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Identity HelpDrop stone in varved clay

8th Sep 2017 21:51 UTCJohn Collins

The photo shows a glacial drop stone (granite) deposited from the bottom of a ancient glacier into a stream flowing over a clay bed. The distortion of the clay deposition around the boulder suggests the stream flow under the ice was from right to left. I found this at a road cut at 46° 26' 55" N, 83° 44' 01" W north of Bruce Mines, Algoma District, Ontario. The size of the clay fragment is 11 x 8 cm and the granite pebble has a diameter of about 2 cm.

1) Do any of you know what the approximate age of this glacial deposition might be?

2) Have I interpreted what I see correctly?

Thanks,

John

8th Sep 2017 22:15 UTCRichard Gunter Expert

Hi John:

You appear to have a sample of the well-known Huronian Age tillite horizon. It is equivalent to much of the Precambrian strata in the Cobalt area and is part of the Southern Geological Province. Given the dynamics of supra-glacial lakes and the very fine-grained size of the glacial sediments I would suggest that it was the drop stone that moved into the sediment and not the sediment moving. Do you have the sample in the orientation you found it or is it upside down? The compression of the sediments would make more sense if they were under the drop stone.

8th Sep 2017 23:15 UTCEd Clopton 🌟 Expert

Cool specimen! I have heard of these but have seldom seen one.

I wait with Richard to find out whether the specimen is posed right side up. I can see it either way, but its appearance would be easier to explain other side up.

9th Sep 2017 00:35 UTCWoody Thompson Expert

Here's a younger dropstone example, where a pebble probably dropped from an iceberg and landed on sea floor muds during the late-glacial marine submergence of lowland areas in southern Maine. The pebble sank a bit into the clay, and younger sediment layers were promptly draped over the top. Some thin sand layers are also seen here.

If this material were lithified and detached, like the Ontario rock, it might be hard to tell "which way was up"! If that sample preserves graded beds, which often are finer upward, that could help. Nice rock in any case!!

9th Sep 2017 00:58 UTCJohn Collins

Hi Richard,

Thanks for your reply.

You can see the actual collecting spot on Google Earth. The specimen was loose at the bottom of the ditch and I don't remember the orientation of the tillite bedding but it is clear that the bedding direction was horizontal as photographed. I think you are correct in thinking that the rock was photographed upside down When looking at the inverted photo, just above and to the right of the granite there looks to be a later and smaller drop stone slightly lighter in coloring from the clay. If the stream direction was from left to right in the inverted photo, then the suspended clay stream lines would match the flow pattern that we see (I think).

When I look at the back of my specimen, the layering is quite regular and is parallel to the undisturbed bedding direction on the pebble side.

Thanks for the historical information about this formation.

Regards,

John

9th Sep 2017 01:04 UTCJohn Collins

Hey Woody,

I do think my specimen was photographed upside down. Oh well.

Regards,

John

9th Sep 2017 01:26 UTCWoody Thompson Expert

Hi John,

That may well be the case (i.e. the rock may be posed upside down). It looks like there's more sediment disturbance just above and to the right of the granite pebble, as seen in the photo.

If this was a lake bottom environment, currents might have resulted from sediment-laden gravity-driven flows across the lake floor, like in front of a delta. However, I don't see evidence of currents in the photo, and it could have been quiet water as Richard proposed.

It's been many years since I saw the Ontario tillites. This kind of stuff is what I do for a living (mapping and researching New England glacial environments), so great fun!

9th Sep 2017 02:12 UTCPaul Brandes 🌟 Manager

John,

Your specimen was indeed photographed upside down by looking at the bomb-sag structure that is to the top and left of the quartzite (not granite) in the sediments. Look at Woody's sample and you see the same structure on the lower left corner of the dropstone. I do agree with Richard in that this is part of a typical Huronian tillite deposit.

9th Sep 2017 16:14 UTCSteve Hardinger 🌟 Expert

POTD! POTD!

9th Sep 2017 17:09 UTCRob Woodside 🌟 Manager

Both of these photos are good POTD candidates . Please upload them.

9th Sep 2017 17:29 UTCDonald B Peck Expert

Both shown (above and below) on the same day for comparison! With an explanation in the call-up window.

9th Sep 2017 22:44 UTCJohn Collins

Bob

I already have: ID# 4AQ-5TE

regards,

John

9th Sep 2017 22:44 UTCJohn Collins

Richard Gunter, Ed Clopton, Woody Thompson, Pauk Brandes:

Thanks for your informative replies. I have encorporated your details into the description of the mindat mineral # 4AQ-5TE.

Check the scene via Google Earth at 46° 26' 55" N, 83° 44' 01" W.

Regards,

John

10th Sep 2017 02:37 UTCWoody Thompson Expert

Hi John et al.,

I'll need to re-scan my 35 mm slide to get better resolution. The photo upload process doesn't accept the low-res version I posted above. Had a long day of field work, so will do this tomorrow.

10th Sep 2017 13:34 UTCJohn Collins

Richard Gunter, Ed Clopton, Woody Thompson, Paul Brandes:

Thanks for your informative replies. I have encorporated your details into the description of the mindat mineral # 4AQ-5TE.

Check the scene via Google Earth at 46° 26' 55" N, 83° 44' 01" W.

Paul, sorry about the earlier typo.

Regards,

John

10th Sep 2017 15:51 UTCWoody Thompson Expert

Hi John,

I uploaded a higher-res version of the Maine glacial dropstone, so it can be compared with your Ontario tillite specimen:

https://www.mindat.org/5KC-NNQ

Cheers,

Woody

10th Sep 2017 16:05 UTCRob Woodside 🌟 Manager

Thanks John and Woody.

10th Sep 2017 16:49 UTCJohn Collins

Thanks Woody for your updated photo. This is all really interesting stuff.

John

10th Sep 2017 21:56 UTCPaul Brandes 🌟 Manager

John Collins Wrote:

-------------------------------------------------------

> Paul, sorry about the earlier typo.

No worries; I've been called much worse in my career.... ;-)

Thanks John and Woody for your contributions.

11th Sep 2017 00:23 UTCHolger Hartmaier 🌟

Attached is a photograph of an exposure of tillite just outside of Cobalt, Ontario, which is accompanied by a sign explaining the geology. It is part of a self-directed geology tour of sites in the Cobalt area.

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