Sunday, March 2, 2014

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.

With the U.S. entrance into World War II, there was an overwhelming and immediate need for war materiel. One of the industrial sites constructed to deal with this demand was the Gopher Ordnance Works in Rosemount. "Site" is a bit misleading:  the ordnance works was to occupy 11,500 acres (more than 46 square km), with more than 850 buildings. Obtaining those acres turned into a court struggle, and in the end the fortunes of war were such that the plant only produced powder from January to October 1945. Most of the land was transferred to the University of Minnesota in 1947 and 1948, where it is now part of UMore Park. Some parts of the works are still present and can be seen from US Route 52 and the vicinity. There's an interesting article on the works from Minnesota History that can be viewed here.

Anyway, Stauffer (1945)'s manuscript was received by the Journal of Paleontology on June 6, 1944, which you might recognize as a date of much more significance than the submission of an article on stromatolites. Work had actually stopped on the works at that time. By the time the article came out in print, in July 1945, the works had only a few more operational months left. We can imagine that a massive earth-disturbing operation like the construction of the Ordnance Works was catnip to a geologist like Stauffer, although of course the nature of the operation may have proved a problem (how often do you go poking around your nearby major war plant, and how impressed is security when you tell them you're looking for fossils?). The breadth of his paleontological interests must be appreciated; we've already seen papers on worm jaws and Ice Age mammals. At any rate, he obtained pieces of stromatolites from the Shakopee Formation of the Prairie du Chien Group, in the Pine Bend area, within Ordnance Works property. The exact location was "near the lower wells, adjacent to the Mississippi River". He named these fossils Cryptozoon rosemountensis.

Stauffer (1945)'s stromatolites; Cryptozoon rosemontensis is 1, 2, 4, and 5. The other two are C. minnesotensis. (and yes, I'm a bit dubious on how useful it is to give stromatolites scientific names)

Stromatolites, if you haven't had the pleasure of making their acquaintance, are layered sedimentary structures formed by the growth of microbial colonies. Sediment would get stuck on the surface of a microbial mat, which would then grow over the debris so they get back to sunlight. Repeat this often enough, and you can get a fairly large layered columnar structure. Cryptozoon rosemountensis, for example, is of respectable size. It takes the form of somewhat flattened hemispherical masses about 18–24 in (45–60 cm) across, with individual "upward bulges" about 0.6 in (1.5 cm) across forming pillars. (there are also several other types of similar structures, such as oncolites, which are spherical, and thrombolites, which a "clotted" internal structure.) Stromatolites and other microbial structures include the oldest fossil evidence for life on Earth. Their fortunes suffered after the appearance of grazing and burrowing organisms, but they can still be found today in places like Shark Bay, Australia. For the Phanerozoic (the Cambrian to the present, about 540 million years ago to now), they are usually interpreted as evidence for shallow water settings with harsh conditions, like high salinity, that prevent colonization by other organisms. It's likely that this was the case for the Prairie du Chien Group and its constituent formations, the Oneota Dolomite and Shakopee Formation, which have very limited fossil assemblages mostly composed of stromatolites and the occasional snail bed.

Stromatolites and other microbial structures are not always easy to see in the field. They don't always look impressive, and they can be confused with inorganic sedimentary features and accidents of weathering. I was able to find a thin bed of small microbial structures in the Shakopee Formation in the Hastings area, probably similar to Stauffer's stromatolites from a few miles upstream. If we're being picky, they lean more to the oncolite side of things than to the stromatolite side, but this is probably a function of environmental conditions.

Click to enlarge; look for the layered features across the middle of the photo, above the quarter.

The piece on the left was loose, so I plucked it for further photography and discovered that stromatolites don't like to be photographed. I got the surface wet and it looked somewhat more impressive. This part of the mat took on an interesting conical shape as it grew, based on the cross-section.

A conical microbial structure from the Shakopee Formation.

References:

Stauffer, C. R. 1945. Cryptozoons of the Shakopee dolomite. Journal of Paleontology 19(4):376–379.

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