Les ammonites du bathonien et du callovien de Thakkhola (Népal central): Biochronologie et intérêt paléobiogéographique

Abstract

A collection of about one hundred ammonites in situ in the Middle Jurassic from the Thakkhola area of Nepal makes possible a more accurate dating of the formations against the standard Submediterranean chronozonation. More particularly, the Himalayan Ferruginous Oolite Formation, usually assigned to Callovian, is pro parte Late Bathonian in age. The Early and Late Callovian are well characterized, but the Middle Callovian has not been recognized. The species coming from the Late Middle Bathonian (Bremeri Zone) - Late Callovian (Athleta Zone) are described: the families Sphaeroceratidae, Pachyceratidae, Perisphinctidae, Reineckeiidae and Oppeliidae are represented. The Late Bathonian faunas (Bifurcatus and Apertus Zones), dominated by the subfamily Macrocephalitinae, are those previously described from Sula Islands, New Guinea and considered until now as endemic in that area. The presence of a diversified Bathonian representative in Nepal gives evidence of an early phase of spread westwards of the subfamily Macrocephalitinae along the Gondwana northern margin from its roots in the south-west Pacific region.