Gondwanan deglaciation and opening of Neotethys: the Al Khlata and Saiwan Formations of Interior Oman

New stratigraphic, petrographic and paleontological data from the Lower Permian of southeastern Arabia help to elucidate the still debated timing of Neotethys rifting and the climatic evolution following the Gondwanan glaciation. The transition from the Al Khlata to the Saiwan Formation, here described in detail, is interpreted to record continental break-up and onset of Neotethyan spreading between northern Gondwana and the Cimmerian terranes at mid-Sakmarian times, as constrained by brachiopod assemblages. The eastern Oman arm of the Neotethyan rift-system instead failed, and sea-floor spreading of the Indo-Madagascar trough was delayed until final fragmentation of Gondwana in the Late Jurassic. Petrographic composition of Lower Permian sandstones documents a distinct change from transitional to dissected rift-shoulder provenance, and widespread exposure of granitoid basement rocks at a late syn-rift to early post-rift stage. Very abundant pore-filling celestite at the Al Khlata/Saiwan transition may have precipitated from circulating brines related to remobilization of Proterozoic salt due to rift tectonics. The sharp base of the Saiwan Formation records a major transgression related to the onset of Neotethyan spreading, final deglaciation and consequent global sea-level rise, as indicated by ravinement surfaces associated with lag deposits with tree trunks, reworked ferruginous pedogenic concretions and phosphate, and by paleoecological analyses. The basal bed of the Saiwan Formation is characterized by the pioneer Pachycyrtella paleocommunity, with Pachycyrtella omanensis displaying the features of a typical paleo-opportunist species: random distribution pattern over a limited area, clustering in groups, numerical dominance ( >85%), suspension feeding, rapid rates of reproduction and growth (r-strategy), early maturity, high mortality rates in the juveniles. The more mature secondary ecological succession developed higher in the Saiwan Formation records maximum flooding and significant climatic amelioration. The Saiwan Formation, at the beginning of the Late Sakmarian, finally sutures the irregular rift-related topography in the rim basin of Interior Oman, and records a shift towards warmer temperate conditions after the end of the Gondwanan glaciation. (C) 2003 Elsevier Science B.V. All rights reserved.