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Irrawaddy Formation
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Irrawaddy Fm base reconstruction

Irrawaddy Fm

Period: 
Neogene, Quaternary

Age Interval: 
Pliocene (Chindwin, Minbu-Salin Basins and Pyay Embayment), Pleistocene (Irrawaddy River Delta), Late Miocene-Pliocene (Pegu (Bago) Yoma-Sittaung Basin (BYSB), Pliocene-Pleistocene in Hukaung Basin (east flank).


Province: 
Myanmar Central Basins

Type Locality and Naming

Chindwin Basin, Named after the Irrawaddy River. In Minbu-Salin Basin, Yenangyaung area, Magwe Division. In Irrawaddy River Delta and Hukaung Basin (east flank). [Original Publication: Barber, A. J., Khin Zaw & Crow, M. J. (eds) 2017. Myanmar: Geology, Resources and Tectonics. Geological Society, London, Memoirs, 48, 219–260; 169-183]

[Figure: Map showing location of sub-basins of the Central Myanmar Basin (after Myint Thein and M. Maung 2017)]


Lithology and Thickness

Coarse-grained sandstone. Mainly sandstones, interbedded with clay lenses, light-yellow, sugary white and buff-colour, massive, fine-grained to pebbly, poorly sorted, it contains silicified fossil wood. It mainly consists of fluvatile sediments, transported from the Eastern Highlands (Shan Plateau), Eastern Himalayas and western ranges (Rakhine Ranges). The Irrawaddy Fm has a measured thickness of c. 2743 m (c. 9000 ft) in the Lawtha syncline. In Minbu-Salin Basin and Pyay Embayment, it is composed mainly of friable sandstones, interbedded with very few clay/mud beds containing silicified wood and vertebrate bones. In Irrawaddy River Delta, it is shallow-marine marl. Mainly fluviatile sediments, this sequence is interspersed with some marine sediments towards the south. In Pegu (Bago) Yoma-Sittaung Basin (BYSB), it is sandstone. A terrestrial unit over 850 m thick of cross-bedded sandstones and gravelly beds. In Hukaung Basin (east flank), cross-bedded sandstones and gravelly beds.

[Figure: (f) Ferruginous beds in the Miocene–Pliocene Irrawaddy Formation, view towards the north, eastern part of Naypyidaw (after Myint Thein and M. Maung 2017)]

[Figure: Stratigraphic succession of the Chindwin Basin (after Than Htut, 2017)]


Lithology Pattern: 
Coarse-grained sandstone


Relationships and Distribution

Lower contact

Conformable with Shwethamin Fm in Chindwin Basin, Unconformable with Obogon Fm in Minbu-Salin Basin, Conformable with Kathabaung Fm in Irrawaddy River Delta, Unconformable with Khabo Sandstone Fm and Unconformable with Upper Unit Fm in Hukaung Basin (east flank).

[Figure: (a) Angular unconformity between Irrawaddy Formation (Upper Miocene–Pliocene) and Khabo Sandstone Fm (Upper Miocene) (after Than et al., 2017)]

Upper contact

This is the youngest Fm in the Chindwin Basin, Minbu-Salin Basin, Pyay Embayment, Irrawaddy River Delta and Hukaung Basin (east flank), in Pegu (Bago) Yoma-Sittaung Basin (BYSB) it conformable with Alluvium,

Regional extent

The Irrawaddy Fm is developed in all the Tertiary sub basins of Myanmar. It occurs extensively throughout central Himalaya.

[Figure: a) Composite stratigraphic correlation of Myanmar Central Basins (after Than et al., 2017)]


GeoJSON

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Fossils

Yields the remains of terrestrial and aquatic vertebrates. In Minbu-Salin basin, Pyay Embayment, Irrawaddy River Delta it contains silicified fossil wood, reptilian and mammalian vertebrate bones. In Pegu (Bago) Yoma-Sittaung Basin (BYSB) the Upper Irrawaddian deposits have yielded a rich assemblage of Early Pleistocene mammals (Colbert 1943; Myint Thein et al. 1982),


Age 

Miocene-Quaternary (underlying Mingin Fm in the main Chindwin Sub-basin is given an age of Late Miocene/Early Pliocene, therefore for generalized lithostrat column, assigned as Pliocene); Pleistocene in Minbu-Salin, Pyay Embayment, Irrawaddy River Delta. Late Miocene-Pliocene in Pegu (Bago) Yoma-Sittaung Basin (BYSB). Pliocene-Pleistocene in Hukaung Basin (east flank).

Age Span: 

    Beginning stage: 
Zanclean

    Fraction up in beginning stage: 
0.0

    Beginning date (Ma): 
5.34

    Ending stage: 
Holocene

    Fraction up in the ending stage: 
1.0

    Ending date (Ma):  
0.00

Depositional setting

Deposited under non-marine condition in Chindwin Basin, in Minbu-Salin basin and Pyay Embayment it is deposited in fluvial, flood basin and marsh environments. In Irrawaddy River Delta it is deposited in Shallow-marine environment. In Pegu (Bago) Yoma-Sittaung Basin (BYSB) the Quaternary deposits blanket the older Fms and were formed mostly in fluvial environments, locally forming alluvial fans (Myint Thein 2002, 2011). In Hukaung Basin (east flank), it is deposited in Fluvial environment.


Depositional pattern:  


Additional Information


Compiler:  

Than Htut, Kapesa Lokho, D.S.N.Raju and Ravi Misra (Than Htut, Myanmar petroleum systems, including the offshore area, Chapter 11 in Barber, A. J., Khin Zaw & Crow, M. J. (eds) 2017. Myanmar: Geology, Resources and Tectonics. Geological Society, London, Memoirs, 48, 219–260)