Quaternary subsurface stratigraphy in the Tronto alluvial plain (Italy): palaeoenvironmental evolution and relationships with sediment geochemistry

The Quaternary subsurface stratigraphy in the Tronto alluvial plain (Marche region, central Italy) is investigated through sedimentological, micropalaeontological (benthic foraminifers, ostracods and calcareous nannofossils) and geochemical analyses of a 60 m long core (TR 1) drilled about 2 km landward from the present shoreline. Elaboration of geochemical data enabled the distinction of sediment provenance (e.g. Tronto River and Adriatic Sea) according to selected geochemical indices. The chronological framework of this succession is mainly based on the analyses of calcareous nannofossils and one AMS 14C date. In the lower part of the core, outer-shelf deposits showing a homogeneous geochemical composition comparable to present day Adriatic Sea, are replaced upwards by alluvial fan gravels, possibly recording a tectonic event, which produced erosion of the underlying marine sediments. Above the alluvial fan deposits, a coastal plain composed of sediments showing Tronto River provenance and characterized by repeated flooding events, was possibly formed in response to small-scale glacio-eustatic cyclicity. Coastal plain deposits are not well dated, however their low thickness and internal arrangement suggest deposition during minor relative sea-level fluctuations, possibly corresponding to those observed in the initial stage of the last deglacial rise in sea level. Consequently the overlying shoreface sediments should be attributed to the lower part of the Holocene and the overall regressive tendency observed in the upper part of the core, including delta plain, swamp and alluvial plain deposits, reflects delta and strandplain progradation. The lack of chronological constraints hampers a precise attribution to these deposits; however the AMS 14C date at 7.95 m core depth confirms the attribution to the Holocene for the upper delta plain deposits. The geochemistry of this part of the core suggests provenance evolution from Tronto River to Adriatic Sea in shoreface deposits, whereas a variable composition is observed upwards, reflecting changes in sediment texture, occurrence of organic-rich layers and diagenetic modifications. The chronological framework of core TR 1 suggests a possible stratigraphic correlation between the upper part of the Qm sequence defined in outcrop successions by several authors, and the outer-shelf deposits from the lower part of the core; the base of the overlying Middle Pleistocene sequence Qc is tentatively correlated with the base of the alluvial fan sediments.