Selected Presentation from the INSTAAR Monday Noon Seminar Series.

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1 Selected Presentation from the INSTAAR Monday Noon Seminar Series. Institute of Arctic and Alpine Research, University of Colorado at Boulder. This seminar presentation has been posted to the internet to foster communication with the science community and the public. Most of the INSTAAR presentations were originally given in PowerPoint format; they were converted to Adobe PDF for posting. You may need to install the free Adobe Acrobat Reader to view these files. These presentations are "works in progress". They are not peer reviewed. They should not be referenced for any kind of publication. Contact the author for proper references and additional information before any use, even for unpublished works such as your own presentations. LICENSING AGREEMENT. Free use of these presentations is limited to a nonprofit educational or private non-commercial context and requires that you contact the author, give credit to the author, and display the copyright notice. All rights to reproduce these presentations are retained by the copyright owner. Images remain the property of the copyright holder. By accessing these presentations, you are consenting to our licensing agreement. 01 Nov Steve Goodbred, State Univ. of New York, Stony Brook, sgoodbred@notes.cc.sunysb.edu "The Ganges-Brahmaputra sediment dispersal system: Controls, responses, and stratigraphy during the Late Quaternary". Seminar given at INSTAAR, University of Colorado. Copyright 2002 Steve Goodbred. All Rights Reserved. Goodbred presentation (3.6 Mb PDF).

2 Selected Presentation from the INSTAAR Monday Noon Seminar Series. Institute of Arctic and Alpine Research, University of Colorado at Boulder Nov Steve Goodbred, State Univ. of New York, Stony Brook, "The Ganges-Brahmaputra sediment dispersal system: Controls, responses, and stratigraphy during the Late Quaternary". Seminar given at INSTAAR, University of Colorado. Copyright 2002 Steve Goodbred. All Rights Reserved. Goodbred presentation (3.6 Mb PDF). Abstract Rivers are the main source of terrigenous sediment delivered to continental margins and thus exert a major control on coastal evolution and sequence development. However, little is known about past changes in fluvial sediment loads despite the recognition of significant variation under changing climatic regimes. In this study we present the first quantified estimate of sediment discharge for a major river system under conditions of an intensified early Holocene monsoon. Development of the Ganges-Brahmaputra River delta began ca yr B.P., when rising sea level flooded the Bengal basin, thereby trapping most of the river s discharge on the inner margin. Chronostratigraphic data from these deltaic deposits are used to calculate the rates of sediment storage on the margin, which provide a minimum estimate of the river s past sediment load. Results reveal that ~5 Å~ 1012m3 of sediment was stored in the Bengal basin from ca to 7000 yr B.P., which corresponds to a mean load of 2.3 Å~ 109 t/yr. In comparison, modern sediment load of the Ganges-Brahmaputra is ~1 Å~ 109 t/yr, ranking it first among the world s rivers and underscoring the significance of a two-fold increase sustained over 4 k.y. Furthermore, the timing of immense discharge in the early Holocene strongly suggests its relation to a stronger than present southwest monsoon in South Asia. Similar patterns of high monsoonrelated sediment discharge have been noted throughout the tropics and subtropics, suggesting a widespread fluviosedimentary response, the potential magnitude of which is showcased by the Ganges-Brahmaputra system.

3 The Ganges-Brahmaputra River Delta: Controls, Response, and Development in the Late Quaternary Steven L. Goodbred, Jr. Marine Sciences Research Center, University at Stony Brook INSTAAR, University of Colorado November 1, 2002

4 Major Rivers of the Himalayan Uplift Annual Sediment Discharge (10 6 t/yr) Indus - 59 (250) Ganges Brahmaputra Irrawaddy Mekong Changjiang Huanghe - (1100)

5 G-B Delta Physiography ~100,000 km 2 of lower floodplain & delta plain Ganges and Brahmaputra Rivers Uplifted sedimentary blocks 2-10 m in Madhupur and Barind multiple Brahmaputra avulsions eastward shift of Ganges Shillong Massif overthrusting Sylhet basin... downthrown block subsiding at 2-4 mm/yr Active shortening at IndoBurman fold belt... Holocene neotectonics Sundarbans world s largest mangrove forest 100 km

6 Swatch of No Ground Canyon

7 Elevation not much height above sea level dark green < 5 m light green < 3 m pale green < 1m 100 km

8 Sediment Dispersal Patterns

9 Sedimentation and Radioisotope Geochronology CHANNEL BRAIDBELT PROXIMAL FLOODPLAIN Fluvial Input Atmospheric Deposition DISTAL FLOODPLAIN BILS (bottomland) Activity (dpm/g) Depth (cm) Pb xs 137 Cs

10 Floodplain Sediment Trapping ~30% of annual discharge stored in delta old coastal plain and delta lobes largely inactive most deposition occurring in landward tectonic basins and floodplains 50(?) 7(?) (?) 86 x 10 6 t/yr

11 Ganges-Brahmaputra Shelf and Subaqueous Delta Three main offshore depocenters: inner shelf clinoform deltafront/ foreset region canyon/off-shelf advection (after Kuehl et al., 1997)

12 Historical Progradation of the Shoreface growth and coalescing of channel-margin bars development of inner shelf sand sheet net of ~7 km 2 new land formed each year` Depth Land 0-2 m 2-4 m 4-6 m 6-8 m 8-15 m (Allison, 1998, JCR)

13 Prograding Shelf Clinoform 5 km 15 m/yr

14 Compound delta: strong coastal shear large sediment load mixed grain-size modern examples: Ganges, Huanghe, Changjiang

15 Offshelf transport: mass wasting into canyon feeder gullies (after Kuehl et al., 1997)

16 Canyon Sedimentation Patterns comprised of turbidite sequence events possibly triggered by cyclones sed. rates >10 cm/yr... up to 50 cm/yr (Michels et al, in press)

17 Ganges-Brahmaputra Sediment Budgets (megatons / yr) Floodplain / Delta Plain Marine Delta Topset Foreset Canyon / Off-shelf Total Discharge t Modern: ? 1000 Mid Holocene: ? 1000? (7-3 ka) data sources: 1 Goodbred and Kuehl, 1998; 2 Allison, 1998; 3 Michels et al., 1998; 4 Goodbred and Kuehl, 1999; 5 Kuehl et al., 1997; 6 Goodbred and Kuehl, 2000.

18 Delta and sequence formation

19 Bengal Basin Borehole Data Goodbred and Kuehl Umitsu study Indian studies misc. unpublished data filled symbols indicate 14 C dating 10 m 50 m 100 m

20 Coastal Plain Stratigraphy depth (m) Umitsu (1993) ,004 Pleistocene Laterite clay silt fine sand medium sand 9876 = calibrated 14 C age Coarse alluvial valley fill Thick, trangressive-phase coastal-plain sequence formed during early Holocene

21 Bengal Basin Borehole Data Goodbred and Kuehl Umitsu study Indian studies misc. unpublished data filled symbols indicate 14 C dating 10 m 50 m 100 m

22 Sylhet Basin Stratigraphy 0 10 Basin Rim 3810 Basin Edge Basin Interior Basin Rim depth (m) Pleistocene Laterite , clay silt fine sand medium sand coarse sand - 10, = calibrated radiocarbon date

23 G-B Delta Margin Sequence sed.. accumulation and thickened sequence in landward tectonic basins very thick TST and alluvial HST Holocene sequence controlled by tectonics, sed supply, and eustacy

24 Late Quaternary Climate: Responses in the Ganges-Brahmaputra

25 Late Quaternary Monsoon Precipitation Last Glacial Maximum Drier than present 18ka 18ka Climatic Optimum Wetter than present 9ka 9ka Present Conditions Arid regions present present (COHMAP, 1988, Science)

26 Ganges-Brahmaputra Shelf at the LGM (18 ka) Oolitic ridges Formation of ooids at G-B rivermouth during lowstand 130 m 140 m Suggests very low river discharge and sediment flux at LGM (Weidicke et al., 1999, Marine Geology)

27 Activation of Bengal Fan with onset of SW monsoon Seismic cross-section of fan channel (Weber et al., 1997, Geology) fan sedimentation and sea level 14 C yr BP (x 10 3 ) Sed. Rate (cm/yr) Rapid sedimentation on delta plain Sea Level (m)

28 25 N 24 Barind Tract Madhupur Terrace Sylhet Basin Tripura Fold Belt Sediment Isopach of Late Quaternary Deltaic Deposits m HIGHSTAND VOLUME: 3.5 x m 3 TRANSGRESSIVE VOLUME: 5.0 x m 3 SEDIMENT LOADS: E 89 Swatch of No Ground 90 Bay of Bengal 0 km ,000 BP 2.3 x 10 9 t/y post-7000 BP 1.0 x 10 9 t/y

29 Ganges-Brahmaputra Sediment Budgets Floodplain / Delta Plain Marine Canyon / Total Delta Off-shelf Discharge Topset Foreset. Modern: ? 1000 Mid Holocene: ? 1000? (7-3 ka) Early Holocene: ? 2600 (11-7 ka) Last Glacial Max: 50? 100? 150? (18-11 ka) data sources: 1 Goodbred and Kuehl, 1998; 2 Allison, 1998; 3 Michels et al., 1998; 4 Goodbred and Kuehl, 1999; 5 Kuehl et al., 1997; 6 Goodbred and Kuehl, 2000.

30 Sources of Immense Sediment Load? From ~ 9 kyr until ~ 5 kyr,, the two [Ganges tributaries] incised m into their former floodplains towards 4.5 kyr until ~ 3 kyr,, the Son and Belan [rivers] aggraded once more, but only to ~ +12 m (Williams and Clarke, 1984, Nature)

31 Bedrock exposure ages ( 10 Be and 26 Al) for Marysandi River valley, Nepal Exposure ages group at 6-8 ka and suggest rapid exhumation of valley sediment fill (~80m) (Pratt et al, in press, Geology)

32 Immense G-B sediment load synchronous with strong monsoon and rapid sea level rise (A. and B. Prins and Postma,, 2000 C. Goodbred and Kuehl, 2000)

33 Tectonic Influences: Ganges-Brahmaputra example

34 Radiocarbon Ages vs.. Sea Level Pleistocene surface 10 Goodbred, Kuehl, 2000 Umitsu, 1987; 1993 Banerjee and Sen, 1986 sidereal years x eustatic sea level sea level mm/yr of subsidence depth below msl (m) 70

35 3-D Stratigraphy of Bengal Basin Dominant Control: fluvial processes tectonics eustacy

36 Brahmaputra channel morphology ( ) effect of 1950 Assam earthquake?

37 Arsenic a mass human poisoning in Bengal

38

39 Araihazar township a case study of 6000 wells Lex van Geen, Yan Zheng, et al., LDEO

40

41 Contrasting Seasons of the Monsoon

42 A Few Slides

43 Delta Plain Vibracore X-radiographs (tidalites) 10 cm 1 meter TOP BOTTOM

44 Sediment Thin-Sections Tidalites and Burrow Cross-bedding Rip-up Clast 1 cm

45 the Ganges Brahmaputra confluence

46 Indo-Himalayan Geologic Subdivisions (after Yokoyama et al., 1990)

47 River Sediment Mineralogy Coarse-grained minerals Fine-grained minerals other (chlorite, mixed layer I/S) kaolinite Epidote:Garnet 2 1 Relative abundance (%) 50 smectite illite 0 Ganges Brahmaputra 0 Ganges Brahmaputra (Heroy, Kuehl, and Goodbred, 2002)

48 (after Derry and France-Lanord, 1997)