I realized shortly before I left the US that I’d written a few posts on what it is that a paleomagnetist does, but nothing about the purpose of our research cruise. I have a couple of days before Expedition 354 starts (I’m spending a few of those in Japan, filling up on ramen before I go!), so I can start with some of the basics now. While the cruise is going on, I’ll get to some more of the specifics while we’re out there.
The Bengal Fan is a huge, thick, wedge-shaped deposit of mud and other sediments that extends from the edge of Bangladesh out into the ocean near Sri Lanka. It is the thickest such deposit in the world. At the sediments’ thickest part – where the Ganges, Brahmaputra, and Meghna Rivers empty into the Bay of Bengal – the fan is over 20 kilometers thick. For comparison, this is at least twice as thick as Mt. Everest is tall! [1] The fan tapers out to the south and also thins out to the east. Where we will be drilling one of our first cores, on the flank of Ninetyeast Ridge, geologists think that the sediments are less than 2 km thick. [2]
Mt. Everest and the Himalaya are themselves part of the reason that the sediments are as thick as they are. As winds blow toward a mountain range, the air is blocked by the mountains and needs to rise. Rising, moist air produces rain. Rain, in turn, fills rivers and causes soil and rock to weaken and eventually slide. This erodes rock from the rainy side of the mountains. The eroded rock and soil has to go somewhere. In the eastern Himalaya, the the mountains’ detritus is funneled into the river systems – the Ganges, Brahmaputra, and Meghna – that feed the Bengal Fan.
But it hasn’t always been this way. The Himalaya were raised to their current, high elevations as India slammed into Asia , starting about 60 million years ago. Before that collision, India was probably shedding sediment off in all directions. Material eroded from India’s upper crust, mixed with ash and sediment from nearby volcanoes, fed an early apron of sediment around India (we think). As the blocks of crust that formed the Himalaya were uplifted, they were ground down by the action of rivers, rain, and landslides, until all of the upper crust was lost. Sediment from the lower crust began to feed the sedimentary system. My impression is that this material built the fan itself. [2]
This is only part of the story. The fan is a huge, complex system, part of an even bigger and more complicated system (the Himalayan uplift). More explanation will come when I have time.
[1] It is also twice as thick as normal ocean crust, and, if I remember my stratigraphy right, over twice as thick as the thickest package of Catskill deltaic sediments in the New York / Pennsylvania area. [2] This is mostly summarized from Curray, J. 2014. The Bengal Depositional System: From rift to orogeny. Marine Geology 352: 59-69, which is a nice introduction to the area.