Syed H. Akhter

Effects of tectonic deformation and sea level on river path selection: Theory and application to the Ganges‐Brahmaputra‐Meghna River Delta

The set of active rivers of the Ganges-Brahmaputra-Meghna (GBM) Delta in Bangladesh overlies an active plate boundary that continually modifies the landscape of the delta by deformation. The response of rivers to spatially variable subsidence, from tectonic tilting or other causes, has been thought to include preferred occupation of regions of higher subsidence. In this paper, we develop further the theoretical framework for analysis of the interplay of tectonics and river dynamics, and apply this model to conditions in the GBM Delta. First, we examine the overall competition between variable subsidence and channel dynamics, and find that tilting in Bangladesh should be strong enough to influence river path selection. We then present new theory for the effect of subsidence that is spatially (not temporally) variable. We find a constant residence timescale on different parts of the delta, and differing frequencies of avulsion to these locations, and describe the effects of incision or floodplain deposition on these quantities. We present estimates of the channel residence timescale of the Jamuna (Brahmaputra) River reconstructed from the lithology, provenance, and dating of sediment cores. We apply our framework to a map of regional subsidence to predict the effects on avulsion for the Jamuna River. Comparison between our predicted (2150 years) and our stratigraphically based estimates of avulsion timescale (1800 years) shows encouraging consistency.

Chapter 32 Heavy Mineral Constraints on the Provenance of Cenozoic Sediments from the Foreland Basins of Assam and Bangladesh: Erosional History of the Eastern Himalayas and the Indo-Burman Ranges

Abstract The Assam-Bengal Basin system, located near the eastern syntaxis of the Himalayas and the northern end of the Indo-Burman Ranges, has received synorogenic sediments of several kilometres thick from these orogenic belts. These deposits provide valuable information on tectonic events, palaeogeography, and evolution of the sedimentary basin. Studies of heavy minerals document temporal variations in detrital compositions reflecting changes in the hinterland. Heavy mineral weight percentages in the Palaeogene and Neogene samples from the Assam Basin vary from negligible to ∼1.5%, and from In the Bengal Basin , Oligocene heavy mineral weight percentages are low (0.2%), and most of the grains are opaque; non-opaque minerals are zircon, tourmaline, and rutile, suggesting intense weathering of the Oligocene sediments. Miocene and younger heavy minerals are much more diverse and include garnet, aluminosilicates, epidote group minerals, pyroxenes, chlorite, hornblende, tremolite-actinolite, micas, prehnite, pumpellyite, and opaques. In contrast to the Assam Basin, provenance-diagnostic minerals from the Cenozoic successions of the Bengal Basin show a distinctive pattern in their distribution, indicating gradual unroofing of the contributing orogenic belts. The presence of blue-green amphiboles in Mio-Pliocene strata from Pakistan, the Assam Basin, the Bengal Basin, and Bengal Fan signal orogen-wide unroofing of arc-type rocks. Modal analysis of framework components and heavy mineral analysis indicate that the sediments in both the Assam and Bengal Basins were derived from discrete sources during the Oligocene. Source areas were the incipient uplifted orogenic belts in the Himalayas for Assam, and the Indian craton for the Bengal Basin. Heavy mineral contents in Miocene and younger successions suggest that both the Bengal and Assam Basins received detritus from orogenic hinterlands, i.e., the Himalayas in the north and the Indo-Burman Ranges to the east. Overall, the Assam Basin appears to represent an earlier and more proximal repository of detritus, shed from the Himalayan convergence, whereas the Bengal Basin was a downstream and somewhat younger depocentre.

Lithologic mapping of a forested montane terrain from Landsat 5 TM image

Abstract Thick forest cover and poor infrastructures are the major hindrances for detailed lithologic mapping in an inaccessible montane landscape. To overcome these limitations, we utilize a Landsat 5 TM image to map lithology using vegetation and drainage pattern as an indicator of underlying rock types in a heavily forested region of the Chittagong Hill Tracts area located in southeastern Bangladesh. We use supervised and unsupervised classifiers for a vegetation-based approach while on-screen digitization is used for drainage patterns-based mapping. Field observations were used for mapping lithology and evaluating accuracy. Overall, our results agree well with the current geologic map and improve it by providing a more spatially detailed distribution of the sandstone and shale. The performances of all approaches are good at the inner and outer flanks of anticlines located in the study area while the drainage pattern mapping performs best at the mid-flank area.

Effect of Elevated Air Temperature and Carbon Dioxide Levels on Dry Season Irrigated Rice Productivity in Bangladesh

Agricultural productivity is affected by air temperature and CO2 concentration. The relationships among grain yields of dry season irrigated rice (Boro) varieties (BRRI dhan28, BRRI dhan29 and BRRI dhan58) with increased temperatures and CO2 concentrations were investigated for futuristic crop management in six regions of Bangladesh using CERES-Rice model (DSSATv4.6). Maximum and minimum temperature increase rates considered were 0°C, +1°C, +2°C, +3°C and +4°C and CO2 concentrations were ambient (380), 421, 538, 670 and 936 ppm. At ambient temperature and CO2 concentration, attainable grain yields varied from 6506 to 8076 kg·ha-1 depending on rice varieties. In general, grain yield reduction would be the highest (13% - 23%) if temperature rises by 4°C and growth duration reduction would be 23 - 33 days. Grain yield reductions with 1°C, 2°C and 3°C rise in temperature are likely to be compensated by increased CO2 levels of 421, 538 and 670 ppm, respectively. In future, the highest reduction in grain yield and growth duration would be in cooler region and the least in warmer saline region of the country. Appropriate adaptive techniques like shifting in planting dates, water and nitrogen fertilizer management would be needed to overcome climate change impacts on rice production.