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GeneralAnother geological puzzle.

3rd Nov 2018 00:19 UTCReiner Mielke Expert

How did these cobbles get there? These are rounded cobbles ( 5-20cm) of various lithologies, everything from quartz, to gabbro, to slate. The ones pictured are mainly granitic. The odd thing is that they are floating in limestone. I was thinking drop stones but ice in an environment that produces limestone doesn't seem likely. Any one have any theories. Thanks

3rd Nov 2018 00:54 UTCPaul Brandes 🌟 Manager

Where is that, Reiner?

Or, what formation is the rock from?

3rd Nov 2018 02:39 UTCLarry Maltby Expert

Reiner,

The only thing that I can think of is that perhaps the rocks were carried by an iceberg and dropped on the sea floor. In that way the sedimentation on the sea floor would not be disrupted and would continue to cover the cobbles and eventually form the limestone. In Glacier Bay I did see rocks being carried by icebergs.

3rd Nov 2018 03:17 UTCBart Cannon

Are you absolutely certain it is a limestone ? I suppose that there could be a limestone captured turbidite.

3rd Nov 2018 06:00 UTCBen Grguric Expert

Could be a so-called Cap Carbonate horizon believed to be associated with Snowball Earth conditions.

3rd Nov 2018 09:32 UTCWayne Corwin

My thoughts would be a huricain or Snowball Earth .

3rd Nov 2018 09:35 UTCSusan Robinson

Several of the stones in the "limestone" matrix are angular and others seem so faint as to be part of the rock enclosing them. Also, the stones do not weather out as they would in a conglomerate and present their whole shapes - they seem more of less part of a horizon that is exposed. If you know the locality of this formation, please post it.

3rd Nov 2018 11:24 UTCReiner Mielke Expert

The outcrop is here: https://www.mindat.org/loc-259398.html it is Ordovician Limestone of the Liskeard Formation.

3rd Nov 2018 11:28 UTCReiner Mielke Expert

"Also, the stones do not weather out as they would in a conglomerate and present their whole shapes" The photo is of a roadcut so weathering has not had time to act. Elsewhere in the same area where it is weathered they do present their whole shape but exposure of this horizon is limited as it is capped by fossiliferous limestone with no cobbles.

3rd Nov 2018 11:36 UTCReiner Mielke Expert

Some sort of Earth Snowball event could explain it. They appear to be drop stones as they are "floating" in limestone. Are there any other examples of this known?

3rd Nov 2018 11:44 UTCReiner Mielke Expert

Also the zone of cobble bearing limestone is relatively thin, less than a meter thick.

3rd Nov 2018 11:47 UTCReiner Mielke Expert

If you go to google maps you can see the entire outcrop on street view here https://www.google.ca/maps/@47.5988683,-79.5905736,3a,75y,90t/data=!3m6!1e1!3m4!1sBejIqNai2DgF9DpstHegBw!2e0!7i13312!8i6656

3rd Nov 2018 12:01 UTCDavid K. Joyce Expert

When I was at mining school, our geology instructor showed us an outcrop of very fine-grained, bedded marine-origin slate that had a granite cobble suspended in the otherwise undisturbed, fine-grained sediment. The ice-drop scenario was the explanation that he led us to in order to explain the presence of the cobble. Somewhere in the Haileybury-Cobalt area.

David K Joyce

3rd Nov 2018 12:12 UTCReiner Mielke Expert

Hello David,

I've seen that but that is much older precambrian rock. The unusual thing with this is that it is in fossiliferous limestone not something that is usually associated with glaciers.

3rd Nov 2018 12:24 UTCPaul Brandes 🌟 Manager

Usually one would need a warmer ocean to get carbonates forming, so the idea of an iceberg dropping its sediment load is plausible. It is not the result of a snowball earth event as the age of the formation doesn't line up.

3rd Nov 2018 12:29 UTCKeith Wood

The common ice drop scenario involves glacially derived icebergs that scavenge up all kinds of rocks then calve off as icebergs and float away dropping them as they melt. However, Reiner is likely correct that icebergs would usually melt before arriving at limestone-forming environments. Also, after such lengthy travel, one would not expect many icebergs to remain close enough together to create a layer a meter thick. It is a challenging conundrum, probably requiring a less likely scenario to explain it.

All I can think of is a large ice sheet like the Antarctic ice sheet that got blanketed with a layer of lithic fragments from a volcanic eruption. Such icebergs are larger, with less surface area, so they melt more slowly. With the lithic ejecta on top of the ice rather than within the ice, the fines could be washed away by rain and melt water. As the iceberg thinned it would break apart, dropping rocks in a local area. This scenario would account for the travel distance of an iceberg source, and the clean horizon.

A lot depends on the speed of currents at the time. Does anyone know if high thermal gradients drive faster ocean currents? If so, times of glaciation events might also have faster currents and be able to deliver icebergs farther than today.

I believe there was a glaciation event in the Ordovician.

I would wager this is a fairly localized occurrence.

Knowing more about the depositional environment of the limestone would help. It is light colored and massive, suggesting a lagoonal facies. If this is the case, perhaps such an iceberg as I described got grounded there and dropped all its stones in that area. Or a strong current delivered several such icebergs in a short while which shed stones while passing through.

Just speculation to open up the discussion. My thinking may be flawed in one or another way. I look forward to the discussion.

3rd Nov 2018 13:07 UTCReiner Mielke Expert

Trouble with the volcanic eruption scenerio is that the cobbles are rounded. There could be a gap in time of millions of years between when the carbonate sediments were deposited and when the ice event occurred. As far as I know carbonate sediment could stay soft for a long time especially if the water became colder and dissolution rather than deposition started.

3rd Nov 2018 13:55 UTCKeith Wood

Cobbles in lithic tuffs are often surprisingly rounded, owing to the abrasion undergone during transport up the volcanic vent. And clearly the clasts are not all rounded based on your pictures. They are at the most subround in shape and at the least, subangular to angular. Their shapes are reasonable for my hypothesis.

Interstitial water in carbonates is almost constantly saturated in CaCO3, leading to relatively rapid lithification, as compared with siliciclastics, but commonly still on the order of hundreds to thousands of years or more. In the lagoonal facies, original bedding is usually poorly preserved, owing to bioturbation. This lends a massive character to the limestone as new beds are mixed with older material and the distinctiveness is lost. The light color is indicative of fully oxygenated conditions unfavorable to the preservation of organic matter.

3rd Nov 2018 14:00 UTCKeith Wood

Also, a millions-of-years time gap is unlikely given the apparent uniform character of the limestone above and below the stony horizon. Non-deposition over millions of years typically results in formation of hard grounds and/or clay layers, which would be apparent, and a clear bedding demarcation, which all seem to be absent.

3rd Nov 2018 14:17 UTCKeith Wood

Also, it may not have been a lithic tuff, as tuff implies inclusion of igneous material such as ash and lapilli that were native to the volcano. A phreatic eruption need not contribute any volcanic material but is just as capable of delivering the mix and character of stones you have noted.

Or there is yet another explanation entirely.

3rd Nov 2018 14:25 UTCReiner Mielke Expert

All the cobbles are rounded to some degree. An eruption would create many angular fragments of which there are none. How do you explain that?

3rd Nov 2018 14:31 UTCKeith Wood

Well, your picture must be very misleading, because there is a quite angular stone in picture 44. Are you ignoring that one?

Have you ever examined a lithic blanket from an eruption? It is surprising the range of shapes found within. There are fewer angular fragments than you would imagine. Also, in the phreatic environment, stones can be milled prior to eruption. But I'll admit there seems to be a shortage here.

3rd Nov 2018 14:42 UTCReiner Mielke Expert

If you look closely at the "angular" fragment you will notice the corners are rounded. There are very few unrounded corners on any of them not something you would expect with a volcanic eruption. Also there are no remnants of volcanos or evidence for volcanoes in the area that would have had to be present at that time.

3rd Nov 2018 15:13 UTCKeith Wood

I would expect the edges to be rounded to some degree. It is still categorized as an angular to subangular clast according to standard guidelines. Look up the chart and see for yourself. And that is why I said "quite angular" and not simply "angular". In all clastic environments, truly sharp edges are very short lived.

The volcano need not have been present "in the area." That is the point. The iceberg could have travelled thousands of miles on a swift current with favorable winds even at 10-15 miles per hour. The Gulf Stream flows at 5.6 miles per hour and is not the fastest current in the ocean even now. There is an African current twice as fast. In a period of glaciation it is possible that high thermal gradients could drive even faster currents. A huge iceberg from an ice sheet would not melt away in 300 hours if it remained in a cool current. The iceberg could travel 4500 miles in two weeks under the right conditions.

I am suggesting a sequence of uncommon events. But they are possible events that account for much of what is observed. I'm open to other explanations but they need to be encompassing in scope.

What's your explanation?

3rd Nov 2018 15:34 UTCKevin Hean

Reiner, I think you answered the question when you said "angular fragment". A Glacier does create rounded boulders but it is also capable of breaking them into fragments, the fragment in question does have one obviously curved side.

3rd Nov 2018 16:32 UTCReiner Mielke Expert

Keeping in mind Occam's razor. A volcano is not necessary to explain the cobbles in ice, simple ablation of rock that a glacier flows across is enough to explain them.

3rd Nov 2018 16:39 UTCRobert Elliott

The clasts appear to be quite well rounded, in particular some 'milky' quartz. To me that suggests abrasion in water rather than a glacial environment. Without knowing the local area at all, I have looked at the geological map and it is apparent that there are sufficient lithologies in the surrounding area to have provided these clasts.

I have seen storm/wave deposits in the limestones of Derbyshire (UK), albeit with clasts of limestone. Could these clasts be from the local area and be related to a high energy event in the Ordovician, which scoured a nearby beach deposit, perhaps a storm? Also if they are dropstones I would expect to see some evidence of impact, which is not apparent in the photographs although I accept that the limestones have been somewhat altered. It is a very interesting geological puzzle!!!!

3rd Nov 2018 16:57 UTCReiner Mielke Expert

"To me that suggests abrasion in water rather than a glacial environment." There is a lot of water abrasion that takes place under glaciers for example eskers. I don't think you can separate the two environments.

3rd Nov 2018 17:08 UTCKeith Wood

Reiner,

In your initial post you were skeptical about the glacial theory. Have you reversed yourself? If so, please explain your newfound belief.

My volcano explanation was to account for a large ice sheet type iceberg, which has different properties in the ocean and a higher likelihood of traveling farther. It was in answer to your concern about getting an iceberg to warmer waters.

I am looking at the pictures in detail using the enlarged views you get when you click on them, and there are many subangular corners to the clasts. Some are rounded, to be sure, but not nearly all.

Bioturbation is likely in a shallow water, poorly bedded, light colored limestone, and may occur around and under dropped clasts. Impact evidence may not be preserved. Notably there is fossil coral in one of the pictures, so bioturbation is a virtual certainty.

3rd Nov 2018 17:12 UTCDoug Schonewald

Wouldn't an ancient river that periodically flooded explain the included cobble (though I imagine a mix of grain size would be involved with a river estuary).

Another theory might be a tsunami situation that drew beach cobble far enough out to sea to drop them onto the ocean floor sediments.

Just jabbering here.

3rd Nov 2018 17:17 UTCKeith Wood

Those high-energy local events would tend not deliver large clasts and omit the small ones. Such events are messy by nature. These stones were likely dropped. We are seemingly only debating the means at this point, since Reiner seems to have quietly come around on the glacial idea.

3rd Nov 2018 17:31 UTCUwe Kolitsch Manager

The geological map (macrostrat) shows that the outcrop is located approximately at the border to Huronian Supergroup rocks. Thus, the glaciation influence is highly likely.

https://en.wikipedia.org/wiki/Huronian_glaciation

"The Huronian glaciation (or Makganyene glaciation)[1] was a glaciation that extended from 2.4 billion years ago (Gya) to 2.1 Gya, during the Siderian and Rhyacian periods of the Paleoproterozoic era. The Huronian glaciation followed the Great Oxygenation Event (GOE), a time when increased atmospheric oxygen decreased atmospheric methane. The oxygen combined with the methane to form carbon dioxide and water, which do not retain heat as well as methane does"

3rd Nov 2018 17:41 UTCKeith Wood

There were no corals in the Huronian. There may be time-equivalent Huronian rocks in the area, but the limestone is Ordovician, so the glacial event involved must also have been Ordovician.

3rd Nov 2018 17:48 UTCUwe Kolitsch Manager

Oops - yes. There is of course a large gap in age ....

3rd Nov 2018 18:12 UTCReiner Mielke Expert

One other problem with a glacial theory. The Liskeard "Formation" is middle Ordovician but the ice age in the Ordovician is thought to be late Ordovician.

3rd Nov 2018 18:15 UTCReiner Mielke Expert

Hello Keith,

I am skeptical of a glacial origin but I really can't see any other way of explaining it so I am going with what is the simplest and most plausable explanation, unless something better comes along. It really needs some detailed study.

3rd Nov 2018 18:23 UTCUwe Kolitsch Manager

What's the nearest geoscience department? They might know about this outcrop or willing to determine the age of the xenoliths.

EDIT: Typo.

3rd Nov 2018 18:37 UTCReiner Mielke Expert

Laurentian University about 200km from there. I will contact them.

3rd Nov 2018 19:03 UTCDon Saathoff Expert

I assume everyone has taken into consideration the location of North America during the middle Ordovician.....equatorial and largely covered by shallow seas? Just beginning to emerge?

3rd Nov 2018 19:27 UTCPaul Brandes 🌟 Manager

If one pulls up Geologic Report No. 144, Geology of the New Liskeard Area, there is only one mention of anything volcanic, and that relates to the Gowganda Formation, a very early Precambrian (Archean) rock unit. To me, any volcanic influence here can be thrown out the window.

Also something no one has mentioned yet, where were the continents during this period? From what I have seen, the area around Cobalt was the edge of an equatorial ocean during the Middle Ordovician, so all the normal nearshore sedimentary facies should apply here. It also nixes the iceberg/glacial hypothesis.

Edit: Sheesh Don, I think you were looking over my shoulder when I posted...... ;-)

3rd Nov 2018 19:33 UTCReiner Mielke Expert

Hello Don,

There was an ice age at the end of the Ordovician and as far as I know North America was still near the equator. So one would have to assume an earlier middle Ordovician ice age with icebergs drifting north from Gondwana towards the equator.

3rd Nov 2018 19:35 UTCReiner Mielke Expert

Hello Paul,

Say we nix those two, what does that leave us?

3rd Nov 2018 20:51 UTCDon Saathoff Expert

No, Paul....inquiring minds stumble along together.....and sometimes trip & fall....

3rd Nov 2018 21:32 UTCKeith Wood

If we are to agree it was icebergs, they need to be large icebergs that could make it to the tropics with some freight. Ice shelf icebergs are the only candidates, and they typically don't have rocks in or on them unless they were added by some process, so far as I know. Hence the volcanic/phreatic idea.

Surely this occurrence has been studied by someone by now. Some library research my end the speculation.

3rd Nov 2018 21:33 UTCPaul Brandes 🌟 Manager

Sorry, I was heading out to dinner so couldn't elaborate any further.

Nathalie has been watching this thread as well. She suggested that maybe we're dealing with a tidal flat situation where the constant movement of water created a winnowing situation that removed smaller sediments, leaving the larger cobbles behind. Usually when one thinks of winnowing however, you're looking at wind removing the finer sediments, but any type of current should be able to accomplish this.

3rd Nov 2018 21:38 UTCKeith Wood

Yes, they could be a lag deposit. But it still had to originate somewhere, no matter how disperse. It only moves the ball back in time, but the problem doesn't go away.

Another thought just came to mind. Was there a karst event at this time? There is one at the end of the Ordovician, but I'm not sure about the middle Ordovician. If there was subaerial exposure there could also have been surficial transport from a local high spot, with all but the larger pieces being eroded away upon inundation. Indicators if that would be terra rosa - red oxidized beds within the formation. It doesn't look like that because the rocks appear to be separate within the limestone. They are matrix supported, but that brings us back around to drop stones.

3rd Nov 2018 21:45 UTCPaul Brandes 🌟 Manager

In the end it could very well be drop stones, but based upon where the continent was at this time, it seems unlikely.

Isn't interpreting geology from a photo fun?? ;-)

3rd Nov 2018 21:50 UTCTom Tucker

Similar rocks occur along Corridor H ( new US highway 48) in West Virginia, in I believe Devonian rocks. I can't find a decent reference just now, but they have raised similar discussions.

4th Nov 2018 04:02 UTCWayne Corwin

Another thought... a large tidalwave, either washed them up, or the returning waters draged then from shore?

4th Nov 2018 04:50 UTCKevin Conroy Manager

An alien rock fight?

4th Nov 2018 04:51 UTCDoug Daniels

Super strong aliens, bored at the time, throwing unwanted specimens out. Yeh, they visited in the Ordovician..... that's the ticket.....

4th Nov 2018 08:09 UTCKevin Hean

Don Saathoff Wrote:

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

> No, Paul....inquiring minds stumble along

> together.....and sometimes trip & fall....

Brilliant Don !!

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