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Bichom Formation
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Bichom Fm base reconstruction

Bichom Fm

Period: 
Permian

Age Interval: 
Early Permian (Sakmarian-Artinskian?)


Province: 
Arunachal Pradesh

Type Locality and Naming

Named after Bichom Valley. [Original Publication: Anon., 1974, Geology and Mineral Resources of the States of India - Assam and adjoining States. Geological Survey of India, Misc. Publ. No. 30, pp. 1-24.]


Lithology and Thickness

Glacial-marine to volcanic-ash Claystone. It is divided into three members: A lower Rilu Member - Diamicite/Pebbly mudstone with minor shales. The Bomte Member - Shale with Calcareous and Phosphatic nodules. An upper Sessa Member - Grey to black tuffaceous shales.


Lithology Pattern: 
Glacial till


Relationships and Distribution

Lower contact

Conformably overlying the Miri Fm.

Upper contact

Conformable contact with the Bhareli Fm.

Regional extent

Easternmost Lesser Himalayas. Gaur-Igo-Basar section, Bichom Valley.


GeoJSON

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Fossils

(Bivalve): Deltopecten sp., Eurydesma aeguale, E. myt il oides, E. Perver sum, E. punjabicum, Myalina sp., Nucula sp., Schizondus sp., Aviculopecten sp., Goniophora sp.

Brachiopods: Buxtonia scabriculus, Chonetes sp., Cyrt el la sp., C. Nagmargensis, Dielasma dadanense, D. biplex, Neospirifer fasciger, Oldhamia sp., Productus purlulosus, Linoproductus cora, Spirifer sp., Spiriferina sp. and Subansiri aranganensis.

Bryozoa: Fenestella sp., Geinitzell sp. Polypora megastroma.

Cephalopods: Pseudogartrioceras sp., Uraloceras siangenses

Conularids: Conularia laskari, panaconularia sp.

Gastropods: Bellerophon sp., Pleurotomaria sp., Straparollus lachiensis, Platyeichum benenssis, P. garuensis, Mourlomia cf. nuda

Palynofloral:

Zone I: Plicatipollenites stigmatus zone – characterised by predominance of radial monosaccate pollens (86%) viz. P. stigmatus, P. densus. It is restricted to the Rilu Member.

Zone: II: Callumispora gretensis Zone – characterised by proliferation of C. gretensis and appearence of Microbacalispora tentula and Indotriradites korbaensis in the basal part of the Bomte Member.

Zone III: Crucisaccites indicus Zone – characterised by appearance of Crucisaccites indicus, Brevit riteles levis and Denisosporites solidus lying in the basal part of the Bomte Member.

Zone IV: Pseudoreticulatispora barakarensis Zone - It represents in the middle part of the Bomte Member and characterised by the dominance of P. barakarensis and Paravesicaspora indica; the latter make their first appearance with Rhizomaspora.

Zone V: Premuspollenites - Rhizomarpora Zone It is in the Upper part of the Bomte Member and characterised by the acme zone of P. obscures, R. indica and R. monosulcata.


Age 

Early Permian. [Temporarily put as Sakmarian-Artinskain here, so the Rilu Member might be coeval with other regional glacial-marine beds.]

Age Span: 

    Beginning stage: 
Sakmarian

    Fraction up in beginning stage: 
0.6

    Beginning date (Ma): 
291.71

    Ending stage: 
Artinskian

    Fraction up in the ending stage: 
0.2

    Ending date (Ma):  
289.07

Depositional setting


Depositional pattern:  


Additional Information


Compiler:  

D.S.N Raju