Afton graptolites revisited

There are two great lost fossil sites in the Twin Cities area. (The Brickyards don't count; they're not lost, they just aren't open to collection.) One is the Johnson Street Quarry, where workers cut into a bed in the Hidden Falls Member of the Platteville Formation that had unusually abundant echinoderms. As described in Sloan et al. 1987: 200, "Sardeson mined out a spot in this unit in the old Johnson Street Quarry in Minneapolis (now filled with garbage, and covered with Interstate 35) that produced about 20 specimens of the starfish Protopalaeaster narrawayi, several specimens of the crinoid Cremacrinus arctus (Fig. 16.2), edrioasteroids, cystoids, brachiopods, bryozoans, molluscs, and graptolites." This is slightly out of date; instead of a dump, there's now a Quarry Shopping Center with a Cub Foods, Home Depot, and Target, although even with all those options you can't get an edrioasteroid there anymore. Regardless of the exact character of the overburden, it seems unlikely that anyone will be doing any paleontological follow-up there anytime soon. The other locality is the Afton graptolite locality in the St. Lawrence Formation. We already had a post on why this locality was important; what I'm curious about is where exactly it was. A locality, even if "lost", had to have been *somewhere*, and apart from the scientific and historic interest, there very well could be similar fossils in rocks nearby. Indeed, Hughes and Hesselbo (1997) reported graptolites in the lowest strata of the St. Lawrence Formation in their Afton section, where collection may have postdated the road work that destroyed the classic location. For some reason, despite its “classic” nature, nobody ever saw fit to just put a pin on the map. What clues do we have?

Arcola Bluffs Day Use Area

It's been a good while since our last St. Croix post, hasn't it? I wanted to let you in on a fun place I recently visited for the first time; Arcola Bluffs Day Use Area. This site is not widely known; there's this article (with much more artistic photos than my own), an NPS cultural landscape assessment from 2018 that weighs in at 204 mb (NPS 2018; absolutely worth it if you want a thorough understanding of the site, and also includes a section on Fairy Falls), and then some short pieces here and there, and that's about it. Visiting it, though, you'll discover great geology, views of the river and the historic Soo Line High Bridge, forest and prairie settings, and some evocative ruins.

Paleozoic Taxa of the St. Croix Valley

Back when I was working on the Saint Croix National Scenic Riverway project, I'd compiled a spreadsheet of all of the fossil genera and species that had been reported from the rocks exposed along the valley. There had been some talk of spinning it off as a separate thing, but that never happened, so I played with the idea of posting it here. I then forgot about it until recently looking through the backlog of half-formed ideas. The spreadsheet itself was all ready to go, so I figured "why not?"

It's pretty simple; a column of numbers so it can be sorted back to the original organization when I'm working, a column for the genus/species, a column for the broad classification, twelve columns for the formations (go here for a refresher; the names are abbreviated for space, but each one has a note providing the full name), a column for references (defined on the "References" tab), and a column for additional notes. If a given species is present in a particular formation, the corresponding cell is marked "Y" and filled blue; if there's some question, the cell is marked "?" and filled yellow. If it's not present, there's a dash and no color fill. To check it out, you can enter here. I would not be surprised if there has been some oversplitting, if for no other reason than the challenges of preservation (we're dealing with a lot of natural molds and casts of partial trilobites in sandstone; fragility and preservation fidelity leave something to be desired).

It's worth mentioning that there is a document that covers some of the same ground, Raasch (1950). In one sense it's more narrowly focused, on Cambrian trilobite biostratigraphy, but in another it's more diffuse, with a larger area of interest.

References

Raasch, G. O. 1950. Zonal range of Croixan trilobite genera in the upper Mississippi Valley. Cambrian Subcommittee Memorandum No. V. Illinois State Geological Survey, Urbana, Illinois.

St. Croix Cambrian trace fossils

Here's something simple: photos of Cambrian trace fossils in the St. Croix Valley. If you would like a refresher on the rocks in question, may I suggest this post? We've already seen some photos of Skolithos burrows in the Mazomanie, a sandstone unit. The finer-grained rocks have other kinds of trace fossils, which makes sense because they represent different environments than the Mazomanie. At the same places in Osceola where Skolithos are found in abundance in the Mazomanie, much different burrows are locally abundant in the overlying St. Lawrence Formation. In the upper part of this formation, where the beds are sandy and can be hard to tell from the Jordan Sandstone (Sardeson 1932), the burrows are much thicker and horizontal.

There are a few in these pieces of float, including just below the scale bar and in the block above and to the left of the central block. The squares in the bar are 1 cm, so the burrows are a bit larger, on the order of finger-sized.

Returning to the same area nearly two and a half years later, I came across these remnants of a disintegrated block. If you expand the photo, you'll see that the largest chunks are those with burrows.

Comparable burrows can be found lower in the formation, in more typical-looking gray-green blocks. I wasn't seeing body fossils, which are reputed to be there, but there were certainly plenty of trace fossils.

This is more like what the St. Lawrence is supposed to look like, and there's another horizontal burrow, a bit smaller than those in the photos above but pretty similar.

One of the finer-grained units intertonguing with the Mazomanie is the Tomah Member of the Lone Rock Formation, another part of the ex-Franconia Formation. The Tomah is the finest-grained part of the Lone Rock Formation and has a tendency to erode into angular chips, blocks, and chunks, usually hand-sized or smaller, often in pastel greens and oranges. Some of the beds are heavily marked by various kinds of trace fossils.

Several different sizes are apparent here.

This one's unusual for having eroded out as a substantial piece.

This chip has traces close to 1 mm in diameter near the top and a thicker trace several mm across near the center, with a "lobed" appearance that may be due to erosion.

The large straight burrow on this piece has a lumpy surface somewhat reminiscent of "corn cob" Ophiomorpha, but not as coarse.

A slice of pizza covered with grains of rice?

I could go on ad nauseum with trace fossil photos from the Tomah (you may already be there), so just one more for the road. As far as I know, nobody has published a detailed analysis of the trace fossils in the Tomah or St. Lawrence, although I can't rule out there being some dissertation or other piece of grey literature I haven't run across. There's certainly quite a lot of these fossils there, though!

This one is dominated by burrows a couple of mm in diameter. Note the long slender trace in the upper center

References

Sardeson, F. W. 1932. Fauna of the Jordan Sandstone. Pan-American Geologist 58(2):103–106.

Ordovician updates

A few miscellaneous items to clear out of my Ordovician inbox, involving new members of Club Late Ordovician, conulariids, and pseudofossils:

Camp Nelson National Monument

Since we were introduced to Club Late Ordovician two years ago, two more parks have joined this exclusive organization for National Park Service units that include fossiliferous Upper Ordovician rocks. One is Saint Croix National Scenic Riverway, thanks to some isolated occurrences of the Platteville and Decorah that were protected from erosion by ancient faulting along the St. Croix Valley. These outliers represent the most northerly and landward outcrops of these formations, and are worth a post of their own at some point. The other new member is the newest NPS unit, Camp Nelson National Monument in Kentucky.

Right on cue, it's a map with a giant caption! (It's the map from the 2016 post with the two new points added, #9 and #14.) 1. Death Valley National Park; 2. Great Basin NP; 3. Yellowstone NP; 4. Grand Teton NP; 5. Bighorn Canyon National Recreation Area; 6. Big Bend NP; 7. Chickasaw NRA; 8. Mississippi National River and Recreation Area; 9. Saint Croix National Scenic Riverway; 10. Pictured Rocks National Lakeshore; 11. Effigy Mounds National Monument; 12. Buffalo National River; 13. Hot Springs NP; 14. Camp Nelson NM; 15. Stones River National Battlefield; 16. Natchez Trace Parkway; 17. Katahdin Woods & Waters NM; 18. Saratoga National Historical Park; 19. Delaware Water Gap NRA; 20. Chesapeake and Ohio Canal National Historical Park; 21. Denali NP & Preserve. The grey blob is the known extent of the Deicke K-bentonite.

Camp Nelson is rather more famous for its Civil War history, but the Camp Nelson area is also notable for its geology. In fact, per Andrews (2005), "Because of the quality of exposures, accessibility, and the importance of the site in research and geologic education, the Kentucky Society of Professional Geologists has named the Camp Nelson area as Distinguished Geologic Site 2."

The park overlays four Ordovician formations, in ascending order the Camp Nelson Limestone, Oregon Limestone, Tyrone Limestone, and Lexington Limestone (Wolcott 1969). They are all examples of your classic marine Ordovician limestones; no points for guessing that brachiopods and nautiloids are implicated in Camp Nelson NM joining Club Late Ordovician. (Not surprisingly, these limestone formations are also karst-formers, and caves and sinkholes are common in the area.) The Tyrone Limestone is known to include the Deicke K-bentonite (Kolata et al. 1996), making it partially correlative in time to the Decorah Shale. Redating of the Middle/Late Ordovician boundary may in fact put all four of these limestones into the Late Ordovician.

As the name indicates, the Camp Nelson Limestone was described from the area of Camp Nelson (Miller 1905); the type locality is a little to the south of the monument, around the area of the Kentucky River crossing. You can also find some nice faulting on the south side of the river (Andrews 2005). The Camp Nelson Limestone appears to be the oldest formation exposed in Kentucky. This was a bit surprising to me, and leads to the reflection that, thanks to accidents of geography and erosion, the bedrock at the surface in the entire state of Kentucky is younger than most of the bedrock exposed in Minnesota.

Hiding conulariids

I really hope to have a more complete update later, but suffice it to say that while tracking down references to a conulariid species from the Grand Canyon, I stumbled across George Winston Sinclair's 1948 thesis on conulariids. In this thesis there is a great deal of information about Minnesota's Ordovician conulariids that Sinclair did not publish in his lifetime, including several unpublished species. There is also the implication that at one time the University of Minnesota collections contained two-to-three-dozen conulariid specimens from the Glenwood Formation on up. This came as a surprise, because I did not recall having seen more than a half-dozen or so on my trips. Several possibilities presented themselves:

• The conulariids were being kept separately from the other fossils when I visited, or were just somewhere I hadn't known to look;
• The conulariids have been lost or stolen: if so, not much more can be added;
• The conulariids passed into Sinclair's hands: this seemed worth following, because Sinclair built up a collection of conulariids and I knew that certain elements of the University of Minnesota collections had gone to others. For example, material from Berkey's Taylors Falls work was donated to the Smithsonian in recent years (Yochelson and Webers 2006).

I checked with folks at the University of Minnesota, University of Michigan Museum of Paleontology, and the Canadian Museum of Nature (the latter two institutions associated with Sinclair), but no luck to date. Hopefully this all works out in the wash. By way of apology, here's a photo of the type specimen of Ctenoconularia obex, one of the few Minnesota species Sinclair ended up publishing.

UMPC 6608, holotype of the conulariid Ctenoconularia obex, from the "Sprechs Ferry" (=lower Decorah) of Minneapolis.

One other conulariid note:
It turned out that one of the other conulariids Sinclair named came from the St. Peter Sandstone of Minnesota: Climacoconus humilis, described in Sinclair (1942) and promptly forgotten by virtually the entire universe (it's based on Carnegie Museum 4753 from Faribault, by the way). Maybe it's just my myopia showing, but it doesn't seem right that it's made almost no impression in the literature, even to be refuted. We are, after all, dealing not only with the St. Peter Sandstone, famous for having practically no body fossils, but also an uncommon and distinctive type of fossil!

Dystactophycus revisited

Back in July 2016, I showed a photo of a strange rock with concentric rings, which provided an excuse to riff on Dystactophycus, a possible trace fossil of a swirling crinoid. At the time, I thought it was unlikely that the photographed rock was Dystactophycus, or indeed of organic origin at all. A few months ago, the collector confirmed my general suspicions: he told me that mutual acquaintances have identified it as a modern fracture pattern, consistent with his later observations of features produced by air hammers punching into the Platteville.

References

Andrews, W. M., Jr. 2005. Geology and the Civil War in central Kentucky: Camp Nelson. Field Trip Guidebook, American Institute of Professional Geologists 42nd Annual Meeting.

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.

Miller, A. M. 1905. The lead and zinc bearing rocks of central Kentucky, with notes on the mineral veins. Kentucky Geological Survey, Lexington, Kentucky. Bulletin 2.

Sinclair, G. W. 1942. The Chazy Conularia and their congeners. Annals of the Carnegie Museum 29(10):219–240.

Sinclair, G. W. 1948. The biology of the Conularida. PhD Thesis. McGill University, Montreal.

Wolcott, D. E. 1969. Geologic map of the Little Hickman Quadrangle, central Kentucky. U.S. Geological Survey, Washington, D.C. Geologic Quadrangle Map 792. Scale 1:24,000.

Yochelson, E. L., and G. F. Webers. 2006. A restudy of the Late Cambrian molluscan fauna of Berkey (1898) from Taylors Falls, Minnesota. Minnesota Geological Survey, St. Paul, Minnesota. Report of Investigations 64.

The League of Saint Croix

If we can have a "Club Late Ordovician", surely we can have something for those National Park Service units with late Cambrian fossils? Of course it can't just be "Club Late Cambrian". Given that this part of the Cambrian is historically known as the Croixan or St. Croixan (Walcott 1912), it seems fitting to work St. Croix in there somewhere. Therefore, I present the League of Saint Croix. If you're working with a name like "St. Croix", you might as well make it sound like some kind of late Middle Ages/early modern period European military order or alliance.

Practical guide to St. Croix Valley sedimentary formations

Now that I've seen a fair amount of the Cambrian rocks of St. Croix National Scenic Riverway and the St. Croix Valley, it seems like a good time to set them out as was done for the MNRRA formations. This time around, we'll go to the base of the Cambrian sequence in Minnesota/Wisconsin and work our way up to where the sequence overlaps with the MNRRA rocks. One day I'll have to get into southeastern Minnesota and complete the Paleozoic sequence with the rest of the Ordovician and the Devonian.

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.

The joys and agonies of other peoples' dissertations

In the hierarchy of information sources, dissertations and theses get a mixed reception. They rank higher than conference abstracts, which isn't saying much; after a few years, even a good abstract is kind of like a tooth that shows you duckbills or tyrannosaurs were present in such-and-such formation, but not much else. They get a bad rap for a number of reasons. Except in rare cases, their authors don't have vast experience writing scientific works, and the proofreeding and editorial processes are not generally as stringent. Speaking for myself, I worked on my thesis like a rat gnawing through concrete, and I wouldn't take back the accomplishment, but at the same time I could do a much, much better job today. I'd bet a lot of other people feel the same way about their graduate work. The quality of scientific review also varies, and is not considered to be of the same quality expected for a peer-reviewed paper, so questionable ideas can sneak through that would be weeded out in other publication venues. Finally, they can be difficult to nigh-impossible to find: the number of publicly available hard copies of a given dissertation or thesis is often one (1), so if the one you're looking for has not been scanned, cannot be purchased, and/or only exists in a university library hundreds or thousands of miles away, it might as well have been lost in the Library of Alexandria for all the good it's doing you. It's not hard to understand why published versions of graduate work are preferred.

If you can get access, though, a dissertation or thesis can be uniquely useful, provided you recognize the faults. They are great for detail. A peer-reviewed publication in a journal is usually on the order of twenty pages or fewer. A dissertation can be much, much longer, and contain the seeds of multiple papers plus stuff that was left out. For example, in my work on Saint Croix National Scenic Riverway geology, I've had access to many relevant graduate works through the University of Minnesota libraries, including dissertations by Clem Nelson (1949) and Bob Berg (1951) that basically reset the paleontology and stratigraphy for the St. Croix Valley. Although the two of them published several papers that incorporated parts of their graduate work (Nelson 1951, 1956; Berg 1953, 1956; Berg et al. 1956; etc.), these publications did not use everything. In particular, if you're just going by post-graduate publications, you only get some of Nelson's and Berg's stratigraphic sections.

Another thing that dissertations and theses are uniquely good for is documenting work from people who either ended up leaving the field or did not publish their results. Not publishing doesn't mean you did a terrible job; it can mean as little as your project's results didn't really have a venue. There are plenty of "Geology of the [insert name here] Quadrangle" projects that haven't been published in journals but are still valuable geologic records. There are a clutch of these at the University of Minnesota for the St. Croix Valley and nearby areas of Washington County for the late 1940s through the 1960s, and they've found their true homes as base data for Minnesota Geological Survey maps. One interesting example is Quaschnick (1959), a thesis on the geology of the Marine [on St. Croix] Quadrangle and the Falls Creek area. Although the Paleozoic rocks of Minnesota are not noted for their structural complexity, there is this one weird area in Scandia where faulting has preserved a thin sliver of St. Peter through Decorah between the expected Upper Cambrian rocks. You can visit part of this area in the Falls Creek Scientific and Natural Area, but don't expect to see much of the geology, thanks to vigorous plant growth. It was published on back in the 1920s (Peterson 1927, itself based on her 1924 thesis), but was still strange enough to warrant a revision. Unfortunately, Quaschnick's results were not formally published, so you have to go to the thesis. Another odd one: a few years ago, I noticed in geological maps of the St. Paul Park quadrangle and Washington County that an outcrop of Platteville Formation was plotted along 70th Street in Cottage Grove, but a personal inspection turned up nothing. This year, while going through Matsch (1962) on the St. Paul Park quadrangle, I found the outcrop described there; whatever it was in 1962, it's simply no longer visible today.

In the end, you still need to keep an eye open when using graduate work. Even apart from errors, often the author will make revisions if they do publish, sometimes substantial revisions. For example, when Nelson published his trilobites (1951), he named a number of new species. If you compare this publication to his dissertation, though, three of the new species have different names and seven new species mentioned in the dissertation were not named in the journal publication, instead being subsumed into other species. There are also some inconsistencies with specimen numbers. (Of course, dissertations and theses don't count for the purposes of formally naming species.)

These two specimens, UMPC (University of Minnesota Paleontological Collections) T6558b and T6558a left to right, were the paratype and holotype specimens of "Stigmacephalus marinensis" in Nelson (1949), but two years later he had second thoughts and identified them as the "paratype" and "holotype" of Stigmacephalus oweni var. B (Nelson 1951).

References

Berg, R. R. 1951. The Franconia Formation of Minnesota and Wisconsin. Dissertation. University of Minnesota, Minneapolis, Minnesota.

Berg, R. R. 1953. Franconian trilobites from Minnesota and Wisconsin. Journal of Paleontology 27(4):553-568.

Berg, R. R. 1954. Franconia formation of Minnesota and Wisconsin. Geological Society of America Bulletin 65(9):857-881.

Berg, R. R., C. A. Nelson, and W. C. Bell. 1956. Upper Cambrian rocks in southeast Minnesota. Pages 1–23 in R. Sloan and G. M. Schwartz, editors. Lower Paleozoic geology of the Upper Mississippi Valley. Geological Society of America, New York, New York. Guidebook Series Field Trip 2.

Matsch, C. L. 1962. Pleistocene geology of the St. Paul Park and Prescott quadrangles (Minn.). Thesis. University of Minnesota, Minneapolis, Minnesota.

Nelson, C. A. 1949. Cambrian stratigraphy of the St. Croix Valley. Dissertation. University of Minnesota, Minneapolis, Minnesota.

Nelson, C. A. 1951. Cambrian trilobites from the St. Croix Valley. Journal of Paleontology 25(6):765–784.

Nelson, C. A. 1956. Upper Croixan stratigraphy, Upper Mississippi Valley. Geological Society of America Bulletin 67(2):165-183.

Peterson, E. 1924. The Cambrian geology of the lower St. Croix valley: Osceola to Stillwater. Thesis. University of Minnesota, Minneapolis, Minnesota.

Peterson, E. 1927. Block-faulting in the St. Croix Valley. Journal of Geology 35(4):368–374.

Quaschnick, R. K. 1959. The geology of the Marine Quadrangle and the Falls Creek area, Minnesota. Thesis. University of Minnesota, Minneapolis, Minnesota.

The limitations of the layer cake

To be perfectly honest, we use simplifications for practically everything. Atoms don't really look like bunches of colorful spheres surrounded by smaller spheres orbiting them. The Earth is an oblate spheroid, which is close to but not quite a sphere. The planets of the Solar System don't have nice circular orbits centered on the center of the sun, lying in a flat plane. The need to simplify complex topics is obvious, both on the grounds of providing what someone needs to know to do something, and tailoring material to what someone can understand. There's a simple version for grade school kids, a more complex version for undergrads, and so on, until you're working professionally, where you've got very detailed models which are still abstractions, only closer (hopefully) to reality. One of these simplifications in geology is "geological formations as layer cakes", where formations maintain their thicknesses and are easily distinguished. The layer cake abstraction is most useful at a local level, in settings where deposition wasn't switching back and forth between different processes and sediment sources. For example, the Ordovician rocks of the Twin Cities fit pretty well. However, the cake starts getting funny-looking as you head into southeastern Minnesota. The photo below is of the Sogn roadcut, where some familiar rocks are exposed.

No, I don't know how to pronounce "Sogn".

At Sogn, though, what we would know as the lower half of the Platteville Formation is absent. Instead, the deposition of the Glenwood Formation persisted much longer (Sloan et al. 1987). Similarly, the Decorah Shale is at its thickest at the Brickyard in Lilydale, but going southeast, the upper part is replaced by the Cummingsville Formation. We can get these shifts in deposition from a number of causes. Sometimes you're looking at the boundary between two different modes of deposition shifting over time (such as a shoreline prograding or regressing). Sometimes there is a tectonic component, such as a basin subsiding. Sometimes the source of sediment changes or runs out.

Here's a more advanced example: the interval long known as the Franconia Formation and now known as the Tunnel City Group is divided into four parts in the St. Croix Valley. These are the Mazomanie Formation and three members of the Lone Rock Formation, from oldest to youngest the Birkmose, Tomah, and Reno members. We've met the Mazomanie Formation before; it's a quartz-rich light-colored very-fine- to medium-grained sandstone with abundant burrows and various forms of cross-bedding. (This of course is also a simplification, boiling down the essence of a rock unit that was deposited across some hundreds of thousands of years over parts of two states.) The Lone Rock Formation is a finer-grained, darker, wormier unit. The Birkmose Member 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). The Tomah Member is a brownish-gray feldspar-rich siltstone and very-fine-grained sandstone with thin interbeds of gray-green shale. Finally, the Reno Member is similar to the Birkmose Member, but somewhat finer-grained and with better defined sedimentary structures. The Mazomanie Formation is a lateral equivalent to most of the Lone Rock Formation. While the Lone Rock Formation was deposited in an offshore setting centered in Minnesota, the Mazomanie was deposited under shallower conditions, and its sediment came from topographic highs to the north and east in central Wisconsin. The two formations intertongue over a wide geographic and vertical range. If you trace the zone of intertonguing, you're seeing deposition fluctuating over time, as pulses of uplift and erosion on the Wisconsin highs sent sand to the south and west. It doesn't look much like a layer cake, at least not a competent example. There are at least three major Mazomanie tongues, plus who-knows-what going on between Franconia and Marine-on-St. Croix. The Tomah seems to go quietly, but the Reno is engaged in some kind of geological close-quarters combat with the Mazomanie.

Schematic colorized version of St. Croix diagram in Berg (1951, 1954), with information from Quaschnick (1959) taken into account for northern Tomah extent. Thick black lines are reasonably certain stratigraphic contacts, thin black lines are inferred, red line are biostratigraphic boundaries, and brown vertical lines show the extent of the measured sections (with the locations identified below the lines). With Berg's Woodhill Member removed (Ironton Sandstone Member of the Wonewoc Sandstone), the Tunnel City Group here is around 52 m (170 ft thick). The rocks continue for a long way south of Afton, but there aren't any good outcrops for a while.

The concept of a simple planar formational contact is in itself a simplification. Sometime you get a nice flat contact between two units. Sometimes you get a contact with vertical relief, because the underlying formation was eroded into hills and valleys before the overlying unit was deposited. Sometimes the contact is arbitrary, because the lower rock type grades into the upper rock type. Sometimes the contact is arbitrary because the two units meet over a zone of alternating beds, due to the two types of deposition switching from time to time. This last kind is what we're seeing here between the Mazomanie and the Lone Rock formations, and if we could see through the ground to get a full picture of what is going on from Taylors Falls from Afton, the contacts would probably look "fuzzy" due to smaller and smaller-scale interbedding.

Finally, I've mentioned a few times how the Franconia Formation was problematic because of mixing rocks with biostratigraphy. Back in the day, people tried to define subunits based on trilobites. Berg (1951, 1954) pointed out that the zones don't actually follow the rocks. When your biostratigraphic formations don't correspond to rock types, it makes it a real pain to try to map. In addition, you have to have both a paleontologist who can identify the relevant species, and well-preserved examples of those species in the rocks you are studying. (Of course, it gets even worse if some significant number of the species you are dealing with are actually minute variations on a single species, but who would ever do that to you?) The red lines in the diagram show that the trilobite zones skew upward going north in the St. Croix Valley. This is not entirely surprising, when you get down to it: the Lone Rock Formation is notable for its glauconite content, which as mentioned is a sign of low sedimentation rate. The Mazomanie Formation lacks glauconite. I'm going to guess that the Mazomanie had a greater rate of sedimentation than the Lone Rock, which would naturally cause the zone boundaries to skew higher where there is more Mazomanie deposition.

References

Berg, R. R. 1951. The Franconia Formation of Minnesota and Wisconsin. Dissertation. University of Minnesota, Minneapolis, Minnesota.

Berg, R. R. 1954. Franconia Formation of Minnesota and Wisconsin. Geological Society of America Bulletin 65(9):857–881.

Quaschnick, R. K. 1959. The geology of the Marine quadrangle and the Falls Creek area. Thesis. University of Minnesota, Minneapolis, Minnesota.

Sloan, R. E., D. R. Kolata, B. J. Witzke, and G. A. Ludvigson. 1987. Description of major outcrops in Minnesota and Iowa. Pages 197–231 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.

Graptolites of Afton

"Saw Clinton R. Stauffer, with a big rock in his hands

Says he found the graptolite site again

Gonna celebrate at Selma's Ice Cream Parlour

Send 'em off to Rudolf Ruedemann

[imitation of the sound of a graptolite]
Graptolites of Afton..."

(I apologize for nothing!)

University of Minnesota Paleontological Collection (UMPC) 4093, a particularly photogenic paratype of Callograptus staufferi, also depicted as Figure 5, Plate 55 in Ruedemann (1933).

Dikelocephalus minnesotensis

Investigating the rocks of Saint Croix National Scenic Riverway (SACN) is a tougher nut than working in MNRRA. Most of the area where rocks are exposed in MNRRA is part of some kind of park (city, state, regional, county, etc.) and generally accessible to the public. Much of the area with outcrops on the St. Croix is private land, and many of the key localities in the literature are now overgrown, destroyed by construction, or are roadcuts next to busy highways. Determining where you are in the strat column is also more difficult. In MNRRA, it's hard to get mixed up if you can tell sandstone from limestone/dolomite and shale. In SACN, you're dealing with several quartz-rich medium to coarse sandstones that tend to look the same, with some intervening shaly, dolomitic, or finer-grained sandy formations, and in the literature practically every single investigator had their own preferred system of names right up until the 1960s. Finally, in MNRRA there are abundant and diverse fossils in the Platteville and Decorah, while in SACN the special of the day is the BLT (burrows, lophophorates [brachiopods and hyoliths], and trilobites) with a side order of mystery snails, and you have to work for everything but the B.

The sweet siren song of the Franconian trilobite.

When brachiopods ruled the Earth

...well, maybe that's an overstatement, but it's a catchier title than "When brachiopods were dominant marine fauna in parts of cratonic North America", right? Our story today goes back to the latter part of the Cambrian, 500 million years ago or so. The Cambrian Explosion had come and gone, the confetti and stray napkins had been disposed of by various wormy things, and in the absence of thumbs to twiddle, there was nothing much to do until the Ordovician Radiation. Many forms of life got bored of waiting and went extinct, or otherwise died out from less frivolous causes, leaving behind a kind of "blah" marine fauna dominated by brachiopods, trilobites, and conodonts. This stretch of time has been called the "Late Cambrian plateau" or, more ominously, a "dead interval".

"We are your masters now! Ha ha!"

Further adventures in the Mazomanie

One of the projects I'm working on concerns the paleontology and geology of Saint Croix National Scenic Riverway, so I've been doing some location scouting to get a feel for the geology. It's not quite as simple as MNRRA, even though I'm still only dealing with a handful of formations and there's not much structural geology to contend with. The main issue is finding access to outcrops. Other complications include sparser fossils and all of these darn Cambrian cratonic sheet sandstones that look about the same.

David Dale Owen and the first geological survey of Minnesota

Although Keating, Featherstonhaugh, and Nicollet made significant contributions to Minnesota geology, the first true geological survey in what is now Minnesota would have to wait until 1847. At this point, the future state was split between Wisconsin Territory and a leftover chunk of Iowa Territory, and with the pending organization of Wisconsin into a state it was actually touch-and-go for a while how the boundaries would fall out. The convergence of St. Croix Valley interests versus the rest of Wisconsin with the old Northwest Territory stipulation that a maximum of five states be made out of the territory, and a dash of underlying slave state versus free state politics, could have led to anything from a super-Wisconsin incorporating much of what is eastern Minnesota to a separate state centered on the St. Croix Valley with Stillwater as the capital (the story can looked at briefly here). Anyway, in 1847 Congress authorized a geological survey in Minnesota and neighboring areas, and appointed David Dale Owen to conduct the work (Hendrickson 1945).

A portrait of Owen, found on p. 206 of Owen (1852).