Bagung Fm
Type Locality and Naming
Named after complete exposures near Bagung village; Thakkhola valley north of Annapurna and south of Tibet.
Synonym: Laptal Fm (proposed by Garzanti, 1999). "The name Laptal, introduced in Kumaon by Heim and Gansser (1939), has been adopted also in Nepal (Garzanti and Pagni Frette, 1991; Cariou et al., 1994) to replace the generic terms ``Upper Lumachelle formation'' (Bodenhausen et al., 1964), "Lumachelle'' or "Lumachelle Formation" (Bodenhausen et al., 1964; Bassoullet et al., 1986; and ohers). .. The term "Bagung Fm.'' of Gradstein et al. (1989) … has to be rejected as a junior synonym." [NOTE: it seems that Gradstein et al., 1989, proceeded Garzanti and Pagni Frette, 1991, therefore Bagung Fm is retained in this Lexicon pending further advice.]
Lithology and Thickness
Rhythmic dark shale and clayey limestone with mollusk coquina layers (storm beds). Thickness about 100 to 120 m. "In the complete Thakkhola section, four parts can be identified: storm-deposited quartzose biocalcarenites prevail in the basal 14.4 m ("lithozone a''), followed by dark-grey bioturbated marls with intercalated arenites displaying hummocky cross-lamination, bivalves, brachiopods and belemnites (44.6 m; "lithozone b''), overlain in turn by predominant light-grey marls (34 m; "lithozone c''); up to 60 cm thick storm-deposited coquina layers with oysters, brachiopods and belemnites are intercalated in the upper 17.7 m ("lithozone d'')".
Relationships and Distribution
Lower contact
Overlies Kioto Limestone Fm (Jomoson Fm)
Upper contact
Sharp contact to the Ferruginous Oolite Fm (which was included as the uppermost member of Bagung Fm by Gradstein et al., 1989).
Regional extent
The equivalent Laptal Fm "exceeds 150 m in the Bhuri Gandaki/Shiar area (Fuchs and Paudel, 1998) … as in relatively distal position in S. Tibet. In more proximal parts of the Indian continental terrace the unit is instead only some tens of metres thick; in Spiti its top is reported to reach only the early Bathonian (Cariou et al., 1996), whereas the formation can be missing altogether in Kumaon (Heim and Gansser, 1939) and Zanskar (Zangla Unit; Jadoul et al., 1985)."
GeoJSON
Fossils
"Lumachelle" layers are rich in belemnites, ostrieds, brachiopods and a few ammonites of Bajocian to middle Bathonian age.
Age
Depositional setting
"A major paleogeographic change was recorded all along the Tethys Himalaya in the mid-Dogger, when the death of the Kioto carbonate platform, marked by a spectacular disconformity in the N.W. Himalaya and by a paraconformity in Nepal and S. Tibet, was followed by deposition on a storm-dominated shelf of interbedded hybrid arenites, marls and ``lumachelle'' layers. … Onset of quartzo-feldspathic detritus, characterized by reduced mineralogical stability with respect to the quartzarenite layers interbedded within the Kioto carbonates, is consistent with rejuvenation of the Indian craton during an extensional tectonic phase, locally associated with mafic volcanism in the distal part of the continental margin (e.g. Reuber et al., 1987; Danelian and Robertson, 1997). … The unconformity at the top of the Kioto Limestone Fm may thus be directly onlapped by transgressive horizons of the succeeding cycle (Ferruginous Oolite Fm), with gaps of various duration representing large parts of the Middle Jurassic."
Additional Information