Saturday, February 9, 2008

Immense submarine debris-flow deposit

A particular interest (professional and otherwise…whatever that means) of mine is flow hydrodynamics. That is to say, how does sediment get transported across the surface of the earth, and what processes control the subsequent deposition of sediments. A recent paper by Talling et al. (2007, Nature v. 450, Nov. 22) is a prime example of how absolutely mind-boggling some of the processes are.

Talling et al. (2007) report on an interpreted submarine debris flow deposit, approximately 1500 km offshore of northern Africa. For those not enamored of the SI, 1500 km is about 930 miles, so you’d want to pack a lunch if you were travelling that far. Anyway, this deposit represents the longest run-out flow ever described on Earth. Neat, huh!?!

What’s really interesting, however, is the interpretation of the origin of this deposit. A submarine landslide, 1500 km shoreward of the eventual deposit, probably produced a kind of sediment-gravity flow called a turbidity current. These turbidity currents are flows that are turbulently supported (ergo the name…), meaning that the motion of the sediment-water mixture keeps the whole flow in suspension. Because of the density of the flow, it hugs the bottom of the seafloor, and moves down-gradient under its own weight. These flows are commonly very erosive, and can travel long distances. Talling et al. (2007) make the interpretation that a turbidity current travelled most of the 1500 km, but as it decelerated, it transformed itself into a debris flow. Debris flows are NOT turbulently supported; rather, they exhibit what’s called laminar flow (so NO turbulence in the flow). This transformation of the turbidity current is what resulted in a debris flow, which was rapidly deposited as a many-meter thick bed 1500 km from the source of the sediment.

An impressive paper, detailing the inherent complexity in unraveling the sedimentological record. And this is just a single event that happened only a couple hundred thousand years ago; now think about strata, many millions of years ago, divorced from their geographic context (in a fold-belt, for instance). What a great challenge!

No comments: