SOIL-LANDFORM DEVELOPMENT OF A PART OF FOLD BELT ALONG EASTERN COAST OF BANGLADESH

Abstract

The investigated Jaldi and Maiskhali structures include a part of coastal region of north-south trending Fold Belt of Bangladesh. It offers an excellent site for studies of soil-geomorphic relationships of undulating coastal regions. Quaternary sea level fluctuations, palaeoclimatic changes and neotectonic activities (folding) have probably influenced significantly the development of soils and landforms of the area. A pedological approach has been taken to unveil the role of these factors in the soil-landform development of the area. Eight soil-geomorphic units have been delineated in the area are: Mainland Higher Hillocks, Mainland Lower Hillocks, Island Hillocks, Proximal and Distal Piedmont Plains, Old and Active Tidal Flats and River Floodplains. A four member (I - IV) Morphostratigraphic sequence for the study area has been prepared based on Luminescence ages of soils. The various soilgeomorphic units included in different member/sub-members are: Member I - River Floodplains and Active Tidal Flats (< 500 yrs); Member II - Distal Piedmont Plains and Old Tidal Flats (1-2 ka); Member III - Proximal Piedmont Plains (6-10 ka) and Member IV - Mainland Higher and Lower Hillocks and Island Hillocks (> 15 ka). Member IV is further subdivided into Sub-member IVa - Island Hillocks (15-18 ka); Sub-member IVb - Mainland Lower Hillocks (23 -25 ka) and Sub-member IVc - Mainland Higher Hillocks (30 - 35 ka). Depending on slope and elevation, Member I of the Morpho stratigraphic sequence forms the 'accreting zone', Member III forms the 'zone of pedogenesis' and Member II occupies a transition zone between the accreting zone and zone of pedogenesis. The Mainland Higher Hillocks, Mainland Lower Hillocks and island Hillocks in Member IV belong to the 'sloughing zone'. The soils of the study area with distinctive pedogenic processes can be broadly divided into the following three groups: (a) Poorly developed soils (Member I) - These are least developed and marked by active sedimentation. Hydromorphism is going on as indicated by diffused mottling, (b) Moderately developed soils (Member II and III) - These soils are marked by mainly translocation of clays as indicated by the presence of thick argilians, ferriargillans and gleyans. Hydromorphic processes are active in these units as indicated by Fe-Mn oxide concretions and Fe-oxide mottles. Mineral alteration is negligible with a relatively stronger alteration in the Member III soils. The clay mineral composition is dominantly inherited from the parent material. The weak alteration of minerals and dominance of 2:1 clays and considerable amount of free iron oxide in the form of Fe-Mn oxide concretions and Fe-oxide mottles suggest that these soils are at the initial stage of weathering i.e. in the fersiallitisation stage of pedogenesis of Duchaufour (1982). (c) Strongly Developed soils (Member IV) - Strong alteration of minerals, the yellowish brown to reddish brown colour of the micromass, dominance of 2:1 and 1:1 clays (illite and kaolinite) suggest that these soils have undergone a ferrugination stage of pedogenesis of Duchaufour (1982). Soils of Member IV exhibit degradation and poor birefringence of argilians and ferriargillans indicating a significant change in conditions of pedogenesis. These soils are characterized by a few orthic Fe-Mn oxide concretions and diffused Fe-oxide mottles. The orthic nature of the concretions and diffuse nature of the mottles suggest their recent formation. These indicate the 'relict' nature of these soils. Field and laboratory analyses suggest that Member II soils have a relatively higher degree of development as compared to Member III soils and this might be attributed to their finer parent material. Among the Member IV soils, Sub-member IVb soils have a higher degree of development as compared to Sub-members IVa and IVc. Lower soil development of Submembers IVa and IVc is probably due to their present position at the top of local catena, which causes lateral movement of eluviated materials, whereas similar process is probably acting only in a minor way on the Sub-member b, as these soils developed on broad flat-topped Mainland Lower Hillocks. Sea level fluctuated several times with some stillstands during the late Quaternary Period. Eustatic stillstands at 35 - 30 ka and 23 - 20 ka may have in affected the palaeodrainage in the study area, made streams to migrate laterally and to form fluvial terraces of the Mainland Higher Hillocks and Lower Hillocks (Jaldi anticline). The age of the Lower Hillock terrace is a little higher than the corresponding eustatic stillstand. This may be due to the fact that timing of stationary sea level in the study area may be slightly different from that in the eustatic curve. The LGM (18 - 15 ka probably) was probably marked by local stillstand of sea and was responsible formation of Island Hillock terrace (Maiskhali island). Deposition of Piedmont zone started at 10 ka and it peaked at 7 ka that coincided with a major transgression in the area. Later a regression at about 1.5-2.0 ka led to the formation of Old Tidal Flats. The tentative uplift rate as estimated from the samples of the Maiskhali structure during the period between 18 ka up to 8 ka is about 4.5 mm/yr and afterwards (8ka -present) the rate of uplift is retarded to about 0.37 mm/yr. The over all uplift rate (18 ka - present) is 2.43 mm/yr. The tentative uplift rate for the Mainland Higher Hillocks (Jaldi structure) is about 7.9 mm/yr during the period 35 - 25 ka. The relative uplift rate for the Mainland Lower Hillock (Jaldi structure) is calculated as 1.5 mm/yr for the period 25 ka - present. The Maiskhali structure is separated from the mainland by a N-S trending fault and this block uplifted at a higher rate as compared to the mainland during the period 18-8 ka. The Late Quaternary Period in the area was marked by two major phases of palaeoclimate. (a) a subhumid to semiarid phase (40 to about 16 ka) (Vishnu-Mittra and Sharma, 1984; Rangarajan *diant 2000) and (b) a hot humid to subhumid phase (16 ka to present) (Sirocko et al., 1993; Sirocko, 1996; Rangarajan*^"''., 2000). The thick argilians and ferriargillans present in Member IV soils were probably formed during the subhumid to semiarid phase. The presence of few orthic Fe-Mn oxide concretions and diffused Fe-oxide mottles indicate their probable formation during the later hot humid to subhumid phase. This humid phase caused degradation of argilians and ferriargillans and their poor birefringence in Member IV soils.