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Earth Science Frontiers, Vol. 17, Special Issue, Aug. 2010 ISSN 1005-2321 Depositional Events and Climatic Signals Recorded in the Condensed Oxfordian Dhosa Oolite of the Kachchh Basin, Western India 290 M. Alberti 1 , F.T. Fürsich 1 , D.K. Pandey 2 , M. Ramkumar 3 1. GeoZentrum Nordbayern, Fachgruppe PaläoUmwelt, Friedrich-Alexander-Universität Erlangen-Nürnberg, Loewenichstr. 28, 91054 Erlangen, Germany (E-mail: matthias.alberti@gzn.uni-erlangen.de) 2. Department of Geology, University of Rajasthan, Jaipur - 302004, India 3. Department of Geology, Periyar University, Salem - 636011, India The Kachchh Basin in western India formed following rifting between Africa and India in the Late Triassic (Biswas, 1982, 1991). An E-W oriented chain of domal structures lies on the Kachchh Mainland between the salt marshes of the Great Rann of Kachchh to the north and the Arabian Sea to the south. These domes offer outcrops from Bajocian to Tithonian in age. The top of the Chari Formation (Callovian- Oxfor- dian) is formed by the Dhosa Oolite member, which received considerable attention (e.g. Singh, 1989; Fürsich et al., 1992) due to its highly complex top unit, the Dhosa Conglomerate Bed. This marker bed which can be traced for over 100 km is highly condensed and contains gaps with several million years of apparent non-deposition. A detailed sedimentological analysis led to a model of the events that took place during these times of non-deposition. Ammonites allowed the establish- ment of a chronostratigraphic framework, while palaeoclimatic reconstruction based on oxygen isotopes and clay mineralogy support the model. Genesis of the Dhosa Oolite During the Late Callovian the relative sea level in the Kachchh Basin fell. This is highlighted by a coarsening of the grain size from silt to sand, as well as by the appearance of sedimentary structures such as cross-bedding. The ferruginous ooids of the Dhosa Oolite member are allochthonous and were transported from their near-shore place of origin into the basin. During the Early Oxfordian formation of the Dhosa Conglomerate Bed started (Fig.1). Large concretions formed in the substrate, which had been bioturbated by crustaceans constructing large Thalas- sinoides networks (A). During an erosional phase (B) the soft sediment was removed and the concretions were left exposed on the sea floor forming a discon- tinuous hardground surface with traces of Thalas- sinoides preserved as negative epireliefs on their upper surfaces. The sea level fell further until the sea floor was subaerially exposed (C). Erosion by meteoric waters produced highly irregular, pitted upper surfaces on the concretions. During a transgressive phase the sea once again covered the basin floor and smaller slabs, pebbles, and ammonites were reworked (D). The fine matrix of the resulting highly fossiliferous conglomerate points to relatively deep water conditions below wave base. Bioturbation led to the incorporation of this matrix into deeper sediment layers. During very high- energy events large concretionary slabs were lifted to swim in the matrix (E). The nature of these events has not been satisfactorily determined yet, but earthquakes in the fault-bordered basin and resulting tsunamis are a reasonable explanation. Signs of synsedimentary tectonics are a common feature. As sediment input remained very low, concentration of fossil remains and re- working, especially of ammonites, continued. During quiet conditions oncoids and microbial-stromatolitic crusts formed in some areas of the basin (F). Finally, the whole unit hardened and locally a thin layer of Fe-oolitic fine sand was deposited on top. Subsequently, the sea level in the Kachchh Basin fell again and the sea floor became once more subaerially exposed (G). Erosion by meteoric waters led to the formation of dissolution cavities which later on were filled with sparitic calcite. The fine-grained sandstone layer present at a few localities developed a pitted, irregular surface similar to that of the concretions formed earlier on (C). After this second phase of subaerial exposure the unit was drowned again (H). Palaeoclimatic signals The recorded sea-level changes in the Kachchh Basin could have been caused either by local (tectonic) or global (climatic) phenomena. To evaluate climatic conditions during the deposition of the Dhosa Oolite member isotope analyses of belemnite rostra have been carried out. Calculated temperatures from the Dhosa Conglomerate Bed are lowest and range between 10.3°C and 11.5°C, which is around 6°C less than published data from Early Callovian belemnites and even around 10°C less than values from Bathonian brachiopods of the Kachchh Basin (Fürsich et al., 2005; all temperatures calculated with the equation given by Anderson and Arthur, 1983, and with δ 18 Osea= -1‰ V-SMOW). This decrease in sea-water temperatures towards the Oxfordian may well be connected to the global climatic change leading to glaciation and a fall in sea-level as proposed by Dromart et al. (2003a, b). While the latter authors suggest the peak of glaciation around the stage boundary, the two subaerial phases of the Dhosa Conglomerate Bed are clearly Oxfordian in age as is shown by ammonite data. Offsets in sea-level changes between the Kachchh Basin and global trends can be explained by tectonics in the basin and the rate of
Earth Science Frontiers, Vol. 17, Special Issue, Aug. 2010 ISSN 1005-2321 subsidence. Palaeoclimatic reconstructions will be completed by isotope analyses of benthic organisms such as brachiopods and oysters as well as by clay mineralogy. These results will allow the evaluation of climatic changes from the Callovian to Kimmeridgian in the Kachchh Basin and will provide insights into the exact development of the global sea-level curve, which is still under debate. While Dromart et al. (2003b), for example, propose a sea-level minimum in the latest Callovian, Wierzbowski et al. (2009) suggest maximum flooding. Key words: Jurassic; India; Palaeoenvironment; Palaeoclimate Fig.1 Simplified model illustrating the formation of, and sequence of events recorded by, the Dhosa Conglomerate Bed in the Kachchh Basin References: Anderson T.F., Arthur M.A. Stable isotopes of oxygen and carbon and their application to sedimentolo-- gical and paleoenvironmental problems. In: Arthur M.A., Anderson T.F., Kaplan I.R., Veizer J., Land L. (ed). Stable Isotopes in Sedimentary Geology. Short Course: Society of Economic Paleontolo- gists and Mineralogists, 1983, 10: 1-151. Biswas S.K. Rift basins in the western margin of India and their hydrocarbon prospects with special refer- ence to Kutch basin. American Association of Petroleum Geologists Bulletin, 1982, 66: 1497- 1513. Biswas S.K. Stratigraphy and sedimentary evolution of the Mesozoic basin of Kutch, western India. In: Tandon S.K., Pant C.C., Casshyap S.M. (eds.) Stra- tigraphy and Sedimentary Evolution of Western India. Nainital: Gyanodaya Prakashan, 1991: 74- 103. Dromart G., Garcia J.P., Gaumet F., et al. Perturbation of the carbon cycle at the Middle/Late Jurassic transition: Geological and geochemical evidence. 291
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