The Paleolake San Nicandro (L'Aquila, central Italy): early stage of continental deposition in the central Apennines

The presence of a Caspiocypris species-flock in the lacustrine deposits of L'Aquila Basin (San Nicandro Fm), which points to a correlation with the Caspiocypris-bearing ostracod assemblage identified in the late Piacenzian-Gelasian of the Tiberino paleolake, is until now the only temporal constraint for the oldest deposits of L'Aquila Basin. We present some independent data (e.g., biochronology, magnetostratigraphy, tephra geochemistry, Sr-Nd- Pb isotope geochemistry, 40Ar/39Ar dating, etc.) to better define the age of the basal infill of the L'Aquila Basin. Above the San Nicandro Fm, the uppermost part of the overlying Valle Orsa Fm contains Candona (Neglecandona) permanenta, which is a marker species for the late Gelasian of the Lake Slavonia succession (Serbia).In these lacustrine deposits, two different magnetozones have been observed: a lower one, with normal polarity and an upper one showing reverse polarity. ChRM with normal polarity directions have been measured in the lower 7 meters of the outcropping San Nicandro Fm, whereas reverse polarities directions have been isolated in the stratigraphic intervals that include the rest of the San Nicandro Fm and the overlying Valle Orsa Fm. In the surroundings of San Nicandro, thin tephra layers are interbedded within the deep-lake deposits (San Nicandro Fm). There, 3 cm-thick (SNC 1) and 7 cm-thick (SNC 2) tephra layers were sampled for geochemical analyses. These tephra are made up by glass shards, clinopyroxene, plagioclase, and minor orthopyroxene. The whole rock samples have an intermediate composition ranging from basaltic andesite to andesite, with slightly variable major and trace element contents. Incompatible trace elements normalised to the primordial mantle display typical subduction - related patterns with troughs at Ta, Nb, and Ti, and peaks at Pb, which are found in both Gelasian calc-alkaline rocks of the Balkan and Italian regions. Sr, Nd, and Pb isotope ratios are well within the 2 Ma old andesitic magmatism found in drill holes below the Phlegrean volcanic fields, suggesting a provenance of the San Nicandro tephras from explosive eruptions from volcanoes of the Neapolitan area in Southern Italy.The new 40Ar/39Ar ages reported herein were obtained via small-population multi-grain analyses of phenocrystic plagioclase from a fine-grained, vitric (almost aphyric) bedded tuff sample (SNC 2). The feldspars were analyzed in small aliquots (~5-6 crystals) by the 40Ar/39Ar technique using two laser-heating methods, multi-crystal total fusion (MCTF) and multi-crystal incremental heating (MCIH). An age-probability density spectrum of the MCTF dating shows that the age distribution is a quasi-symmetric, unimodal population with a weighted-mean age of 1.81 ± 0.13 Ma (1?). The MCIH replicate experiments yielded a quasi-symmetrical unimodal population, with a weighted-mean age of 1.89 ± 0.18 Ma. The final age is taken as the weighted mean of the isochron ages for the MCTF and MCIH experiments, or 1.77 ± 0.15 Ma.In conclusion, all these data point to consider the outcropping lacustrine deposits of the Paleolake San Nicandro as very early Pleistocene in age.