Fossiliferous concretions

from north-central Nepal

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Above: This month we feature concretions of black mudstone / carbonaceous shale:
(1) a 5.0x4.0x3.3 cm example, the core holding the shell and mould of a slightly pyritized ammonite with bifurcating septae, and
(2) a 5.2x4.0x3.3 cm nodule (below, at right), enclosing part-pyritized orthocerid fossil, the "straight-cone" cousin of the more familiar coiled ammonites. The photo above shows the ammonite within and the smooth exterior of the concretion. The image at lower left shows both the fossil and its (concave) external mould. At a quick guess, the ammonites shown here may be referred to the genera Blanfordiceras or Perisphinctes, while the straight-cone example we will call Orthoceras, pending better-informed identifications! Perisphinctes ammonites occurred worldwide, although warm seas were preferred. They tend to be large, often much larger than the samples shown here (Turek et al., 1988, pp.276-277).

"Rock of the Month #139, posted for January 2013" ---

Fossils from the Himalayas are found in artfully broken fine-grained sedimentary nodules. Here we see cephalopod fossils in two concretions, purchased from a Tibetan refugee woman at David's Fall near Pokhara, Nepal in 1990. Such fossils are found and traded along the Kali Gandaki drainage cutting the Himalayas on the west side of the Annapurna massif. The Kali Gandaki traverses the Himalayan chain in the Nepalese district of Mustang, a spectacular and remote region with an exotic landscape and curious archaeology (see, e.g., Finkel and Richards, 2012). The fossils may be referred to as saligrams and are also found southwest of Jomsom, and in Tibet. Saligrams (shaligrams) or sacred stones include a range of awe-inspiring geological wonders, from fossils to gemstones. Indeed, the geological succession in the Garhwal-Kumaun Himalaya includes the uppermost Jurassic, Tithonian (Portlandian) Spiti Shale with its Saligram Member, composed of shales with concretions, which contain cephalopods, bivalves, and other fossils, as illustrated here (Sinha, 1981, pp.137-144). Large gemstones (e.g., ruby) may also be kept as saligrams, and examples with an ancient provenance in the human realm may come under the category of archaeological relics (Rai, 1995)

Jurassic strata with ammonoid faunas are found over a wide range, including Nepal, Pakistan (Spath, 1934), Rajasthan (Dave and Chatterjee, 1996), and especially, though more distant from our present focus, the Kachchh (Kutch) region of Gujarat in western India. The Kachchh region is considered the most important Jurassic locality in the marine faunal province of India and east Africa, and the strata contain ammonites found as far away as Somalia, Kenya and Madagascar.

Ammonoid genera referable to the Spiti shale and equivalents in Himachal Pradesh (Pathak, 1997) include Katroliceras and Aulacosphinctoides (lower Tithonian), Virgatosphinctes and Lemancia (upper part of the lower Tithonian) and Blanfordiceras and Haplophylloceras (upper Tithonian). The Tithonian represents uppermost Jurassic time, circa 151 to 145.5 Ma. In Nepal (Gradstein et al., 1992) black shales of the upper Jurassic Nupra Formation correlate with the regionally extensive Spiti shale. For a detailed discussion of the Jurassic in the Indian subcontinent, see Krishnan (1982), pp.347-369.

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Dave,A and Chatterjee,TK (1996) Integrated foraminiferal and ammonoid biostratigraphy of Jurassic sediments in Jaisalmer basin, Rajasthan. J.Geol.Soc.India 47, 477-490.

Finkel,M and Richards,C (2012) Sky caves of Nepal. National Geographic 222 no.4, 114-135, October.

Gradstein,FM, von Rad,U, Gibling,MR, Jansa,LF, Kaminski,MA, Kristiansen,I-L, Ogg,J, Rohl,U, Sarti,M, Thurow,J, Westermann,G and Wiedmann,J (1992) Stratigraphy and depositional history of the Mesozoic continental margin of central Nepal. Geol.Jb. B77, 3-141.

Krishna,J and Pathak,DB (1991) Ammonoid biochronology of the upper Jurassic Kimmeridgian stage in Kachchh, India. J.Palaeontological Society of India 36, 1-13.

Krishnan,MS (1982) Geology of India and Burma. CBS Publishers & Distributors Private Ltd, New Delhi, 6th edition, 536pp.

Pathak,DB (1997) Ammonoid stratigraphy of the Spiti shale formation in Spiti Himalaya, India. J.Geol.Soc.India 50, 191-200.

Rai,S (1995) Calling all auctioneers. India Today 20 no.12, 39, 30 June.

Sinha,AK (1981) Geology and tectonics of the Himalayan region of Ladakh, Himachal, Garhwal-Kumaun and Arunachal Pradesh: a review. In `Zagros, Hindu Kush, Himalaya: Geodynamic Evolution' (Gupta,HK and Delany,FM editors), AGU/GSA Geodynamics Series Vol.3, 323pp., 122-148.

Spath,LF (1934) The Jurassic and Cretaceous ammonites and belemnites of the Attock district. Palaeontologia Indica New Series XX, Memoir 4, Geol.Surv.India, 51pp.

Turek,V, Marek,J and Benes,J (1988) Fossils of the World: a Comprehensive Practical Guide to Collecting and Studying Fossils. Arch Cape Press, New York, 1990 edition, 495pp.

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Above: A third example of a saligram, with bifurcating septae and evidently flat keel.
Specimen from Doug Astill's collection, acquired by M. Kendall in the Himalayas, circa 1970-1971.

Graham Wilson, 08-10 November 2012, 18-19 December 2012, 01 January 2013, 15 April 2013, 24 October 2014

See an ammonite from Madagascar and a modern Nautilus

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