Late miocene magnetobiostratigraphy of Crete

Abstract

Six Upper Miocene marine clay sections on Crete (Greece) have been subjected to a detailed magnetobiostratigraphic analysis. Six correlatable polarity zones are recognized and these demonstrate the regional synchrony of planktonic foraminiferal biohorizons. By way of correlation to the magnetic polarity time-scale of Lowrie and Alvarez (1981), the Cretan sequence is assigned to polarity chronozones 5 (anomaly 3A) and 6. The new chronology provides an age of 5.6 Ma for the first occurrence datum (FOD) of the Globorotalia conomiozea group in the Mediterranean, an age of 6.0 Ma for the FOD of G. menardii form 5 and an age of 6.6 Ma for the last occurrence datum (LOD) of G. menardii form 4. Correlating the polarity record of the New Zealand Blind River section (Kennett and Watkins, 1974), with the magnetic polarity time-scale provides an age of 6.0 Ma for the evolutionary appearance of Globorotalia conomiozea, which is in complete agreement with the age of 6.1 ± 0.1 Ma given by Loutit and Kennett (1979). The demonstrated diachrony of 0.4 Ma between the New Zealand FOD of G. conomiozea and its Mediterranean counterpart is explicable in view of the different nature of the two events, the one in New Zealand being evolutionary and the one in the Mediterranean migrational. The FOD of the G. conomiozea group in the*Tortonian/Messinian boundary stratotype section coincides with the level proposed by Colalongo et al. (1979) to mark the base of the Messinian. Since the FOD of the G. conomiozea group in Crete and in Sicily are most probably time-equivalent, the age of the Tortonian/Messinian boundary is fixed at 5.6 Ma. The youngest sediments incorporated in this study extend into the Gilbert chronozone and antedate the main evaporitic phase. Consequently, the Messinian evaporitic body is younger than the base of the Gilbert chronozone, the age of which is fixed at 5.3 Ma. Adopting an age of 5.0 Ma for the Miocene/Pliocene boundary would imply that evaporites and post-evaporitic Lago Mare sediments were deposited in some 300,000 years and suggests that in the central parts of the Mediterranean basins evaporites must have accumulated at rates of some 3 m per 1000 years.