Goodbred, Steven L.¹
(1) Stony Brook University, Stony Brook, NY

ABSTRACT: Climate Change Impacts and Signal Preservation in the Ganges-Brahmaputra Delta Sequence

The strong South Asian monsoon and high Himalayan source area supports one of the world's largest riverine sediment loads (~ 1 billion tonnes annually) for the Ganges-Brahmaputra dispersal system. Consequently, an immense delta system has formed in the Late Quaternary. Despite this major forcing by the monsoon system, strength of the precipitation-bearing summer monsoon has varied significantly at orbital and sub-orbital timescales. These variations have left marked imprints on the sedimentology, stratigraphy, and sequences of the Ganges-Brahmaputra delta. During the Last Glacial Maximum, regional aridity suppressed river discharge sufficiently that carbonate ooids formed at the river mouth. Similarly, it does not appear that significant sedimentation occurred at this time on the active channel-levee system of the deep-sea Bengal Fan. Subsequently, the summer monsoon intensified in the early Holocene to its strongest level since at least MIS Stage 5e. Initially recorded on the fan as a sharp increase in sedimentation, the larger sediment load was ultimately trapped in the delta system due to transgression of the shelf. The volume of sediment stored in the delta at this time (11-7 ka) indicate that the rivers were discharging at least 2.3 times their modern sediment load. This climate-driven response led to early delta development under conditions of rapid sea-level rise. Furthermore, apparent balance between sediment flux and sea-level rise at this time favored formation of a thick (to 50 m), aggradational 'transgressive systems tract' that contrasts with traditional views of sequence timing and architecture. Other characteristics of the Ganges-Brahmaputra delta system are also consistent with strong forcing by climate and multimillennial-scale climate changes. Finally, these observed deltaic records are largely coeval with climate, glacial, and fluvial records from the Himalayan catchment.

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