Linwe Fm

Shan South Plateau, the type section of the Linwe Fm is designated to the west of the monastery in Linwe village. The unit is 525 m thick in the type section, 305 m in the Ye-ngan area (Garson et al. 1976), 35–150 m in the Taunggyi town area (Wolfart et al. 1984), 560 m in the Linwe area (Htun Naing Zaw 2005) and 420 m in the Pegin area north of Linwe (Maung Maung 2005). [Original Publication: Barber, A. J., Khin Zaw & Crow, M. J. (eds) 2017. Myanmar: Geology, Resources and Tectonics. Geological Society, London, Memoirs, 48, 317-342]

Synonym: Wabya Fm (formerly)

Clayey limestone. The formation consists of limestone and shale. The lithology varies greatly in most parts of the Shan State South. In the Linwe–Pegin area, the Lowermost part of the unit is dominated by a sequence of purple or grey calcareous shale, with some argillaceous limestone intercalations bearing star-shaped cystoid plates followed, in the Middle part, by medium- to thick-bedded limestones of nodular appearance, often interbedded with the graptolite-bearing shale and micaceous siltstones. In the Upper part, the unit is characterized by a light brown to grey, calcareous, micaceous, bioturbated shale–siltstone–marl sequence, intercalated with greenish-grey argillaceous micritic limestone.

The lithology of the unit in the Myozo area is somewhat different from that of the Pegin-Linwe area. Maung Maung (2005) recognized Volcaniclastic flows in the lower part of the Linwe Fm, the basal layer of which is inversely graded, with large blocks floating in ash. The lithology in the upper part is not much different from that in the Pegin–Linwe and Wabya areas. There are micaceous shales in the Linwe Fm in the Wabya and Phothudaw hill areas, Pindaya township, described in unpublished MSc studies by Chit Sein and Ohn Mar Soe Yin.

The limestone of the Linwe Fm contains some nautiloids and crinoids which are not yet systematically identified. The Linwe Fm has yielded a large number of large crinoid stems in the phacoidal structured limestone, the conodonts Ozarkodina remscheidensis, Ozarkodina sp., Panderodus unicostatus and Dapsilodus obliquicostatus (Maung Maung & Aye Ko Aung 2009) and the nautiloids Michelinoceras spp. The latter and Aka have identified the following graptolites: Normalograptus cf. trifilis, N. normalis, Monograptus priodon, M. elongatus, Campograptus lobiferus, Coronograptus cyphus, C. cf. gregarius, Stimulograptus cf. sedgwickii, Monoclimacislunata, P. regularis, P. variabilis, Rastrites rostrum, R. approximatus, Demirastrites raitzhainiensis, D. triangulatus, Lituigraptus convolutus, Pseudoglyptograptus rigidus, Petalolithus tenuis and P. folium. Six Llandovery graptolite biozones have therefore been recognized in Myanmar (in ascending order): Rhuddanian Coronograptus cyphus Zone; Aeronian Coronograptus gregarius Zone, Demirastrites convolutus Zone and Stimulograptus sedgwickii Zone; and Telychian Spirograptus turriculatus Zone and Monoclimacis griestoniensis Zone. The trilobites Phacops sp. and Calymene sp. were recovered from the silty sandstone.

[Figure: Silurian fossils of the Linwe Formation. (a–d) Michelinoceras sp.: (a) longitudinal section; (b) transverse section; and (c, d) surface specimens; (e) indeterminate dalmanitid; (f) Monograptus priodon (Bronn); (g) Phyllograptus sp.; (h) Normalograptus normalis (Lapworth); (i) Cyrtograptus sp.; (j) Neoglyptograptus sp.; and (k) Pristiograptus variablis (Perner) (after Aung&Cocks, 2017)]

The depositional environment of the Linwe Fm is interpreted as peritidal, protected inner ramp/lagoon, deeper ramp, marginal shoal and offshore crinoidal bank environments (Maung Maung 2005).

Apart from the type locality, the Linwe Fm is fairly widely distributed in Shan State South, including: in Ye-ngan township, Linwe, Pegin, Kyauktaw, Myaing, Alegyaung; in Pindaya township, Shwepahtoe, Kya-in-kan, Konlon, Zawgyi, Myinkyawkan, Thayetpya, Bawzon Taung and Wabya; in Kalaw township–Bawsaing Range, Bawsaing, Naung-Lwe, Kyauktap, Nyaunggaing, Myozo; in Taunggyi township, Taungni and Taunggyi town area; and Méné Taung, Hopong Township.