Permian–Triassic boundary interval in the Abadeh section of Iran with implications for mass extinction: Part 1 – Sedimentology

Abstract

The uppermost Permian strata of the Abadeh section in Iran consist of 56 m of skeletal limestone (Abadehian), that grades upward into 18 m of grey, bioturbated lime mudstone (Djulfian), which in turn grades upward to 18 m of red, nodular wackestone (Dorashamian) containing an abundant pelagic fauna. The overlying lowermost Triassic includes two enigmatic layers at its base. Immediately above the boundary is a 1-m-thick layer of inorganically precipitated synsedimentary carbonate cement. The cement is composed of 10–20-cm-long crystals oriented either perpendicular to the bedding or as dome-shaped features resembling botryoids. About 0.5 m of wackestone overlies the cement layer. The second layer is a 1.5-m-thick grainstone overlying the cement horizon and consists of recrystallised spherical grains (ooids or peloids). The remainder of unit ‘A’ is composed of 100+ m of grey, bioturbated to nodular lime mudstone. The lithofacies succession of the uppermost Permian is interpreted to represent deposition under increasing water depth, related to rising of relative sea level, which led to drowning of the carbonate platform to below storm wave base. Sedimentologic characteristics, lithology, wide distribution, and slow sedimentation rates indicate that strata immediately below the Permian–Triassic (P–T) boundary (Dorashamian interval) were deposited in deep, oxygenated waters. The enigmatic lowermost Triassic synsedimentary carbonate cement and grainstone layers are interpreted to have been deposited in shallow waters, indicating a rapid and major drop in relative sea level at the end of Permian time in this area, followed by a relative sea-level rise during the earliest Triassic. Precipitation of synsedimentary cement immediately above the P–T boundary in Iran and elsewhere (South China) suggests a global change in ocean chemistry. One possible explanation is that dissolved calcium and bicarbonate concentrations in seawater increased sufficiently to promote spontaneous carbonate precipitation, a rare event in the Phanerozoic but common during Precambrian time. Alternatively, a massive heating event could have resulted in CO2 degassing and decreases in oceanic carbonate solubility, both leading to inorganic, synsedimentary carbonate precipitation. Inorganic precipitation of synsedimentary carbonate cement may indicate that biochemical production of carbonate was halted due to the P–T boundary events. Our study indicates that shallow and moderately deep waters during the latest Permian were well oxygenated in the open ocean setting of the central Tethys Sea in Iran. Therefore, P–T mass extinction scenarios that invoke upon widespread marine anoxia should be viewed with caution.