Torlesse Terrane - a podcast by Whitestone Geopark

Geopark - Sasha M Torless Terrane 2019

Real Radio: Time to talk Geopark once again with the geoeducator Sasha Morriss.  You've been on a wee field trip up to Wellington.

Geoeducator: That's right. Yes, I met with Julian Thompson, who's the outreach educator for GNS that's Geological and Nuclear Sciences. And their main office is based just outside of Wellington. And I meet up with him and we did a wee field trip out on the West Coast there to look at some basement rocks. So we had a pretty exciting day scrambling up and down cliffs and got to look at a formation called the Torlesse Terrane. Now, some people might have heard of this, and you'll have to check out our Facebook page a bit later - I'll be putting photos of the trip up on that. So the Torlesse Terrane was a sedimentary formation that was formed alongside Gondwana. So we're talking a long long time ago.  More than 80 000 years ago was when New Zealand started to separate from Gondwana, so it's prior to that. There were sediments that were being washed off Gondwana and deposited. Now these were deposited in a layered way. So in a sedimentary sequence, you would get the heavier quartz sands depositing first and then the finer mud sands so you end up with what's called stratification, which is a big word, meaning 'in lines'. So you would get the quartz sands and then the muds settling later and then you'd get more quartz and more muds. So it forms this record of sedimentary deposition alongside Gondwana. Now that happened along a really wide edge. So in New Zealand, you see these sediments right from Otago right up into the North Island. Really big formation of various ages. There is a younger each and an older age to it.  And it's been metamorphosed to varying degrees through time. So the process of metamorphic process is actually where sediments are changed through heat and pressure.  So if you think of it like a cooker, you get things being compressed and it heats it up a little bit and the sediments start to squish and push some of their water out and they start to change. So the sediments instead of being sand or a mud start to get harder and we get what's called greywacke. So people will find this in our North Otago rivers - the round flat stones are greywacke. And the more that it undergoes this metamorphic process so it gets more heat or more pressure and time is in there as well. Then you get schists or you get gneiss. So depending on how much that rock has changed, how much heat and pressure it's been under determines the end result of the rock. So there's a whole other story we'll talk about another time to do with how we got the Otago and Haast schist. But basically the Torlesse is this greywacke that extends kind of North Otago region (it's a very blurred zone with the schist) and then right up through and into the North Island) and that's what we went to see. Fantastic examples where it's been deformed. So it's been faulted and folded. And you'll see photos of that on Facebook. And it becomes really clear and it almost it looks like a liquorice allsort that's been squished and you can see all the layers in it. Fabulous. So I encourage people to have a look at that and read my wee excert I've got to explain it - you'll see some of our amazing basement rock.

Real Radio: That's fascinating. Good stuff. Thanks very much, Sasha. We'll catch up with you again next time.

Geoeducator: Fabulous. Thank you.

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