphy). Outcrop and well data extended seismic stratigraphy to a finer resolution during the 1980s (sequence stratigraphy). During a fall and rise of sea level, a characteristic package of sedimentary rocks called a sequence is deposited. The sequence is bounded on the top and bottom by major unconformities and often contains lowstand, transgressive, and highstand systems tracts. These are assemblages of sedimentary rocks that were deposited during specific stages of a regression and transgression cycle. Systems tracts can be identified on outcrops, cores, and well logs. The vertical and lateral succession of facies, along with source and reservoir rock locations, can be predicted within each systems tract. Time horizons are formed by the (1) lowstand surface of erosion, (2) transgressive surface, and (3) maximum flooding surface. These either define the border of a sequence or the borders of systems tracts in a sequence. Seismic-sequence stratigraphy is being used to reinterpret many sedimentary rocks and to identify stratigraphic plays.