Sedimentology, stratigraphic architecture and preliminary hydrostratigraphyof the Metaponto coastal planesubsurface (Southern Italy)

The stratigraphic and hydrostratigraphic architecture of the Metaponto coastal plain subsurface (Southern Italy) was obtained from a data set composed of twenty continuously cored boreholes and over 350 drills, 50-120 m deep. In particular, sedimentological, biostratigraphical and chronostratigraphical analyses were performed on four continuously cored boreholes. Furthermore,some profiles, parallel and transversal to the present-day shoreline, permitted correlations between the stratigraphic logs obtained from all boreholes. The overall obtained data allowed the distinction of four types of depositional systems of Middle Pleistocene-Holocene age, including eight facies associations in turn subdivided in seventeen lithofacies. Thanks to the recognition of two discontinuity surfaces of regional extent, the late Quaternary Metaponto buried succession may be subdivided into three units, each made up of different lithofacies. The middle-upper (?) Pleistocene lower unit (“substratum”), at least 60 m thick, is made up of silty and clayey-silty shelf-transition deposits passing upward and landward to sandy and sandy-gravelly deltaic deposits. The upper boundary of this lower unit is represented by a very irregular surface which locally deepens up to 90 m and this can be related to incised valleys formed during time spans of sub-aerial exposition induced by relative fallings and lowstands of the sea level. These “palaeovalleys”, mainly filled by estuarine deposits, developed during two time spans comprised between the Marine Isotope Stage 4 (MIS 4) and the Last Glacial Maximum (LGM). The middle unit, called unit MP1 (Metaponto Plain 1), lying on the substratum discontinuously, is late Pleistocene in age and has a thickness of 15 m. However, this thickness reaches 60 m when a palaeovalley is filled. The unit MP1 is composed in the first case of sandy-gravelly fluvial and/or deltaic deposits; in the second case, the paleovalley is filled with silty-sandy estuarine to deltaic deposits. The upper unit, called unit MP2 (Metaponto Plain 2), lying either on the unit MP1 or on the substratum, is late Pleistocene and Holocene in age. The unit MP2 has a thickness of 30 m. However, when palaeovalleys are filled, thickness may reach 90 m. In the first case it is made up of silty-clay offshore-transition deposits passing upward to silty-sandy deltaic deposits and then finally to sandy fluvial deposits; in the second case, the palaeovalley fill, is composed of sandy-gravelly fluvial deposits passing upward to silty and sandy estuarine deposits. This stratigraphic frame permits us to sketch the hydrostratigraphic picture of the buried Metaponto succession, where units MP1 and MP2, as a whole, correspond to a multilayered aquifer lying on the clayey substratum. Inside each incised valley, this aquifer appears to be laterally confined and vertically partitioned where permeable deposits alternate with low-permeable ones.