Unconformity-Bounded Stratigraphic units (USBs) in an italian alluvial plain area: recognising and dating

We studied the coastal zone of the Tavoliere di Puglia plain, (Puglia region, southern Italy) with the aim to recognise the main unconformities, and therefore, the unconformity-bounded stratigraphic units (UBSUs; Salvador, 1987, 1994) forming its Quaternary sedimentary fill. Recognising unconformities is particularly problematic in an alluvial plain, due to the difficulties in distinguishing the unconformities that bound the UBSUs. So far, the recognition of UBSUs in buried successions has been made mostly by using seismic profiles. Instead, in our case, the unavailability of the latter has prompted us to address the problem by developing a methodological protocol consisting of the following steps: I) geological survey in the field; II) draft of a preliminary geological setting based on the field-survey results; III) dating of 102 samples coming from a large number of boreholes and some outcropping sections by means of the Amino Acid Racemization (AAR) method applied to ostracod shells and 14C dating, filtering of the ages and the selection of valid ages; IV) correction of the preliminary geological setting in the light of the numerical ages; definition of the final geological setting with UBSUs; identification of a “hypothetical” or “attributed time range” (HTR or ATR) for each UBSU, the former very wide and subject to a subsequent modification, the latter definitive; V) cross-checking between the numerical ages and/or other characteristics of the sedimentary bodies and/or the sea level curves (with their effects on the sedimentary processes) in order to restrict also the hypothetical time ranges in the attributed time ranges. The successful application of AAR geochronology to ostracod shells relies on the fact that the ability of ostracods to colonise almost all environments constitutes a tool for correlation, and also allow the inclusion in the same unit of coeval sediments that differ lithologically and palaeoenvironmentally. The treatment of the numerical ages obtained using the AAR method required special attention. The first filtering step has been made by the laboratory (rejection criteria a and b). Then, the second filtering step has been made by testing in the field the remaining ages. Among these, in fact, we have never compared an age with a single preceding and/or following age; instead, we identified homogeneous groups of numerical ages consistent with their reciprocal stratigraphic position. This operation led to the rejection of further numerical ages that deviate erratically from a larger, homogeneous age population which fits well with its stratigraphic position (rejection criterion c). After all the filtering steps, the valid ages that remained were used for the subdivision of the sedimentary sequences into UBSUs together with the lithological and palaeoenvironmental criteria. The numerical ages allowed us, in the first instance, to recognise all of the age gaps between two consecutive samples. Next, we identified the level, in the sedimentary thickness that is between these two samples, that may represent the most suitable UBSU-boundary based on its lithology and/or the palaeoenvironment. The recognised units are: I) Coppa Nevigata sands (NEA), HTR: MIS 20-14, ATR: MIS 17-16; II) Argille subappennine (ASP), HTR: MIS 15-11, ATR: MIS 15-13; III) Coppa Nevigata synthem (NVI), HTR: MIS 13-8, ATR: MIS 12-11; IV) Sabbie di Torre Quarto (STQ), HTR: MIS 13-9.1, ATR: MIS 11; V) Amendola subsynthem (MLM1), HTR: MIS 12-10, ATR: MIS 11; VI) Undifferentiated continental unit (UCI), HTR: MIS 11-6.2, ATR: MIS 9.3-7.1; VII) Foggia synthem (TGF), ATR: MIS 6; VIII) Masseria Finamondo synthem (TPF), ATR: Upper Pleistocene; IX) Carapelle and Cervaro streams synthem (RPL), subdivided into: IXa) Incoronata subsynthem (RPL1), HTR: MIS 6-3; ATR: MIS 5-3; IXb) Marane La Pidocchiosa-Castello subsynthem (RPL3), ATR: Holocene; X) Masseria Inacquata synthem (NAQ), ATR: Holocene. The possibility of recognising and dating Quaternary units in an alluvial plain to the scale of a Marine Isotope Stage constitutes a clear step forward compared with similar studies regarding other alluvial plain areas, where Quaternary units were almost exclusively dated using their stratigraphic position. As a result, they were generically associated with a geological sub-epoch. Instead, our method allowed a higher detail in the timing of the sedimentary processes: for example, MIS 11 and MIS 5.5 deposits have been recognised and characterised for the first time in the study area, highlighting their importance as phases of sedimentation.