New Zealand turbidite field trip – (final) Day 6: Scoured submarine lobe deposits

So, we took a day to drive across from the Taranaki basin to the East Coast basin, the forearc side of the Coromandel arc of the north island of New Zealand.  I wont try to elaborate on the Neogene tectonics of New Zealand – there have been multiple episodes of shortening, extension, strike slip, and subsidence, and there is still much discussion as to the exact evolution.  As we drove, we marveled at the green, mountainous terrain of central New Zealand, and stopped by Lake Taupo to see the caldera lake created by subduction-related arc volcanism.

For the final day of the field trip, we saw turbidites of the Hikuwai sandstone, which are Miocene turbidites exposed along the east coast of the north island. In the title I say ‘scoured lobe deposits,’, but that is just a simple interpretation that probably needs some more thought. We really only got to look at these rocks for a day, so I am cautious.  See the photo below:

Again, I only show a small snippet of the outcrop to allow the student that is working these outcrops to publish them in full detail.  This cliff is about 100 m tall, and the field of view here is about 8 m high.  The nice scour that you see near the base of the cliff is about 1-2 m deep and filled with very similar facies to those outside of the scour.

Now, concave up erosional surfaces are usually interpreted as channels, so why have I called these scours?  And what is the difference between a channel and a scour anyways?  A channel is a long lived conduit for sediment whereas a scour is an ephemeral feature that is eroded and filled by the same sediment gravity flow, or perhaps filled a few flows later.  I call these scours for many reasons, none of which you can see in that photo, but the predominant one is that the erosional surfaces are unorganized, occurring seemingly at random through the sandstone (channels are usually more organized, with predictable facies patterns).

Thanks for reading this field trip log – I know it wasn’t really consecutive, but I had no time and/or no internet during the trip, so I published when I could.  Thanks!

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