New Zealand turbidite field trip – Day 2-3: Submarine lobe deposits
Wow, these rocks are fantastic! The last two days we have been looking at submarine lobes exposed along the Taranaki coast. Lobes are areas downstream of submarine channels where the gradient decreases (i.e. becomes quasi-flat) and turbidity currents deposit sand in an oblate shape emanating from the channel. For a great paper on lobes, see this one. Vertically, in the middle (or axis) of the lobe, there are amalgamated sands with fairly flat bedding contacts like you see in the photo below.
That package is about 5 m thick, and is not a single sedimentation unit (i.e. event) but probably 5-10 beds that are amalgamated into a single thick sand unit. That skinny chap there is about 1.7 m tall.
As you move to the edges of the lobe from the center, the amount of amalgamation decreases, and the vertical succession transitions slowly (typically over a few hundred meters) from a single thick sand unit into interbedded sands and muds, like in the photo below.
There is no scale there, but the thick bed at the bottom is about 1 m thick, and the whole succession is about 8-9 m thick. This locale is interpreted to be at the medial to distal part of the lobe (or later fringe) and as you can see, there is much less erosion, and so mudstones are preserved in between the sand beds. If you took away all the mudstones (light grey), then you would have about the same thickness of sand as in the first photo. Very interesting stuff!
We also saw some mass transport deposits (MTDs), but I will save that for the post of Day 4, as we will see an even more spectacular MTD!