Baud, Aymon

Serveur académique lausannois

23Lucas, S.G. and Spielmann, J.A., eds., 2007, The Global Triassic. New Mexico Museum of Natural History and Science Bulletin 41.LOWER TRIASSIC MICROBIALITES VERSUS SKELETAL CARBONATES,A COMPETITION ON THE GONDWANA MARGINAYMON BAUDGeological Museum, Lausanne, SwitzerlandAs shown by Baud et al. (2005, 2007), a major crisis occurred in Phanerozoic carbonate systems during the end-Permian mass extinction that involved a wholescale change in oceanic geochemistry. The prolific upper Paleozoicskeletal carbonate factory was abruptly replaced by a non-skeletal carbonate factory (Baud, 1998). When preservedbetween the two carbonate systems, the boundary is marked by a post-extinction clay (boundary clay) of latestPermian age (preparvus-meishanensis conodont zone). Microbial communities affected sedimentation in a variety ofnormal marine areas (Baud et al., 1997).On the Cimmerian margin and on the Gondwana margin, from Zagros through Taurus westward, a post-extinc-tion calcimicrobial unit occurs above the extensive Permian skeletal carbonate platform exposed in the shallow, lowenergy post-extinction carbonate ramp with massive thrombolitic mounds and/or stromatolites. To the east, on theGondwana margin in Oman and North India, the calcimicrobial unit is more discrete or absent as shown by the Wasitblock (Krystyn et al., 2003), where the skeletal carbonate is entirely building the basal Triassic limestones. In the SaltRange (Pakistan), with deepening of depositional environment, the basal Triassic crinoid-brachiopod grainstones areprogressively replaced by ammonoid-bivalve packstone-wackestone (Wignall and Hallam, 1993). For these authors,the matrix of the limestone beds in the Ceratite marls is made of  disintegrated shells of the bivalve Leptocondriaminima. A similar situation exists in Kashmir with deeper limestone deposits. There are no visible microbial texturesseen or published in the Lower Triassic of Zanskar and the Spiti areas (Western Tethys Himalaya), but more researchis needed.In a Tethys Himalaya section of Central Nepal, Bassoullet and Colchen (1976) report a planar stromatolite bedof half a meter thickness in their “coupe 2” of Ganesh Himal (probable Dienerian age), between skeletal (bivalves-ammonoids) lime wackestone to packstone beds.To the east, in the Tulong section of South Tibet, isolated stromatolite buildups (cm to dm) have grown on the topof ammonoid wackestone-packstone beds (Smithian age: H.Bucher, oral communication). In this section, the Spathianred ammonoid limestones show levels with stromatactis and thrombolitic texture, but the microbial activity seemsincidental compared to the skeletal carbonate production (mainly ammonoids and bivalves).In summary, in the southern latitudes of the Gondwana margin, in contrast to the low latitudes and equatorialdomains, the environments of deposition are more favorable to the skeletal organisms, which are in competition withthe microbial carbonate factory, but the result is a very low rate of deposition (condensed deposit).REFERENCESBassoullet, J. P. and Colchen, M., 1976, La limite Permien-Trias dans ledomaine tibétain de l’Himalaya du Népal., in Jest, C., editor, Himalaya:Colloques internationaux du CNRS: Sèvres-Paris, CNRS, p. 41-52.Baud, A., 1998, Marine carbonate and siliceous factories: global Changeafter the end of Permian Mass Extinction, in Abstract book, 15th Inter-national Sedimentological Congress: Alicante (Spain), p. 180.Baud, A., Cirilli, S., and Marcoux, J., 1997, Biotic response to mass extinc-tion: the Lowermost Triassic microbialites: Facies, v. 36, p. 238-242.Baud, A., Richoz, S., and Pruss, S., 2007, The lower Triassic anachronisticcarbonate facies in space and time: Global and Planetary Change, v. 55,p. 81-89.Krystyn, L., Richoz, S., Baud, A., and Twhichett, R. J., 2003, A uniquePermian-Triassic boundary section from the Neotethyan Hawasina Ba-sin, Central Oman Mountains: Palaeogeography PalaeoclimatologyPalaeoecology, v. 191, p. 329-344.Wignall, P. B., and Hallam, A., 1993, Griesbachian (Earliest Triassic)palaeoenvironment changes in the Salt Range, Pakistan and southeastChina and their bearing on the Permo-Triassic mass extinction:Palaeogeography, Palaeoclimatology, Palaeoecology, v. 102, p. 215-237.

Lower Triassic Microbialites versus Skeletal carbonates, a competition on the Gondwana margin

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Lower Triassic Microbialites versus Skeletal carbonates, a competition on the Gondwana margin

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