Microbialites and associated facies of the Late Ordovician system in Thailand: paleoenvironments and paleogeographic implications

Abstract

Late Ordovician limestones of the Pa Kae and uppermost Tha Manao Formations (approximately mid-Sandbian to late Katian Stages) of western Sibumasu Terrane in Thailand comprise micritic limestone with abundant bioclasts of fragile fossils, in deep water facies. Both formations have a distinctive Fe–Mn-rich polygonal network vein system containing bioclasts and nodules of micrite. Pa Kae limestones, previously interpreted as rich in microbialites, contain small agglutinated stromatolites but lack calcimicrobes and cements. Unlaminated domes are also present, consistent with the leiolite type of microbial fabrics, other facies are non-microbial micrites. No other microbialite forms were found, despite previous reports of thrombolites and oncolites. The Tha Manao Formation contains no microbialites, evidence that network veins are not genetically related to microbialite growth; in both formations, the network veins formed later. The Thai limestones partly overlap, in age and environmental setting, to mid-Sandbian to early Katian Stage Pagoda Formation (Yangtze Platform, south China), which also possesses a network of veins. A Pagoda Formation sample examined in comparison reveals its veins’ structure to have formed in place by selective replacement of the micritic host rock by opaque matter while enclosed fossils and exotic intraclasts were unaffected. Some authors view the Pagoda Fm and Thai limestones as non-uniformitarian “time-specific facies”. However, they (a) do not fully coincide stratigraphically, and (b) are each diachronous. Furthermore, other time-constrained unusual facies occur in the rock record (e.g., unique microbialite facies in south China after the end-Permian extinction; Ammonitico Rosso facies of Jurassic to Early Cretaceous in Italy) that are not regarded as being non-uniformitarian. Thus, a uniformitarian approach is more appropriate to understand these unusual Ordovician facies, which may relate to early sea-floor partial cementation creating a solid mass that could be fractured to allow pathways of migrating fluids for vein material to develop.