Biostratigraphy, palaeoenvironment and sea level changes during pre-collisional (Palaeocene) phase of the Indian plate: Palynological evidence from Akli Formation in Giral Lignite Mine, Barmer Basin, Rajasthan, Western India

Abstract

Giral Lignite Mine succession (Akli Formation) from Barmer Basin, Rajasthan yielded rich assemblage of dinoflagellate cysts and compared with the global dinoflagellate zonation schemes, and a Danian- Thanetian age is proposed. Within a precise biostratigraphic control, changes in the terrestrial and marine palynomorphs allowed the construction of relative sea level fluctuation curve and the development of depositonal succession in a sequence stratigraphic framework. Based on the lithological and palynological content, the Giral Lignite Mine succession is divided in to five units. Unit 1 (lower 7.8 m) is interpreted as transgressive deposit formed in an open bay setting during rise in the relative sea level representing the Transgressive Systems Tract (TST). The age is Danian on the basis of dinoflagellate cyst assemblages. Unit 2 (16.5 m) consists of 5-6 laterally occurring lignite bands. It is deposited during decrease of sea level and increased detritus supply in essentially salt marsh environment during Highstand Systems Tract (HST). Unit 3 (3.42 m) is shallow marine facies of Thanetian age deposited in a restricted setting represents a Transgressive Systems Tract. Unit 4 (0.9 m) is marked by a return to coastal marsh depositional conditions, possibly representing a Highstand Systems Tract. The erosional surface at the top of Unit 4 is interpreted as a Sequence Boundary. Unit 5 (9.3 m) is highly oxydized and is devoid of any organic matter content. Two major marine flooding surfaces identified in the Giral Lignite Mine succession can be correlated with the Maximum Flooding Surfaces (MFS) at 60.7 Ma and 57.9 Ma corresponding to the eustatic curve. Based on the dinoflagellate biostratigraphy Danian- Thanetian age has been assigned to the lignite bearing succession of Giral Mine. Western Indian margin consist of large-scale lignite deposits in Kutch, Cambay and Rajasthan Basin. The proposed Danian-Thanetian age of lignite bearing succession of Giral Mine is much older than the late Thanetian-early Ypresian age of lignite successions of Cambay and Kutch Basins of Gujarat. The present study suggests a significantly diachronous development of lignite deposits on the western peninsular margin of India.