224 / 2019-06-27 16:39:06

Time specific facies: the red ammonoid limestone at the Paleozoic-Mesozoic turnover and the Tethyan Redox evolution

Neotethys, Gondwana margin, Cimmerian margin, exotic blocks, red ammonoid limestone, Wordian, Changhsingian, Smithian

“Time-Specific Facies” (TSF) refers to unusual facies occurring over large paleogeographic domains during a geologically short time interval. These are generally close in time with important biotic changes and crises, and/or coincide with abiotic changes of large to global scope. Basal Early Triassic shallow water (0-60m) equatorial microbialites is a classic example of such TSF. Red ammonoid-bearing limestone, such as the “Griotte” during the Devonian, the “Han Bulog” during middle Triassic, the “Hallstatt” during the late Triassic and the “Ammonitico Rosso” during intervals of the Jurassic, have a long duration and are not included among TSF. Here, we focus on red ammonoid-bearing limestone around the Paleozoic-Mesozoic turnover, which are much less well known.
The Middle Permian (Wordian) red ammonoid limestone was first described from Sicily, Timor and subsequently from Oman. The paleogeographic distribution of this red limestone is apparently confined to the Gondwana margin of the Neotethys. Recently, we added two new oceanic occurrences of middle Permian red limestone deposited on seamounts, a first one from the Oman Mountains (Buday’ah, BH) and a second one from Arrow Rocks (AR), Northland, New Zealand, thus extending the same TSF from Silcily to Pantalassa and strengthening its relation with the opening of the Neotethys to the East and its western Mediterranean dead-end. Cessation of this TSF coincides with a rise of the CCD manifested by deposition of radiolarian cherts in BH and AR, but with deposition of siliciclastics in other sections.
After a red ammonoid limestone gap, the next TSF episode occurred during end-Permian time on the South Cimmerian margin (central Iran through, north-west Iran, south Armenia). This red “Paratirolites Member”, 4 to 17 m. thick, records a very complete ammonoid and conodont succession of late Changhsingian age. It is capped by the usually red “boundary shales” of very variable thickness and with occasional extreme condensation involving both Permian and Triassic faunas. This TSF ceased abruptly with the global Permian-Triassic boundary global regression, the associated hiatus on shelves and the mass extinction. The “boundary shales”, which may at least partly include part of the PTB global gap, are in turn overlain by light gray platy limestone with sponge-microbial buildups at its base. Further lateral extension to the West or to the East of this end-Permian TSF is not yet known, perhaps due to a preservation bias.
Unknown during lower Triassic Induan times, deposition of red ammonoid-bearing limestone resumed during the Smithian (lower Olenekian) on the Gondwana margin of the Neotethys. It typically occurs as exotic blocks in Oman, the Indus suture zone in the Himalayas, Timor and share the same succession of ammonoid and conodont zones. In West Timor, the Basleo and Noe Tobe exotics where deposited on rifted and drifted off shore seamounts. These are reworked and redeposited within thick series Neogene deep water shales belonging to North Australian accretionary prism. In Oman, this TSF is also found as exotic blocks embedded into late Triassic to Jurassic olistostromes belonging to the Hawasina nappes. Along the Himalayan Indus Suture zone, the Lamayuru exotic block consists of Late Permian to basal Triassic carbonate and volcanic rocks. There, the red ammonoid limestone occurs as fissure fillings cutting through the top of Permian shallow water limestone. According to the literature, other exotic blocks of Smithian red limestone were described from the North India Amlang-La and Kiogar areas of the Western High Himalayan Range. The Smithian TSF is interpreted to have been deposited on isolated seamounts, or on submarine rises and plateaus. Its paleogeographical distribution ranges from the eastern to the western part of the Gonwana margin. However, this Smithian TSF of short duration (<< 1 Ma) is not recorded from the European side, suggesting that new rifts did not open until middle Spathian times in this area. The early and middle Smithian TSF identical successions of ammonoid and conodont zones also indicate that it belonged to a single, much larger biogeographic domain also encompassing classic shelf depositional environments of the Gondwana margin and to a lesser extent, some transit blocks such as South China. But this TSF is so far unknown from the Cimmerian transit blocks.
The pattern in time-space distribution of these red limestones changed from the Spathian onward. They became either locally persistent during long time intervals (e.g. until the end of the Triassic as in Wadi Alwa, Oman exotics, from the late middle Anisian to late Triassic in the classic Hallstatt area, Austria), or became scattered in time and time space. Numerous examples include, for instance, the middle Spathian from Chios Island (Greece) and Karuburun Peninsula (Turkey), part of the middle Spathian from the Luolou Fm in the Nanpanjiang Basin (Yangtse Block, S. China), the mid- and late Spathian from part of Tethys Himalaya, Iran and Albania, the late Spathian to Anisian Han Bulog from Dobrudgea (Romania), the late middle Anisian to early late Anisian from Hydra Island (Greece), the middle and late Anisian Han Bulog from Croatia and the late middle Anisian to Ladinian from the Asklepeion (Peloponesus, Greece). Therefore, from the Spathian onward, red ammonoid-bearing limestones do not fit anymore with the definition of a TSF. They became progressively restricted in their lateral extension throughout the Spathian and reached a minimum of lateral continuity and a maximum of local longevity during Middle and Late Triassic, thus indicating a stepwise geographical partitioning of changes in redox conditions within the Tethys s.l.