Maymyo Fm

Shan North Plateau, Wetwin–Padaukpin area, Pyin Oo Lwin township. Shan Mid Plateau, (Pyin Oo Lwin), Shan South Plateau. This Fm was named the Maymyo Dolomite Fm by Amos (1975) and was included in the Shan Dolomite Group by Brunnschweiler (1970). However, it was renamed the Maymyo Fm by Aye Ko Aung (2012) since no pure dolomites were identified in both megascopic and microscopic studies. No stratigraphical measured section was provided in the previous description of this Fm, but AKA and geology students of Dagon University have since logged the unit where it is most widely distributed in the Wetwin–Padaukpin area, Pyin Oo Lwin township. [Original Publication: Barber, A. J., Khin Zaw & Crow, M. J. (eds) 2017. Myanmar: Geology, Resources and Tectonics. Geological Society, London, Memoirs, 48, 317-342]

Dolomitic limestone. The Maymyo Fm consists predominantly of relict fossil-bearing dolomitic limestones associated with minor calcareous siltstones and shales. The upper Padaukpin Limestone Mbr of La Touche (1913) and the lower Pwepon Limestone Mbr in the unpublished PhD thesis of Khaing Khaing San (2005) are here considered counterparts of the Maymyo Fm. The former consists of light to dark grey dolomitic limestone, commonly associated with loose muddy particles and, in places, interbedded with thin shale. The latter is composed of light to dark grey, thin- to medium-bedded fossiliferous dolomitic limestone and argillaceous limestone with some silty shale interclations. The Wetwin Shale Mbr of La Touche (1913), which was previously known as a separate unit overlying the upper Padaukpin Limestone Mbr in the Padaukpin–Wetwin area, is here considered a similar unit but a different facies of the upper Padaukpin Limestone Mbr. From field mapping of the area, it is suggested that these two units – the Wetwin Shale Mbr and the upper Padaukpin Limestone Mbr – are possibly the same units occurring on opposite limbs of the ‘Wetwin Anticline’. The Wetwin Shale is chiefly composed of yellowish, black shale.

[Figure: Stratigraphical log of the Devonian Maymyo Formation, Wetwun–Padaukpin area, Pyin Oo Lwin township (after Aung&Cocks, 2017)]

Corals and brachiopods are abundant in the upper Padaukpin Limestone Mbr; at the Padaukpin locality, this indicates that the Maymyo Fm is of Eifelian age (Reed 1908; Aye Ko

Aung 1995; Nu Nu Wai 2004; Khaing Khaing San 2005). A substantial brachiopod fauna from Padaukpin, about 12 km ENE of Pyin Oo Lwin (formerly Maymyo township), has yielded many brachiopods including the large fauna described by Anderson et al. (1969). Anderson identified 32 species, all characteristic of the Rhenish–Bohemian Faunal Province which is widespread in Asia as well as Europe.

[Figure: Middle Devonian corals of the Maymyo Formation (‘Padaukpin limestone’), Padaukpin village, Pyin Oo Lwin township. (a–c) Cyathophyllum gigantium Aung: (a) surface specimen; (b) transverse section; and (c) surface specimen; (d) Favosites goldfussi Reed, surface specimen; (e) Phacellophyllum padaukpinensis Aung, surface specimen; and (f) Phillipsastraea hlawaii Aung, surface specimen (after Aung&Cocks, 2017)]

[Figure: Late Devonian ammonoids of the Maymyo Formation, Myogyi area, Ye-ngan township. (a–c) Beloceras shidianense Yang; (d) Tornoceras cf. contractum Glenister (after Aung&Cocks, 2017)]

The sedimentary characters and fossil assemblage suggest the Maymyo Fm was deposited in a very shallow-marine clear-seawater environment, occasionally deepening, under conditions of moderate to strong energy caused by waves or a storm (upper Padaukpin Limestone Mbr–lower Pwepon Limestone Mbr); a small lagoonal environment (represented by the Wetwin Shale) might have been developed near the shoreline (Aye Ko Aung 2012).