(former) Ash beds in St. Paul

Although many Minnesotans are aware of the great Midcontinent Rift that runs across the state, it's less generally well-known that evidence of some of the largest known volcanic eruptions in the history of the planet can be found in the Twin Cities. In fact, you can easily put your hand on a layer of former volcanic ash by taking a short walk at our old friend Shadow Falls Park, at the end of Summit Avenue.

Let's set the scene: First of all, back in the Ordovician, North America was not quite as expansive as it is today. Areas of land that today make up the east and west coasts had not yet merged with the continent. For our purposes, significant chunks of land that today are found east of the Appalachians were then volcanic island arcs, microcontinents, and so forth. Imagine a setting similar to modern Indonesia. While these bits were inexorably closing in on North America by the subduction of oceanic crust, occasionally they would produce giant volcanoes. We have multiple examples of former volcanic ash beds from the Late Ordovician throughout eastern North America produced by these outboard volcanoes. The most powerful of the eruptions sent ash as far as Minnesota and central Oklahoma. Several of these widely recognized beds have been given names. Two of the most famous are the Deicke and Millbrig K-bentonites.

A quick pause for terminology: Bentonite is a type of clay typically formed from the breakdown of volcanic ash. Ash, when you get right down to it, is tiny, tiny shards of volcanic glass (well, there's some other stuff as well, but let's stick with the glass for now). You may be familiar with obsidian, which is a variety of volcanic glass. Volcanic glass is not stable over long periods, and alters to clay minerals. The "K" in K-bentonite refers to the element potassium. Note that these bentonites were frequently described as Middle Ordovician in older literature, but with refinements in the Ordovician time scale would now be considered Late Ordovician in age.

A photograph taken in or before 1996 (presumably in late fall), looking at the upper limestone platform at Shadow Falls Park. The photo is from Kolata et al. (1996: Figure 12) and is included for its historic interest. The caption is "Carimona Limestone (Cm) overlain by the Decorah Shale (Dc) at the Summit Avenue outcrops in St. Paul, Minnesota (locality 89). Meter rule marks the position of the Deicke K-bentonite Bed (D). Warren D. Huff stands at the level of the Millbrig K-bentonite Bed (M)."

Here we are in approximately the same place, July 30, 2016. The same viewpoint is not possible, due to 20 years of erosion. In particular, it appears that there was a dead upright trunk to the left of the outcrop back in the 1990s, and the photographer had a place to stand that was maybe a foot higher. In the absence of the original scales, I have a guitar case and my little hand scale (the little white object hiding out in the Deicke gap, the lower of the two gaps). Without doing some trenching, you'd have no way of finding the Millbrig today. Somebody actually did some trenching around these outcrops a couple of years ago, but you'd hardly know it today.

The Deicke K-bentonite is easy to find at Shadow Falls Park. First, get on the slope below the overlook that leads down to the broad Platteville platform. You'll notice that the upper part of the slope is made up of green-gray Decorah clay, and that about halfway down the slope turns into "steps" of rock. The uppermost thick step is broken up vertically by two recessive layers each a few inches thick. You are looking at the Carimona Limestone Member of the Decorah Shale. The lower of the two recessive beds is the bed we're looking for. This looks like a consistent pattern, at least within a few miles; you can see the same thing across the Shadow Falls ravine and at the overlook north of the Ford Bridge on the St. Paul side. The problem in finding it locally is the usual problem of finding the Decorah in the first place: once you've found the upper Platteville and the lower part of the shale, you should be able to locate the Deicke.

Although the Deicke gets to be substantially thicker to the east, closer to the source, we've still got about 3 in (7 cm) here. It may have been on the order of 11 in (27 cm) thick before compression (Dokken 1987). To give you idea of the scale, the Deicke event may have erupted at least 226 cubic miles (943 cubic kilometers) of ash (Huff et al. 1996), which if you could somehow assemble all the ash together works out to a cube of ash about 6 miles on a side. This does not include lava or any other eruption products. For comparison, Mount St. Helens produced about a quarter of a cubic mile of ash in 1980, and Mount Pinatubo put out about 2.4 cubic miles' worth of material in 1991. The Millbrig is present at Shadow Falls as well, but you'd need to dig to find it: it's up in the shale, which is constantly eroding in its merry way. There has been some interest in correlating this event with another massive eruption of comparable age found in Europe, but this correlation has fallen through; instead, they're just two separate gigantic eruptions. The Millbrig produced at least 362 cubic miles (1,509 cubic kilometers) of ash (Huff et al. 1996). The ages of the eruptions have proven difficult to pin down precisely, although if you go through the literature the majority of the dates for the Deicke and the Millbrig seem to fall between 455 and 454 million years old. [2021/02/17: Update: Metzger et al. 2020 have published new preferred dates of 453.35 ± 0.10 million years for the Deicke and 453.36 ± 0.14 million years for the Millbrig, or basically one right after the other; with dates this close, a slightly older date for the slightly higher Millbrig is within uncertainty.]

There's also some debate about the effects of the eruptions. It is known that there was a global shift from "greenhouse" to “icehouse” conditions during the same time frame, and it's tempting to connect them, but there is no direct evidence to do so (Herrmann et al. 2010). A publication that did attribute the cooling to the Deicke event (Buggisch et al. 2010) was later found to have the eruption plotted too low, removing the correlation (Herrmann et al. 2011; Buggisch et al. 2011). You also might expect that dumping large amounts of ash on immobile bottom-dwelling marine filter feeders would cause extinctions, but evidence have been equivocal. In the Upper Mississippi River Valley, there does appear to be evidence for an extinction associated with the Deicke K-bentonite, as first noticed by Sardeson (Sardeson 1926) and further detailed by Sloan (Sloan 2005; Sloan et al. 1987, 2005). Sloan et al. (2005) reported that 215 of 262 species (82%) ultimately went extinct.

This image, from a couple of years ago, puts the Shadow Falls outcrop in a little more context. You can see the overlook with the obelisk near the top. (It's a different guitar case, but they're about the same size). Again, the Deicke is found in the lower of the two gaps in the central outcrop.

Here we've got the Deicke on the north side of the Shadow Falls ravine. The squares of the scale bar, which is placed next to the Deicke, are 1 cm on a side. (Incidentally, there are some nice shell beds in the underlying Magnolia Member of the Platteville, but it takes a bit of scrambling to appreciate them).

A little farther afield—this is below the overlook across from the Temple of Aaron, and we see the same two recessive beds making gaps.

References:

Buggisch, W., M. M. Joachimski, O. Lehnert, S. M. Bergström, J. E. Repetski, and G. F. Webers. 2010. Did intense volcanism trigger the first Late Ordovician icehouse? Geology 38(4):327–330.

Buggisch, W., M. M. Joachimski, O. Lehnert, S. M. Bergström, and J. E. Repetski. 2011. Did intense volcanism trigger the first Late Ordovician icehouse?: reply. Geology 39(5):e238.

Dokken, K. 1987. Trace fossils from Middle Ordovician Platteville Formation. Pages 191–196 in R. E. Sloan, editor. Middle and Late Ordovician lithostratigraphy and biostratigraphy of the Upper Mississippi Valley. Minnesota Geological Survey, St. Paul, Minnesota. Report of Investigations 35.

Herrmann, A. D., K. G. MacLeod, and S. A. Leslie. 2010. Did a volcanic mega-eruption cause global cooling during the Late Ordovician? Palaios 25(12):831–836.

Herrmann, A. D., S. A. Leslie, and K. G. MacLeod. 2011. Did intense volcanism trigger the first Late Ordovician icehouse?: discussion. Geology 39(5):e237.

Huff, W. D., D. R. Kolata, S. M. Bergström, and Y.-S. Zhang. 1996. Large-magnitude Middle Ordovician volcanic ash falls in North America and Europe: Dimensions, emplacement and post-emplacement characteristics. Journal of Volcanology and Geothermal Research 73(3–4):285-301.

Kolata, D. R., W. D. Huff, and S. M. Bergström. 1996. Ordovician K-bentonites of eastern North America. Geological Society of America, Boulder, Colorado. Special Paper 313.

Metzger, J. G., J. Ramezani, S. A. Bowring, and D. A. Fike. 2020. New age constraints on the duration and origin of the Late Ordovician Guttenberg δ13Ccarb excursion from high-precision U-Pb geochronology of K-bentonites. GSA Bulletin. doi:10.1130/B35688.1.

Sardeson, F. W. 1926. Pioneer re-population of devastated sea bottoms. Pan-American Geologist 46:273–288.

Sloan, R. E. 2005. Minnesota fossils and fossiliferous rocks. Privately published, Winona, Minnesota. Available from the Minnesota Geological Survey.

Sloan, R. E., W. F. Rice, E. Hedblom, and J. M. Mazzullo. 1987. The Middle Ordovician fossils of the Twin Cities, Minnesota. Pages 53–69 in N. H. Balaban, editor. Field trip guidebook for the upper Mississippi Valley, Minnesota, Iowa, and Wisconsin. Minnesota Geological Survey, St. Paul, Minnesota. Guidebook 15.

Sloan, R. E., M. D. Middleton, and G. F. Webers. 2005. Late Ordovician stratigraphy and paleontology of the Twin Cities Basin. Late Ordovician lithostratigraphy and biostratigraphy of the southern margin of the Twin Cities Basin. Pages 105–143 in L. Robinson, editor. Field trip guidebook for selected geology in Minnesota and Wisconsin. Minnesota Geological Survey, St. Paul, Minnesota. Guidebook Series 21.