The Surma group of rocks are well-exposed in the steeper and faulted western flank as well as gentle and wider eastern flank of the asymmetric doubly plunging Sitakund anticline under the Chittagong district from Bariyadhala in the north to Barabkund in the south, which trends regionally in the NNW-SSE directions. Lithostratigraphically, the Bhuban formation consists of alternation yellow to yellowish gray medium to grained sandstone with black shale and siltstone, Boka Bil formation constitutes the medium grained yellow to yellowish gray sandstone-siltstone with bluish gray shale. Eleven facies have been identified with the lithosuccession of this group, which are grouped into turbidite facies association, deep marine basin plain and tide-generated facies association. The sub facies association of tide-dominated delta, estuarine, tidal creek/small estuarine and mud-dominated tidal flat constitute the tide generated facies association. Five parasequences are delineated. The overall depositional environments of the study area vary from deep marine to submarine fan in the Bhuban formation and tidal delta, estuarine channel and tidal flat in the Boka Bil formation at various phases during deposition due to relative sea level rise and fall, sediment supply and basin tectonics.

Recent alluvial sediments are exposed all over Bangladesh, but tertiary sediments are common in the districts of Chittagong and Chittagong hill tracts and Sylhet. The study area is delineated by the latitude 22° 34' N to 22° 41' N and longitude 91° 38' E to 91° 42' E and situated in the northwestern part of Chittagong district covering an area of about 135 sq km within the Sitakund hill range in the Sitakund Upazilla, Chittagong, Bangladesh (Figure 1). The rocks of the upper Bhuban member of Miocene age forms the cliff forming central part of the Sitakund hill range and also in the other hill ranges in the exposed folded areas of the Bengal basin, which are exposed only in road cut, footpath and tributary sections, and these are not easily accessible (Muminullah, 1978; Khan, 1991; and Reimann, 1993). The tertiary sequence of the Chittagong hill tracts and surrounding areas has been widely studied (Akhter et al., 1998; Gani and Alam, 1999; Alam et al., 2003; and Roy et al., 2007). The Miocene marine sediment is believed to extend up to the Dauki fault along the Sylhet-Meghalaya of India-Bangladesh border (Dasgupta and Nandy, 1995; Alam and Ferdous, 1995 and 1996; Akhter et al., 1998; Gani and Alam, 1999; Haque, 2001; and Roy et al., 2006). Detailed sedimentological study of the Surma group of rocks of the Sitakund anticlane is very limited and no one gives paid attention with respect to sequence stratigraphy. Sandstone-siltstone in this group form the reservoir rock for the natural gas of the country. The absence of detailed study has already created some problems in exploration and exploitation of gas from the group. The present work is carried out to study the details of lithology, facies and sequence stratigraphy of the area.

Geologically, the Sitakund hill range is an anticline, which is one of the most prominent structures of the Bengal basin. It is situated in the Bengal foredeep portion of the Bengal Basin, located in the northwestern part of Chittagong district (Curray et al., 1982; and Dasgupta and Nandy, 1995). The Bengal foredeep has two parts, namely, western platform flank and eastern folded flank. The eastern folded flank is again subdivided into two zones, namely, the eastern zone and the western subzone. The Sitakund anticline has the position in the western sub zone of the folded flank, which is considered to be the westernmost continuation of the Arkan-Yoma folded belt. It originates from the deformation by the relative movement of the Indian plate and Burmese sub-plate, since the Sitakund anticline is trending in NNW-SSE direction (Muminullah, 1978; and Reimann, 1993). Most part of the western flank of the anticline is faulted regionally, the alignment of which runs parallel to the NNW-SSE trending regional strike (Figure 1). The rock sequences of the Bhuban, Boka Bil, Tipam Sandstone, Girujan Clay, Dupi Tilla and Dihing formations are exposed in this anticline (Muminullah, 1978; and Reimann, 1993). The formation has an average strike of N 22° W-S 22° E with a higher amount of dip (16°-35°) in the western flank than in the eastern flank (11°-18°).

Earth Sciences Journal, Sequence Stratigraphy, Surma Group, Miocene Marine Sediment, Gross Lithology, Geomorphology, Stratigraphic Succession, Bhuban Formation, Deltaic Environment, Transgressive Systems, Sedimentation Processes, Boka Bil Formation.