Abstract
Linear sand bodies with ridged or mounded morphology are commonly referred to as “sand ridges”. Their origin may reflect a great variety of depositional processes and environments, although many examples from modern shelves include near-shore transgressive deposits formed during the last post-glacial sea level rise. These transgressive sand ridges, however, are not present on all the margins drowned during this interval, indicating that local environmental factors contribute to their formation and preservation. Transgressive sand ridges are recognized on the South Adriatic shelf in water depths between 90 and 120 m, where they overlay a major unconformity originated during the last glacial sea level fall and lowstand. In contrast, they are absent on the Central Adriatic shelf, where transgressive deposits above the glacial unconformity fill erosional scours without forming relief on the seafloor. The transgressive sand ridges on the South Adriatic shelf appear roughly elongated parallel to the bathymetric contour and are locally as thick as 15–20 m. Core data indicate an overall coarse-grained composition, with a relevant bioclastic component. Their limited distribution reflects the importance of local variation in shelf morphology and sediment sourcing at the onset of sea level rise. In particular, their deposition and preservation are favored in less subsiding shelf sectors characterized by higher gradients and tectonic relief, where transgressive erosion and reworking of pre-existing lowstand deposits is also more efficient.
Highlights
South of the MAD, and away from the influence of a large deltaic source, the early unit varies from a few meters thick, flat-tabular unit, to a thin draping lag; locally, it occurs as a transgressive transgressive (TST) unit varies from a few meters thick, flat-tabular to unconformity a thin draping(Figure lag; locally, it places, occurs subtly lenticular, patchy deposit filling erosional scourunit, of ES1
Previous studies report the absence of the lower TST unit in the South Adriatic, suggesting its erosion during a sea level fall associated with the Younger Dryas event, or reduced deposition during the early phase of sea level rise [66]
Sedimentation rates can be highly variable during lowstand to early transgression, and sand supply on the outer shelf may depend on riverine/deltaic sources or on the reworking of older shorelines deposits
Summary
Shelf deposits with overall sandy composition, wedged and/or mounded shape, and along-strike extent over tens of kilometers, are known from ancient and modern continental margins [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15] These deposits include sedimentary bodies formed in different environments by different processes; they range in scale from thick (tens of meters) prograding wedges [16,17] to smaller (1–3 m) bedforms [18]. Following the advent of sequence stratigraphy and the emphasis on sea level change, most “offshore bar deposits” were re-interpreted as “sharp-based” lowstand shorefaces [25,26,27,28]
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