Sunday, March 8, 2020

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.


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.

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