Evidence for ca. 560 Ma Ediacaran glaciation in the Kahar Formation, central Alborz Mountains, northern Iran

Abstract

We report evidence for possible Ediacaran (ca. 560Ma) glaciation from two localities of the late Neoproterozoic Kahar Formation in the central Alborz Mountains of northern Iran. Here, the Kahar Formation consists of about 1000m of siliciclastic sedimentary strata (sub-greenschist), with minor carbonate, which were deposited along the Peri-Gondwanan margin of Iran. The Kahar Formation represents some of Iran's oldest sedimentary basement exposures, but its base is not exposed anywhere. The lowest Kahar exposures in the study localities, which outcrop within anticlinorial cores ~120km apart, offer insights to northern Iran's Neoproterozoic evolution. Distinct diamictite units occur near the base of both sections and are consistent with glaciogenic diamictites on the basis of the following: (1) geometry, (2) apparent widespread lateral continuity, (3) sharp erosional basal contact, (4) and occurrence of striated surfaces in one locality (Kahar Mountain). Their textural and compositional characteristics also provide less diagnostic features. Detrital zircon U–Pb ages from samples underlying and overlying the diamictite units indicate maximum depositional ages of about 560–550Ma. Buff-gray microbial carbonate intervals of 9 to 17m thick, with moderately negative δ13C compositions (−5.4 to −1‰) and 87Sr/86Sr values between 0.70867 and 0.70884 (Kahar Mountain locality), conformably overlie the diamictite units in each locality. These C and Sr isotopic compositions are consistent with the rapid increase in 87Sr/86Sr during the late Ediacaran recovery from the Shuram negative δ13C excursion. Our combined isotope stratigraphy and detrital zircon ages suggest that the Kahar and overlying Soltanieh formations in Iran correlate with the Buah and Ara formations (Nafun Group) in Oman.