Silver Creek Cliff geology

For obvious reasons, this blog has featured plenty of sedimentary rocks, but hardly a crystalline rock (one notable exception being World of Stone, which also allowed me to title a post after an obscure George Harrison song; no particular reason although in hindsight I might have been feeling down at the time). I just got back from a trip to Duluth and the North Shore and discovered my phone had secretly and unexpectedly created this grand panorama of a spectacular roadcut, so I thought I'd mix up the usual topics:

Definitely worth the click to embiggen

This location is the north end of the Highway 61 tunnel at Silver Creek Cliff, where there is a fun roadside pullout with information about the construction of the tunnel (and the former route of Highway 61, which went where the walking path is now and looks like it would have been extremely narrow). What we're looking at is a sequence of events in the old Midcontinent Rift, 1.1 billion years ago. There is an annotated photo showing part of the roadcut here, but most of the different things are easy to spot without too much guidance. On the right, the somewhat pinkish rocks beginning above the road are flows of andesite, a type of volcanic igneous rock. The andesite is cut off by a stark, irregular contact with a dark blue-gray rock; the contact begins near the blue minivan and rises going to the right. The darker rock is diabase, an igneous rock that intrudes into existing rock underground and has a similar composition to basalt. The diabase makes up most of the roadcut, but you may notice a weird "scar" running through it. It begins, from the perspective of the flat image, just right of the right-most metal structure on our side of the road (the one right of the street light pole) and rises to the right until it is lost under vegetation. This "scar" is laced with light-colored rocks. It represents diabase that has been altered by faulting, with new minerals forming in the fault gouge. Thus, the sequence of events is: 1) eruption of andesite; 2) intrusion by diabase; 3) faulting and mineralization within the diabase.

The key part of the panorama, showing the andesite, diabase, and fault.

The Lost Fucoids of Edwin McKee

Okay, that's hyperbole. The fucoids were never lost, they were just overlooked.

Many of you have probably heard of Edwin "Eddie" Dinwiddie McKee, who was the park naturalist at Grand Canyon National Park from 1929 to 1940. He moved on to other positions, but the Canyon was never far behind him. By the end of his career he'd written monographs about almost all of the Paleozoic sedimentary formations of the canyon, missing only the Temple Butte Formation and Hermit Shale/Formation for a complete set. He doesn't seem to have focused much on the Precambrian sedimentary rocks, though. Maybe the logistics of access made them less appealing (you have to go all the way to the bottom to see them, after all). Maybe the relative paucity of non-microscopic fossils made them harder to interpret. Maybe he just wasn't interested in the Precambrian. We do, though, have an interesting assortment of Precambrian rocks collected and briefly described by McKee.

In May 1930, McKee paid a visit to the Chuar Creek/Lava Creek area of eastern Grand Canyon NP, returning with about two dozen specimens from the Dox Formation, part of the Mesoproterozoic Unkar Group (lower chunk of the park's famous Grand Canyon Supergroup). Several had intriguing spindle-shaped surficial pits and ridges. McKee gave a brief description of them in an article about "fucoides" at Grand Canyon (McKee 1932). "Fucoid" goes back to the 19th century proposition that what we now know as invertebrate burrow fossils were actually seaweed fossils. (Usually the plural is "fucoids", not "fucoides") By the early 1930s, this was pretty raggedy, as McKee acknowledged; he was using it as a term of convenience for "any structure of that general nature regardless of its origin." Among the "fucoides" he presented were spindle-shaped markings on two pieces of Dox Formation rock. A careful examination of the photo will reveal that the markings in the piece on the left are incised into the rock, and those on the right-hand piece project out of the rock. They're something like mud cracks, but are certainly not textbook cracks (McKee also notes that there *are* more typical mud cracks in the Dox).

"Grant" is noted NPS photographer George A. Grant. Somewhere there must be a higher-quality version of this photo, but I haven't found it yet (I've seen one of the next figure in the article, but not this one).

This is the last we hear of these Dox "fucoides" from McKee, and probably the last anyone who was not a Grand Canyon National Park museum curator thought about them until 2019, when we began the Grand Canyon National Park paleontological inventory. I was working on the Precambrian paleontology chapter, and I love the historical side of the science, so McKee's article was right up my alley. I included the photo in my draft, and one of the reviewers noted, quite accurately, that there was no scale. Well, I couldn't do much about it (there isn't even a "photo is 2/3rd actual size" or something like that, and my time machine is in the shop, plus I don't do temporal paradoxes). That fall, though, when I was at the park I had the opportunity to visit the museum collections, and there they were. I took them outside to mimic the original photo, which appears to have been taken in front of a step, retaining wall, or something along those lines (of course including a scale bar this time).

It's not quite the same, but you get the idea. The white object is a box I used to provide additional support for the piece on the left, which at some point had been broken in two with the two pieces given different specimen numbers (the new numbers have yellowish backgrounds, while the original numbers have white backgrounds; if you compare this to the original photo, you can see the same spots).

While I was working, a number of other paleontologists were also visiting the collections as part of our National Fossil Day event. Spencer Lucas observed that the specimens looked a lot like microbially induced sedimentary structures (MISS), and asked about writing a short paper about them, to which I agreed (Tweet and Lucas 2021). The specimens themselves turned out to be more complex than is apparent from either photo. They are actually part and counterpart: in the color photo above, the large gray redox spots on the margins of the two pieces are the same spot, and the concave and convex features fit quite nicely when the pieces are lined up properly. Furthermore, there is another piece that belongs to this group, not included in the original photo (perhaps it didn't fit the composition?).

Here's a group shot, with the redox spots lined up and the previously omitted piece included.

The matching surfaces are laced with spindle-shaped features, convex in the upper row in the previous photo, concave in the lower row. Most of them are around 20 to 30 mm long (a little more or less than an inch), about 5 mm across, and with less than 5 mm of relief. A gentle, roughly longitudinal warp is most evident in the two left pieces, which might be a ripple or something similar but is heavily obscured. Flipping them over revealed additional but different features, a combination of blebs and long stringy features.

Figure 5 in Tweet and Lucas (2021). The black arrows in A point to two potential generations of ripples, and the white arrow points to the edge of a bed. B, the lower row in the preceding photo, is particularly marked by equant convex blebs and long stringy features, but there are a few on A as well.

The spindle-shaped features on the part-counterpart surfaces are shrinkage cracks, but as noted they aren't your typical mud cracks. Mud and coarser-grained sediments respond to shrinkage differently; mud grains stick to each other until something gives and a fissure forms, but sand grains slide apart from each other and do not form cracks unless something else interferes. That "something else" can be abiotic or, more frequently, biotic, such as a sticky microbial mat (Seilacher 1999). Something that is counterintuitive about microbially mediated cracks is that they are often preserved as positive features (Eriksson et al. 2007): mat and substrate contract and fissure, sediment fills fissure (either from above or from below due to environmental pressure), mat decays, and presto!—positive cracks! The long linear features in the photo above are similar to a different kind of MISS, in this case rolled-up mat fragments (Donaldson 1967; Eriksson et al. 2007).

This is more of a preliminary assessment than a definitive study. We don't have much to go on for the original geologic context, for one thing, and for another, if you really want to cover all of your bases, you need to take thin sections; MISS and inorganic features can be easily confused. Regardless, the features are highly suggestive of MISS, and the Dox Formation is already known to have stromatolites in other beds (Stevenson and Beus 1982). The Precambrian of the Grand Canyon has produced many features that were originally reported as burrow-like or "fucoidal", but most non-stromatolite features were dismissed as inorganic in the 1960s and 1970s, a couple of decades before the study of MISS took off. I would not be surprised if new study of the various formations of the Grand Canyon Supergroup, attuned to the possibility of MISS, were to uncover them in intervals previously regarded as barren of fossils.

References

Donaldson, J. A. 1967. Precambrian vermiform structures: a new interpretation. Canadian Journal of Earth Sciences 4:1273–1276.

Eriksson, P. G., H. Porada, S. Banerjee, E. Bopuougri, S. Sarkar, and A. J. Bumby. 2007. Mat-destruction features. Pages 76–105 in J. Schieber, P. K. Bose, P. G. Eriksson, S. Banerjee, S. Sarkar, O. Catuneanu, and W. Altermann, editors. Atlas of microbial mat Features preserved within the clastic rock record. Elsevier, Amsterdam.

McKee, E. D. 1932. Some fucoides from Grand Canyon. Grand Canyon Nature Notes 7(8):77–81.

Seilacher, A. 1999. Biomat-related lifestyles in the Precambrian. Palaios 14:86–93.

Stevenson, G. M. and S. S. Beus. 1982. Stratigraphy and depositional setting of the upper Precambrian Dox Formation in Grand Canyon. Geological Society of America Bulletin 93:163–173.

Tweet, J. S., and S. G. Lucas. 2021. Re-evaluation of Precambrian “fucoids,” microbially induced sedimentary structures in the Proterozoic Dox Formation, Grand Canyon National Park, Arizona. New Mexico Museum of Natural History and Science Bulletin 82:427–435.

While taking a walk

Like any good Minnesotan, I appreciate a quality agate, and although I don't do a great deal of agate hunting, I *have* learned how to spot their characteristic luster. (I've never spotted anything big, but it's a neat trick.) When road work began this summer near where I live, I made it a point to observe the disturbed ground when I would take a walk. The glaciers that brought agates into the Twin Cities didn't linger where I am, but you never know. Until a couple of weeks ago, all I saw were chips, including the remains of one that may have been about the diameter of a dime had it not been shattered into shards by being run over (I didn't know they would splinter like that). This time, though, I noticed a red pebble with a cherty sheen on the path. I could tell it hadn't simply gotten paint on it, so I scooped it up to clean it for a closer look. Although it has microcrystalline quartz and red coloration, and certainly looks like it might be a Lake Superior agate from a few feet away, that's not what it turned out to be.

I'm cheating because I haven't yet showed a photo of one of the shorter sides, but this is actually a pebble of Mary Ellen jasper, a stromatolitic jasper that comes from the Biwabik Iron Formation. It's quite a bit older than any of the other fossils that have featured on this blog, coming from a Paleoproterozoic formation about 1.88 billion years old (Jirsa et al. 2008). It gets its name from the Mary Ellen Mine in the Iron Range. How did it get to where I live? Although it could have been a pebble in the subsurface, transported from the Iron Range by glaciers, it may instead have gotten here in gravel being used for the road work. The gravel would have been sourced from a gravel pit, which brings us back to glaciers again because the state's gravel pits are often in glacial deposits. In fact, in some of them you can pick out deposits left by different glaciers moving from different areas. Glaciers that passed through the Superior region brought agates south. Glaciers that passed through the Dakotas scraped up Cretaceous sedimentary rocks. (Cobban and Merewether 1983 commented on a fragment of phragmocone of a Cretaceous ammonite, Prionocyclus novimexicanus, found in glacial drift at the Brickyard.) The glacier that collected this pebble would have gone through the Range before depositing it.

This side of the pebble shows the characteristic red whorls of the stromatolitic layers with dark streaks between them.

In this photo I've put water on the surface, in case that makes it easier to see.

References

Cobban, W. A., and E. A. Merewether. 1983. Stratigraphy and paleontology of mid-Cretaceous rocks in Minnesota and contiguous areas. U.S. Geological Survey, Washington, D.C. Professional Paper 1253.

Jirsa, M. A., J. D. Miller, Jr., and G. B. Morey. 2008. Geology of the Biwabik Iron Formation and Duluth Complex. Regulatory Toxicology and Pharmacology 52(supplement to 1):S5–S10.

The "fossils" of Pipestone National Monument

This is being posted on Halloween, which is fitting for a topic that lingers like a ghost in the literature. Even today, you can still find stray references to the fossils of Pipestone National Monument. "Occasional small trilobites - Lingula, Paradoxides" sounds promising, doesn't it? If you're reasonably familiar with the fossils of Minnesota, you may be wondering why you never heard of this before. There's a pretty good reason: these forgotten fossils are not trilobites or brachiopods, and odds are they are not fossils at all.

World of Stone

This is part of Devil's Parlor in the Minnesota side of Interstate Park, on the St. Croix River. I was at the park on Saturday for a park geology program, and following work I wandered through the Glacial Gardens section. Interstate Park, as its name suggests, has units in two states, in this case Minnesota and Wisconsin. The two parts are state parks, associated with but not part of St. Croix National Scenic Riverway (the Wisconsin side is also the western trailhead for the Ice Age National Scenic Trail, as well as one of the nine parts of the Ice Age National Scientific Reserve). Interstate Park is known for its glacial landforms, imprinted primarily on Precambrian bedrock.