Comments on: Friday Field Foto #47: Paleozoic glacial tillite

By: BrianR

BrianR — Thu, 24 Sep 2009 03:59:12 +0000

John … I’ve only seen these rocks once (for about 20 minutes when I took these photos) … but ‘christie’ who commented above knows quite a bit. I suggest following the link she provided to her blog discussing the Dwyka in more detail.

By: John Bradshaw

John Bradshaw — Wed, 23 Sep 2009 23:58:29 +0000

Hi,
I am writing a paper on some conglomerates in the Permian-Triassic Trinity Peninsula Group of the Antarctic Peninsula. These are not glacial but may be derived from erosion of older galcial deposits, possible marine glacials near the shelf edge (the sediments are probably Triassic). I am looking for information on the sizs and composition of the more common clast in the Dwyka. I see you have photos with large boulders but I can’t tell what they are. Do you have a high proportuion of quartzites and granitoids?

By: christie

christie — Sun, 20 Apr 2008 23:01:11 +0000

Timing of Cape Fold is carboniferous-Permian – initiated during the deposition of the dwyka.

By: BrianR

BrianR — Sun, 20 Apr 2008 17:22:20 +0000

Christie … awesome! Thanks for all the info. One general question, what’s the timing of deformation in the Cape Fold Belt?

By: christie

christie — Sun, 20 Apr 2008 15:30:44 +0000

Sorry I got carried away with the Dwyka thing but one more link…
http://christieatthecape.blogspot.com/2007/09/nerding-out-on-some-old-bugs.html
As I’ve already written about this a bit but had forgotten about it!

By: christie

christie — Sun, 20 Apr 2008 15:21:12 +0000

Hey, glad you all seem to like “dwykite”. The Dwyka Group (which only has one formation, so nearly always called Dwyka) is subdivided in the Laingsburg area of the little Karoo into basically 7 members – not sure if these are similar in the Tanqua Karoo. There are two general lithologies – a “course” (pebble to boulder-bearing) diamictite with massive silty/mud matrix (probable glacial “rock flour” I think. Rarely these have bedding/lamination but generally completely featureless matrix.
1c: Course
basal boulder bed – up to ~1m boulders but mostly 30-60cm, bed is about 1-1.5m thick – origin unknown but it is laterally uniform and continuous for 100s km
2c: similar to 1c but a bit finer grained/ higher % matrix
2f: planar laminated muddy siltstones,
… 3c, 3f, 4c, 4f are lighologically similar to “c” and “f”s down section.
photos of course and fine dwyka on my blog post from last year’s field camp: http://christieatthecape.blogspot.com/2007/09/springboks.html

REally cool oblong concretions – mostly phosphatic core and dolomitic cement mantle – occur in the 3f and 4f.

Regarding Callan’s questions –
there are supposed to be faceted/striated clasts, but I have only seen one (a large boulder). Complicating things – I have documented that the cape-fold-related axial planar pressure solution cleavage, and a possible post-folding fracture cleavage both manifest rather strangely in the dwyka. The cleavages are strong in many clasts but invisible in the matrix. I haven’t thin sectioned the matrix but i think this reflects the mineralogical immaturity at time of deposition, e.g. low in clays. This makes some clasts look faceted – but if you look around carefully at clasts in situ you see that each “faceted” face is parallel to the rest, and parallel to the axial planes of the folds. therefore I suggest that the number of true faceted clasts may be overestimated. However, the composition of clasts (combined with their size) leaves no controversy that they were glacially transported. I desperately wanted to collect a very obvious boulder of Barberton BIF from the dywka at Laingsburg – but it was probably 50kg and not at all close to the road. It had to be transported 1000s kms.

Re clear dropstones – rare, as clasts are common in massive matrix where there is no bedding to show the “drop”. My colleague knows of one fantastic and very large one which he takes the students to see. The “fine” members are planar laminated and I have never seen any deformation (not even bioturbation or soft sed structures) in these members.

It is worth noting that there are reports of a < 1m thick “fold zone” in the shales under the base of the Dwyka. The shales are almost completely unexposed so not much is known about this zone. Could be attributed to shearing by ice or currents, or soft-sed def. during loading. Interesting consequences for interpreting whether ice-sed contact ever occured in the Dwyka basin. I have seen silt-filled mudcracks on the top of the 4f unit; therefore I consider it unconformable. It is overlain by the Prince Albert shales – certainly shallow basin but there is argument whether fresh/marine/ hypersaline…. the Prince Albert is ruthlessly crumpled in the Cape Fold belt (weakest link) and therefore it’s my favorite in the section. (structure candy here: http://christieatthecape.blogspot.com/2007/09/doubly-plunging-overturned-folds.html)

By: Silver Fox

Silver Fox — Sat, 19 Apr 2008 13:20:27 +0000

Whatever you all end up calling it - and sometimes a non-genetic term is best, and dwykite for the new genetic implications sounds good to me - I love the photos, and I collect "breccias" of all types. I guess I'll have to program the new term into Word's spellchecker! (see Kim's post).

By: ChrisR

ChrisR — Sat, 19 Apr 2008 09:36:14 +0000

The Archean section I’m working on contains diamictites, and one of the things I’m interested in are whether they’re glacial or not…

By: Callan Bentley

Callan Bentley — Fri, 18 Apr 2008 19:50:26 +0000

Are there any faceted and/or striated clasts in the mix?

Christie — Why liquid water? Are there clear dropstones?

I love diamictites!

By: BrianR

BrianR — Fri, 18 Apr 2008 17:56:05 +0000

Christie … thanks for the input. That is a good point, we were calling a tillite, but were also discussing how it was interpreted to be subaqueous deposition. Hmmm … dwykite … rolls off the tongue nicely.