Reefal 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: upper Natteik Fm

Limestone. Contains two different but closely associated lithological suites: well-bedded intraclastic limestones and massive organic limestones. It is distributed along a high mountain range with the highest point at 2313 ft, forming an outlier in the Dolomitic Limestone Unit Fm. Details of the lithostratigraphy, palaeontology and sedimentology of this unit have been described by Zaw Win (2008). The intraclastic limestone subunit forms a typical coarsening- upwards sequence. The lower part is occupied by a wellbedded micritic limestone horizon, containing reddish or pinkish chert nodules. This horizon is followed by an interval of massive calcisiltite, which is in turn succeeded by a sequence of thin- to medium-bedded intracalcarenites with occasional intracalcirudite beds. Finally, it passes upwards transitionally into a thick sequence of medium- to thick-bedded intracalcirudite–calcarenite with intercalations of chaotic talus limestone breccias. This subunit is typified by a rhythmic sequence of sharp-based, parallel beds of graded calciruditic–calcarenitic intraclastic limestones and megaslumps, with massive talus blocks of organic limestone. This subunit comprises at least five lithofacies: (1) chertyfied lime mudstone facies; (2) calcisiltic intrabioclastic packstone facies; (3) calcarenitic intrabioclastic packstone–grainstone facies; (4) calciruditic–calcarenitic intrabioclastic packstone facies; and (5) limestone talus breccia (rudstone) facies. The whole facies sequence is characterized by the dominance of intraclastic limestones, composed essentially of limestone intraclasts and abraded skeletal debris, and the turbiditic nature of rhythmic beds, commonly with multiple graded beds and parallel laminations. Occasional slumps and talus blocks form a typical coarsening-upwards rock sequence, developed by sedimentary aggradation and progradation and deposited between a synchronous basinal facies (i.e., Cherty Limestone Unit Fm) in the west and a shelf edge facies (i.e., Massive Limestone Subunit) in the east. The massive limestone subunit is locally fossiliferous with hexacorals and segmented calcareous sponges, which are usually encrusted by calcareous alga, and mostly preserved in growth position. Massive, tan- or grey-coloured, fossiliferous micritic limestone of this subunit in particular include algal oncolites. Between these limestone masses are massive, unfossiliferous, micritic limestone and occasional intraclastic–miliolid limestone.

[Figure: Topographic expression and outcrop nature of the Reefal Limestone Unit. (1) The organic limestones occupy the massive mountain range (in background) while the intraclastic limestones form the flanking spurs (in middle ground). The vegetated plain in foreground is the Dolomitic Limestone Unit (Photo looking east). (2) A weathered outcrop of the graded calciruditic–calcarenitic intraclastic limestones forming a rhythmic sequence of sharp-based parallel beds (after WinZaw et al., 2017)]

Fossils presently identified are: hexacorals, Montlivaltia cf. stylophylloides, M. cf. norica, Thecosmilia cf. eguchi and Thecosmilia spp.; sphinctozoan calcisponges, Paradeningeria alpina and Dictyocoelia sp.; hydrozoans, Spongiomorpha spp.; microproblematica, Cheilosporites tirolensis; and brachiopods, Aulacothyris cf. joharensis and Rhynchonella cf. bambanagensis. Smaller foraminifers, such as Opthalmidium sp., Agathammina spp., Involutina sp., Alpinophragmium perforatum and Trocholina permodiscoides, are also rarely contained in a matrix of massive organic limestone and fine-grained intraclastic limestone.

[Figure: Fossils from the Reefal Limestone Unit (Zaw Win 2008). (1) Montlivaltia cf. stylophylloides (peel-section, ×1); (2) M. cf. norica (peel-section, ×1); (3) ‘Thecosmilia’ cf. eguchi (polished slab, ×1.3); (4) ‘Thecosmilia’ sp. indet. (polished slab, ×1.5); (5, 6) Paradeningeria alpina (polished slabs, ×1.2, ×1); (7) Spongiomorpha sp. (polished slabs, ×1.25, ×0.8); (8) oncolitic algae encrusted with ?hydrozoans (polished slab, ×1.3); (9) Spongiomorpha sp. (polished slabs, ×0.8); (10) Aulacothyris cf. joharensis (ventral view, ×1.3); (11) Rhynchonella cf. bambanagensis (dorsal view, ×1); (12) Cheilosporites tirolensis (polished slab, ×4.5); (13) Alpinophragmium perforatum (thin-section, ×25); (14) Trocholina permodiscoides (thin-section, ×25); (15) ‘Sigmoilina’ sp. (thin-section, ×50); (16) Agathammina sp. (thin-section, ×50); (17) Reophax sp. (thin-section, ×50); (18) ?Glomospirella sp. (thin-section, ×20); (20) Duotaxis sp. (thin-section, ×60); (20) Involutina sp. (thin-section, ×30) (after WinZaw et al., 2017)]

Shallow, warm and clear marine conditions (Zaw Win 2008).

[Figure: Petrographic characteristics and depositional environments of the Reefal Limestone Unit, measured along the western spur of point 2313 m range, in Zaw Win 2004 (after WinZaw et al., 2017)]