Permo-Triassic boundary and Lower to Middle Triassic in South Tibet
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文摘
The abundance of conodonts increases sharply across the Permian/Triassic boundary in South Tibet, and as a consequence Griesbachian assemblages are much better defined than Changxingian ones. The “Otoceras latilobatum bed”, representing the base of the Triassic at Selong, is a condensed biocalcirudite including abundant macrofossils (crinoids, corals, bryozoans, brachiopods) of Permian aspect associated with a varied conodont assemblage (including Hindeodus parvus) of early Griesbachian age. Condensation and reworking at the P/T boundary is interpreted as related to starvation and subaqueous erosion/redeposition during rapid transgression at the base of the Triassic rather than to subaerial exposure during a first-order sea-level drop. All of the Lower Triassic is strongly condensed, particularly in the west (Gyirong and Selong) where total thickness is 6.5m at most, to 8.5m; thickness is more comparable with other Himalayan localities at Tulong, where the Dienerian to Spathian section reaches 20m. As in central Nepal, carbonate intervals dated as early Griesbachian to Dienerian, early to mid-Smithian and latest Smithian to earliest Aegean are separated by two mudrock intervals (locally reduced to millimetric interbeds) dated as late Dienerian or earliest Smithian and late Smithian. Condensed wackestone/packstone intervals were deposited during transgressive stages on the outermost shelf to continental slope. During highstand stages, mudrock intervals slowly accumulated on the outer shelf, whereas only a thin veneer of mud was deposited close to the shelf-break. Oceanic currents, intruding onto the outermost shelf during peak transgressions, are inferred to be the main causes of winnowing and resuspension of mud during the Spathian, when oxidized packstones in “Ammonitico Rosso” facies were deposited at Tulong. Grey limestones locally with bacterial mats or containing agglutinated foraminifers and intercalated black mudrocks indicate instead dysoxic conditions, which were widespread from Griesbachian to Smithian times.