Abstract

This paper deals with the natural radioactivity (NRD) of the Gessoso-Solfifera Fm outcropping in the North Western flank of the Maiella Mts. In the study area, the Gessoso-Solfifera Fm is characterized by sulfate evaporites correlated to the Lower Evaporites in Sicily (Decima & Wezel, 1971, 1973; McKenzie, 1985; Decima et alii, 1988 and to the Vena del Gesso in the northern Apennines (Vai & Ricci Lucchi, 1977; Roveri et alii, 2001; 2003; 2004): Above the Messinian evaporites, brownish marls, micritic-and brecciated-lime-stones rest unconformably on an erosional surface {the intra-Messinian unconformity). The NRD measurements have been carried out in three quarry-sections: 1) the Buzzi-Unicem (Trovigliano); 2) the Lafargessi (Trovigliano); 3) the Colle di Votta (Abbateggio). Combining the stratigraphy of these different sections, a composite stratigraphic column, which starts from the pre-evaporitic euxinic shales and up to the base of the post-evaporitic deposits, can be reconstructed. For each quarry-sections, the field gamma-ray profile has been obtained with a 20 cm regularly-spaced measurement of the gamma ray emission. Field NRD measurements and spectral acquisition have been done by using the portable spectrometer "Rainbow 7010 MCA", equipped with a gamma scintillator having a NaI (Tl) 2"X2" crystal as detector. Different NRD values, in terms both of count per second (Cps) measured in the field and of U <sub>ppm</sub> , Th <sub>ppm</sub> , %K <sub>2</sub> O from laboratory analyses, allow us to recognize some NRD facies, which correspond to the main lithofacies within the Messinian deposits of the Maiella Mts. In terms of NRD values we have been able to distinguish the following facies (fig. 8): a) limestone--20 Cps, 2.58 U <sub>ppm</sub> , 0.33 Th <sub>ppm</sub> , 1.01 %K <sub>2</sub> O; b) massive selenite with carbonate--11.1 Cps, 0.74 U <sub>ppm</sub> , no detectable Th <sub>ppm</sub> , 0.38 %K <sub>2</sub> O; c) massive selenite--7.13 Cps, 0.30 U <sub>ppm</sub> , no detectable Th <sub>ppm</sub> , 0.40 % K <sub>2</sub> O; d) giant selenite--6.95 Cps, 0.07 U <sub>ppm</sub> , 0.08 Th <sub>ppm</sub> , 0.33 %K <sub>2</sub> O; e) euxinic shale--49 Cps, 4.76 U <sub>ppm</sub> , 1.19 Th <sub>ppm</sub> , 13.48 %K <sub>2</sub> O; F) sapropel--33 Cps, 1.04 U <sub>ppm</sub> , 1.52 Th <sub>ppm</sub> , 18.57 %K <sub>2</sub> O; g) branching selenite--8.3 Cps, 0.52 U <sub>ppm</sub> , 0.11 Th <sub>ppm</sub> , 1.16 %K <sub>2</sub> O; h) gypsarenites--9.1 Cps, 0.88 U <sub>ppm</sub> , 0.19 Th <sub>ppm</sub> , 1.16 %K <sub>2</sub> O; i)fine laminated gypsarenites--10.3 Cps, 0.95 U <sub>ppm</sub> , 0.43 Th <sub>ppm</sub> , 5.43 %K <sub>2</sub> O; l) brecciated limestone--42 Cps, 5.29 U <sub>ppm</sub> , 0.21 Th <sub>ppm</sub> , 0.89 %K <sub>2</sub> O. In the gamma-ray profiles (figs. 5, 6 and 7), the NRD signal allows us to distinguish each evaporitic cycle, characterized by low values of radioactivity, separated by sapropel horizons that show high gamma-ray emission. The gamma-ray profile of Colle di Votta section allows us to distinguish the Messinian evaporitic succession from the lower portion of the post-evaporitic Messinian deposits. In fact, the evaporitic and post-evaporitic Messinian deposits are characterized by different background values of the gamma-ray profile, which are about 10 Cps and 20-30 Cps, respectively. In terms of Th <sub>ppm</sub> /U <sub>ppm</sub> ratio, the main lithofacies of the Gessoso-Solfifera Fm show different values (tab. 1 and fig. 9b). Generally, sapropel are characterized by Th <sub>ppm</sub> /U <sub>ppm</sub> z 1. On the contrary, limestones and gypsum deposits are characterized by values of Th <sub>ppm</sub> /U <sub>ppm</sub> <<1. These differences in the Th <sub>ppm</sub> /U <sub>ppm</sub> ratio could be related to changes in the detrital input into the sedimentary basin. In this frame, values of Th <sub>ppm</sub> /U <sub>ppm</sub> <<1 could be referred to decrease in the input of extra-basinal detrital grains and to the formation of authigenic U-minerals (Adams & Weaver, 1958). On the contrary, Th <sub>ppm</sub> /U <sub>ppm</sub> > or =1 could correspond to an increase in the input of detrital grains into the sedimentary basin. Changing of the Th <sub>ppm</sub> /U <sub>ppm</sub> ratio in the stratigraphic section could also indicate alternation between humid (Th <sub>ppm</sub> /U <sub>ppm</sub> > or =1) and arid (Th <sub>ppm</sub> /U <sub>ppm</sub> <<1) conditions. In conclusion, NRD analyses could supply information about several parameters connected with the evolution of sedimentary basin, such as: abundance of organic matter, redox condition, detrital input, climatic changes, etc. Thus, NRD field and laboratory investigations on stratigraphic sections could be a useful tool for basin analysis.

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