Dead in a Ditch
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