5.1.2. Stratigraphic formation of the aquifer
The Bengal Basin of the eastern folded belt portion has exposed the molasses sediments of geosynclinals on the age of Neocene that has consisted of varying proportions on the shale, mudstone, siltstone and sandstone. This lithological succession has been subdivided into Surma (Bhuban and Bokabil), Tipam and Dupi Tila Groups according to the classification reported by Alam (1989). Most of the portion of Bangladesh has covered sedimentary deposits of Tertiary and Quaternary that thicken larger portions to the south. Cox’s Bazar, Ukhia Upazila zone has found in the same category (Curray, 1991). The strata of Cox’s Bazar has considered as the last part of deltaic Bangladesh. Most of the part in the lithology on the study area has covered by sandstone and clay layers (Table 2). The southern part of Cox’s Bazar city (Ukhia Upazila) has consisted of the recent sedimentary deposit of Bokabil Formation. However, majority of this area is covered by Tipam formation. The 2nd lithological zone of aquifer has formed by the Girujan Clay in the Middle Miocene age, but it is a very narrow area of the subsurface (Fig. 2). The 3rd lithological formation of aquifer has covered Tipam Sandstone formation that it has been formed a larger portion and it’s two side has been sealed by the clay layers (Fig. 2). The formation of Tipam Sandstone has unconformable underlined by formation of Boka Bil in a gradual dip low sequence upper the 4th aquifer zone. Here the formation of Boka Bil is considered as the oldest portion of lithological characteristics of the aquifer (Zakaria, Islam, Rahman, & Vol, 2015). From the lithological field observation and data measurement and (Fig. 2(a)), it has been indicated that thickness has expanded of the upper sandstone, Dupi Tila (DT) formation from 20 m to140m (sandstone), the formation of Girujan Clay has expanded from 140m to152m (clay zone). Moreover, the formation of Tipam Sandstone is expanded from 152 m to about 183m (Sandstone with clay). The oldest exposed formation thickness of Upper Boka Bil (siltstone with clay or shale) and Middle Boka Bill (sandstone; fine to medium) has expanded 183 m to 187m and 187 to 244 m, respectively. The last zone (sandstone; medium sandstone and massive) which is sealed by upper clay layers and it has considered as the water-bearing zone or higher permeable zone.