Cherty Limestone Unit Fm

Shan mid-Plateau (Pyin Oo Lwin), [Original Publication: Barber, A. J., Khin Zaw & Crow, M. J. (eds) 2017. Myanmar: Geology, Resources and Tectonics. Geological Society, London, Memoirs, 48, 343-363]

Synonym: lower Natteik Fm

Limestone. It consists characteristically of dark grey or black-coloured, well-bedded micritic limestones, interbedded with pinkish or purplish coloured silicified limestone, or interlayered with black chert layers, patches and nodules. These limestones are dolomitized to varying degrees and extent, grading both vertically and laterally into dolomitic limestone and dolomites that are similar in appearance to those of the Dolomitic Limestone Unit Fm. There are at least four limestone facies of turbiditic lime mudstone and wackestone, with occasional packstone, recognized in the unit. These facies are characterized by the prevalence of turbidite-bedded limestones with sharp, flat planar, parallel beds, forming a series of thinning-upwards sequences; abundant planktonic and nektonic fossils, such as filamentous thin bivalves, calcite-filled casts of radiolarians and ammonites, are jumbled in a lime mud matrix. The limestones of this unit can be considered as distal limestone turbidites.

[Figure: Lithostratigraphic characteristics of some limestone horizons from the Cherty Limestone Unit, in ascending order. (1) Massive microsparitic limestone (lime mudstone) with random dark chert patches. (2) Wavy, medium- to thick-bedded, dark grey limestone (radiolarian wackestone) in the middle part. Dark chert nodules align along the bedding. (3) Flat, planer-bedded filamentous limestone interbedded with thin silicified limestone. (4) Thin- to medium-bedded dark grey limestone. (5) Alternation of cherty limestone (darker and thinner) and micritic limestone (lighter) forming a thinning-upwards sequence of sharp, flat and planer beds. (6) A slab of cephalopod limestone with protrusions of ammonites on weathered surface (after WinZaw et al., 2017)]

In the middle part of the unit, a rich ammonite fauna occurs in a cephalopodal limestone bed. The ammonites identified are Ptychites cf. cochleatus, P. cf. asura, Ceratites (Paraceratites) cf. thuillieri, C. (P.) cf. himalianus, Danubites sp. and Protrachyceras sp. (Zaw Win 1991; Ohnmar Soe Yin 2010). A few straight nautiloid cephalopods (Michelinoceras sp.) are also associated with the ammonites. Halobid-like bivalves are also commonly present in the interbedded filamentous bivalve limestone. Calcite-filled casts of radiolarians occur in dark grey-colored cherty micritic limestone.

[Figure: Ammonite fossils from the Cherty Limestone Unit. Scale bar represents 1 cm. (1, 2) Ptychites cf. cochleatus; (3) Flexoptychites cf. mahendra; (4, 5) Paraceratites cf. thuillieri; (6, 7) Paraceratites cf. himalayanus; (8) Danubites sp.; (9) Frechites cf. tuberculatus; (10, 11) Meekoceras sp.; and (12, 13) Protrachyceras sp (after WinZaw et al., 2017)]

Deposited from waning turbidity currents, in a deeper basinal realm during Middle and Late Triassic times.

[Figure: Petrographic characteristics and depositional environments of the Cherty Limestone Unit, measured along the footpath at the western entrance of deserted Indawgai village, after Zaw Win 2004 in (after WinZaw et al., 2017)]

The Plateau Limestone Gr in the western part of the Shan Plateau was deposited in three different environmental settings, namely: a shallow carbonate shelf for the deposition of the Dolomitic Limestone Unit Fm (=Thitsipin Fm and Nwabangyi Fm or Thigaungdaung Limestone Fm); a deeper basinal area for the deposition of the Cherty Limestone Unit Fm (=part of the Natteik Fm or Kondeik Limestone Fm); and reefrimmed bypass shelf margin for the deposition of the Reefal Limestone Unit Fm (=part of the Natteik Fm). These depoenvironmental environments developed during two depositional phases: the shallow-shelf phase during the Permian–Early Triassic; and the basin-slope-shelf margin phase during Middle–Late Triassic time.