MINERALOGICAL AND GEOCHEMICAL CRITERIA FOR DISTINGUISHING TURBIDITE AND HEMIPELAGIC PELITES - THE MAASTRICHTIAN OF THE NORTHERN APENNINES, ITALY

Sedimentological, mineralogical and geochemical studies were carried o ut on pelitic fractions of an Apenninic deep-sea turbidite sequence (t he Helmintoid Flysch Group) in order to evaluate compositional signals correlated to distinct depositional mechanisms (i.e. sedimentation vs . resedimentation) for deep-sea sediments. The studied sequence (the S olignano flysch, lower Maastrichtian) consists of resedimented silicic lastic, carbonate and composite sediments alternating with thin (few c entimetres thick) beds of hemipelagic sediments low in carbonates. Ail of the sediments were deposited below the calcite compensation depth. X-ray diffraction, thermal (TG, DTG and DTA) and chemical analyses (m ajor and trace elements, carbon and oxygen isotope analyses) were perf ormed on the fine-grained fractions (< 4 and 4 - 10 mu m) in eleven pa irs of closely associated turbidite and hemipelagic mudstones collecte d over a restricted area. Mineralogical and thermal analyses reveal th at turbidite and hemipelagic mudstones differ in both their carbonate and non-carbonate (silicate) components. Turbidite pelites are richer in illite and quartz and contain calcitic carbonates. Hemipelagites ar e characterised by high concentrations of phyllosilicates phases (incl uding interstratified illite-smectite) and Mg-rich carbonates (Mg-calc ites, disordered dolomite). Major and trace element geochemistry of th e carbonates indicates that turbidite carbonates originate from shallo w-water marine sources and confirms the dolomitic and ferroan characte r of the hemipelagic carbonates. Differences in silicate components re flect either differences in the source of elastic material, in the pro duction rate of marine-precipitated phases, or in the multi-stage tran sport and deposition histories of these sediments. Syn-depositional or early, post-depositional oxidation reactions of organic matter (bacte rial?) within the hemipelagic beds are responsible for the conversion of Ca-rich carbonates to dolomitic and sideritic carbonates. The distr ibution patterns of specific redox-sensitive trace elements (Cr, V, Ni , Co, Cu, Zn, etc.) give indications of paleoenvironmental conditions during hemipelagic and turbiditic sedimentation. The element distribut ion in hemipelagites is consistent with early diagenetic element mobil isation related to oxidation of organic matter. (C) 1998 Elsevier Scie nce B.V. All rights reserved.