GEOMORPHOLOGICAL AND PEDOLOGICAL EVOLUTION OF PARTS OF LOWER GANGETIC PLAINS IN WEST BENGAL

Abstract

The Lower Gangetic Plains form one of the most extensive fluvio-deltaic plains of the world. Quaternary climatic and sea level changes and tectonics have influenced significantly the development of landforms and soils of these plains. The present study aims to decipher the roles of these factors on morphogenesis and pedogenesis of the area during the Quaternary Period, using soil-geomorphic approach. On the basis of field observations and laboratory analyses the degree of soil profile development in different soil-geomorphic units were determined. Based on relative degree of development, the soils of the area were divided into five members of a soil-chronoassociation (Mohindra et al., 1992) and have been given the names QGWBl to QGWB5. Various soil-geomorphic units units included in different members are: QGWBl - Ganga Floodplain; QGWB2 - Bhagirathi Plain and Old Ganga Plain; QGWB3 - Barind Tract (Lower Level) and Damodar Terminal Fan; QGWB4 - Bhagirathi-Brahmani Plain, Brahmani-Ajay Plain and Ajay-Silai Plain and QGWB5 - Lateritic Upland. Soils of QGWBl member are least developed and A/C type of horizonation is observed in these soils. The thickness of B horizon increases in general from QGWB2 (40-50 cm) to QGWB4 (80-110 cm) soils. Soils of QGWB4 member are poorly drained, fine textured, vertic intergrade soils with vertical cracks occurring on the gently sloping plains. The soils of the Lateritic Upland occur on an undulating region and they consist of two distinct units. The lower autochthonous unit starts with a lithomarge at the base grading up into pisolitic laterite (200-300 cm thick). The pisolitic concretions have been cemented by iron oxide. The upper allochthonous unit with a maximum thickness of 50-150 cm is comprised of gravelly sand at the top followed by lateritic mudstone which consists of detrital lateritic particles hound by iron oxide. The lower unit occurs only on the lower slopes of the unit. The preliminary Thermoluminescence ages of QGWBl to QGWB4 members are 500 yr, 1-1.5 ka, 3-4 ka and 5-6 ka respectively. Lateritic soils of QGWBB member have been assigned a tentative maximum age of 125 ka. The minimum age is 22 ka that marks the beginning of the Last Glacial Stage. Another unit named the Raghunathganj Surface/Soil has been recognized. It has an undulating surface with a minor surface exposure in the Bhagirathi-Brahmani Plain and occurring at a depth of 1.5 to 3 m under the Brahmani-Ajay and Ajay-Silai Plains. It has a petrocalcic horizon with calcretes developed to stage IV of carbonate morphology of Gile et al. (1966). It has not been considered in the construction of soil-chronoassociation, as it was not studied in detail. A radiocarbon age of 23 ka has been obtained from a calcrete sample from this unit.

The pedogenic processes occurring in the lower autochthonous unit of Lateritic Upland (QGWB5) soils represent intense weathering of terralitisation stage of Duchaufour (1982). The upper allochthonous unit of lateritic soils represents eroded lateritic material from the hinterland and its emplacement in the topographically lower regions due to lowering of the msl as discussed below. This can be called as Laterite-Derived-Facies (LDF) after Valeton and Wilke (1993). The moderately developed soils of the Bhagirathi-Brahmani, Brahmani-Ajay and Ajay-Silai Plains are marked by clay illuviation and transformation of smectite and illite into interstratified clay minerals on a large scale. Additionally they are marked by pedoturbation in the upper part and swelling and shrinking leading to formation of slickensides and stress-oriented coatings. The presence of illuviation features and high amount of 2:1 swelling clays in these soils suggests fersialitisation stage of weathering of Duchaufour (1982). Formation of small amounts of chlorite and illite-chlorite in the surface horizons of QGWB3 soils together with common occurrence of gleyans and flood coatings suggest that illite is being transformed to chlorite through illite-chlorite (e.g. in Barind Tract) and chlorite-smectite (in Damodar Terminal Fan) by ferrolysis in a manner suggested by Brinkman (1969/1970, 1977). The poorly developed soils of the young fluvial plainssuch as the Bhagirathi Plain and OldGanga Plain are marked by hydromorphism and slight clay illuviation The paleoclimatic and sea level changes seem to have played a significant role in the development of landforms and soils of the study area. Three major phases of sea level and climatic changes since Mid Pleistocene can be recognized from eastern coast of India. The transgression of first phase at ca. 125 ka led to the deposition of the parent material for the Lateritic soils (QGWB5). Lateritization started with regression of the sea of this stage on the upland areas with a good drainage under a warm and humid climate. The low msl in the later part of this phase must have caused the formation of the LDF. In the second phase a small transgression at ca. 30 ka with associated rising groundwater must have accelerated lateritization of the autochthonous Lateritic soils in a manner suggested by Valeton and Wilke (1993). Regression at the end of this phase probably led to erosion and formation of Raghunathganj Surface/Soil with petrocalcic horizon during an arid phase. The third phase of transgression at 7-6 ka led to the deposition of sediments of the QGWB3 (Damodar Terminal Fan) and QGWB4 members. As the sea regressed during this phase, freshly deposited sediments were exposed on which pedogenic processes started, as a result soils are younger from west to east in the region east of the Lateritic Upland. This regression has also contributed at least partly to entrenchment of the courses of the Ganga, Damodar, Ajay and Brahmani rivers due to reduced msl. The above discussion also implies the existence of another Quaternary cycle of lateritization different from the Early Tertiary 'Indian Cycle' identified by Valeton and Wilke (1993). The present region lies in one of the most seismically active zones of the world. Mostof the study area overlies the Bengal Basin. Its contact with the Peninsular rocks is along the Chotanagpur Foothill Fault. Within the Basin, three major tectonic units are: Tectonic Shelf in the western part, Barind Tract Horst in the north and the Ganga Fluvio-deltaic Plain (GFDP) Graben in the eastern part. The Tectonic Shelf and Barind Tract Horst are separated by the Ganga-Padma Fault and the Damodar Fault separates the Tectonic Shelf and GFDP Graben. The Medinipur-Farakka Fault within the Tectonic Shelf separates the Lateritic Upland in the west from QGWB4 soils in the east. The Chotanagpur Foothill, Medinipur- Farakka and Damodar Faults are roughly N-S trending with significant reliefs of 250 m, 25 m and 6 m across them respectively and form step faults with eastern sides in each case being downthrown side. The Lateritic soils (QGWB5) and theQGWB4 soils are presently 120-30 mand 50-20 mas! but the sea level '- rose to +25 m and +6 to 10 m above sea level during 125 ka and 7 ka transgressions respectively, suggesting that the Tectonic Shelf has been uplifted significantly more on the western side and less on the eastern side since 125 ka. The total uplift is probably accounted partly by throw across the faults and partly by steep easterly slopes of the plains. The Barind Tract Horst has at least three erosional levels (with varying reliefs of 5 to 20 m) soils developed on each of them. These indicate tectonic uplift in at least three stages and these levels are similar to river terraces. The western boundaries of the (i) Lateritic Upland, (ii) BBP, BAP, ASP Plains and (iii) DTF units are marked by Chotanagpur Foothill, Medinipur-Farakka and Damodar Faults respectively and these faults were the areal limits of sediments deposited at different times indicating their activity at those times. Thus the activity along faults affected sedimentation directly and pedogenesis indirectly. All the easterly flowing rivers in the Tectonic Shelf zone have shifted their courses towards the south within their floodplains indicating southerly tilting of the Shelf in the last 100's of years. The Dwarkeshwar, Silai and Kasai rivers show annular drainage pattern indicating activity of some basal dome in this zone. Also, the courses of the rivers Ganga, Damodar and partly Bhagirithi are confined to faults.