To set the stage, the middle part of the Ordovician in Minnesota and vicinity is not too well known, for the very good reason that there is a notable absence of rocks between the Prairie du Chien Group (Oneota Dolomite and Shakopee Formation) and the St. Peter Sandstone, with a couple of interesting exceptions we'll be getting to. Although neither the Prairie du Chien nor the St. Peter are noted for dateable igneous minerals, we can use the dark and subtle art of conodont biostratigraphy to get a pretty good idea of their ages and the time we're missing. Per Smith and Clark (1996), the Shakopee Formation has Stairsian conodonts, which per Cooper et al. (2012) makes it about about 480 million years old. Meanwhile, the St. Peter Sandstone has conodonts of late Whiterockian age (Chazyan, for those of you who want that detail), latest Middle Ordovician into early Late Ordovician (Witzke and Metzger 2005), approximately between 460 and 456 or so million years ago (Cooper et al. 2012). Thus, we have been left with about 20 million years of zilch between Prairie du Chien stromatolite banks and St. Peter sand dunes, a hiatus reflected by significant erosional relief at the Shakopee–St. Peter contact (Mossler 2008).
So, about those exceptions I mentioned above. Turns out erosion didn't get everything, thanks to a couple of very local basins. During that 20-million-year gap, at least two bolides struck the Upper Midwest, leaving behind two ~6-km craters: one at what is now Decorah, Iowa, the other at Rock Elm, Wisconsin. The Rock Elm crater is thought to have been produced by an object on the order of 200 m (660 ft) across, traveling roughly 30 km/sec, or 65,000 mph, not only scooping out a crater but producing hurricane-force winds 120 km (75 miles) away (Cunningham et al. 2011). The similar size of the Decorah crater suggests similar general parameters. Interestingly, the two impacts are postulated as part of a group of impacts about 467.5 million years ago, from a larger object that had broken up.
|A map of the potential Middle Ordovician impacts in central North America, from Wikipedia. #2 is the Decorah crater and #3 is the Rock Elm crater.|
Of course, the most famous Iowa crater (and the only one known from Iowa until the discovery of the Decorah structure) is the Manson crater, which for a little while was thought to be connected to the end-Cretaceous extinctions before proving to be too old. The Decorah crater is buried and only came to light recently during a geophysical survey of northern Iowa. After the impact, the crater was filled by a shaly unit known as the Winneshiek Shale or Lagerstätte (German for, roughly, "here there be exceptional fossils"), which is generally inaccessible except via wells or damming the Upper Iowa River to reach the one outcrop (Nowak et al. 2017a). It was briefly thought to be an unusual lower facies of the St. Peter Sandstone (Liu et al. 2006), but this has been reconsidered. The Winneshiek, confined as it is to the crater, hosts an unusual fossil assemblage and may represent restricted marine conditions that were inhospitable to the run-of-the-mill Ordovician shallow marine fauna (Lamsdell et al. 2015a). It has produced a variety of fossils, including the early giant (person-sized) eurypterid Pentecopterus decorahensis (Lamsdell et al. 2015a), a more basal cousin to eurypterids and related arthropods (Lamsdell et al. 2015b), various crustaceans (Briggs et al. 2015), algal microfossils (Nowak et al. 2017b), associated remains of big conodonts (Liu et al. 2017), and some brachiopods, mollusks, possible early jawless fish, and such (Liu et al. 2006). (Oh, and these digestive trace fossil things described by Hawkins et al. .) So, with the Decorah crater, not only do you get a crater, you also get a unique and well-preserved fossil assemblage of things that inhabited that crater not long after it formed.
The Rock Elm crater in Wisconsin cannot boast quite the same paleontological pedigree, but it's closer to the surface. Sure, 467-million-year-old craters generally don't look very craterish, but it is possible to tell that something is going on here, unlike the concealed Decorah crater, and it can be documented by geologic mapping (Cordua and Evans 2007). The existence of something geologically odd at Rock Elm has been known for much longer than the Decorah crater. It was included in a thesis (Nelson 1942), but the first significant description in print was in Cordua (1985). 1985 was "pre-Chicxulub" by a few years, and the initial back-and-forth about the feature was marked by hedged bets and alternate interpretations, but opinion has swung to the "bolide impact" interpretation (French et al. 2004). Like the Decorah crater, the Rock Elm crater is filled with rocks limited to just the confines of the crater (not counting messed-up blocks of preexisting rocks), as well-illustrated by the Cordua and Evans map showing a circle of anomalous rocks amid the Prairie du Chien Group. In this case, there is a lower shale unit called the Rock Elm Shale and an upper sandstone unit called the Washington Road Sandstone. The Rock Elm Shale includes fossils of a variety of Middle Ordovician brachiopods, mollusks, trilobites, crustaceans, and biostratigraphically useful conodont and scolecodont elements, indicating it was deposited during the Middle Ordovician (Peters et al. 2002). The Washington Road Sandstone has yielded brachiopods and is often bioturbated (Cordua 1985). It would certainly be interesting if a similar "crater basin" fauna could be found here as at Decorah, but so far no such luck.
Briggs, D. E. G., H. P. Liu, R. M. McKay, and B. J. Witzke. 2015. Bivalved arthropods from the Middle Ordovician Winneshiek Lagerstätte, Iowa, USA. Journal of Paleontology 89(6):991–1006.
Cooper, R. A., P. M. Sadler, F. M. Gradstein, and O. Hammer. 2012. The Ordovician Period. Pages 489–523 in F. M. Gradstein, J. G. Ogg, M. Schmitz, and G. Ogg, coordinators. The Geologic Time Scale 2012. Elsevier.
Cordua, W. S. 1985. Rock Elm Structure, Pierce County, Wisconsin; a possible cryptoexplosion structure. Geology 13(5):372-374.
Cordua, W. S., and T. J. Evans. 2007. Geology of the Rock Elm Complex, Pierce County, Wisconsin. Wisconsin Geological and Natural History Survey, Madison, Wisconsin. Open-File Report 2007-02. Scale 1:12,000.
Cunningham, J., H. A. S. Dolliver, and W. S. Cordua. 2011. Flaming meteors, dark caves, and raging waters; geological curiosities of western Wisconsin. Pages 411-424 in J. D. Miller, G. J. Hudak, C. Wittkop, and P. I. McLauglin, editors. Archean to Anthropocene; field guides to the geology of the Mid-Continent of North America. Geological Society of America, Boulder, Colorado. GSA Field Guide 24.
French, B. M., W. S. Cordua, and J. B. Plescia. 2004. The Rock Elm meteorite impact structure, Wisconsin; geology and shock-metamorphic effects in quartz. Geological Society of America Bulletin 116(1-2):200-218.
Hawkins, A. D., H. P. Liu, D. E. G. Briggs, A D. Muscente, R. M. McKay, B. J. Witzke, and S. Xiao. 2018. Taphonomy and biological affinity of three-dimensionally phosphatized bromalites from the Middle Ordovician Winneshiek Lagerstätte, northeastern Iowa, USA. Palaios 33(1):1–15.
Lamsdell, J. C., D. E. G. Briggs, H. Liu, B. J. Witzke, and R. M. McKay. 2015a. The oldest described eurypterid: a giant Middle Ordovician (Darriwilian) megalograptid from the Winneshiek Lagerstätte of Iowa. BMC Evolutionary Biology 15:169.
Lamsdell, J. C., D. E. G. Briggs, H. P. Liu, B. J. Witzke, and R. M. McKay. 2015b. A new Ordovician arthropod from the Winneshiek Lagerstätte of Iowa (USA) reveals the ground plan of eurypterids and chasmataspidids. The Science of Nature 102:1–8.
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Liu, H. P., S. M. Bergström, B. J. Witzke, D. E. G. Briggs, R. M. McKay, and A. Ferretti. 2017. Exceptionally preserved conodont apparatuses with giant elements from the Middle Ordovician Winneshiek Konservat‐Lagerstätte, Iowa, USA. Journal of Paleontology 91(3):493–511.
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Nowak, H., T. H. P. Harvey, H. P. Liu, R. M. McKay, P. A. Zippi, D. H. Campbell, and T. Servais. 2017b. Filamentous eukaryotic algae with a possible cladophoralean affinity from the Middle Ordovician Winneshiek Lagerstätte in Iowa, USA. Geobios 50(4):303–309.
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