Towards the standardization of sequence stratigraphy: Is the parasequence concept to be redefined or abandoned?

Abstract

After more than thirty years from the introduction of the sequence-stratigraphic concepts (Payton, 1977), and their refinement in the classic SEPM special publication 42 (Wilgus et al., 1988), sequence stratigraphy has becomemoremature and now represents a widely used tool to study sedimentary successions. More recently, review publications (e.g. Miall, 1997; Posamentier and Allen, 1999; Catuneanu, 2006; Catuneanu et al., 2009) have put order among the various sequence-stratigraphic models and attempted to standardize the terminology. Among the historical issues raised by sequence-stratigraphic concepts and terminology, one deals with the high-frequency cycles that compose sequences, which have been named parasequences. A parasequence was defined as ‘a relatively conformable succession of genetically related beds or bedsets bounded by marine flooding surfaces and their correlative surfaces... Parasequences are progradational and therefore the beds within parasequences shoal upward’ (Van Wagoner et al., 1987, 1988, 1990). The flooding surface was defined as ‘a surface across which there is an abrupt shift of facies that may indicate an increase in water depth or a decrease in sediment supply’ (Van Wagoner et al., 1988, 1990). Parasequences may be stacked to form progradational, aggradational and retrogradational parasequence sets, which typify systems tracts composing a sequence (VanWagoner et al., 1990), the latter being defined as ‘a stratigraphic unit composed of a relatively conformable succession of genetically related strata and bounded at its top and base by unconformities or their correlative conformities’ (Mitchum, 1977).