<資料・研究ノート>バタンハリ川流域低湿地の農業景観 : その1.地形と堆積層序

Abstract

This paper aims to describe the ecological settings of coastal swampy lands of Jambi, and the changes taking place in agricultural land use. Part one presents the stratigraphy and geomorphology of the area along one transect which covers about 70km from Jambi to the coast. Borings were made along the transect to a depth of up to seven meters to examine the stratigraphy of the recent sediments. Boring data and small-scale air-photos indicate the presence of the following five zones and 15 stratigraphic types distributed successively from inland to the coast. 1. Zone transitional to the middle reaches. Terrestrial soils have developed on the (1) low terrace, (2) meandering scars, and (3) natural levee, along the Kumpeh river. 2. Central zone covered by swamp forest. This area is mostly (4) peat-capped terrace. A low terrace surface with terrestrial soils is thought to have been transformed into fluviatile swampy lands following the rise in sea level in the postglacial period. Raised, ombrogenous peat plains have developed, sometimes exceeding six meters. The periphery contains an admixture of fluvial deposits from the meandering river course. (5) Natural levees are less developed along the Batang Hari. 3. Zone transitional to coastal zone. Until recently, this was covered by swamp forests, which are now being opened by the government. This area has undergone submersion and emergence most frequently due to changes in sea level. (6) Successive terrestrial soils are noticed to have developed from fluvial deposits on the terrace surface. (7) Mangrove deposits directly covering the terrace surface are most widely distributed. Two strata of different ages are recognized. (8) Another sedimentary phase indicates the intrusion of mangrove on the peat-capped terrace to have taken place much later. (9) Natural levees have developed along the streams which flow through former mangrove deposits. (10) Former beach ridges often outcrop. 4. Coastal zone with fish-bone channel networks. This area has been exploited by local people. (11) Thick sand ridges are advancing offshore, and (12) mangrove deposits have developed on the tidal flat. The present surface is thinly covered by delta topset which comprises humiferous clay and sometimes thin peat. (13) A narrow mangrove belt covers the present coastline. 5. Remnant hill isolated from the middle reaches. This was an island during the period of high sea level in the past. (14) Hillslopes are covered by brush, rubber gardens and fruit trees. (15) Shallow valley bottoms are utilized for bush-fallow cultivation of wet rice in the rainy season. Local people have exploited the coastal belt and succeeded in cultivating the land for plantation of coconut and rice. The government recently started new projects to exploit the zone transitional to the coastal belt. But drainage canals are so deep that the soils become exceedingly dry. Loss of peat soils and release of sulfate acidity take place, which hamper normal crop growth.