Resolving the age of the Mesozoic Kuar Bet Beds (Kachchh, Gujarat, India): A reinvestigation of palaeobotanical and palynological assemblages

摘要

Within the Mesozoic sedimentary sequences of India the age of the Kuar Bet Beds in Kachchh (Gujarat Province) has been hotly contested, with faunal evidence suggesting a Bathonian (Middle Jurassic) age, based on ammonite and foraminiferal occurrences, and palaeobotanical evidence suggesting an Early Cretaceous age, based on the presence of the index fossil Onychiopsis sp. cf. psilotoides (Stokes and Webb) Ward. Previously reported palynological evidence has documented numerous angiosperm pollen taxa, lending further support to an Early Cretaceous age. The present paper reinvestigates the palaeobotanical and palynological assemblages of the Kuar Bet Beds, and discounts previous identifications of Onychiopsis, considering these specimens to represent a new species of the genus Coniopteris, Coniopteris kuarbetensis sp. nov. The plant macrofossil assemblage also contains ovulate scales of Araucarites Presl and rooting organs. Palynological preparations revealed a diverse assemblage that includes ?Sphagnumsporites, Cyathidites, Dictyophyllidites, Todisporites, Concavissimisporites, Klukisporites, Densoisporites, Callialasporites (two sp.), Alisporites spp., ?Podocarpidites, Araucariacites and Ginkgo-cycadophytus, all of which are long-ranging Mesozoic forms that occur in both Jurassic and Cretaceous sediments. Angiosperm pollen has not been observed, and the angiosperm grains reported by Mathur are here regarded as contaminants, introduced during the laboratory treatment of rock samples. Refuting the presence of Onychiopsis and of angiosperm pollen from the Kuar Bet Beds discounts an Early Cretaceous age, with the plant fossils and palynological assemblage being more consistent with a Middle Jurassic age, in general agreement with faunal data. However, the precise position within the Middle Jurassic remains uncertain with palaeobotanical and palynological results being most comparable with a late Middle Jurassic (Bathonian or Callovian) age while faunal evidence supports an early Middle Jurassic (Bajocian) age.