摘要
In the Bay of Bengal,the Andaman and Nicobar Islands represent part of the Burma-Sunda-Java subduction complex.The Islands are composed of sediments ranging in age from Jurassic to Recent,represented by ophiolites,flysch sediments,along with deep marine sediments scraped off from the subducting plate.The stratigraphic succession that overlies meta-sedimentary and ophiolite suites consists of turbidite and non-turbidite sequences,along with thick-bedded nannofossil chalks.The present study describes ichnofabrics of chalks from the Inglis Formation(Early to Middle Miocene).These chalks are highly to moderately bioturbated and comprise several levels of ferruginised layers as weak discontinuity surfaces.The studied section shows the recurring occurrence of ichnotaxa belonging to Asterosoma,Chondrites,Cladichnus,Ophiomorpha,Palaeophycus,Planolites,Taenidium,Thalassinoides,and Zoophycus.Sediments are represented by Bioturbation indices varying between BI-2 to BI-5,represented by(a)light coloured trace fossils in dark sediment(LID ichnofabric)and(b)dark coloured trace fossils in light sediment(DIL ichnofabric).Ichnofabric analysis suggests multiple colonization,complex tiering,and multilayer tiering.The LID ichnofabric exposed at Kalapathar reveals three tiers,a diverse shallow tier and a moderately low diverse middle and deep tiers.At the Lacam Point Section,in contrast,the LID ichnofabric is represented by condensation of the tiers and the absence of shallow tiers.The DIL ichnofabric at the Kalapathar Section seems to be more expanded and is represented by four tiers with extensive bioturbation.Ichnofabric analysis supports deposition of the chalk sediments in a lower bathyal paleoenvironment and suggests that organic matter,pore water,and bottom-water oxygenation were the main controlling factors.Thus,the ichnofabric analysis of the EarlyMiddle Miocene Inglis Formation gives first-hand information regarding the poorly known chalk facies of the Andaman and Nicobar Islands pre-Bengal fan stage of the Indian plate.
