Signal Propagation From the Himalayan-Sourced Ganges-Brahmaputra Rivers to the Deep-Sea Bengal Fan From Detrital Zircons

Abstract

The Himalayan-sourced Ganges-Brahmaputra river system and the deep-sea Bengal Fan represent Earth’s largest sediment-dispersal system. This presentation summarizes a detrital zircon U-Pb (DZ) provenance record from the Bengal Fan from cores collected during IODP Expedition 354, with coring sites located 1350 km basinward of the shelf margin. Samples were collected from medium- to fine-grained turbidite sand and, based on shipboard biostratigraphic analyses, our samples are middle-late Miocene to late Pleistocene in age. Each sample was analyzed by LA-ICPMS at the Arizona Laserchron Center, with an average of n=270 concordant U-Pb ages per sample. We use these data to evaluate the influence of allogenic controls vs. autogenic processes on signal propagation from source-to-sink.

The DZ record of the Bengal Fan clearly records the strong tectonic and climatic forcing associated with and inherent to the Himalayas and Ganges-Brahmaputra system: even after up to 2500 km of river transport, and 1350 km of transport by turbidity currents, the DZ record faithfully represents Himalayan source terrains. However, the sand-rich turbidite part of the record is biased towards glacial periods when rivers extended across the shelf in response to climate-forced sea-level fall, and connected directly with slope canyons and the shelf margin. Moreover, specific U-Pb populations record the rapid transfer of signals from key events in the evolution of the Brahmaputra part of the system: among these are the mid-Miocene integration of the Brahmaputra with the Asian plate based on the presence of Mesozoic Gangdese arc zircons in Bengal Fan samples as early as 10 Ma, and the rapid Plio-Pleistocene incision through the eastern Himalayan syntaxis, which produced zircons from decompression melting with U-Pb ages <10 Ma: grains with U-Pb ages of 4-10 Ma were exhumed and appear in the Bengal Fan by ca. 2.4 Ma. Last, only part of the Bengal Fan DZ record represents either a Ganges or Brahmaputra provenance. Instead, most samples represent varying degrees of mixing of sediments from the two systems: this mixing, or the lack thereof, represents the fingerprint of autogenic avulsions on the delta plain that result in these two giant rivers delivering sediment separately to the shelf margin, or merging together as they do today.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018