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Sisne Formation
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Sisne Fm base reconstruction

Sisne Fm

Period: 
Permian

Age Interval: 
earliest Permian (Asselian-Sakmarian?)


Province: 
Nepal Lesser Himalaya

Type Locality and Naming

Tansen and lower Kali Gandaki region


Lithology and Thickness

Pebbly mudstone (glacial-marine), then claystone: "The Sisne Formation comprises a thick succession of diamictite (300 m) in its lower levels, followed upwards by sandstones (80 m) and another diamictite sequence (80 m). Its upper part [Ritung Bioturbated Mudstone Member] is made up of black shale with some diamictite beds and an approximately 100 m thick highly bioturbated top sequence of alternating sandstone and mudstone. The maximum thickness of the Sisne Formation is 1,020 m (Sakai 1983, p. 34). … Most of the diamictite is unstratified and unsorted, and contains a structureless clayey matrix with randomly scattered clasts of varying sizes and shapes. Most of the clasts are of granite, gneiss, limestone, dolomite, and sandstone."


Lithology Pattern: 
Glacial till


Relationships and Distribution

Lower contact

Major unconformity (ca. 1 billion years) at the disconformity onto the underlying Kerabari Fm (Kali Gandaki Gr; early-middle Proterozoic) with 20-cm-thick conglomerate (mainly derived from the eroded Kerabari Fm).

Upper contact

Regional extent


GeoJSON

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Fossils

"From some floats of shale, derived from the upper part of the Sisne Formation, Sakai (1983, p. 40) discovered various bryozoan fossils belonging to the genera Fenestella, Polypora, and Acanthocladia. Because the genera Fenestella and Polypora range in age from the Ordovician to Permian, whereas the distribution of Acanthocladia is known to span from the Carboniferous to Permian, Sakai assigned a Carboniferous to Permian age for the Sisne Formation.'


Age 

Put here as Early Permian to be consistent with estimated age spans of other regional glacial-marine "pebbly mudstone".

Age Span: 

    Beginning stage: 
Asselian

    Fraction up in beginning stage: 
0.5

    Beginning date (Ma): 
296.21

    Ending stage: 
Artinskian

    Fraction up in the ending stage: 
0.5

    Ending date (Ma):  
286.91

Depositional setting

"Most of the Sisne Formation is inferred to be of glacial origin. But the occurrence of thick sandstone beds with climbing ripple lamination and scour-and-fill marks, and the presence of thin conglomerates exhibiting imbricate pebbles indicate the influence of water current. The bioturbated upper member seems to have originated in a shallow marine (tidal flat) environment (Sakai 1983, p. 39)."


Depositional pattern:  


Additional Information


Compiler:  

Descriptions from Megh Raj Dhital (2015, Geology of Nepal: Regional Perspective of the Classic Collided Orogen, Springer Publ., 499 pp.). Age spans estimated from correlation diagrams in Martin, A.J. (2017, "A review of Himalayan stratigraphy, magmatism, and structure", Gondwana Research, 49: 42-80; http://dx.doi.org/10.1016/j.gr.2017.04.031), but these are often controversial.