Ammonoid Faunas from Northwestern Guangxi (South China): Taxonomy and Biochronology

Smithian (Early Triassic) ammonoid faunas from northwestern Guangxi (South China): taxonomy and biochronology Arnaud Brarard and Huao Bucher SmithianBlackwell Publishing Ltd (Early Triassic) ammonoid faunas from northwestern Guangxi (South China): taxonomy and biochronology ARNAUD BRAYARD AND HUGO BUCHER Brayard, A. & Bucher, H. 2008: Smithian (Early Triassic) ammonoid faunas from northwestern Guangxi (South China): taxonomy and biochronology. Fossils and Strata, No. 55, pp. 1–179. ISSN 0024-1164. Intensive sampling of the Luolou Formation in northwestern Guangxi (South China) has led to the recognition of several new ammonoid faunas of Smithian age and to the construction of a new biostratigraphical zonation for the Smithian in the palaeoequatorial region. These faunas significantly enlarge the scope of the Smithian Stage, and the new zonal scheme facilitates correlation with other mid- and high-palaeolatitude faunal successions (i.e. British Columbia and Siberia). In ascending order, the new biostratigraphical sequence comprises: the Clypites sp. indet. beds of uppermost Dienerian age, the Kashmirites kapila beds, the Flemingites rursiradiatus beds, the Owenites koeneni beds, and the Anasibirites multiformis beds of Smithian age. Thus, the Smithian of this palaeoequatorial region now includes a newly introduced lowermost subdivision (i.e. the Kashmirites kapila beds) that may approximatively be correlative of the Hedenstroemia hedenstroemi Zone of British Columbia and Siberia. Likewise, the newly introduced uppermost subdivision is equivalent to the Anawasatchites tardus Zone of British Columbia and Siberia. Fourteen new genera (Guangxiceltites, Weitschaticeras, Hebeisenites, Jinyaceras, Xiaoqiaoceras, Nanningites, Wailiceras, Leyeceras, Urdyceras, Galfettites, Guangxiceras, Larenites, Guodunites and Procurvoceratites) and 36 new species (Kashmirites guangxiense, Hanielites gracilus, H. angulus, Xenoceltites variocostatus, X. pauciradiatus, Guangxiceltites admirabilis, Weitschaticeras concavum, Hebeisenites varians, H. evolutus, H. compressus, Jinyaceras bellum, Juvenites procurvus, Paranorites jenksi, Xiaoqiaoceras involutus, Wailiceras aemulus, Leyeceras rothi, Urdyceras insolitus, Rohillites bruehwileri, R. sobolevi, Galfettites simplicitatis, Pseudoflemingites goudemandi, Guangxiceras inflata, Anaflemingites hochulii, Arctoceras strigatus, Hemiprionites klugi, Inyoites krystyni, ‘Paranannites’ ovum, P. subangulosus, P. dubius, Hedenstroemia augusta, Cordillerites antrum, Pseudaspenites evolutus, Guodunites monneti, Procurvoceratites pygmaeus, P. ampliatus and P. subtabulatus) are described. ᮀ Ammonoids, Early Triassic, Luolou Fm, northwestern Guangxi, Smithian, South China. Arnaud Brayard [[email protected]], UMR 5125 PEPS CNRS, France; Université Lyon 1, Campus de la Doua, Bât. Géode, 69622 Villeurbanne Cedex, France; Present address: [[email protected]], LMTG, UMR 5563 CNRS – Université Toulouse – IRD, Observatoire Midi-Pyrénées, 14 Avenue Edouard Belin, F-31400 Toulouse, France; Hugo Bucher [[email protected]], Paläontol- ogisches Institut und Museum der Universität Zürich, Karl-Schmid Strasse 4, CH-8006 Zürich, Switzerland. Introduction data within the Flemingitan and Owenitan subdivisions of the biostratigraphical scheme of Spath (1934). The marine Permo–Triassic boundary record is well Since Chao’s studies, few papers have been published preserved in South China and has attracted the on the Early Triassic ammonoids of South China, attention of many scientists ever since the Meishan and none has reassessed the Smithian ammonoid section was chosen as the global stratotype section succession from northwestern Guangxi. and point (GSSP) (Yin et al. 1996, 2001). Moreover, In order to better understand the dynamics of the Lower Triassic marine sedimentary formations are biotic recovery following the Permo–Triassic mass also widespread in the Guangxi and Guizhou provinces extinction, Early Triassic palaeobiogeographical and of South China. The pioneer contributions of Chao diversity studies are now receiving more attention (1950, 1959) first documented the occurrence of rich (e.g. Brayard et al. 2004, 2006, 2007b; Galfetti et al. Early Triassic ammonoid faunas in northwestern 2007b,c). From this perspective, the South Chinese Guangxi. Chao (1950, 1959) directly integrated his record is of prime importance. Indeed, marine Early © 2008 The Authors, Monograph compilation © 2008 The Lethaia Foundation 2 A. Brayard and H. Bucher FOSSILS AND STRATA 55 (2008) Triassic palaeoequatorial sections are few (see Brayard part of the South China Block during the Early et al. 2006), thus further emphasizing the importance Triassic. Sedimentary deposits in this province, of the abundant ammonoid data from South China. which is also known as the Nanpanjiang Basin, con- Moreover, the widely accepted palaeoequatorial sist of clastic and carbonate rocks deposited in a position of the South China Block during the Early deep-water basin with a few, smaller, isolated carbonate Triassic makes it a key biogeographical reference, platforms distributed within Guangxi and Guizhou since the majority of recent works dealing with Early provinces (Lehrmann et al. 2003). The basin is Triassic ammonoids are from either from mid- or bordered to the north and west by the large Yangtze high-palaeolatitudinal settings (e.g. Dagys & Ermakova carbonate platform, and Lower Triassic deposits are 1990; Tozer 1994). generally distributed along the northern and western Our investigations in northwestern Guangxi have edges of this platform. revealed several new Early Triassic faunas. In this paper we mainly focus on the taxonomy and biostratigraphy of the Smithian ammonoid faunas, as well as their The Luolou Formation implication for low palaeolatitude diversity and The Luolou Formation, which represents an important biochronology. Global correlations across latitudes part of the Early Triassic outer platform facies in and between both sides of Panthalassa are also discussed, southern Guizhou and northwestern Guangxi, was as are new palaeobiogeographical and phylogenetic first well described by Chao (1950, 1959). Its lower implications. part mainly consists of dark grey, thin limestone beds alternating with dark shales, whereas its upper part is composed of massive, grey, nodular limestone. Overlying the Luolou Formation is the +1,000-m-thick Geological framework Baifeng Formation of Anisian age, which consists mainly of clastic turbidites. This influx of terrigeneous General context material suggests a drowning of the basin at that time, Southwestern Asia is a complex collage of orogenic and thus, a concomitant modification in directions belts and successive accretions of Gondawana-derived or rates of convergence between the South and North continental blocks (Boulin 1991). China is tectonically China blocks (Gilder et al. 1995). complex and composed of several blocks that travelled across the Tethys during the Permian and Triassic (Enkin et al. 1992). Southwestern China is Sections from northwestern Guangxi located at the junction of the Chinese, South-East All the studied sections are located in northwestern Asian, and Indian plates, as well as the Tibetan block Guangxi (Fig. 1). The Luolou Formation is located (Fan 1978), but the kinematic and temporal frame of on the periphery of the Carboniferous–Permian their amalgamation is not well understood (Gilder karstic massifs, and often in fault contact with Middle et al. 1995). The South China Block and North China Triassic siliciclastics. Classic sections such as Tsoteng, Block were located in the low latitudes of the eastern sampled by Chao (1950, 1959), were resampled in Tethys during the Early Triassic. Palaeomagnetic order to obtain a precise, detailed record, since data indicate that the South China Block was situated bed-by-bed sampling was not systematically utilized near the Equator (Gilder et al. 1995). for the original collections. The type section near Luolou is now poorly exposed and therefore, was not resampled. With the exception of Tsoteng, all sampled Triassic deposits in South China sections (Figs 2–12) yielding numerous fossiliferous Marine Triassic sediments, represented by carbonate layers were first correlated by means of lithological platforms and deep-water facies, are widespread in or marker beds (e.g. the 3 m thick limestone band the South China Block and other neighbouring containing the Flemingites rursiradiatus beds), which blocks. They are especially well developed and then facilitated the construction of a composite range exposed in Tibet, Qinghai, Yunnan, Guizhou, chart for ammonoid faunas (Fig. 13). Guangxi and Sichuan (Hsü 1940, 1943; Chao 1959; The Smithian lithological succession is very similar Wang et al. 1981). Generally, in the South China within the Jinya and Leye areas and can be easily Block, the Lower Triassic and Anisian are represented summarized (see also Fig. 14). Dark shales alternating by marine deposits, while the Upper Triassic is com- with thin, laminated micritic limestone beds (Figs 3– posed of continental sediments (Tong & Yin 2002). 5, 14 for details; Galfetti et al. 2007a) characterize the Marine deposits in northwestern Guangxi belong lower portion of the Smithian. These recessive rocks to the Youjiang Sedimentary Province, which was are usually only partly exposed and ammonoids are FOSSILS AND STRATA 55 (2008) Smithian ammonoids from Guangxi 3 Fig. 1. Location map of sampled sections (Tsoteng, Jinya, Waili, Shanggan,

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