A 1700 m thick succession, cropping out near Pignone (SP), was sampled, focusing on the sediments with finer grain size (pclites), the coarser sediments being already studied by other Authors. The Pignone section represent most of the Macigno'Formation, a turbiditic deposit developed during the Upper Oligocène, during the early stage of formation of the Northern Apennine. The mineralogical analyses were performed mainly on the bulk rock by X-ray diffraction methods. The mineralogical association includes quartz, feldspars, mica-illite and chlorile. Usually calcite is absent or a few, indicating the absence of carbonates in the source areas; only toward the upper part of the section, where true hemipelagic sediments occur, calcite is more abundant. Interstratified chlorite-smectite occur in the lower part of the section. From the base to the top of the section the relative mineral abundance of chlorite, mica-illite and feldspars varies abruptly, as well as the crystallinity index of mica-illite minerals (with better crystallised terms toward the top of the sec-tion). Other mineralogical peculiarities are due to the low crystallinity value of quartz and to the abundance of the mineral (with pseudo fibrous morphologies) within a narrow stratigraphie interval. The quartz enrichment and the low crystallinity values are typical of the biogenic supply, but further confirmations of this hypothesis are needed. The vertical distribution of smectite-bearing minerals and of the crystallinity index of mica-illite, is not compatible with the diagenetic evolution of deep buried siliciclastic successions, therefore the abrupt changes affecting the vartical variation of the minerals abundance, are interpreted as the effect of variations in the detrital supply, rather then implying post-depositional transformations developed during deep burial. In particular the occurrence of chloritesmectite in the lower part of the succession is consistent with volcaniclastic sedimentation in coeval sediments of the Northern Apennine, mainly with the occurrence of biotite-rich sediments.
Significato della composizione mineralogica dei sedimenti pelitici nella sezione di Pignone (SP), Formazione del Macigno (Oligocene Superiore).
Tateo F
1998
Abstract
A 1700 m thick succession, cropping out near Pignone (SP), was sampled, focusing on the sediments with finer grain size (pclites), the coarser sediments being already studied by other Authors. The Pignone section represent most of the Macigno'Formation, a turbiditic deposit developed during the Upper Oligocène, during the early stage of formation of the Northern Apennine. The mineralogical analyses were performed mainly on the bulk rock by X-ray diffraction methods. The mineralogical association includes quartz, feldspars, mica-illite and chlorile. Usually calcite is absent or a few, indicating the absence of carbonates in the source areas; only toward the upper part of the section, where true hemipelagic sediments occur, calcite is more abundant. Interstratified chlorite-smectite occur in the lower part of the section. From the base to the top of the section the relative mineral abundance of chlorite, mica-illite and feldspars varies abruptly, as well as the crystallinity index of mica-illite minerals (with better crystallised terms toward the top of the sec-tion). Other mineralogical peculiarities are due to the low crystallinity value of quartz and to the abundance of the mineral (with pseudo fibrous morphologies) within a narrow stratigraphie interval. The quartz enrichment and the low crystallinity values are typical of the biogenic supply, but further confirmations of this hypothesis are needed. The vertical distribution of smectite-bearing minerals and of the crystallinity index of mica-illite, is not compatible with the diagenetic evolution of deep buried siliciclastic successions, therefore the abrupt changes affecting the vartical variation of the minerals abundance, are interpreted as the effect of variations in the detrital supply, rather then implying post-depositional transformations developed during deep burial. In particular the occurrence of chloritesmectite in the lower part of the succession is consistent with volcaniclastic sedimentation in coeval sediments of the Northern Apennine, mainly with the occurrence of biotite-rich sediments.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
