INTERFLUVES OF THE SOUTHERN GANGA PLAINS, INDIA: ATTACHED AND DETACHED FLOODPLAINS IN THE LATE QUATERNARY

Major rivers of the southern Ganga Plains (Himalayan Foreland Basin) occupy narrow, incised valleys bordered locally by 30 m cliffs and separated by wide interfluves that are too elevated for inundation by the present-day rivers. Interfluve landforms include planar floodplain surfaces with soil development, small plains-fed rivers, lakes, eolian mounds and extensive tracts of badland ravines. Sea-level fluctuations have not affected stratal patterns, as the area lies 1000 km inland.

At Bithur on the Ganga and along the Sengar River, 15 m cliff sections commence with thick floodplain muds with carbonate nodules and a slight to strong pedogenic overprint. The muds are overlain by lacustrine and eolian sheets of silt and sand, with incised channels fills of reworked carbonate gravel. Radiocarbon dates suggest that floodplains were attached to the major rivers prior to 10-15 ka B.P. but later became detached and were degraded or accumulated local lacustrine and wind-blow sediment. Comparison with late Quaternary marine and continental records of monsoon intensity suggest that detachment took place during a period of increased monsoon precipitation. Discharge of Himalayan and cratonic rivers probably increased relative to sediment load, leading to incision and a reduced frequency of floodplain inundation. Along the Yamuna, topmost cliff strata include 10 m ravine fills of colluvium, currently undated.

In 25-33 m cliffs along the Yamuna and Betwa, reworked gravel lenses, sand and gravel channel bodies, and carbonate cementation mark prominent erosional discontinuities at lower levels. The discontinuities can be traced for >2 km within thick floodplain deposits, and are interpreted as older interfluve surfaces and strata (probably >100 ka B.P.). The origin of these discontinuities is uncertain. Carbonate veins that cut strata below but not above one discontinuity suggest a link between floodplain degradation and tectonic events; others may reflect climatic change.

The discontinuity-bounded units indicate a pattern of alternate floodplain aggradation and degradation. Such "A/D rhythms" may characterise alluvial successions in continental settings, especially along the cratonic margins of foreland basins where subsidence rates are modest.