A Comparative Study of Ichnofossils from Upper and Middle Bhuban Unit of Bhuban Formation (Surma Group), Aizawl, Mizoram to Decipher the Depositional Environment

Abstract

Objectives: The aim of the present study is to interprete and compare depositional environment of Upper and Middle Bhuban rocks from its trace fossil content. Methods: The trace fossils collected from the Upper and Middle Bhuban Formation are studied group wise up to species level for their systematic paleontological description with the help of type material and available literature in the laboratory. Distribution pattern of trace fossils in the sediments such as orientation, diversity, mode of preservation, relationship with the host sediments are also collected in order to interprete the depositional environment. Findings: Ichnological analysis of Upper Bhuban and Middle Bhuban Unit has been attempted to decipher the palaeoenvironment. The Upper Bhuban Unit are represented by a diverse lithofacies comprises of sandstone, shale and silty-sandstone with the presence of wavy sedimentary structure and parallel lamination. A total of 12 ichnospecies found in Upper Bhuban Unit belongs to Skolithos, Cruziana and mixed Skolithos – Cruziana ichnofacies. Phycosiphon isp. is reported for the first time in North-East India. Middle Bhuban Unit exposed along Zuangtui section comprises ~40m thick sequence of sandstone, siltstone and shale.17 ichnospecies belonging to Skolithos, Cruziana, Teredolites and mixed Skolithos – Cruziana ichnofacies has been identified along this section. Diplopodichnus is reported for the first time from the Surma rocks of Mizoram as well as other Miocene successions in India. Presence of Teredolites ichnofacies indicates marine margin environment, Skolithos ichnofacies indicate sandy shifting substrate and high energy conditions in foreshore zone while Cruziana ichnofacies indicate unconsolidated, poorly sorted soft substrate and low energy condition in the shoreface/offshore zone. Novelty: Present study corroborate that Upper Bhuban rocks were deposited under shallow marine environment while Middle Bhuban rocks were deposited in a deeper, quiet water environment.