From Firestone 2007:
“Toon et al. suggest that an impact capable of continent-wide damage requires energy of 10^7 megatons, equivalent to an impact by a 4-km-wide comet . Although an impactor that size typically leaves an obvious large crater, no such late Pleistocene crater has been identified. The lack of a crater may be due to prior fragmentation of a large impactor, thereby producing multiple airbursts or craters. Hypervelocity oblique impact experiments (Peter. H. Shultz , unpublished data) indicate that a low-impedance surface layer, such as an ice sheet, can markedly reduce modification of the underlying substrate if the layer is equal to the projectile’s diameter. These results suggest that if multiple 2-km objects struck the 2-km-thick Laurentide Ice Sheet at 30°, they may have left negligible traces after deglaciation. Thus, lasting evidence may have been limited to enigmatic depressions or disturbances in the Canadian Shield (e.g., under the Great Lakes or Hudson Bay), while producing marginal or no shock effects and dispersing fine debris composed of the impactor, ice-sheet detritus, and the underlying crust.”
A couple of those high velocity impact experiments can be seen in the YouTube video below. Please pay particular attention to the experiment about halfway through that simulates an oblique angle high velocity impact into an ice covered surface. That’s the “unpublished data” mentioned in Firestone 2007.
Did you notice the randomized character of the sub-ice target surface in that ice sheet shot? With those ice sheet impact experiments in mind, I’m thinkin’ that folks who’re looking for nice orderly, and round, craters and impact structures in The Great lakes Region, and Canada as the residual planetary scarring, and ‘smoking gun’ of the YD impact event after the ice melted in the Great Lakes region are chasing red herrings. The laurentide Ice Sheet was up to two kilometers thick. So even a direct high velocity impact there as large as the one that produced Barringer Crater in Arizona probably would’ve left no trace in the sub-ice surface. And I’m betting that none of the ice sheet impacts left a recognizable crater. But that’s not to say that there is no residual planetary scarring to be found.
I’ve watched every available YouTube video of the ice sheet impact experiments Dr Schultz conducted at the HVGR, and there is a place in the great lakes region that seems to fit the description given above, and the post impact target surfaces of those experiments, to perfection. And the number, and scale of those features is also consistent with multiple 2-km+ objects
It’s called “The Patterned Peatlands of Minnesota” There’s a very good book with that title by H. E. Wright et. al. And I’ve been interested in the place since the first time I saw it in a satellite image in 2008. I realize that intuition, or a gut feelings have a tenuous place at best in science. But there was just something about the shape and directionality of those patterns that does not jibe at all with the idea that they were created by the normal linear passage of glacial ice during the last ice age, or by normal riparian processes during the past 13,000 years or so since. In order for them to have formed as a result of erosion you need moving, flowing water. And the water in the Patterned Peat Lands is some of the most stagnant wetlands on Earth.
If you read the book above you’ll see that the hydrology, and biota of the area has been very thoroughly studied. And many cores have been taken of the deposits of peat. So the formative ecological processes that created the bogs, and how old the peat deposits are, is very well understood indeed. Unfortunately, the same cannot be said for the late Pleistocene, early Holocene geomorphology that created the strangely shaped depressions that have no inflow or outflow channels, and that the PPL are setting in to begin with. Writing about the environment aspects of the place only describes it in an ecological framework. And the simple fact is that the formation of peat since the LIS retreated only filled the depressions with stagnant, anoxic, water and ancient dead vegetable matter that isn’t able to decay. It couldn’t have formed them. Something else had to have done that.
Early on a friend of mine in the Air Force reserve was able to log a LIDAR flight over the area as a training mission. But in the long run his contribution was more of a confusing hindrance than help. Because unfortunately, while his skills as a pilot operating a LIDAR equipped aircraft may be unmatched, he wasn’t so sharp at interpreting the data that equipment was collecting. According to him the image he’d provided was a false color image depicting altitude that graded from darker = lower, to lighter = higher. But what my pilot friend wasn’t clear about was the reflective effect that the water in a wetland environment would have on the LIDAR data.
It was almost impossible to conceive of any normal geologic process that could leave such strangely shaped raised terrain features after an ice age. But it was also impossible to imagine, and think trough, a plausible way for them to be deposited as raised features left on the sub-ice surface by an ice sheet impact event. I didn’t believe for a minute that glaciers could do such a thing. After all, they haven’t done so anywhere else on Earth. But no amount of grappling with it could come up with a way I was completely comfortable with for an impact event to do it either.
The enigma was too much, my overwhelming curiosity got the best of me, and I just had to go there personally and see the place for myself.
Thanks to the generous support of Quantum Future Group I was able to take off on a road trip to go and spend a few weeks on the ground there. I was able to figure out that in this case his explanation was ass backwards, and that in fact in his image Lighter = deeper, and wetter; Darker = higher, and dryer. So what I had at first perceived as something that had been somehow deposited on top of the sub-ice surface was in fact where something had gouged into that surface. A few weeks on the ground there also convinced me more than ever before that the place is indeed the remaining scars of a major impact event into the ice of the LIS. But the mechanism of the event must have been vastly different, and probably more violent, anything I was imagining before going there.
I still couldn’t imagine a way for the linear movement of an ice sheet to gouge randomly flowing depressions, and patterns into the hard and archaic igneous bedrock of the Canadian Shield; much less a linear gouge that’s perfectly perpendicular to that flow of ice. But when considering them as locations where some of the energy of a very large multi fragment impact event that was violent enough to do continent-wide damage penetrated the ice and gouged into the sub-ice surface, it’s not hard to work out a plausible way to get those randomized patterns at all.
A few multi-gigaton, low angle, impact induced, hydrothermal explosions blowing the Laurentide Ice Sheet to smithereens can explain most of the randomized and flowing patterns. And for the most dramatic depression of them all, namely the arrowhead shaped gouge just west of the patterns, and north of the Red Lake Indian Reservation, all we need is for some of the kinetic energy of one of the larger fragments to have penetrated all the way through the 2-km+ ice of the LIS.
The rock composition of the bedrock for the general area of the black arrowhead shaped bog depression is labeled on the USGS’s geologic maps as: "Para gneiss and schist-rich migmatite -grades into undivided meta sedimentary rocks." And the meta sedimentary rocks are further described as: ‘Meta sedimentary rocks-undivided-greywacke, slate, local units of conglomerate, arentite, graphic slate, fine-grained felsic volcanogenic, and volcaniclastic rocks, lean oxide iron-formation and its metamorphic equivalent. Includes the Knife Lake Group and the Lake Vermilion Formation in northeastern Minnesota.’
Migmatite is a rock of both metamorphic, and igneous origin that exhibits characteristics of both rock types. It forms under extreme temperature conditions during metamorphism through the heating (but not quite melting) of rocks in the presence of a lot of water, and where partial melting occurs in pre-existing rocks. Migmatites aren’t crystalized from a totally molten material, and are not generally the result of solid state reactions. But are composed of a new material crystalized from incipient melting, and an old material that resisted melting. I’m wondering if that might be exactly the kind of rock we should expect to see in the burn scars of impact induced hydrothermal explosions in the Laurentide Ice Sheet. And I’m especially suspicious of the words “Volcanogenic”, or “Volcaniclastic” anywhere in the Canadian Shield because except for some dyke activity that dates to about a million years or so, the Canadian Shield isn’t supposed to have seen any volcanic activity in more than 2.5 billion years.
So I can’t help but wonder how long it’s been since that stuff was in a molten state. But if the area is in fact a location where some of the kinetic energy made it all the way through the ice sheet without being converted to heat, there should be some shock metamorphism to be detected in some of the quartz grains.
One of the things that going there personally and exploring the place for myself has taught me is that if you want to get a good handle on interpreting the true color images from Google Earth it helps to pay attention to the time of year in the image date at the bottom of the page as it will have a huge bearing on what kind of terrain and flora is indicated by any given color.
For example, the GE image above showing a close up of the distal, or western end of the arrowhead shaped depression of the Big Red Lake Bog is actually from two different dates, and seasons, a couple of years apart, and stitched together by GE into an almost cloudless image. The left half is from May 8, 2013. And the right half is from Sep 9, 2011. So on the left we see the flora of very early spring. The last frost is behind them. But it’s still too early in the season for the deciduous trees to be showing any leaves yet. The native grasses haven’t awakened yet either. So for the most part the only green in the left half of the image is that of lowland evergreen forests. On the right we see the flora of late summer. So everything on that side is green and verdant. And from high up it’s difficult to tell the difference between one shade or the other. But the real clue is in the huge dead zone visible on both sides of that image, where little to nothing can grow or live at all, no matter what time of the year it is.
In both halves of the image the areas that grade from a dark tan to light brownish grey in the patterns are floating bog conditions that would make anything described in a J.R.R.Tolkien novel pale in comparison. The inner areas of that big Arrowhead shaped bog are almost completely inaccessible except by some sort of specialized, floating track vehicle. But the old timers will tell you that the bog surface in anywhere but the arrowhead shape you see above consists of a mat of anywhere from 18” to 36” of vegetable matter that moves like a giant water bed. Below that you’ll find anywhere from 3 to 15 feet of almost completely anoxic water the color of bad black coffee. And finally a deep layer of peat before coming to the early Holocene sediments, and finally the hard igneous rock of the basement bottom. It’s all a dead zone. And the reason that even grasses have difficulty there is the lack of oxygen in the water. There aren’t even any game trails across it. And the pilot I met while there, and who flew me over the area, said jokingly as we flew over it that there probably weren’t even any mosquitoes living down there because there wasn’t any blood for them to feast on.
The large arrowhead shape you see in the images becomes more pronounced, and visible in wetter years. And the black color is more of that dark stagnant water. But that bog is the deepest depression of them all with the exception of upper, and lower Red Lakes. The local folks told me that 2013 was the second of the two driest years in living memory. So when flying over it in late September there was very little of the black, open water showing at all, just a vast, wet, brownish tan dead zone.
The view below was taken from about 2,500 feet up looking east from the west end of the large arrowhead shaped bog
Some of the older locals who’ve lived in the area for a lifetime, and who’ve explored the wildlife management area it sits in, estimate that at it’s deepest point on the east end it could be as much as 60 feet to the basement rock of the bottom. And in an area that’s mostly as flat as a parking lot, a 60 foot deep depression is extremely dramatic relief. You’ll find the material that was excavated from it in an ejecta layer extending out to the sides, and the distal, or west, end that’s thick enough to support the evergreen forests surrounding the bog; and high enough above the extremely shallow water table of the region for them to thrive.
Another set of features you’ll notice in Google Earth is the set of lines incised into the terrain that form a grid of rectangles, each 1 mile wide by 2 miles long. It seems that back around the turn of the century some bright boy in Washington got the bright idea to dig ditches to drain the swamps, and hopefully turn the area into good arable farmland. They gave a lot of farms away on a first come, first serve basis back then. And it could have worked too. Whole new communities were thriving for a while. But never underestimate the power of the mighty phuque ups in Washington to tax a really good idea into oblivion. The ditch tax they levied on those fledgling farmers struggling to start up and run a small family farming business was too much. And when they couldn’t pay their ditch taxes the government took their farms. Then without a tax base, there were no more funds for ditch maintenance. The ditches all became clogged with overgrowth. And the swamps, and mother nature took over again. Uncle Sam finally threw in the towel and the Minnesota Department of natural resources resolved to call it the Red Lake Wildlife Management Area forevermore.
If you get a wild hair to wander up there and have a look at the place like I did the thing to keep in mind is that while they don’t show up on any road maps of the area, every one of those old ditches is paralleled by a perfectly good access rd.
The center of the Big Red Lake Bog is extremely difficult to get to, even in a good year. So even if I had had the kind of equipment required to get a core down to the bedrock of the bottom while I was there, I’d have had to wait and come back in the winter when the trails around it can be reached by snowmobile in order to haul that equipment out there. But I reasoned that since the slightly raised areas with evergreen forests surrounding that bog have all the characteristics of an ejecta curtain around an impact structure then, if it is the work of an early Holocene impact event, a soil specimen from the edge at the west end of that ejecta curtain might be just as informative. And in fact, while I have not had the chance to look at it in detail, it was encouraging to notice that the small rocks, and gravels in the sample I dug up there do not have the rounded, tumbled appearance of normal glacial til. These little rocks are all sharp, and jagged like freshly crushed rock. I can’t wait to get some of this stuff under a good microscope!
So I’ve got some of it. I was also able to get at an exposure in a ditch at a construction site on a farm about 20 miles from there where we can see the transition from the sub-ice clays of the LIS to the sandy lacustrine conditions of the Lake Agassiz sediments exposed, and presented like a prize. And I was thinking that if the area was hit by an impact event at the end of the last ice age, then some of the debris of that event would have been deposited in the transition layer between them. So I took some of that with me too. And one of these days when I’m done with this walkabout I hope to look for more clues in that stuff under a decent microscope. Not to mention getting them into someone’s lab for a more thorough analysis if anything interesting turns up under that microscope.
But that’ll have to wait. Because my next stop will be a couple of weeks at Big Bend Ranch State Park, Texas to look at the burn scars of a completely different kind of catastrophe.
I’ll post a lot on that place before it’s over too. But in the meantime, like an ass into the desert go I forth to do my work.