The composition of the subsurface Neogene shales of the Surma group from the Sylhet Trough, Bengal Basin, Bangladesh

Nineteen shale samples from six wells penetrating the Neogene Surma Group in the Sylhet Trough in northeastern Bangladesh were analyzed by X-ray diffraction, X-ray fluorescence spectrometry and direct current plasma optical emission spectrometry. Mineralogically, the subsurface Neogene shales of the Surma Group comprise predominantly quartz, plagioclase, illite, chlorite, kaolinite, with lower amounts of K-feldspar, dolomite and smectite, and minor to trace amounts of calcite and siderite. Significant variation in the content of K-feldspar, smectite and illite–smectite mixed layer clays with depth has been observed. K-feldspar has not been noted below the depth of about 3200 m, and smectite has not been observed deeper than 3000 m. It is inferred that illite–chlorite, quartz and feldspar are predominantly detrital in origin, reflecting a fast-rising source terrain with no intense weathering activity. Geochemically, the mean major element composition of the shales is similar to that of the average shale with the exception of the CaO content, which is lower here. The low CaO content could be due to lower carbonate and total organic content. The samples are enriched in Sc and Cu and are depleted in Be, Sr and La. Other trace elements have values close to the average shale. The geochemical characteristics suggest that the Neogene shales of the Surma Group were derived from an active margin setting related to a recycled source.