Two Approaches to Sequence Stratigraphy

Abstract

Abstract Sequence stratigraphy is a stratigraphic discipline in which the defined stratigraphic surfaces represent either breaks in deposition or changes in depositional trend. Two approaches for defining sequence stratigraphic surfaces and units have evolved, with one being inductive and the other deductive. The empirical, inductive approach defines surfaces based on observable, physical characteristics. Five surfaces, subaerial unconformity (SU), unconformable shoreline ravinement (SR-U), slope onlap surface (SOS), maximum regressive surface (MRS), and maximum flooding surface (MFS), are used to define inductive sequence units and to construct a correlation framework. Two different types of sequences (depositional T-R, genetic stratigraphic) and two systems tracts (transgressive systems tract, regressive systems tract) are employed in the inductive approach. In contrast, the deductive approach defines sequence stratigraphic surfaces in terms of theoretical events on a base-level curve (e.g., start base-level fall). Three deductive surfaces (SU, MRS, and MFS) used for unit definition and correlation are the same as those employed by the inductive approach. Two surfaces, not recognized in the inductive approach, are also used to delineate units and for correlation. Notably, both are chronostratigraphic surfaces which display no diagnostic physical characteristics. These surfaces include the basal surface of forced regression (BSFR), the depositional surface at the start of base-level fall, and the correlative conformity (CC), the depositional surface at the start of base-level rise. A sequence is defined as a succession of strata deposited during a full base-level cycle and associated change in accommodation or sediment supply. Recognition of the five surfaces has allowed the definition of four types of sequences (depositional sequence type 1 and 2, T-R, genetic stratigraphic), with each type being divided into four systems tracts (lowstand, transgressive, highstand, and falling stage). The decision as to what approach to use—inductive or deductive—depends on the robustness of the available data and whether the BSFR and CC can be recognized with reasonable objectivity and consistency over the study area. Caution must be exercised to avoid the pitfall of trying to force fit data into the deductive approach by interpreting an inappropriate surface (e.g., facies change) as either the BSFR or CC. In many situations, especially when seismic data are not available, it is not possible to recognize and correlate the BSFR and/or the CC with objectivity. In these cases, the inductive approach is required for unit delineation and correlation.