As with the MNRRA formations, we're covering a fairly narrow span of time. The Cambrian formations were all deposited between about 500 to 491 million years ago based on biostratigraphic correlations. This includes some unconformities. One other note: I'm working from the Minnesota side of the St. Croix River, and I'm most familiar with the Minnesota names. Mossler (2008) harmonized the stratigraphic nomenclature of Minnesota's Paleozoic rocks with the schemes used in neighboring states, but there is still one difference: the Minnesota Geological Survey uses lithological terms in formation names, while the Wisconsin Geological & Natural History Survey doesn't. The upshot is slightly different names. For example, the units called the Jordan Sandstone and Oneota Dolomite on the Minnesota side of the river are called the Jordan Formation and Oneota Formation on the Wisconsin side. There isn't really a practical difference; the names just look different. In ascending order, the rock units we're most concerned with are the Mount Simon Sandstone, Eau Claire Formation, Wonewoc Sandstone, Tunnel City Group, St. Lawrence Formation, and Jordan Sandstone.
Mount Simon Sandstone and Eau Claire FormationThe two oldest Cambrian units of the St. Croix region are also the most elusive. The Mount Simon Sandstone is, by reputation, a medium- to coarse-grained gray quartz sandstone with cross-bedding (Mossler 2008), but it is atypical where we're looking (Mossler 1987). Below Taylors Falls it is only found in deep wells and along the Willow River near Hudson (Mossler 2008). Above Taylors Falls, Atwater and Clement (1935) and Nelson (1949) described isolated outcrops as far north as Sandrock Cliffs as the Mount Simon, but without fossils or other distinctive characteristics, the sandstone outcrops could belong to any of several other quartz sandstone formations, up to and including our old friend the St. Peter Sandstone (Mossler 2008). I drove up to Sandrock Cliffs this summer and found the bluff about as advertised: unfossiliferous, white to gray, medium- to coarse-grained sandstone, featuring some cross-bedding and the occasional thin bed of pebble gravel. This is more or less your basic Lower Paleozoic quartzose sandstone, and could indeed represent the Mount Simon Sandstone or something else. In places where everyone agrees that the rock is the Mount Simon Sandstone, brachiopods, trilobites, and trace fossils have been found, but no luck where I was. The sandstone is the kind that's really useful for industrial purposes, which currently is frac sand. In fact, if you're looking at satellite images, it's very obvious where the boundary of St. Croix National Scenic Riverway is in the Sandrock Cliffs area, because there's a sand mine on the other side. (Given that most of the units we're talking about here are friable [crumbly] sandstones, their main utilities for Homo sapiens are as aquifers and as sources of sand for various industries.)
|Sandstone at Sandrock Cliffs. Some beds have better cross-bedding than others.|
The Eau Claire Formation pulls a similar trick. It makes guest appearances at a few places in the valley, some from Franconia to the St. Croix Falls Dam (Baker et al. 1989; Yochelson and Webers 2006), others as far up-river as Rush Creek (Atwater and Clement 1935). However, it doesn't show up as a complete section (Nelson 1949), and most of the known outcrops are on private land or are difficult to access (Mossler 1987). About the best there is in the area is the 76 ft (23 m) at Little Falls Dam near Hudson (Nelson 1949). It is heterogeneous, and often either fine-grained thin-bedded shaly sandstone with interbedded shale, or dark gray calcareous sandy shale (Nelson 1949). It's supposed to be the most fossiliferous Cambrian unit in Minnesota (Mossler 2008), but fossils are limited to certain beds (Twenhofel et al. 1935). The fossils include brachiopods, trilobites, aglaspidids, and archaeostracan crustaceans (Nelson 1949). Both the Mount Simon and the Eau Claire used to be members in the Dresbach Formation, back when we had the Dresbach, Franconia, and Trempealeau Formations. Those three names are no longer used for formations, but they still are sometimes used in terms of age (e.g., the Dresbachian stage).
|Haven't had one of these before: This is a fragment of an aglaspidid from the Eau Claire Formation of Taylors Falls in the collections of the University of Minnesota (UMPC GB16350).|
Wonewoc SandstoneThe thing about the Wonewoc is that technically speaking, it is composed of two disconformable members, more closely related to the underlying and overlying formations. However, they're more or less indistinguishable in outcrop unless you have Super Sandstone Vision. The lower part, the Galesville Member, used to be called the Galesville Formation (or Sandstone, or Member of the Dresbach Formation), while the upper part, the Ironton Member, used to be the Ironton Formation (or Sandstone, or Member of the Franconia Formation, or Woodhill Member of the Franconia Formation... I could go on like this for longer than I'd like to think). Anyway, the Galesville tends to be better sorted and have purer sandstone than the Ironton (Mossler 2008), and the Ironton is more fossiliferous than the Galesville. Together they make up something like 72 ft (22 m) of fine- to coarse-grained cross-bedded quartz sandstone in the St. Croix Valley.
The easiest place to see the Wonewoc Sandstone is in the cuts on the east side of Second Street north of Interstate 94 in Hudson, in the vicinity of the junction with Coulee Road. This includes the northwest nose of the ridge that makes Birkmose Park. If you're stopped at the Dairy Queen, there's the Cambrian. (Note: given the number and distribution of ice cream places in the St. Croix Valley, it may be possible to build a geological field trip around them.) In terms of fossils, the Wonewoc does best with brachiopods and burrows.
|Part of the big Wonewoc cut in Hudson, with my yellow field book in there for scale. Yes, it looks a lot like the Sandrock Cliffs sandstone, or some outcrops of the Mazomanie Formation, or some outcrops of the St. Peter Sandstone. It happens.|
The Mill Street Conglomerate, which we visited here and here, is usually considered a facies of the Wonewoc (Mossler 2008), but strictly speaking the unusual environmental conditions that produced the conglomerate were replicated in different places at different times, so not every instance of Mill Street-like conglomerate is necessarily Wonewoc.
Tunnel City GroupWe checked in briefly with the complicated Tunnel City Group (née Franconia Formation) back in July. To recap: there was a marine basin on the south and west (our south and west, not the Cambrian south and west) and an eroding sand source on the north and east. The marine basin sediments became the Lone Rock Formation, and the tongues of sand became the Mazomanie Formation. Within the Lone Rock Formation, changes in sediment supply give us three distinct members, the basal Birkmose Member, the middle Tomah Member, and the upper Reno Member. Fossils in the Tunnel City Group are usually given as including brachiopods, trilobites, trace fossils, and some other odds and ends (snail-like fossils, conodonts, etc.) (Mossler 2008), but by far the most abundant are the burrows, and the best-studied are the trilobites, which are particularly well-documented in Clem Nelson's work (Nelson 1949, 1951).
The Mazomanie is probably the easiest formation to observe in the valley. Excellent outcrops requiring nothing more than the ability to use stairs can be seen at the Boom Site in Stillwater and Cascade Falls in Osceola, and the outcrops at Interstate State Park can be reached via a slightly greater number of stairs. (Okay, a lot of stairs, but come on! It's not like there's a Cambrian seafloor delivery service, and it's a nice walk.) If you're driving into Taylors Falls from the south on Highway 8, the big roadcut is almost all Mazomanie Formation, with a bit of Birkmose at the base, but I wouldn't recommend stopping and having a look. You can get a pretty good sense of the division between the two using the road imagery on Google Maps: the Mazomanie is the steep bare wall and the Birkmose is obscured by vegetation. The Mazomanie in the valley is up to 115 ft (35 m) of light-colored fine- to medium-grained sandstone, often with interesting sedimentary structures, thickest near Taylors Falls and splitting into several tongues going south (Mossler 2008).
The funny thing about the Mazomanie is that I'm starting to think I can identify hardened blocks of it by touch, perhaps due to some combination of grain size, roundness, and mineralogy, as well as porosity. It's hard to describe, but a chunk of Mazomanie with a bit of hardening has got a slightly "styrofoam" texture, and is much lighter than it looks. There's something visually distinctive about it, too.
For the Lone Rock Formation, the Birkmose Member is the unit directly above the Wonewoc Sandstone at Birkmose Park. Per the work by Berg (1951, 1954), the whole Tunnel City Group in its various manifestations can be seen along Coulee Road, but erosion and pernicious vegetation are hiding significant portions of the section now. Rehashing the "layer cake" post, the Birkmose is a "greenish-gray very-fine to fine-grained sandstone, with a lot of feldspar and glauconite grains (glauconite being a green mineral that likes to form on marine bottoms with little sedimentation)". It can be 20 to 50 ft (6 to 15 m) thick, and is frequently lacking bedding features due to heavy burrowing (Mossler 2008). It is also not especially photogenic, as rocks go.
The Tomah Member is best seen in Marine on St. Croix, at the low elevations near the river, such as around Mill Creek (it's the fine-grained siltstoney stuff). It's a mix of feldspar-rich siltstone, very fine sandstone, and thin shale interbeds. It differs from the Birkmose and Reno members by being brownish or grayish instead of glauconitic green, and it differs from the Mazomanie by being much finer grained. It's about 23 to 27 ft (7 to 8.2 m) thick and is kind of the "runt" of the Tunnel City Group in the valley, because it pinches out partway up the valley and isn't all that thick or well exposed to begin with (Mossler 2008).
|The Tomah tends to be made up of inch-scale beds that weather into flat blocks, as here at Marine on St. Croix.|
Finally, the Reno Member can reputedly be seen in outcrop at several areas in the valley; I saw it along Stagecoach Trail near Afton. It looks a lot like the Birkmose, but it doesn't get as coarse-grained (granted, we're talking very-fine-grained sandstone versus fine-grained sandstone) and has better sedimentary structures. It can be essentially as thick as the Mazomanie, up to 110 ft (33.5 m) (Mossler 2008).
|A float chunk of the Reno with some burrows (the orangish tubular features), seen in Afton.|
St. Lawrence FormationThe St. Lawrence Formation can be difficult to meet in person. You can see it pretty well at Cascade Falls in Osceola, where it looms over the falls, and there are some outcrops in the vicinity are somewhat more accessible (trying to climb the falls is not recommended). I've seen some small areas of outcrops in Afton as well, but regrettably no graptolites. At these locations it crops out as fine-grained pale greenish or grayish rocks in beds on the order of an inch thick.
|Beds of the St. Lawrence near Cascade Falls in Osceola.|
The St. Lawrence Formation is noted for including some of the few carbonate rocks in the Cambrian sequence of Minnesota and Wisconsin, but in the St. Croix Valley the carbonate facies is absent. Instead, we get the siltstone facies, which is interpreted as more shoreward marine (Mossler 2008). The two facies are sometimes defined as two members, designated the Lodi Member (siltstone) and Back Earth Member (carbonate or dolomite) (Nelson 1956), but the facies don't always play nice (McGannon 1960; Hughes and Hesselbo 1997), and so member names aren't used by the Minnesota Geological Survey (Mossler 2008). Osceola is one area that proves annoying, because the upper St. Lawrence bears a striking resemblance to the overlying Jordan Sandstone (Sardeson 1932). The formation is around 33 ft (10 m) thick in the valley, about a quarter of the thickness it has in southeastern Minnesota (Mossler 2008). My own inspection notwithstanding, it has a pretty diverse fauna as far as the Upper Cambrian goes, with the usual brachiopods, trilobites, and trace fossils joined by hyoliths, various and sundry snails and snail-like mollusks, aglaspidids, archaeostracans, graptolites, and conodonts in the St. Croix Valley area (Nelson 1949; Hughes and Hesselbo 1997; Runkel et al. 2007).
Jordan SandstoneThe last Cambrian unit in Minnesota is the Jordan Sandstone, deposited a few million years before the end of the period; we get a geologically brief "Scene Missing" before the Prairie du Chien Group of the Early Ordovician. As the name suggests, the Jordan Sandstone is made of sandstone, 77 to 97 ft (23 to 30 m) of it in the valley (Nelson 1949). You get two flavors: fine- to coarse-grained quartz sandstone, and very-fine-grained feldspathic sandstone with some shale and siltstone. The quartzose facies is sometimes called the Van Oser Member, and the feldspathic facies is sometimes called the Norwalk Member, but as with the St. Lawrence Formation, the "members" present problems in mapping (basically a bunch of tongues of sandstone; Runkel 1994a), so the Minnesota Geological Survey doesn't use them (Mossler 2008). Overall it's a shallow marine sandstone, with the sea becoming shallower over time, until the uppermost part of the formation has tidal features (Runkel 1994b; Byers and Dott 1995; Tape et al. 2003). It's not noted for its fossils; you get some brachiopods, gastropods, trilobites, and trace fossils (Nelson 1949; Tyler 1956), with some conodonts for the microfossil folks (Runkel et al. 2007).
The Jordan Sandstone is well-exposed at Eagle Bluff in Osceola (if you don't mind hiking up a talus slope), at Fairy Glen in Stillwater (if you don't mind some narrow bluff trails often hidden by enthusiastic plant growth), and some other places that happen to be road cuts. The roads aren't super busy, but narrow road shoulders and other pesky logistical and personal safety concerns may prove problematic.
|This was different, for the Jordan: a pocket of fine-grained greenish-gray rock sandwiched by the sandstone, seen at Eagle Bluff.|
Prairie du Chien Group and higherJust like the MNRRA corridor, we next get the Oneota Dolomite and Shakopee Formation of the Prairie du Chien Group, and the sequence would go on through the St. Peter Sandstone, Glenwood Formation, Platteville Formation, and Decorah Shale if those units weren't all more or less eroded from the valley. About the best we can do are some outliers of St. Peter Sandstone in the vicinity, and some unique fault blocks near Scandia (Peterson 1924, 1927; Quaschnick 1959). The fault blocks are significant as the most northerly and shoreward outcrops of the St. Peter–Decorah rocks (Quaschnick 1959).
|A loose stromatolitic Prairie du Chien block observed in Afton State Park, with some modern greenery as well.|
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