An insight into the Eocene tide-dominated estuarine system: implications for palaeoenvironmental and sequence stratigraphic interpretations

摘要

This article describes a complete sedimentary succession of an ancient macrotidal tide-dominated estuarine system based on the detailed outcrop study. The Eocene siliciclastic sedimentary facies of Ameki Group in the southeastern Nigeria provides a record of the sedimentary response to an initial regression, followed by marine incursion (transgression) into the Niger Delta Basin. These sedimentary successions are analogues to the subsurface petrolific Niger Delta lithostratigraphic units. Seven facies associations (FA 1 to FA 7) are documented in the study area and the sediments are interpreted as fluvial channel, tidally influenced fluvial channel, tidal channel, tidal flats, supratidal, tidal sand bar and estuarine embayment (open estuarine) deposits. The occurrence of low diversity ichnofaunal assemblages and/or localised high-density monospecific ichnofossil assemblages indicates brackish-water condition typical of estuarine settings. The suites of assemblages include Scoyenia, Skolithos, Cruziana, mixed Skolithos-Cruziana, Glossifungites, Psilonichnus and Teredolites ichnofacies. A complete depositional sequence is encountered in the Eocene Ameki Group which consists of the lowstand, transgressive, highstand and falling stage systems tracts. This depositional succession was most probably controlled by relative sea level changes, sediment supply, accommodation and regional tectonics which affected the development of Niger Delta Basin.