Turbidite sequence located in SE Bangladesh

S.H. Akhter, A.H. Bhuiyan
University of Dhaka
Dhaka, Bangladesh
M. Hussain, M.B. Imam
King Fahd University of Petroleum and Minerals
Dhahran, Saudi Arabia

The basal part of the Miocene Surma Group as exposed at the Sundari Chara outcrop section at the axial part of Sitapahar anticline, Chittagong Hills, southeastern Bangladesh, shows facies associations indicative of a turbidite origin. A total of five turbidite cycles including a complete Bouma sequence have been recognized.

The Surma Group was earlier interpreted to be of deltaic to shallow-marine origin. The recognition of the turbidite in the basal part of the Surma Group will require revision of the existing depositional and tectonic models of the Bengal basin. In addition, this will add to a better understanding on the hydrocarbon generation and accumulation potential in the basin.

Introduction

The importance of turbidites as a potential hydrocarbon source and reservoir has now been widely recognized in global hydrocarbon exploration and production.¹

In recent years, many oil companies around the world have accelerated their efforts in locating deepwater plays. Pettengill1 noted a three-fold increase in cumulative ultimate discovered turbidite reserves since 1970.

The Mio-Pliocene Surma Group in the Bengal basin is a monotonous siliciclastic sequence comprising alternating beds of sandstone and shale. The sequence is considered deltaic to shallow marine.^{2 3 4}

More than 20 gas fields have been discovered in the Surma Group in Bangladesh. This article for the first time presents evidence indicating deepwater origin of the basal part of this sequence.

Facies, facies association

Five well-defined cycles representing Bouma sequence⁵ were observed at Sundari Chara section, Sitapahar anticline, southeastern Bangladesh ( Fig. 1 [118,453 bytes]).

The cycles (Fig. 2 [112,783 bytes]) show an overall fining-upward grain-size profile. The contact between the units within the cycle is gradational, while intercycle contacts are sharp.

Cycle I, which represents a complete Bouma sequence (T_a-e; Fig. 3 [160,246 bytes]) comprises graded sandstone, plane parallel laminated sandstone, ripple-laminated very-fine sand and siltstone, parallel laminated siltstone, and dark grey to black highly indurated fissile shale. Other cycles are incomplete and represented either by T_acde, T_ade, or T_ae units (Fig. 2).

T_a: Light grey, medium to coarse grained, graded-bedded sandstone up to 2 m thick. The unit is underlain by dark grey to black finely laminated shale often showing an erosional contact with the overlying sandy unit. Occasionally, this facies contains mud clasts at the basal part. This unit was observed in four of the five cycles identified in the section.

T_b: Light grey to bluish grey medium to fine-grained, plane parallel-laminated sandstone ranging in thickness from 0.3 to 0.5 m. The basal part of this facies is occasionally dark grey to black in color. The contact between this unit and the underlying sandstone unit is gradational. This unit was observed only in the basal cycle (Cycle V).

T_c: Light grey, ripple-laminated, very fine sandstone to siltstone with bluish grey clay intercalation. The ripples are short, asymmetric, and often show convolute structures (Fig. 3). The thickness of this unit ranges from 0.5 to 0.7 m. The unit shows gradational contact with the underlying parallel-laminated sandstone. This unit was observed only in the lower two cycles.

T_d: Light grey to greyish white parallel-laminated, very fine sandy or silty clay and siltstone with occasional very thin and irregular clay intercalation. The thickness of the facies ranges from 0.25 to 0.5 m. This unit shows a sharp contact with the overlying mudstone unit. The unit was recognized only in the lower three cycles.

T_e: Dark grey to black, indurated, and thinly laminated shale up to 3 m thick. The basal part of the unit consists of dark grey silty mud which grades to black shale at the top. It shows sharp erosional contact with the overlying graded sandstone. This unit was observed in all five cycles and is interpreted as the pelagic mud unit in the Bouma sequence.

Discussion

The Bengal geosyncline is a huge depositional system that extends from the Assam basin in the north through the Bengal Delta basin in the middle to the Bengal Deep Sea Fan complex far south into the Bay of Bengal.^{6 7 8}

The collision of the Indian Plate with the Eurasian Plate and the Burma Subplate during early Oligocene time resulted in major uplift of the Himalayas to the north and Indo-Burman range to the east.^{3 4 9 10} Paul and Lian¹¹ suggested that the Bengal basin came into existence during this period.

The growth of the Bengal Delta was initiated by the huge influx of sediments brought by the rapid erosion of the rising Himalayas during Oligocene which caused the sea to retreat farther south as the river systems developed a megadelta complex. This resulted in accumulation of a thick sequence of alternating arenaceous and argillaceous sediments of the Surma Group representing complex interfingering of deltaic sub-environments with minor and intermittent shallow marine facies.^{2 12} The progradation of the delta to the south resulted in a diachronic unit of clastic wedge including the Surma Group, progressively younger to the south.^{13 14}

In the Bangladesh part of the Bengal basin, the Surma Group is 3,000 to 5,000 m thick and divided into the Bhuban and Bokabil formations.¹² The sequence is fairly well exposed in the eastern fold belt of the Bengal basin including the southeastern part of Bangladesh. This article, for the first time, documents Bouma sequences in the basal part of the Surma Group (Bhuban formation) indicating a deepwater origin of these sediments.

The occurrence of a deepwater turbidite in the lower Bhuban formation and deltaic nature of the overlying sediments indicate a similar tectono-depositional setting to the present day whereby a delta front (the Ganges Delta) progrades towards a deep marine turbidite facies (Bengal Deep Sea Fan complex). Based on overall lithologic characteristics, Dasgupta et al.¹⁵ also favored a deepwater origin for the basal sediments of the Oligocene Barail Group as exposed in the Cacher district, Assam in the north.

The recognition of the turbidite sequence below the deltaic to shallow marine deposits in the Miocene sequence in the Bengal basin will require major revision of the existing tectonic and depositional models of the Bengal basin. In addition, the revised tectonic and depositional model will offer a better optimism with respect to the hydrocarbon potential in the area. The turbidite sequences in the Surma Group may serve not only as a marine organic source but also as a reservoir for hydrocarbon accumulation, a perspective that has never been considered for the region in the past.

Acknowledgment

The authors are grateful to Prof. M. Hoque and Dr. A.M. Nazre Sobhan of the Department of Geology, University of Dhaka, for their encouragement and critical suggestions during the study and preparation of the report. They are also indebted to Dr. David M. Rohr of Sul Ross State University and Dr. Harry Shaw of Imperial College for their review and constructive criticisms in upgrading the manuscript.

References

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The Authors

Syed Humayun Akhter is an associate professor of geology at Dhaka University, Bangladesh. He has worked on a variety of research projects including tectonic evolution and depositional environment of the Neogene clastic sequences in the Bengal basin. He received a PhD degree in geology from Indian Institute of Technology, Kharagpur, India. E-mail: [email protected]
Anwar Hossain Bhuiyan has conducted extensive field investigation in the foldbelt area of Bangladesh. His research interest includes petrology and diagenesis of the Neogene clastic sediments of the Bengal forebelt. He is a lecturer in geology at and holds an MSc degree in geology from Dhaka University.
Mahbub Hussain is an associate professor in the department of earth sciences at King Fahd University of Petroleum and Minerals, Dhahran, Saudi Arabia. His research interests include diagenesis, hydrocarbon source bed study, and brine evolution in evaporitic settings. He holds an MS degree from Acadia University, Canada, and a PhD degree in geosciences from the University of Texas at Dallas.
Badrul Imam specializes in reservoir sedimentology and diagenesis of clastic sequences. He is an associate professor of earth sciences at King Fahd University of Petroleum and Minerals, Dhahran, Saudi Arabia. He received MSc and PhD degrees in geology from Imperial College, University of London.