Silurian Stromatolites in the Grand Rapids Uplands
On the long drive northward from Winnipeg to Grand Rapids, I always look forward to seeing one area not far beyond where the road rises onto the Grand Rapids Uplands. After the monotony of the “great bog” north of St. Martin Junction, the curves and slopes make a very welcome change. When I first visited Grand Rapids 20 years ago, this area had been freshly burned; it was a blackened waste similar to the one that followed the larger fire north of Grand Rapids in 2008.
The ever-changing appearance of the burn is also a great relief to the monotony, as over the years we have been able to watch saplings grow into trees, scorched trunks slowly replaced by a new forest. The regenerating plants attract wildlife, and we have seen deer, foxes, and a variety of birds there. One fall day, we even had a lynx pad across the empty Highway 6 in front of us – a rare sight indeed!
The reality of constant landscape change was brought home last autumn as I again drove over the southern end of the Uplands, this time with Dave Rudkin and Michael Cuggy. We were surprised to see that, in a place where there had been roadwork in recent years, the ditch had since been stripped completely bare by local floodwaters. What had been gravel the previous summer was now a gleaming white dolostone pavement. It didn’t appear to extend very far but it did look interesting, and we made a mental note of the location as we zoomed past. Somewhere to stop on the return drive from William Lake, if time and weather permitted.
Remarkably, both time and weather did permit on this occasion. After parking on the shoulder, we stepped down into the beautiful dry ditch. And there we saw a world of stromatolites – nothing but fossilized microbial mat structures covering the full extent of the bedding plane. How could it be that an ancient seafloor was entirely covered with cyanobacteria, those single-celled organisms that used to be called blue-green algae? Aren’t stromatolites supposed to be rare in any rocks that date from after the end of the Precambrian, their growth limited by gastropod grazing and seafloor burrowing by other creatures? And where are the snails and the other varied marine invertebrates, which can be seen in so many of the Paleozoic limestones elsewhere in this region?
Like the nearby burn, the ditch itself also tells part of our Earth’s story of ever-changing environments. This dolostone exposure may be small in area, but it represents a very interesting interval in geological time. In Manitoba’s sedimentary succession, stromatolites are very widespread in some parts of the Early Silurian Interlake Group. Some of the stromatolite-rich intervals were traditionally assigned to the Inwood Formation, but it has since been recognized that stratigraphic correlation of this unit is difficult, and former “Inwood” rocks are generally placed in the Moose Lake and Atikameg formations.*
Regardless of the technical correlation issues, the blooming of Early Silurian stromatolites in the Manitoba part of the Williston Basin was probably related to global patterns of extinction and evolution. The Early Silurian was the time immediately after the Late Ordovician mass extinction, the first of the “big five” extinctions in the history of life. During an interval about 443-445 million years ago, many families of marine life became extinct, some of them members of groups familar to fossil collectors: trilobites, brachiopods, and bryozoans. This extinction was associated with a glaciation on the South Pole, in a place that is now the Sahara Desert (I kid you not; you could look this up!). In Manitoba, fabulous Ordovician life forms such as the giant coral colonies in Tyndall Stone, and our beloved Churchill giant trilobite Isotelus rex, were replaced in the Early Silurian by . . . not much at all.
The post-extinction faunas around here are, as might be expected, not very diverse. But their lack of diversity can be compensated by remarkable abundance, such as in the nearly monospecific assemblages of the brachiopod Virgiana decussata, which can be seen near Churchill and along the riverbank near Grand Rapids. These stromatolites are also remarkably abundant: they were clearly the dominant life form in this area’s warm sea when the sediments we see were being laid down. Perhaps these stromatolites did so well because the water was too salty or too hot for gastropods and other invertebrates to thrive, but stromatolite blooms also occurred in other places following mass extinctions, to the extent that their abundance may actually be a diagnostic character for marine ecosystems that have been greatly disturbed.
Cyanobacteria are always with us, and maybe all it takes is the removal of grazers for them to return to Precambrian-like dominance. Which makes me wonder: how will the stromatolites do during the next few millennia? Not too well, I hope.
But enough of these morbid thoughts. In the present day, Dave, Michael, and I admire the ditch stromatolites. At first glance these are monotonous, but upon inspection they show considerable variation in size, shape, and surface texture. We snap a few photos and then it is back into the truck, wheels crunching on the gravel as we accelerate onto pavement and toward the “great bog.” Homeward.
* See Bezys and McCabe, 1996, Lower to Middle Paleozoic Stratigraphy of Southwestern Manitoba.
© Graham Young, 2014
Churchill River, Manitoba: August 24, 2014
Heading south from Churchill, the helicopter follows the river for the first 30 kilometres above its mouth. Then we head off overland, taking a straight-line shortcut instead of duplicating the river’s long dogleg. “Overland” is, perhaps, a bit of a misnomer, since the surface we pass over must be at least 20 percent open water, with much of the rest consisting of bog and moss, but as far as I know “overtundra” and “overmuskeg” are not words. Anyway, we fly low across this strangely-coloured otherworldly landscape for many kilometres, before rejoining the river’s course.
Coming over the steep bank, we can see that the river is still broad, but it is very different from the estuary you see at Churchill. Here the water rushes over its bed, with many treacherous shallows, boulders, and long stretches of rapids. After a stop to survey the cliffs beside Bad Cache Rapids, we are picked up again for the hop to Portage Chute. One hundred and twenty-five kilometres from the Town of Churchill, this will be the farthest we go during our several days of helicopter work in August, 2014. Around the steep rapids of Portage Chute we can see no landing place (the name “Portage Chute” apparently means “falls requiring a portage”), but a little way downstream a flat platform of bedrock extends from the cliffs on the river’s northwest bank. It is a perfect natural helicopter landing pad, and Frank quickly sets down. Once he gives us the all clear signal, we pile out and gather the packs, tools, and shotgun (never forget the shotgun, as there is still a risk of meeting polar bears even this far inland).
Portage Chute represents the beginning of Ordovician geology for this part of the Hudson Bay Lowland. South and west of here, everything is Precambrian for hundreds to thousands of kilometres. In fact, the granitic rock on which the helicopter rests is Precambrian in age, while the limestone cliffs beside us are Ordovician (Portage Chute Formation, Bad Cache Rapids Group, Katian [Upper Ordovician, in the range of 450 million years old]). We are standing on a great unconformity, one of the most spectacular geological contacts in the world. Our feet are on a surface that was eroded for more than a billion years, starting in a time when there was no complex life on this planet, while our hands can reach out and touch bedrock that was deposited as carbonate sediment on an ancient tropical seafloor during a time when marine life was reaching the peak of its first great diversification.
It is a magical place for a geologist, but it is also spectacularly beautiful, and for a few minutes I just stand there drinking it in. Not only is the exposure of the unconformity here almost too wonderful for the average geologist to believe, but it extends as far down river as I can see. In fact, Precambrian rock makes up the riverbed all the way to Bad Cache Rapids, some 20 kilometres away, while the Ordovician cliffs extend much farther than that!
Those Ordovician cliffs are also special for a paleontologist, because their mottled limestones hold wonderful examples of the fossil assemblage that has been sometimes called the “Arctic Ordovician fauna.” This biota includes a great variety of groups, and it is notably defined by receptaculitids, large gastropods such as Maclurina manitobensis, and cephalopods. In just an hour or so on the site, I am able to collect very good examples of all of these, and also corals (Calapoecia, Catenipora, Manipora, Palaeophyllum), brachiopods, and parts of trilobites; a rich haul!
The Arctic Ordovician fauna is widespread in central and northern North America; its best known occurrence is in the Tyndall Stone (Red River Formation, Selkirk Member) quarried at Garson, Manitoba, but it also occurs in places as widespread as New Mexico, Montana, the Canadian Arctic islands, and Greenland. The name “Arctic Ordovician fauna” was proposed by Dr. August Foerste in the 1930s as a shorthand for those Ordovician fossils in central North America that do not belong to the “Richmondian fauna” typical of the Cincinnati Arch area, and the concept was expanded upon by Dr. Sam Nelson in the 1950s.
Interestingly, Nelson carried out his early field research in 1950-51 along this same river we are visiting today, and his publications list a great variety of fossils from this very site.* This place is so inaccessible that I doubt anyone has done serious fossil collecting here since Nelson’s visit more than sixty years ago, which probably explains why there are so many fossils for us to find! With another hour or two working through these scree slopes, I’m sure we could easily equal his total of 46 species from the upper part of the Portage Chute, without even bothering to go down river to other sites such as Bad Cache Rapids. Still, the hour of collecting has given me a very good set of samples to take back to the Manitoba Museum, and perhaps we will return next summer for another visit.
Listening to the deep rumble of the water as it rushes past over the rapids, I consider Nelson’s traverse of this river with considerable admiration, since he did the great majority of it by canoe or on foot (he had to walk along this particular stretch of rapids). But now it is time to wrap up my fossils, and get them labelled and bagged before they go into the back of the AStar. Our fieldwork is ridiculously easy by comparison with Nelson’s, and I especially appreciate the padded seat as I nestle in by the window for our return to Churchill, with thoughts toward plans for a shower and a hot meal back at the Churchill Northern Studies Centre.
Still, miles to go, and all that tundra, river, and coast to contemplate. Will we see any bears this evening?
* See Nelson, S.J., 1963, Ordovician paleontology of the northern Hudson Bay Lowland, Geological Society of America Memoir 90, 152 p.
© Graham Young, 2014
There is really no way of knowing what the media, scientific or otherwise, will grab onto.
A week ago, at the Geological Society of America meeting in Vancouver, I presented a descriptive talk that may well have been the simplest I had ever given to a scientific audience. We had just listened to a series of presentations, many of them by students and postdocs, which incorporated considerable amounts of “big science”: sophisticated imaging techniques, chemical analyses of fossil preservation, or multivariate statistical studies of large numbers of specimens.
When I got up in front of the same audience, I was a bit worried because I realized that my talk could have been just as easily presented in 1914 as 2014: I was describing a single specimen, illustrated with photographs. Nineteenth century Natural History, really. But it was such a strange specimen that it seemed worth presenting, and as it turned out, the reporter from Science News thought so too (here is his short article, freshly out).
Those of you who visit this page occasionally will know that I am, perhaps, obsessed with the topic of jellyfish fossilization. In addition to ongoing detailed work on Ordovician-age jellies from the William Lake site in Manitoba, I have been collaborating with my colleague James Hagadorn of the Denver Museum to figure out the global fossil record of medusae.
Jellyfish are rarely fossilized; many things have been published as fossil jellies, but few of those actually are preserved medusae. So we have been working through the world literature of all papers published describing “fossil jellyfish.” As we have studied the literature, James and I have determined which museums hold collections that need to be examined, and when the opportunity arises we will go and spend a day or two on a collections visit. When necessary, we will borrow material for further study: some of the fossils are easily interpreted, but others are problematic.
The presentation in Vancouver was about one of the specimens we had found in the huge collections of the Field Museum in Chicago. Preserved in a slab of the Carboniferous Mecca Shale from Indiana, it looked like a blob of pure white quartz sand surrounded by thinly bedded black shale. Except this was a sand blob with tentacles.
Spending many days with the specimen, photographing it in every possible way (double polarized photography is our friend!), we were able to recognize many features that allowed us to identify it as a chirodropid cubozoan (a group of box jellyfish still abundant in modern oceans). It is the same age as the box jellies in the well-known Mazon Creek Lagerstätte of Illinois, and it is very similar to the Mazon Creek box jelly Anthracomedusa turnbulli, but preserved in a very different way. We explained its unusual preservation in this manner: A body of pure quartz sand is very unusual in the middle of a black shale bed; this resulted from sediment rafting by the jellyfish, a process analogous to ice rafting. The medusa apparently stranded on a beach, ingested sand as it attempted to free itself, and then was washed or rafted into a lagoon where it was buried in anoxic mud.
The scientific manuscript describing this remarkable fossil is almost complete. Maybe this publicity will motivate me to get it out the door and move on to the next batch of “fossil jellyfish”!
© Graham Young, 2014
October 17, 2014
Why does it seem to rain every time I go for a walk in Vancouver? There has been precipitation on every day I have spent there on recent visits, even if it is sometimes just what the Irish would term a “grand, soft day.”
For those of us who live elsewhere, this could be looked at as fair recompense when the place is so unfairly blessed with topographic splendour. But maybe it is also a gift from the weather gods, an appeasement that permits us to remain contented with our own climates and locations. Maybe Vancouver rain is specially arranged for the visitor’s benefit, with the warm coastal sun scheduled to re-appear minutes after the departure of our flights eastward. If you, like me, are “looking forward” to a prairie winter, then it would be simply unbearable to think that Vancouver could ever get a string of autumn days of perfect crystal clarity, as well as those mild winter temperatures. Far better for us to always see this place in the rain, and to think that the residents must be miserable on account of the constant drizzle and mist, which are surely interrupted only by the occasional downpour.
Anyway, the rain does give vibrancy to the colours of plants and stones, even under such a flat grey sky. And I didn’t really mind getting a bit wet, since I was headed to the Vancouver Aquarium to commune with the jellyfish and comb jellies . . .
© Graham Young, 2014
In late August, I had what could be called the trip of a lifetime: I was invited by colleagues at the Manitoba Geological Survey to travel with them to examine sites up the Churchill River, far from the more accessible Hudson Bay coast. With several days of helicopter time, we were able to visit many sites along the river and its tributary creeks. It was a tremendous experience to spend time in such a remote, stunning, and almost unspoiled landscape.
I returned with a wealth of fossil specimens for the collections of the Manitoba Museum, and also with a wealth of photographs. I have every intention of sharing a sampling of these, but circumstances have conspired against me as I have been fully occupied with other matters. Now that I have a bit of time, I am beginning the posting process.
If you are interested, the first batch of photos, providing a general overview of the Churchill River, can be found on my Museum page here.
More to come!
Churchill, Manitoba: August, 2014
After three days of helicopter-based collecting up the Churchill River, we were planning to start today on a few days of fieldwork close to Churchill. The helicopter work was strenuous at times, with slogging up creekbeds and carrying fossil samples up steep slopes, so I was looking forward to driving to some of our study sites. The first of these would be the numerous outcrops and quarry pits spread through the large cove that we call Airport Cove (for the obvious reason that it is immediately north of Churchill Airport).
Yesterday evening we took a drive to town, and on our way back we were going past the cove when a fellow headed the other way motioned to us to stop and exchange information (they do this often here). He told us that there were four polar bears in the cove. Bears are seen frequently around here at this time of year, but nevertheless, four at once is not all that common. We were excited about seeing the bears, of course, but I also greeted this news with some trepidation, thinking what it would be like to be on foot in the cove under such conditions.
We were able to watch all four bears, but the photos here are of the one that we viewed closely over quite a period of time. He was a huge animal, and very lively in the cooler weather at this time of year. We were entranced to watch him stalk a goose, and then chase and catch it before it could take flight! His tremendous speed and agility were wonderful to see in a creature that weighs as much as a holstein cow, but frightening to think about in the context of fieldwork at any distance from the vehicle.
Fortunately today worked out well. We saw three bears (again), but one was from the safety of the vehicle, and the others, a mother and large cub, were a safe distance away. Still, we will remain vigilant for the remaining work days up here – we may spend evenings looking for bears to watch, but in the day we will be watching for bears!
© Graham Young, 2014
Victoria Beach, Lake Winnipeg, 2014
Along Lake Winnipeg in July it felt like business as usual. At Victoria Beach we rode our bikes down narrow gravel lanes through swarms of dragonflies and clouds of midges, basking in the perfect warm air and golden sunlight. The shop, bakery, tennis courts, and playground were busy. But what about the beach? If you listened to the residents, beyond the usual small talk, this question remained: what about the beach? And what about the scarps behind it?
Manitoba’s lakes have been having issues of one sort or another for decades. Most summers Lake Winnipeg has trouble with algal blooms as a result of increased nutrient flow from farms and cities, though the extent of this problem varies from year to year. In 2011 the shores of Lake Manitoba were devastated by high lake levels, after Assiniboine River flooding resulted in heavy use of the Portage Diversion channel. This year water has again been very high on Lake Manitoba and Lake Winnipeg, after heavy precipitation in the late spring and early summer.
On Lake Winnipeg, cottagers always reckon that their shorelines suffer from increased erosion because Manitoba Hydro keeps water levels high, the better to power dams on the Nelson River system. This could certainly be a cause, particularly when this high lake level is augmented by unusual rainfalls. But there is another factor, a geological one, which most people ignore because of its subtlety and complexity.
The land of Manitoba has been slowly rising for the past 10,000 years or so, as the crust rebounds from being pressed down by glacial ice that was hundreds or thousands of metres thick at the height of the last ice age. The ice melted away from south to north, so the southern part of the province is now closer to equilibrium and is rising slowly. In the north, the rebound is still more rapid (even farther north, near Churchill, the rate of rise is close to a metre per century!).
Postglacial rebound may seem like an interesting if remote phenomenon, but its effect “on the ground” is this: Lake Winnipeg’s outflow is at the northern end of the lake, and since this is the part that is rising more rapidly, the basin is gently tilting southward (imagine what happens as you tip a dish full of water). The lake is gradually moving across the flat lands at its southern end, reclaiming the marshes of the Red River Delta and the beachfronts of Ponemah and Matlock. Given enough time (before this land is again subject to large-scale glaciation), the lake will surely arrive where downtown Winnipeg stands today.
But this will take centuries or millennia. Meanwhile, whenever there is high water the lake will take advantage of it, moving just a bit farther southward and chewing away at the wonderful shorelines of Grand Beach and Victoria Beach. These shores are soft, being composed largely of sediment left behind by that glacial ice. The waves readily remove sand and gravel from the beaches and scarps, leaving behind only the larger boulders, and the shoreline continues to retreat. From a human standpoint this seems unfair and a great shame, but we can only hope to slow down nature. It will always win in the end.
A lot of detail on the geology of the Lake Winnipeg basin was published in: Lake Winnipeg Project: cruise report and scientific results; Todd, B J (ed.); Lewis, C F M (ed.); Thorleifson, L H (ed.); Nielsen, E (ed.). Geological Survey of Canada, Open File 3113, 1996, ; 656 pages, doi:10.4095/207501
© Graham Young, 2014