DEPOSITIONAL PATTERNS OF DEEP MARINE (?) NEOGENE SURMA SEQUENCES IN THE SITAPAHAR ANTICLINE, CHITTAGONG HILL TRACTS, SOUTHEASTERN BENGAL BASIN

The Neogene clastic strata are very thick in the Chittagong –Tripura Fold Belt along the eastern margin of the Bengal basin. Detailed fieldwork was carried out for facies and facies sequence analysis of these successions within the southeastern fold belt of the basin and samples were analyzed for compositional and nannoplankton studies. The exposed Neogene succession represents an overall basin-ward progradation from deep marine (suggested by the presence of zoophycos lagerstätte) through shallow marine to fluvial environments. According to regionally correlatable erosion surfaces the entire succession (3500+ m thick) in the Sitapahar Anticline can be grouped into three composite sequences; C, B and A, from oldest to youngest .

Composite sequence C (1128+ m) begins with deep-water base-of-slope clastics identified by thin packages of turbidities (mostly partial Bouma sequences) together with some slump and debris-flow deposits contained within thicker intervals of hemipelagic mudstones. Composite sequence B (1293+ m) characteristically depicts tide-dominated open-marine to coastal depositional systems with evidence of cyclic marine regression and transgression. Thick conglomerate bed passing upward to coarse to very fine sandstones with large- to small-scale trough cross-stratification. The top of the composite sequence B is marked by a pronounced erosion surface indicating the final phase of marine regression followed by the gradual establishment of fluvial depositional systems represented by composite sequence A (1080+ m).

Compositionally, the sequences are dominantly quartzolithic (~Q₆₈F₁₀L₂₂), with a “recycled orogenic” provenance. Preliminary nannofossil study on these sequences did not yield any significant data or any age-diagnostic species. A sequence stratigraphic approach has been adopted to interpret the basin-fill history with respect to relative sea-level changes; and to subdivide the rock record into several sequences and units (systems tracts and parasequences) based on identified bounding discontinuities. Although the presence of zoophycos is a strong indication for these units to be marine but the lack of dateable nannoplanktons questions that validity. Further study including seismic data analysis and precise biostratigraphic zonations are needed.