Gibling, Martin R.¹, S.K. Tandon², R. Sinha³, M. Jain⁴
(1) Dalhousie University, Halifax, NS
(2) University of Delhi, Delhi 110007, India
(3) Indian Institute of Technology, Kanpur 208016, India
(4) Risoe National Laboratory, Roskilde, Denmark

ABSTRACT: Sequence Stratigraphy of the Southern Gangetic Plains, India: Quaternary Alluvial Sequences Created by Climate Change and Tectonism

The sequence-stratigraphic paradigm of base-level and accommodation control explains the architecture of many coastal deposits, but has failed to provide a unifying explanation for alluvial architecture. The southern Gangetic Plains of India lie near the cratonic margin of the Himalayan Foreland Basin, 1500 km inland and 130 m above sea-level -- beyond range of sea-level control. Discontinuity-bounded sequences that reflect alternate floodplain aggradation and degradation characterize the Quaternary alluvium in this region of modest subsidence and strong climatic signature. Although climatic and discharge changes and valley incision and filling are implicated, the sequences are not suitably described through base-level and accommodation scenarios.
The Ganga and Yamuna rivers presently occupy narrow incised valleys separated by wide interfluves too elevated for inundation. In 30 m cliffs below the modern interfluve, floodplain muds are capped by lacustrine and eolian silt-sand sheets, with incised channels of reworked carbonate gravel and 10 m colluvial ravine fills. Age dates suggest that floodplains were attached to the major rivers during the 40-80 ka period of relatively high monsoon precipitation (Marine Isotope Stages 3-5). The floodplains detached as precipitation decreased into the Last Glacial Maximum (MIS 2) and locally at 9-12 ka (MIS 1-2). Ganga and Yamuna valley sands are <10 m thick.
Extensive discontinuities lower in the cliffs show reworked gravel lags, small sand/gravel channel bodies, irregular paleosurfaces cemented by carbonate, and paleo-ravine fills. These interfluve levels (>100 ka?) have carbonate veins and dewatered sands, suggesting that tectonic activity controlled interfluve development or coincided with climate change.

AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004.