There is only one rule for a park to join Club Late Ordovician: it has to have fossils of Late Ordovician age. Of course, the database is only as good as our records, so there are probably some we've missed outright, some that have Late Ordovician fossils that nobody's noticed yet, and some that have Late Ordovician fossils we only have parsed to Ordovician or, heaven forbid, accidentally misfiled as Middle Ordovician. The Middle–Late Ordovician boundary has shifted some over the past few decades, and in fact the Late Ordovician fossils of the Twin Cities were long assigned to the Middle Ordovician. The current tally is 18 parks with reasonably well-established Late Ordovician fossil records, and one more that's on the bubble (Denali NP&Preserve). This total is middling; it beats the heck out of our Paleocene and Silurian totals, for example, but is easily beaten by the Early and Late Cretaceous.
In terms of quantity, diversity, history of study, just about everything really, the Twin Cities can take some hometown pride, because MNNRA looms over the others. The Late Ordovician is simply not a fertile chunk of time for NPS units, at least in our current knowledge. There are some significant Cambrian and Early or Middle Ordovician sites, and some significant Devonian sites, but MNRRA is bearing the Late Ordovician load with relatively little assistance. It's just one of those quirks of geology and geography. One particular hole is the absence of anything in the Cincinnati region; there is William Howard Taft National Historic Site in the Mount Auburn area with what appears to be some bluffs, but no one has reported fossils from it. The north end of Natchez Trace Parkway (and Natchez Trace National Scenic Trail) cuts through part of the Upper Ordovician rocks of Tennessee, and I wouldn't be a bit surprised to learn that there's much more there than we have listed. This is something that's true in general of many shallow marine Paleozoic formations that crop out in parks. Often records drawn from the literature focus only on biostratigraphically important fossils, so we may just get lots and lots of brachiopods, and varying numbers of trilobites, mollusks, and microfossils depending on the when and where. At this time, outside of MNRRA, I'd say our best records of Late Ordovician fossils in the parks have come from Chesapeake & Ohio Canal National Historical Park and a couple of places that should be familiar by now: Chickasaw National Recreation Area and Death Valley National Park. A couple of others could have been added, and I'm sure we can agree to disagree if necessary about significance, some Middle versus Late stuff, and so on.
Chickasaw NRA is probably the most comparable to MNRRA at this point for Upper Ordovician fossils. Actually, technically speaking Effigy Mounds National Monument and Saint Croix National Scenic Riverway are ideally most comparable, because Effigy Mounds NM has most of the same formations and Saint Croix NSR also has some overlap, but Effigy Mounds NM has not been explored in terms of paleontology and the Upper Ordovician rocks of Saint Croix NSR are very limited in extent and have not produced fossils. So, we'll stick with Chickasaw NRA. This site is handy because it looks like we can tie in with the bentonites (the old ash beds) in the Decorah. Investigations in the Ordovician rocks of Oklahoma indicate that the margins of the bentonites are also present there, in the upper Bromide Formation. The Bromide and the overlying Viola Group are exposed in and around Chickasaw NRA, making that part of the column in the recreation area about the same age as the Decorah in MNRRA. On my visit to the recreation area and its surroundings, it didn't seem to me that the bottom fauna was as diverse as in the Twin Cities, and the echinoderms were notably different. These differences could be a function of different regional faunas, different ecological settings, the limited time we had for exploration, or a combination.
The bentonites allow us to make a number of correlations across the eastern United States. Looking to Tennessee, we can tie into the Nashville Basin part of Natchez Trace Parkway through the Carters Formation, which includes both the Deicke and Millbrig K-bentonites, and see that the Ridley Limestone of Stones River National Battlefield is slightly older. In the east, the Martinsburg Formation of Chesapeake & Ohio Canal NHP has the Millbrig K-bentonite near its base, which is found in the lower Decorah. In the area including Buffalo National River in Arkansas, the St. Peter Sandstone and overlying Plattin Limestone (named for Plattin Creek and coincidentally equivalent to the Platteville) are present, but it appears that unconformities mess with the overlying rocks so that there isn't a straight-up Decorah equivalent within the national river itself. Going west, we unfortunately lose the bentonites, so we can't make the same direct correlations, but the Bighorn Dolomite of the Yellowstone region is somewhat younger than the bentonites, while the Ely Springs Dolomite and equivalents in the Great Basin and Mojave regions are known to include Upper Ordovician fossils.
So, what was this Late Ordovician world like? First off, at press time the Late Ordovician began 458.4 ± 0.9 million years ago and ended 443.8 ± 1.5 million years ago, giving us a slice of about 15 million years. What we know as North America was hanging out mostly south of the Equator and rotated clockwise compared to today. It was in the process of a hostile takeover of some small slices of crusts, island arcs, microcontinents, and so on, arriving along what is today the East Coast. The process of continental growth fueled the giant volcanoes that produced those convenient bentonites. To (over)simplify things, lands east of the Appalachians are remnants of the various bits that got stuck or shoved onto the continent during the Paleozoic. (Incidentally, that means the settled areas of the Thirteen Colonies are imports.)
Sea level started off quite high, and without the Rockies or the Appalachians, much of North America became submerged, creating the warm shallow seas where our bottom-dwelling faunas lived. These faunas, as we've seen with the Twin Cities, were loaded with solitary corals, bryozoans, brachiopods, bivalves, snails, trilobites, and echinoderms, under the watchful eyes of nautiloids. They were the result of an earlier Ordovician radiation that tidied up the survivors of the freewheeling Cambrian; with some changes here and there (more sea urchins, more diversified corals, ammonites, diversified fish, etc.), the Late Ordovician marine fauna is similar to what we get for the rest of the Paleozoic. Part of why it's not exactly the same occurs in the latter part of the Late Ordovician. At around 445 million years ago, the world entered a glacial phase. Glaciation was not a big thing in North America itself, being at the Equator, but the draw-down of sea level as water became ice disrupted the existing marine communities. The glaciation just happens to coincide with a major extinction, affecting a number of groups that lived in warm shallow marine waters. The lower sea levels also give us a weaker geologic record for the end of the Ordovician into the Silurian, because there wouldn't have been as great an area for new rocks to be deposited, and old rocks would have been exposed to erosion.
|Partying like it's the Mohawkian, all day and all night, at Club Late Ordovician|
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.