Stratigraphic architecture and spatio-temporal variability of high-frequency (Milankovitch) depositional cycles on modern continental margins: An overview

In contrast with the theoretical and overall scale-independent model developed to predict composite stratigraphic cycles and sequences, the Middle-Upper Pleistocene stratigraphic record from numerous shelf settings shows a subdued signature of the higher-frequency (20 ka) Milankovitch cyclicity. However, when detectable, the 20 ka architectural pattern is characterized by a relatively greater spatial and temporal variability compared to 100 ka sequences. This fact likely reflects the increasing importance of factors controlling the depositional environment (e.g., seafloor morphology, oceanographic regime, sediment input and dispersal, etc) with respect to sea-level change during the shorter intervals encompassed by 20 ka cycles. On this basis, two end-member cases have been distinguished, depending on the generation or not of high-frequency sequence boundaries, a prerequisite to qualify the higher-frequency motifs as depositional sequences. These two end members are comprehensive of the highly variable patterns displayed by Middle-Upper Pleistocene shelf sequences, which in turn reflect the interplay between the geological setting, the dominant sea-level control and the effective response of sedimentary systems. The variable stratigraphic patterns of Milankovitch cycles represent conceptual and practical constraints as to the classification of high-frequency Quaternary sequences under the general schemes of the standard sequence stratigraphy model.