Dzong Fm
Type Locality and Naming
Upper-middle formation in Chukh Gr.
Synonym: Tangbe Fm ("The poorly defined "Tangbe formation'' of Bodenhausen et al. (1964; see also discussion by P. Bordet, p. 121) is broadly equivalent to the Dzong Formation.")
Lithology and Thickness
- "Subdivided into two members ("lower green sandstones'' and "upper black shales''), separated and capped by glaucony-bearing intervals. The about 300 m thick "lower green sandstones'' consist of up to fine-grained greenish volcanic arenites locally yielding abundant bivalves or ammonites of early Aptian age (Bordet et al., 1967), interbedded with dark mudrocks particularly in the lower part, horizons of glaucony-bearing arenites and rare grey mudstones. The about 100 m thick "upper black shales'' … largely
consist of black mudrocks; intercalated in the lower part are very fine-grained bivalve-bearing sandstones with hummocky cross-lamination or lithoclastic lags yielding ammonoids of Albian age."
Relationships and Distribution
Lower contact
Underlain by Kagbeni Fm
Upper contact
Overlain by Muding Fm. "The formation is capped by 25 m thick glauconitic greensands with intercalated glaucony-bearing marly limestones of late Albian age ("Glauconitic Horizon'' of Premoli Silva et al., 1991); this major drowning horizon can be traced all along the Himalaya (Garzanti et al., 1989; Garzanti, 1991b, 1993a,b).
Regional extent
"The source rocks were comparable with the alkalic Aulis Volcanics of the Lesser Himalaya (Sakai, 1991), although the Rb/Sr biotite-feldspar age of 96.7 +/-2.8 Ma obtained for these trachytes and trachyandesites (Arita et al., 1991) is slightly too young (corresponding to the latest Albian/Cenomanian)."
"Mixing of volcanic and partly recycled quartzose to quartzo-feldspathic continental block detritus characterizes the whole of the Tethys Himalayan succession from S. Tibet, where intraplate volcanism started possibly even earlier than the Jurassic/ Cretaceous boundary (Jadoul et al., 1998), to the Spiti- Zanskar Synclinorium, where it was recorded only at Albian times (Garzanti, 1993a,b). Diachronous effusion of intraplate alkali basalts and more felsic differentiates on the southern margin of Neotethys, in a vast area from offshore S.E. Africa to N.E. India and offshore N.W. Australia (e.g. Vallier, 1974; Baksi et al., 1987; von Rad et al., 1992), is related to a period of extensional tectonism, when deep-seated faults propagated through the continental crust and tapped magma sources in the upper mantle. These tectonic and magmatic events accompanied the successive detachment of India from Africa, Australia and Madagascar, and marked the initial opening of the Indian Ocean and the final disintegration of the Gondwana superplate. Volcaniclastic sedimentation ended synchronously in the late Albian all along the southern margin of Neotethys from Zanskar to Nepal, where the terrigenous Chukh Group was mantled by condensed glauconitic arenites and mudstones yielding planktonic foraminifers of the R. subticinensis Subzone; "Glauconitic Horizon'' of Premoli Silva et al., 1991)."
GeoJSON
Fossils
Age
Depositional setting
"The distal deltaic to shelfal, fine-grained volcanic arenites, locally glauconitic, are capped by a glauconitic arenite horizon, sharply followed by dark shales locally containing ammonoids of Albian age. Abundant volcanic detritus indicates renewed magmatic activity of intermediate to felsic character, fading out at the top of the unit, where an increase in detrital feldspars and hypabissal grains suggests erosion of the volcanic pile."
Additional Information