Differential severity of Permian–Triassic environmental changes on Tethyan shallow-water carbonate platforms

Abstract

Significantly different Guadalupian–Late Triassic patterns are observed in the evolution of attached and isolated carbonate platforms of the southwestern Tethys (Oman Mountains). Close to the rim of the attached Arabian platform, carbonates of the Saiq and Mahil Formations reveal an almost complete Permian–Triassic sedimentary record. Guadalupian–Changhsingian 3rd order sequences consist of fossiliferous transgressive systems tracts and monotonous highstand systems tracts with mud/wackestone and coral bafflestone. The youngest Changhsingian beds are bioturbated floatstone with crinoids, sponges and bryozoans. All sediments indicate a healthy, tropical carbonate production. Above, a unique facies change begins with a pyrite-encrusted omission surface. Greenish mudstone rich in authigenic pyrite infills the relief of the unconformity and is overlain by clastic sediment and by laminated, microbialite-bearing carbonate. Unfossiliferous sediments and seafloor cements indicate a change in carbonate production towards abiotic processes. Prevailing anoxic conditions were interrupted by seven oxic event beds, as indicated either by low-diversity and small-sized ichnotaxa or by shell beds with low-diversity bivalve and crinoid assemblages. By comparison with published data, the described sedimentary sequence can be assigned to the Changhsingian–earliest Griesbachian. Beginning probably with the Anisian, bioturbated Griesbachian–Dienerian recovery period and the unconformity below to the latest grain-supported sediment textures mark the return to biogenic tropical carbonate production under oxic conditions. The Middle–Late Triassic carbonate platform consists of stacked high-frequency shallowing upward cycles. By contrast, carbonate production of Neo-Tethyan isolated platforms was discontinuous and interrupted by a large gap. Guadalupian deposits of the Al Jil Formation consist of bioclastic limestone typical of a tropical carbonate production. The uppermost bed, an impoverished bioclastic packstone capped by an unconformity, marks the onset of platform drowning which resulted from the end-Guadalupian mass extinction. Above, a polymict breccia witnessed rift pulses of the Neo-Tethys. The overlying pelagic mud- and packstone contains radiolarians and rare foraminifera of Lopingian age, and overlying microbialites. In the Carnian, tropical shallow-water carbonate production restarted with a low-relief platform and culminated in a Norian–Rhaetian reef-rimmed platform. Stacked Lofer cycles dominated the inner platform of Jebel Kawr (Misfah Formation). We here propose a differential onset and severity of the Late Permian mass extinctions for carbonate platforms. On the Arabian Plate, tropical carbonate production collapsed after the end-Lopingian mass extinction and was replaced by microbialites and sea-floor cements during the earliest Triassic. After approximately six million years, tropical shallow-water carbonate production resumed in the Middle Triassic. Neo-Tethyan isolated platforms drowned shortly after the end-Guadalupian mass extinction and did not recover before the Late Triassic. Absence of shallow-water limestone suggests that carbonate production of isolated platforms ceased for about 30 million years, a period exceeding the recovery of most marine ecosystems.