Characterization of green clay concretions from the Tonggao Formation, South China: Mineralogy, petrogenesis and paleoenvironmental implications 1 Matthew R.M. Izawa, Roberta L. Flemming, Renbin Zhan, and Jisuo Jin Abstract: Enigmatic millimetre-scale micro-concretions with pseudocrystal faces and dominated by green clay minerals occur in unfossiliferous siliciclastic mudstone of the Lower Ordovician (479.0–466.0 Ma) Tonggao Formation, South China. The fossil-free mudstone unit is associated with local biodiversity decline. The mineralogy and mineral chemistry of these concretions were unknown previously, and this study comprises a preliminary investigation. The concretions are dominated by Fe-rich phyllosilicate minerals including glauconite and clinochlore, with minor quartz and traces of magnetite. The tex- tural relations between the micro-concretions and the surrounding matrix, and the preservation of original mudstone lamina- tions within the concretions, point to an origin during early diagenesis. The mineralogy and chemistry of these concretions are consistent with an origin in a restricted, hypersaline, relatively deep-water environment, in accordance with stratigraphi- cal and paleonotological data. These micro-concretions provide clues for a stressed environment with poor water circulation and anomalies of salinity and oxygen. Résumé : Des micro-concrétions énigmatiques d’échelle millimétrique, avec des faces pseudocristallines et dominées par des minéraux argileux verts, se trouvent dans des mudstones siliclastiques non fossilifères de la Formation de Tonggao de l’Ordovicien inférieur (479,0–466,0 Ma), dans le sud de la Chine. L’unité mudstone sans fossile est associée à un déclin de la biodiversité locale. La minéralogie et la chimie des minéraux de ces concrétions n’étaient pas connues antérieurement et la présente étude comprend une investigation préliminaire. Les concrétions sont dominées par des minéraux phyllosilicates riches en Fe, incluant la glauconite et le clinochlore, avec un peu de quartz et des traces de magnétite. Les relations textura- les entre les micro-concrétions et la matrice environnante ainsi que la préservation des laminations originales du mudstone à l’intérieur des concrétions signalent une origine au début d’une diagenèse. La minéralogie et la chimie de ces concrétions correspondent à une origine dans un environnement aqueux limité, hypersalin et relativement profond, ce qui concorde avec les données stratigraphiques et paléontologiques. Ces micro-concrétions fournissent des indices qu’il s’agissait d’un environ- nement perturbé avec une mauvaise circulation d’eau et des teneurs anormales en sel et en oxygène. [Traduit par la Rédaction] Introduction The Lower Ordovician (479.0–466.0 Ma) Tonggao Formation in Guizhou, South China (Fig. 1), contains abundant micro- concretions of clay minerals, which occur in the largely unfossiliferous units of siliciclastic deposits (mudstone, silt- stone, and sandstone) of the formation (Zhan and Jin 2008). These micro-concretions have been a lithological puzzle for geologists mainly because they have the shape of individual crystals, with sharply defined “pseudocrystal faces and an- gles”, ranging from ∼0.2 to 2.0 mm in size (Fig. 2). Their composition, however, is predominantly a mixture of clay minerals. For this reason, survey geologists in the past have provisionally treated the concretions as pseudomorphs of evaporite crystals and dubbed them “green spots” character- istic of the formation. The occurrences of the green-spotted strata are regionally consistent and have been used as marker horizons for local geological mapping. Despite their abundance in the Tonggao Formation, these nor similar micro-concretions do not seem to have been re- ported or studied in previous literature. In the Tonggao For- mation, the lack of black shale or abundant Fe-sulfides indicates that conditions were probably not anoxic, and the lack of evaporite deposits precludes the possibility that the ba- sin experienced salina conditions. These general observations Received 4 December 2011. Accepted 14 June 2012. Published at www.nrcresearchpress.com/cjes on 17 August 2012. Paper handled by Associate Editor Marc R.M. Bustin. M.R.M. Izawa, R.L. Flemming, and J. Jin. Department of Earth Sciences, The University of Western Ontario, 1151 Richmond St., London, ON N6A 5B7, Canada. R. Zhan. State Key Laboratory of Palaeobiology and Stratigraphy, Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences, Nanjing 210008, China. Corresponding author: Matthew R.M. Izawa (e-mail: matthew.izawa@gmail.com). 1 National Natural Science Foundation of China 40825006. 1018 Can. J. Earth Sci. 49: 1018–1026 (2012) doi:10.1139/E2012-040 Published by NRC Research Press Can. J. Earth Sci. Downloaded from www.nrcresearchpress.com by Shanghai International Studies University on 06/06/13 For personal use only.