Submarine landslide deposits – a spectacular outcrop example from Japan
I tweeted an image that got a lot of attention the other day and wanted to follow it up with a quick post describing the deposit. The back story is this: Lesli Wood, a submarine landslide expert, showed an image at a recent conference that is a spectacular example of a mass transport deposit (MTD), or more simply, a submarine landslide deposit. Landslides occur on land (example video), causing plenty of infrastructure damage and other problems. While they are difficult to view and visualize, landslides also occur on the seafloor, causing massive reorganization of the seafloor that can generate tsunamis. In fact, the Storegga slide that occurred offshore Norway about 6000 B.C. likely killed many Europeans.
Due to their volume and size, submarine landslides are usually characterized best with seismic reflection data – the map and cross section below from this paper by T.M. Alves. The map shows large blocks of rock that have been broken apart and transported downslope (from left to right) and the cross section shows what the internal character of those blocks are. Note the discordant nature of the blocks, very similar to the image above of the outcrop, with rocks in all directions.
Outcrops usually are too small or not well enough exposed to view these types of features, but the outcrops created by road making on the Boso Peninsula in Japan are definitely good enough. This paper by Yamamoto et al (download the paper here)describing these outcrops has a very nicely drawn diagram that demonstrates the deposit. There are probably two landslide deposits that are stacked here, with a turbidite separating them (grey layer in the middle of the diagram).
Here is another nice photo from a field symposium website showing a closeup of the deposit.