New perspectives on offshore groundwater exploration through integrated sequence-stratigraphy and source-to-sink analysis: Insights from the late Quaternary succession of the western Central Adriatic system, Italy

Abstract

Sequence stratigraphic concepts have a variety of applications well beyond hydrocarbon exploration. Through coastal plain-to-shelf stratigraphic correlation of Last Glacial Maximum deposits from the Central Adriatic area, we tested a source-to-sink approach for exploring offshore groundwater reserves stored within the lowstand systems tract. Above an erosional unconformity (sequence boundary) formed at the Marine Isotope Stage 3–2 transition in response to sea-level fall, lowstand fluvial gravel-sand bodies, up to 20 m thick, can be tracked continuously downstream, from the coastal-plain paleovalleys to the shelf, 30 km away from the modern shoreline. The LST is overlain by a mud-dominated wedge (TST + HST) made up of alluvial, estuarine and delta plain deposits in lateral transition to thick shallow-marine and prodelta clay successions.Using three catchment-to-shelf transects, 35–70 km long, we document the separation between potential reservoir/aquifer units (LST), primarily made of coarse-grained (porous) deposits, and the overlying, laterally continuous seal (TST + HST), which mainly includes fine-grained (low permeability) estuarine to marine sediments. Thickness maps of reservoir/aquifer and seal units provide a three-dimensional view of the stratigraphic architecture and of accumulation patterns at the systems tract scale. Lowstand fluvial deposits spread across a 5600 km2 wide area of the western Central Adriatic shelf, with average thickness of about 10 m. North of the Meso-Adriatic Deep (MAD), two major depocenters, up to 60 m thick, reflect the local highest fluvial sediment load that correlates, further offshore, to the lowstand Po Delta. West of the MAD, LST deposits, up to 25 m thick, were nourished by Apennine rivers. In the southern area, lowstand deposits are <10 m thick. The LST is overlain across the entire western Central Adriatic shelf by an up to 80 m-thick succession of TST + HST fine-grained deposits.A first assessment of sediment volumes provides a value of 130 km3 for TST + HST and 57.2 km3 for LST. Sediment provenance analysis delineates the contribution to the shelf of individual detrital sources (Apennine rivers from the west, Po River from the north), offering a powerful tool in quantifying sediment fluxes (about 52 km3 from the Apennines catchments and 5 km3 from the lowstand Po system).As a whole, the application of sequence stratigraphic concepts led, for the first time, to the identification of a potential groundwater reservoir stored beneath the western Central Adriatic shelf. This LST aquifer possibly contains about 13.85 km3 of groundwater (the salinity of which is unknown), and is vertically confined by a thick, low-permeability unit (i.e., TST + HST) that might have prevented salt-water intrusion into the underlying aquifer. The documented stratigraphic continuity likely makes this offshore aquifer an actively recharging system, with important implications for possible future sustainable exploitation.