Dolomitic 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: Thitsipin Fm

Dolomitic limestone. The Dolomitic Limestone Unit Fm is composed essentially of a monotonous sequence of dolomitic limestones and dolomites, with less dolomitic interbeds of coralline limestone, bioclastic limestone and limestone breccia in the lower part, horizons of foraminiferal (miliolid) limestone and fusuline limestone in the middle and an occasional interval of burrowed limestone in the upper part. A persistent horizon of stromatolitic limestone forms the upper boundary of the unit.

[Figure: Lithostratigraphic characteristics of limestone horizons from the Dolomitic Limestone Unit. (1) Bioclastic limestone with detached rugose corals randomly embedded in the matrix; (2) coralline limestone composed of waagenophyllid rugose corals (scale in millimetres); (3) foraminiferal limestone with profuse number of Shanita amosi, a miliolid foraminifer (weathered surface, scale in millimetres); (4) foraminiferal limestone with profuse number of Shanita thawtinti, a miliolid foraminifer (polished slab, scale in millimetres); (5) outcrop of wavy-bedded burrowed limestone; and (6) outcrop of stromatolitic laminated limestone (after WinZaw et al., 2017)]

The coralline limestone from the lower part of the unit yields some waagenophyllid and tabulate corals such as Wentzelella cf. timorica, Ipciphyllum spp., Polythecalis sp. And ‘Syringopora’ sp. (Sinopora sp.). They occur in association with fenestellid bryozoans, undifferentiated calcisponges, a few brachiopods and foraminifers (e.g., Multidiscus cf. padangensis). Fusulines such as Pseudofusulina sp. and Yangchienia sp. are also present in the crinoidal bioclastic limestones that are associated closely with the coralline limestone. A remarkable foraminiferal limestone horizon from the middle portion of the unit carries a distinctive fauna, consisting essentially of miliolid foraminifers (Shanita amosi, Hemigordiopsis spp., Baisalina pulchra, Agathammina sp.), other smaller foraminifers (such as Cribrogenerina sp., Pachyphloia spp., Globivalvulina sp., Tetrataxis sp.) and a few dasycladacean algae (Mizzia sp., Macroporella sp.). Solitary rugose corals occur as detached bioclasts. Details of this foraminiferal fauna have been described by Bronnimann et al. (1978) and Whittaker et al. (1979). In the present study, a Middle Permian (Murgabian = Wordian age) index fusuline foraminifer, Neoschwagerina craticulifera, is found rarely in this Shanita amosi fauna. On the other hand, no fossils have been recognized from the upper portion of the unit because of their destruction by widespread intense dolomitization.

[Figure: Fossils from the lower part of the Dolomitic Limestone Unit. (1) Schwagerina sp. (thin-section, ×15); (2) Multidiscus cf. padangensis in bioclastic limestone (thin-section, ×15); (3, 4) undifferentiated waagenophyllid corals in argillaceous bioclastic limestone (weathered surface, ×0.25, ×1.5); (5, 6) coiled cephalopods in argillaceous bioclastic limestone (weathered surface, ×1.5); (7, 8) waagenophyllid corals, Ipciphyllum sp. And Wentzelella sp., in coral boundstone (weathered surface, ×1.5); and (9) an undifferentiated calcareous sponge (weathered surface, ×0.2) (after WinZaw et al., 2017)]

[Figure: Foraminifers and calcareous algae from the middle part of the Dolomitic Limestone Unit. Specimens 1–14 are from the foraminiferal limestone. (1, 2) Shanita amosi (equitorial section, axial section, ×12); (3) Hemigordiopsis renzi (axial section, ×12); (4) Baisalina sp. (equatorial section, ×20); (5) Cribrogenerina sp. (longitudinal section, ×30); (6) Pachyphloia sp. (longitudinal section, ×35); (7) Frondina sp. (longitudinal section, ×35); (8) Nankinella sp. (subsagittal section, ×25); (9) Tetrataxis sp. (longitudinal section, ×25); (10) Hemigordius sp. (axial section, ×12); (11) Agathammina sp. (equatorial section, ×20); (12) Neoschwagerina craticulifera (transverse section, ×20); (13, 14) Mizzia sp. (transverse section, longitudinal section, ×12); (15) Shanita thawtinti (equatorial section, ×10); (16, 17) Neoschwagerina margaritae (axial section, transverse section, ×12) (after WinZaw et al., 2017)]

A total of nine carbonate facies types, typically coralline boundstone and stromatolitic bindstone, benthic foraminifera bearing packstone–wackestone and some lime mudstones, are recognized in the unit. On the basis of stratigraphic, palaeontological and sedimentological characteristics, the Dolomitic Limestone Unit Fm can be interpreted as deposited on a carbonate shelf with scattered small patch reefs, under shallow, warm marine conditions, with normal to slightly high salinity. The facies sequence as a whole exhibits a shallowing-upwards sequence, bounded by stromatolitic limestone of intertidal algal-mat facies developed during a relative fall in sea level.

[Figure: Composite section, showing the petrographic characteristics and depositional environments of the Dolomitic Limestone Unit, after Zaw Win 2004 in (after WinZaw et al., 2017)]