Mineralogical and geochemical criteria for distinguishing turbidite and hemipelagic pelites — the Maastrichtian of the northern Apennines, Italy

Abstract

Sedimentological, mineralogical and geochemical studies were carried out on pelitic fractions of an Apenninic deep-sea turbidite sequence (the 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 Solignano flysch, lower Maastrichtian) consists of resedimented siliciclastic, carbonate and composite sediments alternating with thin (few centimetres thick) beds of hemipelagic sediments low in carbonates. All of the sediments were deposited below the calcite compensation depth. X-ray diffraction, thermal (TG, DTG and DTA) and chemical analyses (major and trace elements, carbon and oxygen isotope analyses) were performed on the fine-grained fractions (<4 and 4–10 γm) in eleven pairs of closely associated turbidite and hernipelagic mudstones collected over a restricted area. Mineralogical and thermal analyses reveal that turbidite and hernipelagic mudstones differ in both their carbonate and non-carbonate (silicate) components. Turbidite pelites are richer in illite and quartz and contain calcitic carbonates. Hemipelagites are characterised by high concentrations of phyllosilicates phases (including interstratified illite-smectite) and Mg-rich carbonates (Mg-calcites, disordered dolomite). Major and trace element geochemistry of the carbonates indicates that turbidite carbonates originate from shallow-water marine sources and confirms the dolomitic and ferroan character of the hernipelagic carbonates. Differences in silicate components reflect either differences in the source of clastic material, in the production rate of marine-precipitated phases, or in the multi-stage transport and deposition histories of these sediments. Syn-depositional or early, post-depositional oxidation reactions of organic matter (bacterial?) within the hemipelagic beds are responsible for the conversion of Ca-rich carbonates to dolomitic and sideritic carbonates. The distribution patterns of specific redox-sensitive trace elements (Cr, V, Ni, Co, Cu, Zn, etc.) give indications of paleoenvironmental conditions during hernipelagic and turbiditic sedimentation. The element distribution in hemipelagites is consistent with early diagenetic element mobilisation related to oxidation of organic matter.