Namhsim Sandstone Fm
Type Locality and Naming
Pegin-Linwe area, Ye-ngan township, Shan State South, La Touche (1913) first gave the name Namhsim Beds to rocks in the gorge of the Namhsim River. It is appropriate to use this unit in the Silurian of Shan State South, since a sandy siltstone unit bearing Silurian trilobites and brachiopods was found in the Pegin-Linwe area, Ye-ngan township, Shan State South by AKA and Dagon University students in 2002–04. However, the Namhsim Sandstone Fm has not been found in the area between Mandalay and Pyin Oo Lwin, where deposition of the Nyaungbaw Fm continued into the Early Devonian. [Original Publication: Barber, A. J., Khin Zaw & Crow, M. J. (eds) 2017. Myanmar: Geology, Resources and Tectonics. Geological Society, London, Memoirs, 48, 317-342]
Lithology and Thickness
Sandstone. The lower part of the Namhsim Sandstone Fm is a sandy shore deposit, sometimes conglomeratic at its base. The coarse conglomerates of the basal bed contain boulders of the Precambrian Chaung Magyi quartzite or, where conglomerates are absent, the basal beds are of coarse sandstone, heavily feldspathic and with a purplish or bluish grey colour. That sandstone gradually becomes less feldspathic and passes up into a finer, brown variety, with very fine, hard, white, quartzose layers. The upper part consists of sandy marls with layers or lenticles of very hard compact limestone. Although this subunit is thin, it is remarkably persistent up to the mouth of the Pangyun River.
Relationships and Distribution
Lower contact
Conformable with Linwe Fm
Upper contact
Conformable with Zebingyi Fm
Regional extent
This Fm is developed in Shan South Plateau. Which can therefore be correlated questionably based on the fossil assemblages with the Late Silurian Taungmingyi Orthoquartzite Member of Shan State South (Myint Lwin Thein 1973). Myint Lwin Thein (1973) suggested that the Taungmingyi Member should be considered as part of the Namhsim Sandstone Fm on the basis of its lithological similarity and stratigraphical position.
GeoJSON
Fossils
The Namhsim Sandstone has yielded the actinozoan Lindstroemia cf. subduplicata; fenestellid bryozoans; the brachiopods Mimulus aunglokensis and an indeterminate dalmanelloid; the bivalve Pterinea konghsaensis; and the trilobites Encrinurus konghsaensis, Calymene sp., Cheirurus cf. bimucronatus, Phacops shanensis and Trachyderma cf. squamosa. The assemblages indicate a Wenlock or Ludlow age for the Namhsim Sandstone Fm. (Pascoe 1959)
Age
Depositional setting
The lithologies of the Namhsim Sandstone Fm clearly show that the lower part was deposited in an area close to a coastline in a sea shallowing rapidly to the NW in the direction of the old Chaung Magyi land-mass, while most of the upper part appears to have been deposited under deeper water.
Additional Information