GEOMORPHOLOGICAL AND PEDOLOGICAL STUDIES OF THE GANDAK MEGA-FAN AND ADJOINING AREAS IN THE MIDDLE GANGETIC PLAINS, INDIA

Abstract

The Gandak river with the mean annual of flow 51.7 cubic km forms an elongated megafan in the Middle Gangetic Plains, measuring about 100 km x 240 km and trending NW-SE. Based on studies by airphotos and Landsat imageries, field check and field and laboratory investigations of soils from the the Gandak megafan and adjoining areas, the following ten major soil-morphological units are recognised: Gandak Floodplain, Younger Gandak Plain, Older Gandak Plain, Oldest Gandak Plain, Old Ghaghra Plain, Old Rapti Plain and Ganga-Ghaghra Interfluve Plain and floodplains of large rivers like Ganga, Ghaghra and Rapti. Study of field and laboratory characters of soils have been used to determine the degree of development of soil profiles on different geomorpholgical units and prepare a soil chronosequence. The soil chronosequece has five members, QGD1-QGD5, QGD1 bening the youngest. Soils of different geomorphic units included in various members are : QGD1 (? <500 BP) - Floodplains of Gandak, Ganga, Ghaghra and Rapti, QGD2 (? >500 BP) - Younger Gandak Plain, QGD3 ( 2500 BP) - Older Gandak Plain and Old Rapti Plain, QGD4 (? 5000 BP) - Oldest Gandak Plain and QGD5 (10,000 BP) - Old Ghaghra Plain and Ganga-Ghaghra Interfluve Plain. Due to gentle eastward tilting of the Gandak megafan, the river Gandak has shifted about 80 km from west to east in the II last about jfuOO years leading to the development of a soil chronosequence on its megafan, somewhat similar to terrace soil chronosequences developed close to some rivers due to tectonic uplift of the areas. The Gandak river has plastered its megafan even in the northern portion, where braided stream was active and is still active due to slow shifting and high silt content in the load of the river in contrast to usually much coarser sediments deposited by the braided streams (Miall, 1977). Soils of the Ganga-Ghaghra Interfluve Plain (tha oldest soils in the area) record a climatic event - an. extremely dry period coinciding with the late phase of a glacial period with a maxima at about 18000 BP and developed during the period 9-11 ka. This resulted in accumulation of salts and pedogenic lime in the soils of this unit. Some regressive pedogenesis, in region of this unit close to Ganga and Ghaghra Floodplains, in the form of leaching out of salts and translocation of pedogenic calcite from B horizon to lower parts in C horizon also took place about 8ka time due to amelioration of climate. Major processes of formation of soils are decalcification and accumulation of calcium carbonate, weathering of primary minerals, displacement of some constituents active in the differentiation of horizons like plasma fabric separation and illuviation of clays and organic matter and accumulation of sesquioxides. Ill Some significant clay mineral transformations related to age and changes in environment are noted on the Gandak megafan in us-rie soil moisture regime. Chlorite changes to vermiculite, which also decreases drastically in the older soils. Kaolinite and interstratified kaolinite-smectite tend to appear in the older soil of the megafan with acidic environements. Association of chlorite with pedogenic lime is also noted in Ganga-Ghaghra Interfluve Plain. A major tectonic feature of the area- a floodbasin in the lower part of the Younger Gandak Plain and its eastward continuation between the Burhi Gandak and the active Gandak Floodplain /Ganga Floodplain, with a high rate of subsidence is recognised. Other structural features recognised are Rapti Fault, Gandak Fault, and Ghaghra-Ganga Fault, which are probably shallow in nature and unrelated to any basement features. These are similar to growth faults of the deltaic environment and have been active during the period less than 10,000 BP. Also the Himalayan Front Fault, separating the geomorphological systems of the Gangetic Plain and Siwalik Ranges is delineated.