Nyaungbaw Fm

Shan Mid Plateau, (Pyin Oo Lwin). The type section of the Fm is not yet defined and the thickness of the unit is estimated to be a few thousand metres, as seen in the section just west of the custom gate near the junction of Zebingyi and Mandalay–Lashio road. La Touche (1913) first used the name Nyaungbaw Limestone, but described the limestone as a local unit without giving a formal definition for the limestones exposed in the neighbourhood of Nyaungbaw village beside the cart-road between Mandalay and Pyin Oo Lwin. The Burmese National Committee (IGCP 1980) used the name Nyaungbaw Fm informally. The Nyaungbaw Fm is widely distributed in Pyin Oo Lwin township, although it is very restricted in Shan State North (La Touche 1913, p. 122). A thick sequence is seen in Pyin Oo Lwin township on the Mandalay–Pyin Oo Lwin road between the 21st Milestone and the junction with Zebingyi village; on the Mandalay–Lashio railway line near Luwa Taung, west of Zebingyi, to the east between Zebingyi and Kyaukpyado; and east of Kangyigon, Nyannyintha and north of Pyin Oo Lwin, Naungkangyi area. [Original Publication: Barber, A. J., Khin Zaw & Crow, M. J. (eds) 2017. Myanmar: Geology, Resources and Tectonics. Geological Society, London, Memoirs, 48, 317-342]

Limestone. The Nyaungbaw Fm consists of limestones which are medium to thick bedded, red, chocolate-brown passing into grey or blue, light green, ashy-grey and sometimes purple in color. Interbedded with the limestones are subordinate bands of red clay. The limestones themselves usually contain a large amount of argillaceous matter and show phacoidal or nodular structure.

The limestones of the Nyaungbaw Fm have yielded: the crinoids Scyphocrinus sp. (a calyx, ossicles and stemjoints) and Camarocrinus asiaticus; the bryozoan Diplotrypa palinensis; and the nautiloid Michelinoceras sp. The graptolite– shale horizons of the Nyaungbaw Fm contain two graptolite zones: a lower zone which is the Coronograptus cyphus Zone of Llandovery age represented by C. cyphus itself, Demirastrites triangulatus, Climacograptus rectangularis, Glyptograptus cf. tamariscus, Petalolithus folium and Diplograptus sp.; and an upper zone which is the Cyrtograptus murchisoni Zone of the Wenlock, which has yielded Cyrtograptus cf. murchisoni and Monograptus priodon (Hla Thein 1991). Reed (1913) published the species Camarocrinus asiaticus within a crinoid genus earlier known in America from the uppermost Silurian, and which suggested to Pascoe (1959) that part of the Nyaungbaw Fm may be of Late Silurian age. That was confirmed by the find of three useful specimens of the conodont Pseudoneotodus sp. characterized by short, stout cones with a deep basal cavity, which were first recovered from a limestone horizon approximately 200 m above the Camarocrinus asiaticus-bearing limestone in west Pyintha village, Pyin Oo Lwin township, by Aka and a field party in 2013.

[Figure: Silurian fossils of the Nyaungbaw Formation: (a, b) non-ammonoid cephalopods, 5 km east of Wetwin village, northern Shan State; (c) Camarocrinus sp. cf. asiaticus Reed, Yemeye, east of Nyaungbaw village, Pyin Oo Lwin township, Mandalay Division; (d) Cyrtograptus sp. cf. murchisoni Lapworth; and (e) Coronograptus cyphus Lapworth, 2 km east of Okpo village, Pyin Oo Lwin township, Mandalay Division (after Aung&Cocks, 2017)]

The depositional environment of the Nyaungbaw Fm is interpreted as a shallow-marine carbonate platform with a ramp and lagoon from the evidence of mixed clastic carbonate sediments and the graptolite-bearing shale unit.