Discontinuity-Bounded Alluvial Sequences of the Southern Gangetic Plains, India: Aggradation and Degradation in Response to Monsoonal Strength

Abstract

Discontinuity-bounded late Quaternary sequences in the southern Gangetic Plains (Himalayan Foreland Basin) reflect floodplain aggradation and degradation in response to forcing by the powerful Southwest Indian Monsoon. The major Himalayan and cratonic rivers in this area occupy narrow, incised valleys and do not inundate the adjoining broad interfluves, which display soil development, small plains-fed rivers, lakes, eolian deposits, and badland ravines. However, these areas formerly experienced active alluviation because thick floodplain muds underlie the interfluve. An age model suggests that interfluve areas near the major rivers aggraded periodically between about 27 ka and 90 ka (Marine Oxygen Isotope Stages 3–5). They subsequently degraded or accumulated sediment only locally, probably reflecting decreased monsoonal precipitation around the Last Glacial Maximum (Marine Isotope Stage 2), when major river valleys moved to an underfit condition. Increased precipitation during the 15 to 5 ka period of monsoon recovery probably increased discharge and promoted incision and widespread badland formation. Discontinuities at deeper levels reflect earlier degradation phases, and are marked by reworked gravel lenses, thin channel bodies, and carbonate-cemented surfaces. Carbonate veins (infilled joints) that cut strata below but not above one discontinuity suggest some local tectonic influence. There is no evidence that sea-level fluctuations influenced stratal patterns in this elevated, inland region. Discontinuity-bounded sequences linked to climate change may characterize alluvial successions in many continental settings, especially along the cratonic margins of foreland basins, where subsidence rates are modest.