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Ammonoid zonation in the Oxfordian of the Gondwanian Tethyan margin developed in Kachchh, India

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Krishna J. , Pandey B. , Ojha J.

tom Vol. 4, no. 1

181-181

The Kachchh Basin in the extreme west of India has been considered the most prospective region of the Gondwanian Tethyan margin for the development of an independent Jurassic ammonoid zonal framework. In furtherance of our earlier realizations of independent ammonoid zonal schemes in the Kachchh of Callovian, Kimmeridgian and Tithonian stages, we here present the ammonoid zonal formulation in the Kachchh Oxfordian into a succession of 7 zones that is exclusively developed in Perisphinctidae. The older 3 zones (Bernensis Zone, Obliqueplicatum Zone, and Indogermanus Zone) are developed in the Lakhapur section in the distal exposed part of the basin with good density, diversity and frequency of ammonoids. The 4th zone – the Orientalis Zone which includes the 1st order MFS (maximum flooding surface) is the richest in ammonoids of the entire Kachchh Jurassic in the proximal most exposed part of the basin at Kantkote. The younger 3 zones (Subevolutum Zone, Kantkotensis Zone, and Wagurensis Zone) also developed in the Kantkote-Bharodia section in the proximal exposed part are ammonoid scarce in view of their location on the margin along with stratigraphic position above the 1st order MFS. All the Kachchh Oxfordian 7 zones are precisely correlated with the European Tethyan standard on the basis of common or similar Peltoceratinae (Peltoceratoides in Early and Gregoryceras in the Middle Oxfordian), Perisphinchinae (Properisphinctes and Alligaticeras in Early, Perisphinctes and Larcheria in Middle, Dichotomoceras in early Late, and Pseudorthosphinctes, Orthosphinctes and Idoceras in late Late Oxfordian). The geologically interesting and eventful Kachchh Oxfordian includes the 1st order MFS of the Toarcian-Albian sequence in the younger part of the late Middle Oxfordian Orientalis Zone (equivalent of the European Transversarium Zone, Schilli Subzone, Subschilli Horizon), which subdivides the Kachchh Oxfordian into two altogether contrasting sedimentation regimes with markedly revealing litho-biofacies and environmental frameworks. The Early and Middle Oxfordian until the close of Schilli Subzone time is in extremely slow sedimented, condensed to starved, fining, thinning and deepening upward in irregularly based, lensoidal, pebbly/nodular/conglomeratic, hard grounded, mixed carbonate-siliclastic ammonoid rich facies with increase in the share of carbonates and also in the reworking of pebbles/nodules. There is decrease in clastics, also in presence and size of physical structures and energy framework from margin to basin as also upward. Paleontologically, there is increase in the ammonoid density, diversity and frequency, share of European Tethyan elements, also of relatively deeper water sphaeroceratids, phylloceratids, lytoceratids upward and also from margin to the basin. In contrast, from the start of Rotoides Subzone time to near the close of Oxfordian, the ca 280 m thick sedimentary succession present only in the margin in relatively rapidly sedimented, coarsening, thickening and shallowing upward, is scarce to nearly devoid of ammonoids and other macro-invertebrates.