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Earth Science Frontiers, Vol. 17, Special Issue, Aug. 2010 ISSN 1005-2321 The Impact of Calcareous Nannofossils on the Pelagic Carbonate Deposition from Late Tithonian (Jurassic)-Early Berriasian (Cretaceous) Boundary 276 Xuejuan Da 1* , Xianfeng Tan 1 , Haisheng Yi 2 , Lan Chen 1 1. Chongqing University of Science and Technology, Chongqing 401331, China (E-mail: crossrain59@163.com) 2. Chengdu University of Technology, Chengdu 610059, China Calcareous nannofossils were important producers of pelagic carbonates in Mesozoic. The carbonate deposition in pre-Jurassic time mainly occurred in epicontinental seas. Substantial amounts of deep-sea carbonate are unknown from the Palaeozoic and the early Mesozoic. The Jurassic-Cretaceous boundary interval is characterized by a wide spread increase of the pelagic carbonate production and major changes in the palaeoenvironmental conditions. In late Tithonian and early Berriasian times, before planktonic foamini- fera occurred in rock-formation proportions, calcareous nannofossils were the most important carbonate contri- butors to deep-sea sediments. In Amdo 114 section of the Qiangtang Basin, northern Tibet, the lower part of this section is composed of sandstone, sandy gravel and siltstone, whereas the middle part is limestone, biosparite, and silty limestone. The lower and middle parts are ~295 m thick with abundant bivalves (Pectinidac, Ostrcidac, etc.) and brachiopods (Burmirhynchia sp.). The upper part of the section was divided into two members: the lower member (~90 m thick) consists of well developed grey-green siltstone, calcareous fine-grained sandstone with a few intercalated argillaceous siltstone and silty mudstone, and the upper member consists of grey-dark, medium-heavy layer microcrystalline limestone, medium-thin layer marls and biosparite interbeds, partly as argillaceous and/or silty limestone with bivalves Buchia, Chlamys, Posidonia plus some signi- ficant ammonite fossils. These ammonites mostly pertain to Virgatosphinctinae subfamily (including Aulacosphinctes, Virgatosphinctes), Berriasellinae subfamily (including Blanfordiceras), and Spiticera- tinae subfamily (including Spiticeras). Above- described ammonite fossils are some typical molecules of late Tithonian to early Berrisian. Except for abundant ammonites, calcareous nannofossils were found in the Amdo 114 section. Employing microscope work, the first discovery of the family Watznaueriaceae has been made, Watznaueria fossacincta is distinguished from Watznaueria bri- tannica. From the biostratigraphy and chemostrati- graphy, the sediments are characterized by paucity of assemblages, prevalence of coccospheres, and a re- latively high organic content. There are two classes of preservation: poorly and moderately preserved assemblages. Total nannofossil abundance and species diversity show lower values within calcareous beds, compared to the argillaceous. Assemblage composition appears quite different in the marls and argillaceous rocks compared with the limestones. Fossil abundance and preservation are partly determined by diagenesis, selective dissolution and sedimentary environment. Various authors have inferred the lithogenetic importance of calcareous nannofossils of the Early Jurassic. Limestone-marl alternations have been inter- preted as the result of productivity cycles of schizo- sphares and coccolithophorids producing fluxes of biogenic carbonates to the sedimentary reservoir. However, it remains difficult to find a mechanism to explain why both coccoliths and Schizosphaerella spp. are systematically found in higher quantities in marls than limestone. Our work concerns the quantification of pelagic carbonate production in late Tithonian-early Berriasian, and the comprehension of the main factors controlling production and transfer of pelagic carbonates to the sedimentary reservoir. According to Price (1999), the latest Jurassic was an episode of cold and subfreezing polar conditions with possible polar ice sheets approximately about one-third of the size of those at the present day. During the Late Jurassic to earliest Cretaceous interval the eustatic sea level reached its maximum in the early Tithonian, probably caused by increased seafloor spreading and rifting of the Pangaea continent. Both Haq et al. (1987) and Hallam (1988) agree that the Tithonian was marked by a sea-level high followed by a drop across the Jurassic-Cretaceous boundary. This long-term sea-level fall was probably initiated by decreasing spreading rates causing numerous semi- restricted epicontinental seas, which in turn favored the evolution of endemic taxa. Provincialism in Tithonian- Berriasian times has been proposed for many marine floral and faunal groups on a global scale, e.g. belemnites and ammonites, with endemisms peaking in the Berriasian. Similar patterns are also reflected by calcareous nannofossils, which exhibit a distinctive latitudinal distribution. Acknowledgements: Supported by Natural Science Foundation Project of CQ CSTC (Grant No. CSTC, 2009BB7383) and National Natural Science Foundation of China (Grant No. 40972084).
Earth Science Frontiers, Vol. 17, Special Issue, Aug. 2010 ISSN 1005-2321 Key Words: Jurassic-Cretaceous; Carbonates; Calcareous nannofossils; Northern Tibet References: Erba E., Castradori D., Guasti G., et al. Calcareous nannofossil and Milankovitch cycles: The example of the Albian Gault Clay Formation (southern England). Palaeogeography, Palaeo- climatology, Palaeoecology, 1992, 93: 47- 69. Huang J.J. Nature of the Qiangtang Basin and its tec- tonic evolution. Journal of Geomechanics, 2000, 6(4): 58-66 (in Chinese with English abstract). Li C., He Z.H., Yang D.M. The Problems of Geological tectonics in the Qiangtang Area, Tibet. Global Geology, 1996, 15(3): 18-23 (in Chinese with English abstract). Mattioli E. Nannoplankton productivity and diagenesis in the rhythmically bedded Toarcian-Aalenian Fiuminata section (Umbria-Marche Apennine, central Italy). Palaeogeography, Palaeoclimato- logy, Palaeoecology, 1997, 130: 113-133. Mattioli E., Pittet B. Contribution of calcareous nannoplankton to carbonate deposition: A new approach applied to the Lower Jurassic of central Italy. Marine Micropaleontology, 2002, 45: 175- 190. Noël D., Busson G. L'importance des schizospères, stomoisphères, Conusphaera et Nannoconus dans la genèse des calcaries fins pèlagiques du Jurassique et du Crètacè infèrieur. Sci. Gèol. Bull., 1991, 43: 63-93. Pittet B., Mattioli E. The carbonate signal and calcareous nannofossil distribution in an Upper Jurassic section (Balingen-Tieringen, Late Ox- fordian, southern Germany). Palaeogeography, Palaeoclimatology, Palaeoecology, 2002, 179: 71- 96. Yi H.S., Lin J.H., Zhao B., et al. New biostratigraphic data of the Qiangtang area in the northern Tibet Plateau. Geological Review, 2003, 49(1): 59-65 (in Chinese with English abstract). 277
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