GEOMORPHOLOGICAL AND PEDOLOGICAL EVOLUTION OF PARTS OF BANGLADESH PLAINS

Abstract

The Lower-Ganges-Brahmaputra-Meghna river system occupies most of Bangladesh and forms one of the world's largest fluvio-deltaic plains. Neotectonism and Worldwise Quaternary palaeoclimatic and sea level changes have influenced significantly the development of landforms and soils on these plains. Based on the remote sensing techniques, field checks, soil morphology and laboratory investigations of soils from parts of Bangladesh plains, twelve soil-geomorphic units have been recognized. Studies of field and laboratory characters of soils helped to determine the varying degree of soil development on the different soil-geomorphic units which has been used to group and rank the soibiof the different soil- geomorphic units into five members of a soil chronoassociation. These members are (i) QGB1 (<1500 Yrs. B.P.) comprising the soils of the Brahmaputra Floodplain and Young Ganges Deltaic Plain, (ii) QGB2 (2500 - 3000 Yrs. B.P.) comprising the soils of the Tista Fan, Old Tista Plain and Moribund Ganges Deltaic Plain, (iii) QGB3 (3000-4000 Yrs. B.P.) which includes the soils of the Old Brahmaputra Plain, Sylhet Depression and Old Meghna Deltaic Plain, (iv) QGB4 (5000-6000 Yrs. B.P.) which includes the soils of the Lower Atrai Depression and (v) QGB5 (>15000 Yrs. B.P.) comprising the soils of the Barind Tract, Madhupur Tract and Lalmai Hills. The soil-forming processes are found to be different on the three major landforms, namely (i) the fluvio-deltaic plains, (ii) the palaefluvio deltaic uplands and (iii) the low-lying depressions. The

fluvio- deltaic plain soils are marked by moderately thick argillans (40-lOOfim), hydromorphism and ferrolysis of soils and partial decalcification of the upper horizons and reprecipitation of calcite in the lower horizons. The palaeofluvio-deltaic upland soils are marked by sesquioxide-rich (relict red) and polygenetic soils as evidenced from three major phases of pedogenesis such as (i) alternating wet and dry phase of the Late Pleistocene time promoting widespread translocation of fine clay (<.2 fim) and formation of many thick argil lans (50-250miti), (ii) dry phase at 7000-8000 Yrs.B.P. promoting the development of calcrete of stage III and (iii) wet and humid phase since the Mid-Holocene period leading to the hydromorphism/ferrolysis of soils both on the upland and lowland areas. Also, these upland soils have reached the fersiallitic and ferruginatic stages of pedogenesis of Duchaufour (1977) on the level areas and margins of the uplands respectively. The depression soils are marked by the general segregations of iron oxides throughout the solum, thick ferriargillans (-80 fim) and flood coatings. The clay minerals such as illite, kaolinite, chlorite, vermiculite, montmorillonite and mixed layered minerals vary with increasing degree of pedogenesis and profile sites. Chlorite amount decreases with increasing degree of pedogenesis and it becomes totally absent in the oldest soil chronoassociation member (QGB5). Kaolinite shows an increasing trend with increasing degree of pedogenesis. The poorly drained profile sites are marked by the formation of montmorillonite and it is generally preceded by the formation of vermiculite. Neotectonism has influenced significantly the development of landforms and soils in the study area in the following manners: (i) If a tectonic block is uplifted/tilted gently, the rivers flowing over it shift away and the zone of sedimentation becomes a site of pedogenesis, (ii) If a certain block subsides slightly it becomes a waterlogged depression allowing hydromorphism of soils but higher subsidence allows sedimentation over the block and (iii) Rapid uplift of the tectonic block promotes partial erosion but slow rate of uplift facilitates good profile development. The shift ings of most of the rivers such as the Ganges and the Tista river systems towards the east and the southeasterly subsidence of the Barind and the Madhupur Tracts are attributed to the overall subduct ion of the Indian plate beneath the Burmese plate. The observed three cases of gradual changes in the degree of soil development are: (i) Rate of sedimentation/subsidence decreases from the proximal to distal part of the Tista Fan. This is accompanied by an increase in the degree of soil development, (ii) The Ganges Delta is prograding southward with contemporaneous increase in rate of sedimentation in the direction of progradation. However, there is a decrease in the degree of soil development towards the south and (iii) The tilting of the Ganges Deltaic Plain Block towards the east/southeast has caused easterly/southeasterly shifting of the the Ganges river. This has also resulted in the decrease of soil development from the northwest to southeast. The above soil development relationship can be called as pedofacies after Brown and Kraus (1987). The last one is related to active tectonics and may be called as tecto-pedofacies after Kumar et al. (in press). vi Also, the palaeoclimatic and sea level changes have affected significantly the development of landforms and soils in the study area. The monsoon climate passed through (i) a very arid phase at about 22000-18000 yrs. B.P. when the sea level fell down to -130m in the Bay of Bengal allowing the coastline to move over 100 km south than at present and (ii) a very humid phase at about 12500-10500 yrs. B.P. which led to the increase rainfall and run-off in the region (Cullen, 1981). At 6000-7000 yrs. B.P. the transgression of sea swept over many parts of the study area. Except the upland areas, the soil formation started with regression of sea since 6000 yrs. B.P. in the region. Corresponding to the climatic and sea level changes three major phases (e.g. alternating wet and dry, dry, and wet phases) of pedogenic processes have been recognized as discussed earlier.