Pliensbachian nannofossils from Kachchh: Implications on the earliest Jurassic transgressive event on the western Indian margin

Abstract

The oldest rocks within the Kachchh Basin belong to the sediments of Kaladongar Formation exposed in Kuar Bet, Pachchham Island (western India). The Formation’s lowest unit, the Dingi Hill Member has yielded a moderately diversified calcareous nannofossil assemblage that includes the marker species of Lotharingius contractus and Triscutum sullivanii of late Early Aalenian age associated with reworked species of Biscutum finchii, Bussonius prinsii, Crucirhabdus primulus, Crepidolithus pliensbachensis, Discorhabdus criotus and D. striatus suggesting an age spanning NJ4a to NJ7 Zones (Early Pliensbachian, Tethyan ammonite Jamesoni Zone to Middle Toarcian, Variabilis Zone). Additionally, samples from four other Kachchh domal localities (Kachchh Mainland: Jara, Jumara and Habo and the Island belt, Waagad) have also yielded reworked Pliensbachian-Toarcian age (~183 Ma) nannotaxa viz. Crepidolithus granulatus, Diductius constans, Mazaganella protensa, Mitrolithus elegans, Parhabdolithus liasicus, Similiscutum orbiculus, and Triscutum tiziense. This nannotaxa age is much earlier than the ammonite-based Earliest Bajocian date (~171.6 Ma) based on the presence of ammonite Calliphylloceras heterophylloides (Oppel). Additional reworked assemblages have been studied from the Callovian sediments at Jara, the Bathonian-Callovian sediments at Jumara, the Callovian sediments at Habo and the Oxfordian-Kimmeridgian sediments at Wagad. The present data, thus, indicates the presence of a marine connection at least since the Pliensbachian in Kachchh. It is proposed that both global eustatic rise and local tectonics were responsible for this Pliensbachian inundation of the Kachchh Basin. Similar record of Late Pliensbachian age reworked nannofossils have also come from the Masirah Island of the Sultanate of Oman and of ?Aalenian-Bajocian age nannofossils from Kuwait, lending credence to our findings and of the presence of at least Pliensbachian-Toarcian age sediments in Kachchh.