Original article An enigmatic piece of amber in the Oligocene of the Assam-Arakan Basin (Eastern India) § Vincent Girard a,b, *, Ignace Salpeteur c , Suryendu Dutta d , Hugues Bauer c , Srinivasan V. Raju e a Centre de Bio-Arche ´ologie et d’E ´ cologie (UMR 5059 CNRS/EPHE/INRAP), Institut de Botanique, Universite ´ Montpellier 2, 163, rue Auguste-Broussonet, 34090 Montpellier, France b Universite ´ Montpellier 2, place Euge `ne-Bataillon, 34095 Montpellier, France c BRGM, 3, avenue Claude-Guillemin, 45060 Orle ´ans cedex 2, France d Indian Institute of Technology, Powai, Mumbai 400 076, India e Center of Excellence for Energy Studies, G.S. Road, Rukminigaon, Guwahati 781022, India 1. Introduction Amber is a fossil plant resin and scientists always tended to identify their botanical origin. Go ¨ ppert (1846) already identified the plant that secreted Baltic amber and named it as Pinites succinifer (later called Pinus succiniferus). Recent advances in spectrometry allow the development of physical characterisation of amber and confirmed that amber originates from various kinds of plants such as conifers and Fabaceae (Langenheim, 2003). These works highlighted that amber corresponds to polymerised plant resins; nevertheless, precise studies dealing with all the phenom- ena that occur during the fossilisation and diagenesis/metamorph- ism of the resin are still lacking. The sample studied here helps to clarify this, and brings new data to the study of fossilisation processes of amber. Amber studies increasingly developed over the last 20 years, and many new outcrops have been found over the world (Penney, 2010). If until the 2000s amber research mostly focused on European and North American outcrops, recent works proved that amber can also be found in other part of the world. Recent examples are the discoveries of Cenozoic ambers in Amazonia, Australia and India. Antoine et al. (2006) described the arthropods and microfossil assemblages found in Middle Miocene amber from Amazonia. Hand et al. (2010) mentioned the discovery of an important amber outcrop in the Northern part of Australia (Cape York Peninsula). Recently, important accumulations of Eocene amber age have been described from the western part of India (Rust et al., 2010; Dutta et al., 2011). All these discoveries highlight one common characteristic of the amber deposits: amber-bearing layers never suffered metamorphic, or even anchimetamorphic conditions. In 2007, the BRGM (France) and the Directorate General of Hydrocarbons (India) performed a preliminary resource estima- tion of oil shale deposits associated with the Namchick-Namruk Makum coalfield in Assam (North-Eastern part of India) with the local support of MECL (Mineral Exploration Company Ldt.). During Geobios 48 (2015) 1–8 A R T I C L E I N F O Article history: Received 7 April 2014 Accepted 3 December 2014 Available online 23 December 2014 Keywords: Taphonomy Metamorphism Amber preservation Oil shale Makum coal field Assam-Arakan Basin Oligocene Barail Group A B S T R A C T Oil shale investigations in eastern India led to the discovery of an amber-filled vein within a rock that suffered sub-anchimetamorphic conditions whilst this material has never been found elsewhere than in low-grade diagenetic rocks. This discovery questions amber resistance to the increase of temperature and pressure. Our investigations indicate that a solid resin and incompletely polymerised amber are able to melt at a temperature of about 100–150 8C. It appears that the studied amber nodule melts during the increase of the diagenetic grade. Chemical analysis demonstrates that such event do not modify the chemical signature of the amber and microstructure shows that cooling was probably slow, allowing the acquirement of microstructure similar to a normal amber piece excepted the high abundance of minute pyrite crystals. ß 2015 Elsevier Masson SAS. All rights reserved. § Corresponding editor: Gilles Escarguel. * Corresponding author. Centre de Bio-Arche ´ ologie et d’E ´ cologie (UMR 5059 CNRS/EPHE/INRAP), Institut de Botanique, Universite ´ Montpellier 2, 163, rue Auguste-Broussonet, 34090 Montpellier, France. E-mail address: vincent.girard@univ-montp2.fr (V. Girard). Available online at ScienceDirect www.sciencedirect.com http://dx.doi.org/10.1016/j.geobios.2014.12.001 0016-6995/ß 2015 Elsevier Masson SAS. All rights reserved.