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.