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

This study is a review of Quaternary Milankovitch cycles as recorded on modern continental shelves worldwide. On the background of the many existing examples, we focus on selected case studies from Mediterranean (Adriatic and Tyrrhenian seas) and Atlantic (gulfs of Mexico and Cádiz) margins that represent general and peculiar characters of Middle-Upper Pleistocene sequences forming under the control of composite 100 and 20 ka Milankovitch cycles. The most pervasive stratigraphic pattern displayed by shelf deposits is referred to the 100ka sea-level cycle, and consists of fairly uniform depositional sequences mostly composed by regressive deposits forming during the falling limb of the sea-level curve. These are generally classified as Falling Stage Systems Tract (FSST) and display a regressive facies architecture reflecting the dominant control of sea level. On many margins, FSST units may show slight though significant differences with respect to Highstand Systems Tract (HST) and Lowstand Systems Tract (LST) regressive units, more closely reflecting environmental changes and the local variability of depositional systems. 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 100ka 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 20kacycles. 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-frequencymotifs as depositional sequences. These two endmembers 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.