Le facies evaporitiche della crisi di salinità messiniana: radioattività naturale della Formazione Gessoso-Solfifera della Maiella (Abruzzo, Italia centrale)

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-limestones rest unconformably on an erosional surface (the intra- Messinian unconformity). The NRD measurements have been carried out in three quarrysections: 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″×2″ crystal as detector. Different NRD values, in terms both of count per second (Cps) measured in the field and of Uppm, Thppm, % K2O 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 Uppm, 0.33 Thppm, 1.01 % K2O; b) massive selenite with carbonate – 11.1 Cps, 0.74 Uppm, no detectable Thppm, 0.38 % K2O; c) massive selenite – 7.13 Cps, 0.30 Uppm, no detectable Thppm, 0.40 % K2O; d) giant selenite – 6.95 Cps, 0.07 Uppm, 0.08 Thppm, 0.33 % K2O; e) euxinic shale – 49 Cps, 4.76 Uppm, 1.19 Thppm, 13.48 % K2O; f) sapropel – 33 Cps, 1.04 Uppm, 1.52 Thppm, 18.57 % K2O; g) branching selenite – 8.3 Cps, 0.52 Uppm, 0.11 Thppm, 1.16 % K2O; h) gypsarenites – 9.1 Cps, 0.88 Uppm, 0.19 Thppm, 1.16 % K2O; i) fine laminated gypsarenites – 10.3 Cps, 0.95 Uppm, 0.43 Thppm, 5.43 % K2O; l) brecciated limestone – 42 Cps, 5.29 Uppm, 0.21 Thppm, 0.89 % K2O. 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 Thppm/Uppm ratio, the main lithofacies of the Gessoso- Solfifera Fm show different values (tab. 1 and fig. 9b). Generally, sapropel are characterized by Thppm/Uppm ≥ 1. On the contrary, limestones and gypsum deposits are characterized by values of Thppm/Uppm <<1. These differences in the Thppm/Uppm ratio could be related to changes in the detrital input into the sedimentary basin. In this frame, values of Thppm/Uppm <<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, Thppm/Uppm ≥ 1 could correspond to an increase in the input of detrital grains into the sedimentary basin. Changing of the Thppm/Uppm ratio in the stratigraphic section could also indicate alternation between humid (Thppm/Uppm ≥ 1) and arid (Thppm/Uppm <<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.

È stata misurata la radioattività naturale (NRD) di tre successioni stratigrafiche del Messiniano nella Maiella nord-occidentale (Trovigliano-Abbateggio, PE). Complessivamente, le tre successioni analizzate (cava Buzzi-Unicem; cava Lafargessi; cava Colle di Votta) comprendono tutto il Messiniano evaporitico e la base di quello postevaporitico. I differenti valori di radioattività misurati corrispondono alle principali litofacies che caratterizzano la Formazione Gessoso- Solfifera affiorante nell’area di studio. In particolare, il profilo dell’emissione gamma, acquisito sul terreno, mostra picchi di attività in corrispondenza delle argille euxiniche, alla base della successione evaporitica, e in corrispondenza degli orizzonti sapropelitici intercalati alle bancate di gesso. All’interno di queste ultime, la bassa radioattività tende ad aumentare in presenza di componente carbonatica, che può costituire la matrice delle bancate di gesso e/o può essere presente in livelli o strati. I valori maggiori di radioattività sono stati misurati nei depositi messiniani post-evaporitici, in corrispondenza di marne laminate di colore marrone e di calcari brecciati. Il contributo dei radioelementi naturali (238U, 232Th e 40K) varia nelle diverse litofacies analizzate. In particolare, mentre i sapropel sono caratterizzati da rapporti Thppm/Uppm ≥ 1, i gessi e le facies carbonatiche hanno rapporti Thppm/Uppm <<1. I diversi rapporti Thppm/Uppm sono stati messi in relazione con la variazione dell’apporto detritico nel bacino di sedimentazione e con la possibile presenza di minerali autigeni dell’uranio.