Unveiling the Ancient: Reconstruction of Paleoenvironment of the Mio-Pliocene Sediments through Micropaleontological Analysis

This study aims to reconstruct the paleoenvironmental condition by studying the taxonomy and ecology of palynomorphs and diatoms and their dominance-diversity of the Mio-Pliocene sediments of Shahbazpur gas field, Bhola, Bangladesh. Seven sediment samples from different well depths have been collected and prepared for micropaleontological analysis following standard methodology. After observing the temporary slides of palynomorphs under the transmitted light microscope, all the imaged samples have been identified and an inventory has been prepared with all the taxonomical, morphological characteristics and ecological records. Data has been scrutinized for paleoenvironment and Alpha diversity by using POLPAL Diagram, and PAST 4.03 software respectively. The study found that the sediment samples contained 43 identified species of varying ecology, indicating, a fresh to brackish water marshy-estuarine, marginal area of land and sea of the transitional environment to a very early stage of the formation of the delta, tidal flat, mangrove swamp, terrestrial and freshwater floodplain environment in humid climates. In addition, evidence of grazing mammals was found at depths of 2593.21m to 2593.23m and 2591.95m to 2591.98m. Furthermore, evidence of grazing mammals and high production of herbaceous plants due to heavy monsoon rain were identified at depths of 2591.52m to 2591.55m. The ecological complexes of microfossils indicate shallow marine to fluvial-deltaic conditions with the indication of continuous shoreline regression towards the sea, developing mangrove swamps with some terrestrial flood plain area. Statistical analysis expresses shallow marine to the transitional environment in the deeper part of the well and transitional to a fluvial-deltaic condition in the shallower part of the well. Dominance is low to moderately high; the Simpson diversity index exhibits a low to moderately high degree of diversity and the Shannon-Wiener diversity index designates very low diversity. Therefore, the study describes the paleoenvironmental situation of the Shahbazpur area during the Mio-Pliocene time.