The Eocene Sylhet Limestone of Jaflong and Adjoining Areas, Sylhet: an Endangered Geoheritage in Bangladesh

Abstract

This study deals with the identification, characterisation and evaluation of the Jaflong, Eocene Sylhet Limestone (and areas surrounding the outcrop around Sylhet, Bangladesh) as a geoheritage as well as a potential geopark. The Eocene Sylhet Limestone is exposed only in the eastern bank of the Dauki River, Jaflong, in the vicinity of the Dauki Fault zone at the southern fringe of the Shillong Plateau, and outcrops are highly endangered. These outcrops and associated stratigraphic units have a great geo-scientific and educational importance. Typically, the limestone beds are variable and include micrites, dismicrites, intramicrites, pelmicrites, sparites, intrasparites and pelsparites. Geochemical characteristics suggest that the limestone is a very pure calcite-rich carbonate, indicating a low energy, shallow marine depositional environment. Nummulitic limestones (48–50 Ma) present an abundant and diverse larger foraminifer assemblage with genera such as Nummulites Lamarck, 1801; Discocyclina Gumbel, 1868; Alveolina d’Orbigny, 1826; Operculina Silva Manso, 1836; Assilina d’Orbigny, 1839; and a sparse but diverse ostracod assemblage. The foraminifer assemblage has greater biogeographic affinities with Eocene Tethyan and Indopacific than the ostracod assemblages. This limestone also contains a very rich and diversified dasycladalean algal assemblage. The Jaflong Eocene Sylhet Limestone outcrop and surrounding areas score as rank III in an open system of validation of their geological heritage significance, indicating regional importance and including indicial, symbolic, documental and scenic qualities. The results and level of rank derived from different quantitative and qualitative assessments of the geosite also indicate categorisation as an excellent geoheritage. The Jaflong area, with nearby Tamabil and Jaintiapur, also includes most of the geological features necessary for establishing a geopark as the studied geoheritage demonstrates sufficient scientific, scenic and educational content. However, the main feature of the area, the Eocene Sylhet Limestone, is in an extremely vulnerable condition, which provides a strong justification for establishing protection as a geoheritage. It is crucial, therefore, that a legal base for conservation is established as well as a conservation unit or site is delineated.