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Subsurface Lithofacies Analysis of Miocene Deltaic Sediments in the Himalayan±Bengal System, Bangladesh

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Subsurface Lithofacies Analysis of Miocene Deltaic Sediments in the Himalayan±Bengal System, Bangladesh ELSEVIER Sedimentary Geology 123 (1999) 239±254 A paleo-Brahmaputra? Subsurface lithofacies analysis of Miocene deltaic sediments in the Himalayan±Bengal system, Bangladesh Ashraf Uddin a,b,Ł, Neil Lundberg a a Department of Geology, Florida State University, Tallahassee, FL 32306, USA b National High Magnetic Field Laboratory, Florida State University, Tallahassee, FL 32306, USA Received 30 January 1998; accepted 22 September 1998 Abstract The Bengal foreland basin contains a succession of up to 16C km of dominantly deltaic deposits, eroded from the eastern Himalayas and the Indo±Burman ranges and carried by major river systems similar to the present-day Ganges and Brahmaputra. Analysis of electric logs and core descriptions acquired during oil and gas exploration in Bangladesh allows construction of lithofacies maps, which constrain depositional patterns of Miocene strata. Compilations of sand thickness and sand=shale ratio of the Miocene Surma Group show that Lower to Middle Miocene strata of the Bhuban Formation accumulated in a large, elongate trough. Sand thickness and percentage both decrease markedly away from this depocenter, which describes a large-scale bend, running initially westward from Rashidpur (northeast Bengal basin) and curving southward toward the Bengal fan. Middle to Upper Miocene strata of the Boka Bil Formation show a similar geographic trend in deposition of coarsest and thickest sediment, but the major depocenter had shifted northward relative to that of the Bhuban Formation by some 30 km, passing near Beani Bazar. These trends suggest that deltaic deposits of the Surma Group ®lled the Sylhet trough of the northeast Bengal basin from the east. Published seismic data from western Bangladesh show that additional large channels also contributed materials to the Bengal basin from the northwest during the Late Miocene, but these channels resulted in very little accumulation in the northwestern part of the basin, probably due to restricted subsidence of underlying continental crust. This study suggests that there was a major drainage system similar to the modern Brahmaputra River during Miocene time, which carried orogenic sediments eroded from the uplifted terranes of the eastern Himalayas and Indo±Burman ranges to the eastern Bengal delta. 1999 Elsevier Science B.V. All rights reserved. Keywords: Bangladesh; deltaic sedimentation; electric logging; Himalayas; lithofacies; paleogeography 1. Introduction ®ll that is more than 16 km thick (Hiller and Elahi, 1984; Reimann, 1993). It is ®lled mainly by oro- The Bengal basin of Bangladesh includes one of genic sediment derived from the eastern Himalayas the largest delta complexes in the world, covering an to the north and the Indo±Burman ranges to the east area of more than 200,000 km2 with a sedimentary (Fig. 1). These deposits record uplift and exhuma- tion of mountain belts formed by the ongoing India± Ł Corresponding author. Tel.: C1 (850) 644-5860; Fax: C1 (850) Eurasia collision. The bulk of the deltaic deposits are 644-4214; E-mail: [email protected] Miocene and younger (Fig. 2). Upper Eocene and 0037-0738/99/$ ± see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S0037-0738(98)00134-1 240 A. Uddin, N. Lundberg / Sedimentary Geology 123 (1999) 239±254 Fig. 1. Map showing major tectonic elements of the Bengal basin. This map also shows location of wells for which geophysical data were analyzed in this study, and locations of outcrop stratigraphic sections depicted in Fig. 3. Hinge zone separates shallow Indian platform from the thick Bengal foreland basin and eastern fold belt. Abbreviations: DF D Dauki fault; IC D Indian craton; MH D Mikir Hills. Oligocene sediments comprise relatively thin mud- Lundberg, 1998a). Miocene sediments of the Surma stone and quartzose sandstone units derived mainly Group, however, consisting of the Early to Middle from cratonic sources of the Indian block (Uddin and Miocene Bhuban Formation and the Middle to Late A. Uddin, N. Lundberg / Sedimentary Geology 123 (1999) 239±254 241 Fig. 2. Stratigraphic framework of the Bengal basin. Note that Miocene sediment thickness is considerably lower in the northwestern part of the basin (Indian platform), which is underlain by continental crust (basement contact not exposed or penetrated in remainder of basin). Abbreviations: CFB D Chittagong fold belts; ST D Sylhet trough. Data sources: Khan and Muminullah (1980), Ahmed (1983), Reimann (1993), and ®eld work by A. Uddin. Miocene Boka Bil Formation, make up thick and international oil and gas agencies. Seismic re¯ec- widespread accumulations of mudstone and quart- tion data have outlined the overall framework of zolithic sandstone derived from the neighboring oro- the delta as well (e.g., Hiller and Elahi, 1984; Salt genic belts. These units have been penetrated by et al., 1986; Lindsay et al., 1991; Radovich et al., exploratory wells, mainly by Bangladesh Oil, Gas, 1995). These seismic re¯ection surveys have been and Mineral Corporation (BOGMC, formerly known carried out mostly in northeastern and northwest- as `Petrobangla') and Bangladesh Petroleum Explo- ern Bangladesh. These data have re®ned current ration, Inc. (BAPEX), with assistance from various understanding of the geometry, nature, and relation- 242 A. Uddin, N. Lundberg / Sedimentary Geology 123 (1999) 239±254 ships between different tectonic domains. Analysis 2.1. Tectonic setting of these seismic data has also led to interpretations of paleogeography and sedimentation patterns of the The basin has two broad tectonic provinces: (1) delta. thin sedimentary strata overlie rocks of the In- Despite its size and record of collisional history, dian craton in the northwest and west, in a part the Ganges=Bengal delta has not been studied in of the basin known as the `Indian platform'; (2) detail compared to other major deltas. Even the off- and very thick basin ®ll overlies deeply subsided shore equivalent strata in the Bengal deep-sea fan basement of undetermined origin in the south and have been studied more intensively (e.g., Naini and east (Bakhtine, 1966; Khandoker, 1989). These two Leyden, 1973; Curray, 1994). The basin is rich in provinces are separated by a northeast-trending hinge hydrocarbons, with estimated reserves of 400 billion zone (Fig. 1), with Indian continental crust extending m3 of natural gas and 40 million barrels of con- beyond the hinge zone toward the southeast (Khan- densate. Studies of the Miocene deltaic sediments doker, 1989). Sediment thickness reaches more than to date have largely focused on hydrocarbon po- 12 km within 200 km southeast of the hinge zone tential (i.e., Brunnschweiler, 1980; Lietz and Kabir, (Paul and Lian, 1975). Thick deposits of the basin 1982); in contrast, processes of deltaic sedimentation ®ll have been uplifted signi®cantly in two areas: (1) and Miocene paleogeography have not yet been well along the north and eastern margins of the Sylhet documented. trough in northeastern Bangladesh; and (2) along In this study, subsurface lithofacies maps of the Chittagong fold belts of eastern Bangladesh. The the Miocene Surma Group have been constructed, Chittagong fold belts comprise tight NNW-trending based on analysis of geophysical logs from wells folds along the eastern edge of the foredeep. The in Bangladesh. These data are used to constrain Sylhet trough, also known as the Surma basin, is depositional patterns of deltaic strata in the Ben- important for its hydrocarbon reserves and exhibits a gal basin. These data have also allowed us to trace large, closed negative gravity anomaly (80 mGal; a Miocene major drainage system developed as a Brunnschweiler, 1980; Salt et al., 1986; Johnson and result of orogenic uplift of the eastern Himalayas Nur Alam, 1991). In the Chittagong fold belts, com- and the Indo±Burman ranges. This paleogeographic plexity of the folds increases eastward, merging with information, in addition to being useful for hydro- the Indo±Burman ranges. carbon exploration, can provide constraints on the geomorphic evolution of the developing foreland in 2.2. Stratigraphy the least-known part of the Himalayan system. The stratigraphy of this region was initially es- tablished by work on exposures in the eastern fold 2. Bengal basin belts, by results of exploratory drilling in the 1930s and by lithostratigraphic correlation to type sections The Bengal basin has developed largely over a in neighboring Assam, northeastern India (Fig. 1; remnant-ocean basin and the rifted eastern conti- Evans, 1964; Holtrop and Keizer, 1970; Khan and nental margin of India (Graham et al., 1975), with Muminullah, 1980). The present stratigraphic frame- continental crust underlying at least the northwestern work of the Bengal basin has been re®ned by sub- portion of Bangladesh (Khan and Agarwal, 1993). sequent studies of palynology (e.g., Baksi, 1962, The basin extends southward into the Bay of Ben- 1972; Chowdhury, 1982; Uddin and Ahmed, 1989; gal toward the Bengal fan, and is bounded by the Reimann, 1993), micropaleontology (Ahmed, 1968; Shillong Plateau, a Precambrian massif, to the im- Banerji, 1984), and seismic stratigraphy (Salt et al., mediate north and by the Himalayas to the distant 1986; Lindsay et al., 1991; Radovich et al., 1995). north. Exposures of the Indian craton form the west- Because this basin has been studied far less inten- ern boundary of the basin and the Indo±Burman sively than parts of the western Himalayan foreland ranges form the eastern boundary (Fig. 1). deposits, we provide here a relatively in-depth out- line of stratigraphic relations. A. Uddin, N. Lundberg / Sedimentary Geology 123 (1999) 239±254 243 Pre-Tertiary stratigraphic units of the Bengal to about 600 m, and the lower part of the unit in this basin are known only from the northwestern part area is partly marine (Biswas, 1961; Banerji, 1981). of the basin (Fig. 2). There, a Precambrian basement The upper part of the Kopili Formation grades into complex made up of gneisses, schists, amphibolite, brownish siltstone and off-white sandstone with local diabase, migmatite, granite, granodiorite, and quartz carbonaceous streaks.
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