The Lower Paleozoic Stratigraphy of Western Part of the Southern Shan State, Burma

Abstract

Rocks of all periods of the Lower Paleozoic are exposed at the western part of the Southern Shan State, typically at the Pindaya and Bawsaing (formerly known as Mawson) ranges . The Cambrian rocks are recently discovered, the Ordovician and Silurian rocks have been systematically restudied and grouped into formal lithostratigraphical units. The Cambrian (Upper), Molohein Group proposed here as a new lithostratigraphic unit, is essentially made up of clastic sediments, and composed of slightly metamorphosed mica­ ceous, fine-grained, pinkish to brown sandstones, and light-colored quartzites as principal rock types, and coarse-grained, pinkish sandstones, grits, greywacke, conglomerates and dolomites as minor rock types. These rocks are exposed as the cores of Pindaya Range and Hethin Hill in Bawsaing Range. The discovery of Saukiella and related genera from the mica­ ceous sandstones enabled the assignment of the unit as Upper Cambrian. The thickness of the group is about 3,500 feet. The lower boundary of the unit in contact with the Chaungmagyi rocks of the pre-Cambrian age (La Touche, 1913) is unconformable, while the upper bound­ ary in contact with the lower boundary of the Lokepyin Formation (Ordovician) is grada­ tional. The Ordovician rocks of the Southern Shan State can conveniently be grouped into the Pindaya Group which includes the Pindaya Beds and Mawson Series of Brown and Sondhi (1933). The Pindaya Group, herein, could be differentiated into four newly proposed forma­ tions, viz., (from lowest to uppermost), Lokepyin Formation (essentially containing grey siltstones) , Wunbye Formation (essentially containing bedded limestones with burrowed structures and interbedded grey siltstones), Nan-on Formation (essentially containing yellow to buff color siltstones and mudstones, and Tanshauk Member (containing purplish shales and siltstones) of Nan-on Formation. The Silurian rocks are grouped under the newly proposed Mibayataung Group in whose contentions are Brown and Sondhi's Orthoceras Beds, Graptolite beds of Wabya, Kyauktap, and Mibayataung, Tentaculites beds, and other unnamed beds that occur between the Pin­ daya Group and Plateau Limestone. The Mibayataung Group is differentiated into Linwe Formation, Wabya Formation and Taungrningyi Member. Linwe Formation, a formal lithostratigraphic unit, proposed for substitution of Ortho­ ceras Beds of older usage, is made up of phacoidally textured, pink, purple and grey lime­ stones, calcareous mudstones, silty shales and shales. Michelinoceras in the limestones, and graptolites in the shales, are universally present. The Wabya Formation, overlying the Linwe Formation, contains the profusion of graptolite fauna. The shales are typically soft to sub­ indurated, micaceous, grey, light grey to light buff in color; black indurated shales and slates, which often pyritiferous are also common. Taungmingyi Member is introduced as a member of Wabya Formation, containing lens-shaped bodies of white quartzose sandstones in the shales, occurred near the top of the formation. Mineralization, especially lead and zinc, occurs in the rocks of the Pindaya Group. Vol­ canic activities, as evidenced by the occurrence of rhyolites , rhyolitic tuffs, ash and bentonitic beds within the Wunbye Formation and at the upper horizon of the Wabya Foramtion, could have been sporadically active between the Ordovician and Silurian Periods.