Stromatolites in situ

A few weeks ago, during the elongated-dry-autumn-with-long-nights that has been substituted for winter in these parts this year, I visited a stromatolite patch in the Prairie du Chien Group and took the usual digital heap of photographs. The photos aren't quite as sharp as I would have liked, but they show a variety of aspects of stromatolites both up close and in situ, unlike the usual circumstances of getting only one of those two properties.

First off, here's a general idea of what we're dealing with:

Click to embiggen, as usual; the thing about stromatolites is sometimes they're more apparent at a distance, and sometime they're more apparent when your nose is practically on them. In this photo, there are columns a couple of centimeters across in the lower part, leading to a small shelf with a knobbly surface representing the tops of said columns, followed by an interval of more obscure growth.

The prolific interval was up to about three quarters of a meter thick, with some significant variation, microbial mounds not being big on standardization. Within this interval it was possible to see where a mound's growth had been cut off, or changes between narrow columns and broader stacks.

Here we have a broad mound of numerous coalesced small centers that is cut off starkly about two-thirds of the way up the photo.

Another example where there appears to be discontinuity between the growth lower in the photo and that in the upper part.

This tall mound is fairly broad at the base, then goes into narrower columns, then appears to show column consolidation near the top.

Part of why the photos may have lacked some clarity is the weathering of the surfaces (and the bright light). The stromatolites had a sort of artistic appearance in places, a bit like fingerprints in rock. I could see paintings of these done with heavy strokes to emphasize the tactile appearance of weathering layers.

There's a certain melted quality to this exposure.

I'm not sure what happened here, but this one looks like it's breaking up (and there seems to be a big rounded pebble in the upper left).

Another interesting feature was the occasional exposure of the top surface of a stromatolitic interval. Given the preponderance of smallish columns, it should not come as a surprise that such surfaces are knobbly.

A close look will reveal the more or less concentric layers of individual columns that have been truncated by weathering.

Finally, here's a surface showing columns that apparently grew out laterally. We usually think of the original microbial colonies growing vertically, to reach the light, but again microbial mounds aren't big on standardization, and will grow as conditions influence them. (Or maybe this mound was simply knocked over at some point; unfortunately, we're missing most of it.)

Preservation is different in this one as well, with layers still being evident but expressed less colorfully.

Hunting the wild stromatolite at Vermillion Falls

The gorge at Vermillion Falls, as seen before, is cut through quite a bit of the Prairie du Chien Group. Therefore, it is entirely reasonable to suppose that somewhere in all of those oodles of outcrop, there are some stromatolites. It should be just a matter of walking down and having a look, right? We-e-ll, easier said than done. A few factors are pushing against going from "predicted" to "established":

1) The classic booby prize of vertical exposures—sure, there's a lot of surface, but you don't get to see most of it up-close;

1b) Furthermore, for many of the places where you *can* examine the walls, you can't change your vantage point because backing up a foot or two puts you a foot or two lower or in open space (and that never helps). What makes this annoying with stromatolites is they can be expressed in various scales, and what you can't see with your nose on the rocks may be perfectly apparent from a couple of arm's lengths away except for that whole "absence of footing" thing, or a promising feature may disappear as you clamber up the slope to inspect it;

2) The surfaces have been fried to a crisp by weathering, which for our purposes disguises the fine layering and other subtle features.

The existing literature is not especially illustrative. Stauffer and Thiel (1941) included a section taken at the railroad bridge (now a footbridge), which describes the walls as 58.5 ft (17.8 m) of Shakopee Dolomite over 1 ft (0.3 m) of Root Valley Sandstone over 47 ft (14 m) of Oneota Dolomite, with nary a stromatolite mentioned (to be fair, the lithological descriptions are also very slim). The site has made it into a few theses and other student papers (Shea 1960; Squillace 1979; Robins 2005), but that seems to be about it. Maybe this limited documentation is a recognition of the above limitations by wiser heads than mine, but I have the faith of a gambler that something will turn up if I look long enough, so down we go back into the gorge.

Besides, why would I want to sit around inside when I could see things like this?

My strategy involves *not* looking directly for stromatolites, at least not the classic laminated features. I would never see them here except on fresh breaks. Instead, I'm looking for a couple of other features: bedding planes that are undulating rather than horizontal, and anomalously recessive intervals (I've seen elsewhere that stromatolitic intervals may be relatively erosion-prone compared to non-stromatolitic intervals). These two features should be visible even through the stain of weathering that began before the most recent Ice Age.

Down near the falls, in some of the lowest accessible beds, something very promising is apparent.

Left

Center

Right

We've definitely got an undulating surface that is appropriately stromatiform* in appearance, as well as being within an interval that is recessed. (In fact, the interval above is recessed in comparison to the interval above that, so this is not for people who don't like rocks poised above their heads [or, for that matter, people who have a phobia of birds pooping on them, because there are pigeons roosting in cavities above].)

*Google says it can come up with about 328,000 hits for "stromatiform". What it doesn't tell you is how many are useful and how many are just dictionaries, rhyming words, anagrams, pronunciations, and SEO things seeking to boost traffic with lists from dictionaries.

A little closer, between two apparent mounds

So stromatiform...

Obviously the next thing to do is to is to get close and look for finer details. That's where things become less clear. There are, conveniently, some fresh surfaces (not made by me!), and they do show alternating compositions. They are, however, rather thick layers.

Layers in one fresh surface.

Not quite as fresh, but you get the idea.

Layers on a spalled piece, frozen to the underlying moss.

I'd been hoping to find nice stacks of small columns beneath the undulating layers, but they aren't appearing (yet, at least). If we compare to Logan et al. (1964) and May et al. (2012), the latter also dealing with large Prairie du Chien stromatolites, it's as if we've skipped the columnar "Cryptozoon" stage and gone straight to broad "Collenia". This may say something about the local environment, or it may say that I'm just getting excited about some inorganic stromatiform-producing process.

Then there's whatever the heck is this. Bedded sedimentary rocks don't do things like this without a good reason.

If the issue is not immediately apparent, check the annotated version below and click to expand as necessary.

Too bad it was on the other side of the gorge. There's probably a way down somewhere...

References

Logan, B.W., R. Rezak, and R. N. Ginsburg. 1964. Classification and environmental significance of algal stromatolites. The Journal of Geology 72(1):68-83.

May, S. L., L. E. Davis, and D. G. Brown. 2012. Algal stromatolites in the Willow River Member of the Lower Ordovician Shakopee Formation near Chatfield, Minnesota, USA. The Compass: Earth Science Journal of Sigma Gamma Epsilon 84(1, Article 6):42–48.

Robins, C. 2005. The geology of the New Richmond Sandstone. Senior Integrative Exercise. Carleton College, Northfield, Minnesota.

Shea, J. H. 1960. Stratigraphy of the Lower Ordovician New Richmond Sandstone in the Upper Mississippi Valley. Thesis. University of Wisconsin, Madison, Wisconsin.

Squillace, P. J. 1979. The geology of the New Richmond Member of the Shakopee Formation (Lower Ordovician), Upper Mississippi Valley. Thesis. University of Minnesota, Minneapolis, Minnesota.

Stauffer, C. R., and G. A. Thiel. 1941. The Paleozoic and related rocks of southeastern Minnesota. Bulletin 29. Minnesota Geological Survey, St. Paul, Minnesota.

Stromatolites in the snow

To close out 2021, here are some stromatolites in the snow.

A big stromatolitic eye; no scale bars today (I couldn't reach this one anyway, but it's a couple of feet across).

At this location there's a large wall of Prairie du Chien Group rocks that was exposed a few years ago during the construction of a bike trail. The bike trail descends from north to south, exposing more of the section as you go until a few dozen feet of the Lower Ordovician are visible. There is a distinct stratigraphic break near the top of the section, and just above this break is a stromatolitic interval with some large colonies, like the eye-shaped example above. I haven't seen any definite fossils below the break, but there might be some (there are numerous small voids, at least some of which could represent dissolved shells). There are some nice sedimentary structures, though, such as cross-bedding and planar bedding (sand grains in the carbonate).

The center of this photo shows a sort of bi-lobed pillow that smooths out into a single mound. It is not far from the eye-like stromatolite in the previous photo. In both cases, the large colony is about a foot above the stratigraphic break.

There are also some blocks that have fallen. In fact, I suspect that the stromatolitic interval is more prone to shedding blocks than the rest of the interval, because the stromatolitic interval has more opportunities for fracturing.

The green rocks of home revisited (now with stromatolites!)

Almost exactly five years ago I posted on a visit to Cottage Grove Ravine Regional Park. I was in the area and thought I'd stop again. There's been a lot of work at the park since then, and I wanted to check that the abandoned stream channel and dry waterfall were still visible. Also, over the past five years I've become much better at finding stromatolites, so I thought I'd try my luck.

Were the landforms still there?

That's a dry waterfall, all right. The lowest interval seems to have some weathered cross-bedding; perhaps a somewhat sandier bed in the Prairie du Chien?

I'd say yes, the old channel is still there.

How about stromatolites?

We have stromatolites!

That's the stuff.

Separation has occurred within a stromatolitic interval here.

At the beginning, I was seeing them in loose blocks, which led to the question "where are they coming from?" This was more complicated. The stromatolitic blocks were much lighter-colored and less weathered, indicating fresh faces. There were no obvious fresh faces on the surrounding walls, though. Had large stromatolitic blocks broken up in the dry stream bed fairly recently? Or were these "plants", Prairie du Chien blocks that had been brought in for landscaping and culvert riprap, then declared surplus and deposited here? The walls were not helping.

Features like the wavy lines give a general biotic impression, but I'd really want to see some light-colored patches indicating fallen blocks to cinch a relationship.

Fine lines are visible behind the dull gray and moss.

Then I saw this enormous fallen block.

Do you see them? They're near center. If not, hold on.

There they were.

How about now?

A proverbial bull's eye.

This block is too large and too well-mossed to have been placed here in the past few years. Clearly at least some of the stromatolites are local. I'm still wondering where the loose "clean" blocks came from, though.

Barn Bluff

One of the outstanding geological sites in southeastern Minnesota is Barn Bluff (He Mni Can to the Dakota, La Grange to the early French explorers*) in Red Wing. The bluff is a bedrock island oriented roughly east–west, adjacent to the Mississippi River just above the Lake Pepin section. If you were to have gone back to the early postglacial period, it would have been a literal island thanks to meltwater filling the Mississippi River valley.

*All of the names are kind of prosaic, actually. "He Mni Can" is "hill, water, wood", "La Grange" or "Lagrange" is "the barn" for its general shape, and the English name is just a translation of the French.

The view of the south side of the west end. Observant eyes may notice that the color of the lower outcrops change near the left side of the picture, from orangish on the left to banded green on the right. Read on for why!

While taking a walk

Like any good Minnesotan, I appreciate a quality agate, and although I don't do a great deal of agate hunting, I *have* learned how to spot their characteristic luster. (I've never spotted anything big, but it's a neat trick.) When road work began this summer near where I live, I made it a point to observe the disturbed ground when I would take a walk. The glaciers that brought agates into the Twin Cities didn't linger where I am, but you never know. Until a couple of weeks ago, all I saw were chips, including the remains of one that may have been about the diameter of a dime had it not been shattered into shards by being run over (I didn't know they would splinter like that). This time, though, I noticed a red pebble with a cherty sheen on the path. I could tell it hadn't simply gotten paint on it, so I scooped it up to clean it for a closer look. Although it has microcrystalline quartz and red coloration, and certainly looks like it might be a Lake Superior agate from a few feet away, that's not what it turned out to be.

I'm cheating because I haven't yet showed a photo of one of the shorter sides, but this is actually a pebble of Mary Ellen jasper, a stromatolitic jasper that comes from the Biwabik Iron Formation. It's quite a bit older than any of the other fossils that have featured on this blog, coming from a Paleoproterozoic formation about 1.88 billion years old (Jirsa et al. 2008). It gets its name from the Mary Ellen Mine in the Iron Range. How did it get to where I live? Although it could have been a pebble in the subsurface, transported from the Iron Range by glaciers, it may instead have gotten here in gravel being used for the road work. The gravel would have been sourced from a gravel pit, which brings us back to glaciers again because the state's gravel pits are often in glacial deposits. In fact, in some of them you can pick out deposits left by different glaciers moving from different areas. Glaciers that passed through the Superior region brought agates south. Glaciers that passed through the Dakotas scraped up Cretaceous sedimentary rocks. (Cobban and Merewether 1983 commented on a fragment of phragmocone of a Cretaceous ammonite, Prionocyclus novimexicanus, found in glacial drift at the Brickyard.) The glacier that collected this pebble would have gone through the Range before depositing it.

This side of the pebble shows the characteristic red whorls of the stromatolitic layers with dark streaks between them.

In this photo I've put water on the surface, in case that makes it easier to see.

References

Cobban, W. A., and E. A. Merewether. 1983. Stratigraphy and paleontology of mid-Cretaceous rocks in Minnesota and contiguous areas. U.S. Geological Survey, Washington, D.C. Professional Paper 1253.

Jirsa, M. A., J. D. Miller, Jr., and G. B. Morey. 2008. Geology of the Biwabik Iron Formation and Duluth Complex. Regulatory Toxicology and Pharmacology 52(supplement to 1):S5–S10.

Stromatolite Sunday

I happen to know a place along a shore where the riprap includes a small percentage of stromatolitic blocks, and given the nice weather on Saturday, I thought I'd stop by and take some pictures. I have not asked about the source, but I have a pretty good idea that the provenance is one or more quarries in the area working the Shakopee Formation (Prairie du Chien Group). The rocks look like the Shakopee Formation (including that which is exposed in outcrop nearby), and there are no fossils beyond stromatolites and burrows. Stromatolites can be subtle features, but not these examples. Not only is the layering very distinct, once you get used to the appearance of the stromatolitic blocks it is possible to reliably pick them out from the other blocks at a distance of a few meters/yards. The simplest way to describe it is that the non-stromatolitic blocks are sandier and thus reflect light differently. Here's a side of a stromatolitic block, as you might see it as you approach.

Since these are domes, way-up is toward the top of the photo.

You'll see what looks like numerous parallel series of stacked parentheses. Let's zoom in on them:

Note that the column near the center splits into two smaller columns going up.

Enhance!

There's been just enough weathering to make the layers stand out nicely.

The stacks aren't necessarily separate columns, because if you look closely you can see that layers can continue from stack to stack, but "column" is a handy quick descriptor. In life, the colony would have had a lumpy upper surface composed of a number of distinct but connected domes. We can see this in upper and lower surfaces of stromatolitic blocks.

The upper surfaces of stromatolite blocks (A) are lumpy, showing the tops of the small domes. Sometimes there is a nice bottom surface that's essentially a negative of the upper surface (B). (These are definitely hollows, but your eyes may interpret the photo as domes.) Bottom surfaces can be easier to appreciate than the sometimes subtle upper surfaces.

In this case, weathering has partially removed some of the uppermost layers, "dissecting" the tops of the domes.

This photo, taken along the edge between an upper surface and a side, shows how the stacks translate into domes.

I am completely comfortable interpreting these lumpy stacked colonies as Stauffer's Cryptozoon rosemontensis, which he named from the Shakopee Formation not too far away. Almost all of the stromatolitic blocks I saw fall under this category, but there were a couple of examples that did not have stacks, instead having large hemispherical structures. These are much more typical of a variety named Cryptozoon minnesotensis.

These two photos show a much different flavor of microbial colony immortalized as stromatolites. A shows multiple lobes, while most of B is one big dome. These are in the vein of Cryptozoon minnesotensis.

Some of the blocks are well-preserved, even though they are now exposed to the elements. Others are now weathering, breaking down either by spalling at the tops of the dome stacks or by the stacks themselves beginning to fall out. The difference may have to do with how much sand was incorporated into the colony.

This group of photos shows a heavily eroded block, with columns beginning to spall out. A gives the overall appearance. B is a close-up of several such columns. The pockmarks mark where sand grains have eroded from the layers. C shows a closer view of pockmarks.

The Prairie du Chien Group is not noted for its non-stromatolitic fossils, although I have seen some snails in the Shakopee elsewhere. These blocks did not change that overall impression. There were a couple of cases of burrows or things that look very much like burrows, though.

A couple of examples of burrows or burrow-like features. A has a long feature from the lower left to near the center, plus other shorter similar features. B has what looks like a web of burrows.

Thanksgiving Leftovers

You're probably pretty busy this weekend. How about something light, like some photos? These all come from a few site visits over October and November, taking advantage of the very pleasant autumn weather conditions in the Twin Cities metro.

This and the next photo come from the U.S. Route 10 roadcuts, in the Shakopee Formation (Prairie du Chien Group). All of the little stone rainbows are small domed stromatolites. At very close range, you can distinguish between layers that are "crystalline", so to speak, representing minerals deposited by the microbes, and layers of sand (a grain or two thick). There is a band populated by these small stromatolites about as thick as the area photographed here that extends for at least a few tens of meters. (I do not recommend casual visits along this busy road; the couple of times I've stopped have been Sunday mornings.)

Ordovician microbial mats, featuring the Gopher Ordnance Works

Here's a little curiosity, bringing together a short-lived ordnance plant, Early Ordovician sea scum, and Clinton R. Stauffer, who seems to have become a patron saint of this blog through the first couple of months. Let's deal with the ordnance plant first.