Ordovician geology of the Sibumasu Block, SE Asia

Abstract This paper describes the Ordovician stratigraphy of the Sibumasu Block, which formed part of equatorial peri-Gondwana during the early Paleozoic, and summarizes the palaeoenvironmental conditions that prevailed during that time. Upper Cambrian sequences are commonly dominated by coarse-grained clastic rocks of shallow-marine origin. The lithology of the Lower Ordovician varies from region to region, with fine-grained clastic rocks in the Shan Plateau, shallow-marine clastic rocks in northern to western Thailand and shallow-marine carbonate rocks in southern Thailand to northwestern Malaysia. Middle Ordovician strata consist entirely of shallow- to deeper-water carbonate rocks, with the exception of turbidites in midwestern Malaysia. Upper Ordovician rocks show an overall deeper environment, and the uppermost Ordovician consists of pelagic and deeper, fine-grained clastic rocks. On the basis of palaeobiogeographical data, the Sibumasu Block was adjacent to Western Australia throughout the early Paleozoic. Upper Cambrian rhyolitic rocks in the Shan Plateau and Lower–Middle Ordovician volcanics in midwestern Malaysia may have formed as a result of tectonomagmatism associated with the subduction of the Proto-Tethys Ocean plate into the northern Gondwanan margin. The close similarity of Late Ordovician fossil assemblages from Sibumasu and South China may be due to a sea-level rise during this time.