Allogenic and Autogenic Signals in the Stratigraphic Record of the Deep-Sea Bengal Fan

Mike Blum,Jarrett Cruz,Kimberly Rogers,Yani Najman,James Gleason,Lyndsey Fox

doi:10.1038/s41598-018-25819-5

Abstract

The Himalayan-sourced Ganges-Brahmaputra river system and the deep-sea Bengal Fan represent Earth’s largest sediment-dispersal system. Here we present detrital zircon U-Pb provenance data from Miocene to middle Pleistocene Bengal Fan turbidites, and evaluate the influence of allogenic forcing vs. autogenic processes on signal propagation from the Himalaya to the deep sea. Our data record the strong tectonic and climatic forcing characteristic of the Himalayan system: after up to 2500 km of river transport, and >1400 km of transport by turbidity currents, the U-Pb record faithfully represents Himalayan sources. Moreover, specific U-Pb populations record Miocene integration of the Brahmaputra drainage with the Asian plate, as well as the rapid Plio-Pleistocene incision through, and exhumation of, the eastern Himalayan syntaxis. The record is, however, biased towards glacial periods when rivers were extended across the shelf in response to climate-forced sea-level fall, and discharged directly to slope canyons. Finally, only part of the record represents a Ganges or Brahmaputra provenance end-member, and most samples represent mixing from the two systems. Mixing or the lack thereof likely represents the fingerprint of autogenic delta-plain avulsions, which result in the two rivers delivering sediment separately to a shelf-margin canyon or merging together as they do today.

Highlights

The Himalayan-sourced Ganges-Brahmaputra River system (GB) and the deep-sea Bengal Fan represents Earth’s largest modern-day source-to-sink sediment-dispersal system
A total of 1.7 kilometers of core was obtained from the 7-site transect, resulting in recovery of abundant turbidite sand and silt ranging in stratigraphic age from ~18 Ma to the present (Figs 1, S1,S2, Tables S1 and S2)
We collected 25 unconsolidated samples of Bengal Fan turbidite sand and silt from IODP 354 cores, with samples ranging in age from Early Miocene to mid Pleistocene (Tables 1, S1 and S2), and two samples from the modern Ganges and Brahmaputra Rivers some 100 km upstream from their confluence

Summary

Introduction

The Himalayan-sourced Ganges-Brahmaputra River system (GB) and the deep-sea Bengal Fan represents Earth’s largest modern-day source-to-sink sediment-dispersal system. As used here these are: (1) the Tethyan Himalaya Sequence (THS), comprised primarily of Paleozoic and Mesozoic sedimentary rocks, with leucogranites of Neogene age[11]; (2) the Greater Himalaya Sequence (GHS) is separated from the THS by the South Tibetan detachment, a normal fault, and is comprised of Late Neoproterozoic to Ordovician high-grade metamorphic and plutonic rocks, with leucogranites of Neogene age; (3) the Lesser Himalaya Sequence (LHS) is separated from the GHS by the Main Central Thrust, and is comprised of Paleoproterozoic and older metasedimentary and igneous rocks; and (4) the Sub-Himalaya is separated from the LHS by the Main Boundary Thrust, and is comprised of mostly Neogene foreland-basin sediments (Fig. 2). The THS, GHS, and LHS dominate the geographically-defined Tethyan, Greater (or Higher), and Lesser (or Lower) Himalaya, respectively, but the geographic Greater Himalaya includes THS rocks as structural outliers, and the geographic Lesser Himalaya includes THS and GHS structural outliers

Methods

Results

Discussion

Conclusion