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 exhumation 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

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. diorite (Rahman and Sen Gupta, 1980; Zaher and The Oligocene Barail Formation was deposited Rahman, 1980; Khan, 1991) makes up part of the In- during a major marine regression that exposed most dian craton. It is overlain by an ¾1-km-thick Gond- of the `Indian platform' of the Bengal basin. The wanan bituminous coal formed during the Permo± Barail Formation comprises a variably thick se- Carboniferous in intra-cratonic, fault-bounded basins quence of medium- to coarse-grained sand and (Sengupta, 1966; Zaher and Rahman, 1980). This sandstone intercalated with siltstone, mudstone, and coal-bearing sequence is overlain by about 500 m of intraformational conglomerate. Sand-rich units are Cretaceous ¯ood basalts, the Rajmahal=Sylhet Traps, brightly colored (yellow, yellow±brown, red±brown, which are equivalent to the Deccan Traps of western light to dark pink, and cherry red), massive to thickly India and which are composed of hornblende basalt, bedded, and locally cross-laminated. Interbedded olivine basalt, and andesite (Khan and Muminullah, siltstone is gray and compact; mudstone is gray 1980). Associated `trapwash' deposits are composed and locally laminated and rippled; and conglomerate of red ferruginous mudrock, claystone, and green is reddish brown. Barail rocks exposed along the and red sandstone (Khan and Muminullah, 1980). northern fringe of the Sylhet trough in the foothills Paleocene to Early Eocene shallow-marine to ma- of the Shillong Plateau and near the Dauki fault zone rine deposits of the Cherra Formation have been range from 800 m (Johnson and Nur Alam, 1991) recovered in drill holes in the northwest platform re- to 1600 m (Ahmed, 1983) thick (Figs. 2 and 3a). gion (Khan and Muminullah, 1980) and are exposed Thicker deposits have been encountered in boreholes at a single locality, in a south-dipping block near in the Chittagong Hills tracts. Barail equivalents of the Shillong Plateau in the northwestern part of the the Bogra Formation (Khan and Muminullah, 1980) Sylhet trough (Zaher, 1970). The Cherra Formation, have been penetrated in the northwestern (platform) 169 m to 360 m thick, consists mainly of poorly part of the Bengal basin, where they are less than sorted sandstone, mudrock, and fossiliferous marl, 200 m thick and composed of mostly ®ne-grained, with some carbonaceous material and impure lime- light-gray to gray sandstones, alternating with silt- stones (industry data, reported by Reimann, 1993). stones and dark-gray calcareous and carbonaceous Overlying the Cherra Formation is the Middle mudrocks. The Barail Formation has been inter- Eocene Sylhet Limestone, a 250-m-thick nummulitic preted as deposited in mainly tide-dominated shelf unit interbedded with minor sandstones, which is environments (Alam, 1991), whereas the Bogra For- broadly equivalent to similar shallow-marine car- mation represents estuarine, deltaic to high-energy bonates that developed across much of the region coastal conditions (Banerji, 1984). south of the present-day Himalayas (Sengupta, 1966; The Lower to Middle Miocene Surma Group, con- Banerji, 1981). The Sylhet Limestone is overlain by sisting of the Bhuban Formation and the overlying an argillaceous unit, the Upper Eocene Kopili For- Boka Bil Formation, was deposited during repeated mation, which consists of dark-gray to black, fossil- transgressions and regressions. These widespread iferous, well-indurated mudstone, interbedded with units together reach ½4 km thick in the eastern fold calcareous bands, fossiliferous glauconitic sand- belts (Figs. 2 and 3) and the deeper part of basin, and stones, and limestones (Figs. 2 and 3a). The Kopili an equivalent unit in the northeasternmost Indian plat- Formation ranges from 40 to 90 m thick across form (the Jamalganj Formation) ranges from 150 m most of the Bengal basin, and the lithological and to 1.3 km thick (Fig. 2). Of the three units within the fossil content indicate paralic (brackish±marshy) de- Bhuban Formation, the lowermost and uppermost are positional environments (Uddin and Ahmed, 1989; mainly light-gray to light-yellow siltstone and ®ne- Reimann, 1993). In the Shillong Plateau region, how- grained sandstone, alternating with bluish-gray, bed- ever, the Kopili Formation reaches a thickness of up ded mudrock, whereas the middle unit is composed 244 A. Uddin, N. Lundberg / Sedimentary Geology 123 (1999) 239±254

Fig. 3. Stratigraphic columns of (a) the Sylhet trough. A. Uddin, N. Lundberg / Sedimentary Geology 123 (1999) 239±254 245

Fig. 3 (continued). (b) Chittagong Hills areas, representing the most complete exposed Miocene sections in the Bengal basin. Locations shown in Fig. 1; data sources as in Fig. 2. mainly of bluish to yellowish-gray silty and sandy The Surma Group is unconformably overlain by mudstone. The Boka Bil Formation is generally com- the Upper Miocene to Pliocene Tipam Group, composed of alternating mudstone, siltstone, and ®ne- to posed of the Tipam Sandstone and the Girujan Clay. medium-grained sandstones of various colors, com- The Tipam Sandstone typically consists of yellowish monly ripple cross-laminated. The middle part of the brown to orange, coarse-grained, cross-bedded, mas- Boka Bil is more arenaceous and forms natural gas sive sand and sandstone with pebbles of crystalline reservoirs in the Bengal basin. The top of the Boka Bil and sedimentary rocks, with common carbonized is pelitic, and is popularly known as the `Upper Ma- wood fragments (including petri®ed tree trunks) and rine Shale' (Holtrop and Keizer, 1970), marking the coal interbeds. Where present, the Tipam Sandstone last marine transgression in the Surma Group. John- ranges from 76 m to 2565 m thick, with maximum son and Nur Alam (1991) have interpreted Bhuban thicknesses located in the Sylhet trough, but it is sequences as prodelta and delta-front deposits of a absent in the northwestern part of the basin. The large mud-rich delta system similar to the modern coarse grain size, large-scale sedimentary structures, Ganges=Bengal delta (Fig. 1). Boka Bil sediments and clay balls suggest that the Tipam Sandstone was were deposited in subaerial to brackish environments, deposited in ¯uvial systems, interpreted by Johnson based on mudcracks and pollen types (Hystrichos- and Nur Alam (1991) to have been bedload-domi- phraedis; Holtrop and Keizer, 1970). nated braided-¯uvial systems. The Girujan Clay is 246 A. Uddin, N. Lundberg / Sedimentary Geology 123 (1999) 239±254 mostly composed of brown, blue, purple, and gray collected from the well logs serve to de®ne over- mottled clay, ranging from 168 m to 1077 m thick, all trends in Miocene deposition across the basin. with greatest thicknesses again in the Sylhet district From the electric logs, total thicknesses have been (Khan, 1978). The Girujan Clay accumulated in sub- calculated for each formation, as well as aggregate aerial conditions as lacustrine and ¯uvial overbank thicknesses of sand intervals, aggregate thicknesses deposits (Reimann, 1993). of mud intervals, and aggregate thicknesses of inter- The Plio±Pleistocene Dupi Tila Sandstone un- vals of mixed sand and mud. Calculated thicknesses conformably overlies the Tipam Group, and is best have been combined to construct thickness, percent- represented in the Sylhet trough where it comprises age and ratio maps for paleogeographic analysis. a sandy lower unit and an argillaceous upper unit Intervals of mixed sand and mud are either thinly (Hiller and Elahi, 1984). The lower unit is com- interbedded sand and mud or intermixed sediment posed of gray to yellow, red, pink, purple, yellow (sandy mud, clayey or silty sand) that cannot be to off-white, medium- to coarse-grained, massive to subdivided on the basis of electric logs. Hence these trough cross-bedded sandstone with pebbles of sed- data, which make up 25% of the Bhuban Formation imentary and crystalline rocks. The upper unit of and 21% of the Boka Bil Formation, respectively, the Dupi Tila Sandstone, which has not been iden- have not been used in the construction of lithofacies ti®ed elsewhere in Bangladesh, consists of ®ne- to maps. In northwestern Bangladesh, Miocene stratig- medium-grained siltstone with intercalations of mot- raphy is not well de®ned because the Surma Group tled clay horizons. Silici®ed wood and lignitic layers is not readily differentiated in this area. Thicknesses, are common in this ®ner unit. The thickness of the however, are far less than those in the eastern and Dupi Tila Sandstone ranges from 92 m in northwest southern parts of the basin (Fig. 2). Bangladesh to 2393 m thick in the Sylhet trough (Khan, 1978). The ®ning-upward sequences of the Dupi Tila Sandstone, with alternating channel and 4. Results and discussion ¯oodplain deposits, have been interpreted as mean- dering river deposits (Johnson and Nur Alam, 1991). The ten wells studied are from northeastern, cen- Younger Pleistocene deposits have been identi®ed tral, and southeastern Bangladesh (Fig. 1). Most of only locally as relatively thin subaerial deposits un- these wells have proven reserves of natural gas, sev- conformably overlying the Dupi Tila Sandstone. eral are producing gas, and one (Hazipur-1, located on the hinge zone) exhibited an oil show (Reimann, 1993). 3. Methods The most important observation drawn from the subsurface data (Table 1) is that sand thicknesses and This study is primarily based on electric logs percentages are consistently higher in the eastern (spontaneous potential and resistivity) collected dur- Sylhet trough, near Beani Bazar-1X and Rashid- ing oil and gas exploration at ten representative wells pur-2, than elsewhere in the study area. Sand per- by BOGMC (Fig. 1). These wells were selected centages decrease away from this area in almost because they penetrate the entire Surma Group. all directions for both formations (Figs. 4 and 5). In general, older and younger units were either The only other locality in which the Bhuban Forma- not penetrated or not logged. Well compilation re- tion has a comparably high sand content is at the ports for each well have also been consulted. These Hazipur-1 well, in the hinge zone (more than 200 km reports include composite logs, core descriptions, west from the Beani Bazar-1X area. At Hazipur-1, foraminiferal and palynological biostratigraphic re- however, sand content in the Boka Bil Formation is ports, and lithostratigraphy. Data from other wells quite low, less than half of the average for the for- and outcrops have also been reviewed. Although mation. Although sand percentages for the Bhuban the well logs themselves, proprietary biostratigraphy, Formation in the Beani Bazar-1X and Rashidpur-2 and seismic data have not been released for publi- areas (eastern Sylhet trough) are close to the average cation by BOGMC and BAPEX, the data we have formation values (21%), sand content in the Boka A. Uddin, N. Lundberg / Sedimentary Geology 123 (1999) 239±254 247

Table 1 Subsurface lithofacies data of the Miocene Bhuban and Boka Bil formations, Bengal basin, Bangladesh

Well name Total Sand Sand Shale Shale Mixed Mixed Sand=shale thickness thickness % thickness % thickness % (m) (m) (m) (m) Boka Bil Formation Hazipur-1 854 89.6 10.5 523.4 61.3 241 28.2 0.17 Chaatak-1 610 109.2 17.9 308.8 50.6 192 31.5 0.35 Atgram-1X 1171 114.8 9.8 892.9 76.3 163.3 13.9 0.13 Beani Bazar-1X 1009 538 53.3 308 30.5 163 16.2 1.75 Rashidpur-2 1424 474.2 33.3 740.8 52 209 14.7 0.64 Titas-1 1530 471.5 30.8 873.6 57.1 184.9 12.1 0.54 Kamta-1 1710 375.4 22 988 57.8 346.6 20.2 0.38 Bakhrabad-1 1210 152.5 12.6 605 50 452.5 37.4 0.25 Begumganj-1 1100 357 32.5 516 46.9 227 20.6 0.69 Semutang-1 1280 414.7 32.4 698.1 54.5 167.2 13.1 0.59 Average % 25.51 53.7 20.79 0.549 Bhuban Formation Hazipur-1 883 218.3 24.7 474.3 53.7 190.4 21.6 0.46 Chaatak-1 1052 205.8 19.6 379.4 36.1 466.8 44.3 0.54 Atgram-1X 1920 288.9 15.1 1193.5 62.1 437.6 22.8 0.24 Beani Bazar-1X 471 111 23.6 207 43.9 153 32.5 0.54 Rashidpur-2 1680 468.9 27.9 773.2 46 437.9 26.1 0.61 Titas-1 1396 291.8 20.9 704.3 50.5 399.9 28.6 0.41 Kamta-1 874 227.3 26 472 54 174.7 20 0.48 Bakhrabad-1 1068 190.1 17.8 700.7 65.6 177.2 16.6 0.27 Begumganj-1 1076 118.4 11 637.7 59.3 319.9 29.7 0.19 Semutang-1 1666 350 21 1105.3 66.3 210.7 12.6 0.32 Average % 20.76 53.75 25.48 0.406

Bil Formation is dramatically higher than average in the Boka Bil Formation (698 m) in the southeastern these areas (26%; Table 1). The lowest sand=shale area (Semutang-1, northern Chittagong). ratio for the Boka Bil Formation (0.13) is at the Atgram-1X well in the extreme northeastern corner 4.1. Depositional patterns of Bangladesh. Sedimentation patterns changed in detail between Sand thickness and sand=shale ratio maps the Early Miocene and the Late Miocene, but overall (Fig. 4a,b, Fig. 5a,b) of the ten wells analyzed depositional patterns remained similar. In particu- suggest the following depositional patterns for the lar, sand percentages and thicknesses at locations of sediments of the Bhuban and the Boka Bil forma- most eastern and southern wells increased from the tions. The Miocene deposits in the northwestern part Bhuban Formation to the Boka Bil Formation (Ta- of the Bengal basin are very thin relative to other ble 1). For example, the sand percentage of Bhuban parts of the basin, presumably due to limited sub- strata at Beani Bazar-1X (24%) is less than half sidence in this area. The exposed Indian shield is that in the Boka Bil Formation (53%), and the over- located not far from here, and the area is ¯oored by all thickness of the Bhuban Formation is less than buoyant continental crust as opposed to the thinned half that of the Boka Bil Formation. In the hinge continental and oceanic basement in other parts of zone (Hazipur-1), in contrast, the sand percentage the basin. Also, the northeastern part of the basin in the Bhuban Formation (25%) is over twice that was affected by tectonic loading, both from the east of the Boka Bil Formation (11%). Shale thickness (Indo±Burman ranges) and the northeast (eastern Hi- decreases considerably from the Bhuban (1105 m) to malayas) which caused nearby parts of the basin to 248 A. Uddin, N. Lundberg / Sedimentary Geology 123 (1999) 239±254

Fig. 4. Subsurface lithofacies maps of the Bengal basin showing trends in clastic sediment accumulation in the Lower to Middle Miocene Bhuban Formation. (a) Isopach map of cumulative sand thickness (in meters). (b) Facies map of sand=shale ratio. (c) Trends of sand deposition inferred for the Early to Middle Miocene. A. Uddin, N. Lundberg / Sedimentary Geology 123 (1999) 239±254 249

Fig. 5. Subsurface lithofacies maps of the Bengal basin showing trends in clastic sediment accumulation in the Middle to Upper Miocene Boka Bil Formation. (a) Isopach map of cumulative sand thickness (in meters). (b) Facies map of sand=shale ratio. (c) Trends of sand deposition inferred for Middle to Late Miocene; note overall similarity to trends inferred for the Early to Middle Miocene (see Fig. 4c). 250 A. Uddin, N. Lundberg / Sedimentary Geology 123 (1999) 239±254 subside further, helping to accommodate huge thick- river system, draining from the Indo±Burman ranges nesses of Miocene sediments of the Surma Group. straight to the Bengal fan through the northern Trends from the lithofacies maps of the Miocene Chittagong Hills tracts (Figs. 4 and 5). These two Surma Group in Bangladesh suggest that deltaic Miocene streams were likely distributaries of a ma- deposits ®lled the Sylhet trough of the northeast jor river system that transported orogenic sediment Bengal basin from the east, and that the source ter- into the paleo-Bengal fan. The lithofacies maps also ranes of this sediment included the north-trending suggest that the Shillong Plateau, an uplifted massif Indo±Burman ranges directly adjacent to the east of Precambrian basement rocks to the north of the and the eastern Himalayas. This is consistent with Bengal basin (Fig. 1), was not a source for Surma preservation of thick orogenic sequences (Rao, 1983) Group deposits. This ®nding is consistent with the in Assam, northeast India (Fig. 1). The consistent in- results of petrologic studies of sediments from the crease in sand percentages from the Bhuban to Boka Sylhet trough, which suggest that the plateau was up- Bil formations in most of the eastern wells suggests lifted during Pliocene time (Johnson and Nur Alam, progradation of the delta and=or westward migration 1991). of the Indo±Burman ranges as a proximal source. These Miocene lithofacies data from the eastern The lowest sand=shale ratio for the Boka Bil and northern Bengal basin are consistent with petro- Formation (0.13) is at the Atgram-1X well, in the logic records in the Bengal basin sediments of ero- extreme northeastern corner of Bangladesh. This sional unroo®ng of the nearby orogenic belts. Clear well is located remarkably close (about 30 km) to evidence of this unroo®ng has been documented in the Beani Bazar-1X area, which has the highest Miocene through Pleistocene strata in the Bengal sand=shale ratio (1.75). There is no obvious expla- basin, by both sandstone modal analysis (Uddin and nation for this anomaly; it may be due to structural Lundberg, 1998a) and heavy-mineral data (Uddin reasons, although no major fault has been recog- and Lundberg, 1998b). The ®rst clearly orogenic de- nized that might have juxtaposed the rocks of these tritus appears in lowermost Miocene strata. These two areas. The area of the Atgram-1X well may sands have substantial lithic components and both represent an interdistributary setting between major plagioclase and potassium feldspars, as well as a channels, one of which clearly drained through the host of diverse heavy minerals, including aluminosil- Beani Bazar area. icates, pyroxenes, amphiboles, and tourmalines, sug- Depocenters in the Bengal basin did not migrate gesting exhumation of metamorphic terranes from signi®cantly during the Miocene, suggesting that the adjacent orogenic belts to the east and north. there was no major shift in the location of maximum Relatively high sand content in the Bhuban For- subsidence during that time (Uddin, 1990). The im- mation in the Hazipur-1 area suggests that, like the mediate source of these sediments remained almost ¯uvial network of the present-day Ganges, the north- constant, because the depocenters for the Early to western part of the Bengal basin was also drained by Middle Miocene Bhuban Formation and the Middle stream systems during the Early to Middle Miocene. to Late Miocene Boka Bil Formation are only ¾30 There was shifting of one such channel in the Late km apart. This insigni®cant shift of depocenters may Miocene, as seen by the lower sand percentage in the be attributed to channel shifting (avulsion) of a ma- Boka Bil Formation. Input of detritus from the north- jor Miocene river. Considering the Oligocene and west has also been suggested by Salt et al. (1986), Miocene stratigraphy of Assam, immediately to the Alam (1989), and Lindsay et al. (1991). Seismic northeast, it is likely that this major river originated stratigraphy reveals that there were major sandy tidal near the eastern Himalayan syntaxis. mega-channels (8±10 km wide and 100 m deep) in Increases in sand percentages from the Bhuban to the southwestern part of the Bengal basin between the Boka Bil formations in northeastern Bangladesh 10.5 and 5 Ma (Late Miocene; Lindsay et al., 1991). may re¯ect westward advance of the Indo±Burman These data suggest that this area was an active part orogenic front toward the Sylhet trough. The litho- of the Ganges±Brahmaputra delta, in which a dis- facies maps also suggest the presence of another tributary channel shifted through the Hazipur area. A. Uddin, N. Lundberg / Sedimentary Geology 123 (1999) 239±254 251

4.2. Regional implications zone), where long-term accumulation was restricted by very limited subsidence. Subsurface lithofacies maps show that Lower Middle to Upper Miocene strata of the Boka Bil to Middle Miocene sand-rich deltaic strata of the Formation show a geographic trend similar to that Bhuban Formation de®ne a large, elongate depocen- shown by the Bhuban Formation in deposition of ter. The maximum grain size and sand thickness de- coarsest and thickest sediment, but the depocenter scribe a large-scale meander westward from Rashid- had shifted northward by about 30 km, passing near pur (northeast Bengal basin) and curving southward Beani Bazar. As in the time of deposition of the toward the Bay of Bengal and the Bengal fan. Bhuban Formation, there is also a second sediment This sediment lobe can be traced down to south- distribution system that passed southward through ern Bangladesh. A second lobe distributed sediment the northwest Chittagong Hills tracts in southeastern southward through the Chittagong Hills of southeast- Bangladesh. Sand content in depocenters of the Boka ern Bangladesh, where sand contents and thicknesses Bil Formation is generally higher than that in the are high relative to the surrounding regions. There Bhuban Formation. was likely a third major stream in the northwestern These data, taken together, suggest that there was Bengal basin (in the Hazipur area, near the hinge a major paleochannel, similar to the present-day

Fig. 6. Traces of drainage systems inferred from subsurface facies analysis. Paleo-Brahmaputra drainage is hypothesized to have originated close to inferred location of eastern Himalayan syntaxis (on the basis of regional stratigraphic relationships) draining much of the Assam valley to enter the northeastern Bengal basin. Deltaic deposition apparently prograded southwestward between advancing deformation fronts of Himalaya and Indo±Burman orogens. Present-day locations of rivers, coastline, and Main Boundary Thrust shown for reference. 252 A. Uddin, N. Lundberg / Sedimentary Geology 123 (1999) 239±254

Brahmaputra, that ¯owed from near the eastern Hi- gal basin from the northwest, with little net sediment malayan syntaxis. This paleo-Brahmaputra system preservation. Geographic trends during the Middle apparently drained through the upper Assam valley to Late Miocene are very similar to those of the and reached the eastern part of the Sylhet trough to Early to Middle Miocene, although sand content enter the Bengal basin, and eventually drained to the generally increased through the Miocene. This inves- Bengal fan (Fig. 6). These data also show that rocks tigation suggests that a major paleo-drainage system of the Shillong Plateau were not a source of the ¯owed from the eastern Himalayan syntaxis, drain- Miocene deltaic sediments of the Bengal basin, con- ing through present-day Assam and bringing oro- sistent with inferred Pliocene uplift of the Shillong genic detritus into the Sylhet trough (Surma basin) Plateau (Johnson and Nur Alam, 1991). The Indo± from the east. The Indo±Burman ranges probably Burman ranges probably contributed sediment into contributed sediment into the Chittagong area, but the Chittagong area, but the major sediment source the major sediment source for Miocene strata of for Miocene strata of the Bengal basin was likely the the Bengal basin was likely the early uplifts of the early uplifts of the eastern Himalayas. eastern Himalayas. The southeasternmost portion of the Bengal basin contains perhaps the only intact portion of the remnant Tethyan oceanic basement in the Himalayan Acknowledgements collision (Curray et al., 1982; Klootwijk et al., 1992; Beck et al., 1995; and others). The collision prop- The authors thank the Honorable former Minister agated from west to east (Burch®el, 1993; Rowley, for Energy and Mineral Resources of the Govern- 1996), beginning during the Paleocene in Pakistan ment of Bangladesh and the former Chairman of (Beck et al., 1995). Therefore, at the time signi®cant BOGMC for permission to analyze subsurface logs uplifts began in the western Himalayas, the region to from BOGMC and BAPEX. BAPEX, and especially the east was occupied by a shallow sea or only shal- M. Moinul Huq and Nazim U. Ahmed, who provided low uplifts. Initial uplifts in the eastern Himalayas valuable logistical assistance. Andrew Miall, Allen and the Indo±Burman ranges in the Oligocene, and Clark, Leroy Odom, Jim Tull, and Paul Ragland more intense uplift in the Miocene, apparently fun- have provided constructive comments on an earlier nelled orogenic detritus along a major drainage (pa- version of this paper. We thank NSF (grants INT- leo-Brahmaputra?) through the remnant ocean basin. 9300295 and EAR-9419086), Geological Society of Miocene depocenters in the Bengal basin migrated America, Sigma Xi, and the Department of Geol- from east to west and from north to south, toward the ogy at FSU for ®nancial support. This paper was Bay of Bengal, as underthrusting of India beneath improved by thoughtful reviews by Peter DeCelles, southeast Asia along the present-day Java trench and George Postma, and Michael Zaleha. its northern extension continued.

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