Introduction. The chronometric calibration of the standard global chronostratigraphic scale has undergone a major revolution during the first half of this Century, and is still showing substantial changes in the last few decades (e.g., Vai, 1975; Harland et al., 1982; Menning, 1988; Odin, 1994). Particularly relevant are the variations in chronometric estimates of Neogene stage boundaries such as the Tortonian/Messinian (T/M), and Miocene/Pliocene boundaries, which have resulted from improved high-resolution and interdisciplinary stratigraphic research. The most dramatic change occurred at the transition from the Kulp (1960, 1961) scale (with a figure of slightly more than 13 Ma) to the Phanerozoic Time Scale (PTS) 1964 of Harland et al. (1964) in which to the base of the Pliocene was assigned an age of about 7 Ma. This is remarkable when compared with the chronometric estimate for the base of the Miocene, which is practically unchanged since at least Holmes (1947) (see Harland et al., 1982). The later stratigraphic use registered a trend toward even younger age estimates, up to 6.2 Ma, mainly based on both the assumption that the duration of the Messinian Age should be very short, and the first attempts at calibrating the T/M boundary, taken as the first appearance datum (FAD) of Globorotalia conomiozea, with the magnetostratigraphic time scale (Ryan et al., 1974; Hsti, 1986b). The youngest figure in this trend was reached by Langereis et al. (1984) with 5.6 Ma. The once widely used time scale of Berggren et al. (1985b) suggested an age estimate of 6.5 Ma. In such a state we considered it to be highly desirable to search for possible direct dating of the T/M boundary in biostratigraphically well studied, continuous, and complete marine sections in the Italian Peninsula, which as a whole (Northern Apennines, Calabria and Sicily) is the place of all the historical stage stratotypes for the Middle Miocene and the Pleistocene. New, detailed stratigraphic and tectonic surveys of the Vena del Gesso Basin (Northern Apennines, Romagna region, Imola to Faenza area), aimed at resolving the still open problems of the Mediterranean salinity crisis (Selli, 1960; Vai and Ricci Lucchi, 1977), allowed the recognition of a sequence of thin volcanogenic biotite-rich horizons. They are intercalated within an increasingly anoxic mudstone sequence spanning the T/M boundary across the entire basin (Castellari, 1989; Calieri, 1992). Actually, the first record of a stratigraphically undifferentiated Tortonian volcano-sedimentary layer, found almost at the top of the Marnoso-Arenacea formation immediately west of the Faenza-Imola area, dates back to Gandolfi et al. (1983). In this paper we report and discuss in some detail the preliminary results published by Vai et al. (1992, 1993), and compare them with some new measurements closely bracketing the T/M boundary, thus allowing a cross check of the original geochronologic results. In the meantime similar volcano-sedimentary layers have been found in the same time interval some 100 km to the east (Coccioni et al., 1992) and some 300 km to the south (D. Cosentino, pers. commun., 1993), suggesting a regional importance for this volcanic cycle. Independent dating of biotite-rich horizons from the original locality of Gandolfi et al. (1983) showed a good agreement (Odin et al., Chapter E6) with the dating by Vai et al. (1993). Our radiometric results have been obtained in the frame of integrated and detailed litho- and cyclostratigraphic (Vai, Chapter E3), biostratigraphic (benthonic and planktonic Foraminifera, and calcareous nannofossils), mineralogic, isotopic, and magnetostratigraphic assessments (Negri and Vigliotti, Chapter E4). Before listing our analytical results, we consider a present review of the previous, most important chronometric calibration of the T/M boundary, which we consider not only useful but crucial for a correct understanding of the new calibration provided by our measurements.

New radiometric datings bracketing the tortonian/messinian boundary in the romagna potential stratotype sections (Northern apennines, Italy)

Laurenzi MA;Tateo F;
1997

Abstract

Introduction. The chronometric calibration of the standard global chronostratigraphic scale has undergone a major revolution during the first half of this Century, and is still showing substantial changes in the last few decades (e.g., Vai, 1975; Harland et al., 1982; Menning, 1988; Odin, 1994). Particularly relevant are the variations in chronometric estimates of Neogene stage boundaries such as the Tortonian/Messinian (T/M), and Miocene/Pliocene boundaries, which have resulted from improved high-resolution and interdisciplinary stratigraphic research. The most dramatic change occurred at the transition from the Kulp (1960, 1961) scale (with a figure of slightly more than 13 Ma) to the Phanerozoic Time Scale (PTS) 1964 of Harland et al. (1964) in which to the base of the Pliocene was assigned an age of about 7 Ma. This is remarkable when compared with the chronometric estimate for the base of the Miocene, which is practically unchanged since at least Holmes (1947) (see Harland et al., 1982). The later stratigraphic use registered a trend toward even younger age estimates, up to 6.2 Ma, mainly based on both the assumption that the duration of the Messinian Age should be very short, and the first attempts at calibrating the T/M boundary, taken as the first appearance datum (FAD) of Globorotalia conomiozea, with the magnetostratigraphic time scale (Ryan et al., 1974; Hsti, 1986b). The youngest figure in this trend was reached by Langereis et al. (1984) with 5.6 Ma. The once widely used time scale of Berggren et al. (1985b) suggested an age estimate of 6.5 Ma. In such a state we considered it to be highly desirable to search for possible direct dating of the T/M boundary in biostratigraphically well studied, continuous, and complete marine sections in the Italian Peninsula, which as a whole (Northern Apennines, Calabria and Sicily) is the place of all the historical stage stratotypes for the Middle Miocene and the Pleistocene. New, detailed stratigraphic and tectonic surveys of the Vena del Gesso Basin (Northern Apennines, Romagna region, Imola to Faenza area), aimed at resolving the still open problems of the Mediterranean salinity crisis (Selli, 1960; Vai and Ricci Lucchi, 1977), allowed the recognition of a sequence of thin volcanogenic biotite-rich horizons. They are intercalated within an increasingly anoxic mudstone sequence spanning the T/M boundary across the entire basin (Castellari, 1989; Calieri, 1992). Actually, the first record of a stratigraphically undifferentiated Tortonian volcano-sedimentary layer, found almost at the top of the Marnoso-Arenacea formation immediately west of the Faenza-Imola area, dates back to Gandolfi et al. (1983). In this paper we report and discuss in some detail the preliminary results published by Vai et al. (1992, 1993), and compare them with some new measurements closely bracketing the T/M boundary, thus allowing a cross check of the original geochronologic results. In the meantime similar volcano-sedimentary layers have been found in the same time interval some 100 km to the east (Coccioni et al., 1992) and some 300 km to the south (D. Cosentino, pers. commun., 1993), suggesting a regional importance for this volcanic cycle. Independent dating of biotite-rich horizons from the original locality of Gandolfi et al. (1983) showed a good agreement (Odin et al., Chapter E6) with the dating by Vai et al. (1993). Our radiometric results have been obtained in the frame of integrated and detailed litho- and cyclostratigraphic (Vai, Chapter E3), biostratigraphic (benthonic and planktonic Foraminifera, and calcareous nannofossils), mineralogic, isotopic, and magnetostratigraphic assessments (Negri and Vigliotti, Chapter E4). Before listing our analytical results, we consider a present review of the previous, most important chronometric calibration of the T/M boundary, which we consider not only useful but crucial for a correct understanding of the new calibration provided by our measurements.
1997
0-444-82498-7
Tortonian/Messinian boundary
stratotype section
40Ar-39Ar dating
Northern Apennines
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/20.500.14243/233582
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