High-resolution sequence stratigraphy of clastic shelves VIII: Full-cycle subaerial unconformities

The interplay between high-frequency relative sea-level changes, such as those connected to late Quaternary glacio-eustasy, and regional uplift usually produces staircases of marine terraces along coastal areas. These particular settings are characterized by variable topography and gradient of the landscape due to both tectonics and wave erosion, which result in the development of subaerial unconformities during both transgressive and forced regressive stages of the high-frequency relative sea-level curve. In particular, the increased topographic gradient due to tectonic uplift results in a coastal profile steeper than the transgressive shoreline trajectory and the consequent formation of a retreating coastal cliff fronted by a transgressive wave ravinement surface, whereas subaerial exposure occurs landwards. In contrast, the relatively low-gradient wave ravinement surface formed during transgression seaward of the coastal cliff affords the deposition of a highstand shallow-marine prograding wedge characterized by a relatively low-gradient top, gentler than the forced regressive shoreline trajectory, resulting in the formation of a new subaerial unconformity at the top of highstand and forced regressive deposits during relative sea-level fall. The overall uplifting conditions favor the persistent development of the already formed transgressive and forced regressive subaerial unconformities also during the highstand and lowstand stages of the high-frequency relative sea-level curve, excepting for the accumulation of continental deposits just seaward of the uplifted paleo-coastal cliffs. The repetition of such a motif for several high-frequency relative sea-level changes will result in the formation of a long-term, higher rank subaerial unconformity expanding seaward and recording the uplift of the coastal area. The way in which the subaerial unconformity develops in contexts dominated by regional uplift and superimposed glacio-eustatic sea-level changes is not described by classic sequence stratigraphic models and this needs to be taken into account in sequence stratigraphic analysis.