Development of Oxfordian (Early upper jurassic) in the most proximally exposed part of the Kachchh basin at Wagad outside the Kachchh Mainland

Abstract

Over a century and half the Kachchh Oxfordian ammonoid record has remained restricted to Early and early Middle Oxfordian, that too in extremely condensed/reworked/starved sedimentary facies. In this backdrop we here discuss the Middle and Late Oxfordian ammonoid faunas recently discovered in Wagad (Krishna et al. 1994a, b). Among these the mid Middle Oxfordian stratigraphic interval is found exceptionally rich in ammonoids at Kantkote - the richest ever discovered in the Kachchh basin. It is made up of a 10 m thick succession of 13 ammonoid levels. Farther up after a thick ammonoid-devoid sediment interval appear again a few rather ammonoid scarce levels of late Late Oxfordian age with doubtful extension into early Early Kimmeridgian. The Oxfordian/Kimmeridgian boundary may be included in the latter of the two ammonoid bearing intervals in the Orthosphinctes/Lithacosphinctes levels. The rich mid Middle Oxfordian ammonite fauna inspite of being known for over a century from loose imprecise, unlocalised collections has been stratigraphically precise and dated for the first time in the relatively unexplored, stratigraphic section of Wagad. The ammonoid abundant mid Middle Oxfordian Transversarium Zone succession in addition of the restricted Indo-East African mayaitins and widely distributed perisphinctins includes a few examples of the Mediterranean Gregoryceras gr. fouquei, Euaspidoceras and Taramelliceras and suggests maximum flooding and eustatic rise in the Kachchh Mesozoic in the Middle Oxfordian Schilli Subzone of the Transversarium Zone. Another significant aspect of this fauna is the marked continuous presence of the compressed platyconic densely costate lithacoceratins (Larcheria and Discosphinctes) stocks in parallel with the true perisphinctin and mayaitin lineages almost althrough the Middle Oxfordian (Krishna et al. 1994a, b, 1995, 1996a). The mayaitins are for the first time precisely ranged up to the Transversarium Zone. The basin margin Oxfordian ammonoid succession at Wagad shows recurrence only of small portion in its basal part of the previously known Oxfordian section of the Kachchh Mainland (e.g. the common Perisphinctes-Epimayaites assemblage of bed G Lakhapur). The remainder ca 200 m thick Oxfordian at Wagad is interpreted here to correspond to the non-depositional submarine stratigraphic gap elsewhere in the relatively deeper parts of the basin from the later part of the Middle Oxfordian Plicatilis Zone to the early Early Kimmeridgian Hypselocyclum Zone.