Federico Lucchini

Geochemical and mineralogical variations as indicators of provenance changes in Late Quaternary deposits of SE Po Plain

Combined geochemical and mineralogical characterisation of mudstone-prone successions can provide suitable indicators of sediment provenance. A case study from the Late Quaternary depositional sequence of southeastern Po Plain shows that major changes in geochemical and mineralogical composition can be ascribed to changes in sediment dispersal patterns occurring during the last 30 ka in response to fluctuating sea level. The Po River, with a mixed contribution from the Alpine and Apenninic chains and its southern tributaries flowing from the Apennines, acted as the major feeders for the study area during the Late Quaternary. Chromium (and nickel) among trace elements, and serpentine (and dolomite) among minerals appear as the major provenance indicators for the study succession, allowing distinction of an Apenninic (chromium- and serpentine-poor) province from a mixed Alpine–Apenninic, Po-related (chromium- and serpentine-rich) province. The relatively low Cr/Al2O3 and serpentine/silicate ratios recorded in sediments of late Pleistocene age (<30 ka BP), which accumulated at the last glacial maximum, suggest that during lowstand times and early stages of transgression, the southeastern Po Plain was beyond the Po River influence, being fed uniquely by rivers of Apenninic provenance. Increasing Cr/Al2O3 and serpentine/silicate ratios, recorded within back-barrier transgressive deposits (8–10 ka BP) in the Comacchio and Ravenna sectors, reflect a major change in sediment supply taking place during the Holocene, when partially barred environments largely connected with the open sea became established in the study area, and Po-derived detritus was delivered to estuaries and lagoons by littoral drift. Maximum chromium and serpentine values are recorded at peak transgression and during the following sea-level highstand (6 ka BP–Present) when sediment supply exceeded the rate at which new accommodation space was created, leading to extensive Po-delta progradation. The drop in Cr/Al2O3 and serpentine/silicate ratios recorded in some cores at top of the Holocene succession reflects the local reestablishment, behind the prograding Po-delta complexes, of an alluvial plain drained by rivers of Apenninic provenance as a result of distributary channel switching and delta lobe abandonment. Geochemical and mineralogical analyses show negligible compositional variations between Pleistocene and Holocene deposits landward of the line of maximum marine incursion especially close to the basin margin. In these instances, sample composition throughout the entire stratigraphic succession clearly suggests an ongoing supply from the adjacent Apenninic chain. Comparison with sand compositional analyses from major sand bodies of the same stratigraphic succession highlights the complementary but fundamental role of geochemical and mineralogical characterisation of clays for provenance research.

TEPHRA LAYERS IN LATE QUATERNARY SEDIMENTS OF THE CENTRAL ADRIATIC SEA

Abstract Magnetic susceptibility curves of ten cores, collected in the western Adriatic shelf and in the Mid-Adriatic Deep, allowed us to recognize thirty tephra layers whose age, based on physical and biostratigraphic data, encompass a time interval ranging from less than 70 ka to the Present. Chemical features of the tephra, defined by SEM–EDS analyses performed on glass shards, indicate alkaline affinity, mainly with K/Na>1, and trachytic compositions with two minor benmoreitic and phonolitic groups. These characteristics are consistent with Campanian and subordinate Etnean provenances. Because of chemical and age constraints the glass shards can be referred to six tephra layers already described in the literature, such as C20, C14 (Citara/Ischia), C10 (Campanian Ignimbrite), C2/NYT (Neapolitan Yellow Tuff), AMS/PF (Agnano Monte Spina/Phlegrean Fields) from Campanian area, and Y1 from Etna volcano, indicating an age ranging from about 70 to 4 ka. AMS/PF and C2/NYT, dated 4.4 and 12.3 ka respectively, are the most widespread tephra beds in the study cores. The occurrence of AMS/PF, never recorded in marine sequences up to now, represents a very good marker for Holocene Adriatic sediments; C2/NYT already recorded in the central Tyrrhenian Sea, in lacustrine sediments from southern Italy and in the Campanian area, is a good stratigraphic marker for inter-sea and sea–land correlations. Y1 tephra, dated 14.2 ka, also occurs in on-land sediments of central Italy and in marine sediments from the Tyrrhenian and Ionian seas: it now represents one of the most widespread stratigraphic markers in the central Mediterranean region.

Geochemistry of Oligocene–Miocene sandstones of the northern Apennines (Italy) and evolution of chemical features in relation to provenance changes

Integrated petrographic and geochemical criteria for provenance determination in clastic sedimentary rocks are applied to four flysch formations of Oligocene–Miocene age in the northern Apennines (Italy). Major and trace element geochemistry of 60 sandstones for the Macigno Formation, Monte Modino Formation, Monte Cervarola Formation, and Marnoso-arenacea Formation is presented. Geochemical criteria of discrimination based on element ratios (Y/Ni, Cr/V, Rb/Sr and Cu/Al2O3) allow distinction of the Macigno, Modino, Cervarola and Marnoso-arenacea. The differing geochemical features of the four flysch formations suggest a decrease of ultramafic and metamorphic supply and increase of carbonate and intrusive rock fragments from Macigno to Cervarola, and a sharp increase of carbonate/siliciclastic ratio at the transition to the Marnoso-arenacea Formation. All this provides significant insights and additional constraints about the chronostratigraphic evolution of sediment supply in the northern Apennines foredeep from Oligocene to Tortonian times: the decrease of detritus supply from the Adriatic Plate and the increase in felsic detritus from the Alps become apparent after the Oligocene–Miocene transition; the carbonate influx from E and SW starts from the Lower Miocene; the increasing importance of the Apennine Chain input, with the recycling of turbiditic successions and rocks from Ligurian units, is testified by the geochemical features of the Marnoso-arenacea Formation. On the whole, the applicability of the geochemical techniques to clastic sedimentary rocks is confirmed, but also the complementary character of petrographic and geochemical methods in provenance and evolution studies is emphasized.

Chemostratigraphy of Lago Albano sediments (Central Italy):geochemical evidence of palaeoenvironmental changes in lateQuaternary

The results of the geochemical investigation on bulk sediment fromthree cores collected in Albano crater lake (Central Italy) are reported, andthe main markers of the palaeoenvironmental changes in the last 26ka are discussed. The sediment of Lago Albano consists of greysilt and mud, and is made of volcanogenic, calcareous, siliceous and organicmaterial. Some tephra layers provide a chronological framework for thesequence. The chemical features of the sediments are strongly impacted by theAlban Hills volcanism (Roman volcanic area), but there are also strong changesof organic/inorganic matter ratios, owing to variations in biologicalproductivity and terrigeous supply from the catchment. Six chemostratigraphiczones record the main steps of the transition from the cold and dry climate ofGlacial Maximum to the warmer and more humid Holocene climate. The most usefulgeochemical indices are: biogenic silica, CO2, Br and organiccontent (OM) for biological productivity; Al, Y, CIA (Chemical Index ofAlteration), Al/Rb, Ti/Zr and Y/Al ratios for terrigenous clasticmaterial; OM/Al ratio for organic/terrigenous ratio; S/Fe andMo/Fe ratios and Authigenic U for redox diagenetic conditions of the lakesediment. The geochemical records agree rather well with those of thelithological and paleomagnetic studies, and despite the information obtainedare less detailed than those acquired by the study of biological remains, thepalaeoenvironmental significance inferred is consistent. A comparison of thechemostratigraphic results of Lago Albano with those obtained on coevalsediments from Central and Southern Italy lakes supports the role of thegeochemical investigation as important complement to more sophisticatedtechniques in the palaeoenvironmental reconstructions.

An example of flow differentiation: clinopyroxenite of the Predazzo igneous complex (north Italy)

Abstract Chemical, modal and mineralogic data and textural observations suggest that the small mafic-ultramafic body at Predazzo, consisting of olivine-bearing gabbro, clinopyroxenite and quartz-bearing gabbro, is the result of a flow differentiation mechanism acting on a mush formed of 60% solid plus 40% liquid. Such a mechanism was capable of producing the concentration of the early formed crystals ( cpx + mt ) towards the center of the body (clinopyroxenite) with segregation of two portions enriched in liquid toward the margins (olivine- and quartz-bearing gabbros).