I’m a Geologist, but my emphasis is sedimentology and stratigraphy, not petrography and mineralogy. Plus I research rocks of the western US now, which are in general much younger than the Michigan area – but I grew up in MI, so I will do my best, and there are few things I can say that might help. Like most questions asked of Geologists you are going to get a far more boring and wordy response than you signed up for... First off, the color red, red = oxidized iron almost 100% of the time - just think about rust on old cars. You take iron and oxygen and your get shades of dark red rust, add water which acts as a weak acid, and you speed up the process, add salt and it goes even faster. So parts of the red/brown clasts contain iron bearing minerals. When I look at this I see 2 different major compositions. 1) The angular (very angular) dark red to brown clasts, probably shale to slate or mudstone, to maybe sandstone if the grains are large enough and 2) Off-white to light yellow matrix. The white matrix looks to be quartz which is a common form of silica – common in almost every rock. The other option is calcite, but calcite weathers and is usually softer, so soft that large rocks with a pure calcite matrix are far less common than quartz. Without scratching the rock, or the ability to drop dilute amounts or hydrochloric acid on it, I cannot tell if its quartz or calcite, but quarts seems the most reasonable. The most interesting part of this rock are the dark red angular clasts. The rock itself is rounded, probably been tumbling around in the lake (or river) for longer than most people can conceive and transported some distance from where it was found, but the inside is not rounded. This tells me it formed and was cemented into a rock very fast (in geologic time not human time) – there was little time for those red clasts to get exposed and rounded. Later it was broken up again and transported as a new rock made up of even older angular clasts. Our general classification for this type of rock is called a “breccia” – Google it, you’ll get the full definition and more images. If the clasts inside were rounded, instead of angular, we’d call it a “conglomerate” – both are types of sedimentary rocks. Breccia’s can form is a multitude of geologic conditions and nothing is really diagnostic, but in general they usually represent a high-energy, or tectonic event – something happened to deviate from the normal day-to-day. The clasts are not aligned – some a parallel, some are perpendicular to each other, which add to this interpretation. This is why it appears almost shattered inside. If I had to guess, I think this is a "fault breccia" – again Google this and look at the images. The age of the rock and what geologic formation it came from is the hardest to nail down. Knowing that it comes from northern Michigan helps a little. Michigan is in the middle of the Michigan Basin, a very big Paleozoic sedimentary basin with old rocks, again Google “Michigan Basin” – think of big giant bowl over 100’s of mines. The middle of Michigan is the high point (center) where the youngest rocks are Jurassic in age (200-145 million years old). As you move N-S-E or W you are getting older and older in age. The northern shore of Superior is Silurian (420-440 million years old) and eventually as you go north into Canada you hit the Precambrian basement or shield terrains which are a couple of billion years old. Since this is a sedimentary rock and contains clasts of older sedimentary rocks, the best you can do to bracket the age as being no older than Cambrian (550 million years old) and even this is poor because it was transported via water, so it could have come from hundreds of miles away. I have no idea if this is a common local rock and if it has a local name – like the Petoskey stone as an example. Petoskey stones are old limestone chunks that are made up of rugose corals, which have been rounded and weathered on the shorelines, they are Devonian in age (420-360 million years ago). The name is just a local name, not a geologic name. We’d call it something lengthy and confusing and far less interesting than just a “Petoskey stone”. I hope this helps, I have no real answers and I probably just created more questions, which is all we really do as Geologists - bore you to death on what it's not, but never really tell you what it is.
ha ha, yeah well I know a lot of useless science. I'm in Denver, been here ever since Missoula. Its a great town, but getting a little too hipster, over-priced crazy. If another micro brewery opens I may go postal, but we love it none the less...
YES! Another Denver-ite! Welcome to Hubski! There are a few of us mile-high folk on hubski. We keep threatening to meet up, but weather has foiled us a few times. But soon enough it will happen. I'll keep you in the loop if you're interested. Missoula.... did you study at (the other) U of M?
Tofer, your replies to mk here are fantastic, thank you for coming by the site. Don't be a stranger. Also, I don't want you to confuse Hubski user "steve" with me, "thenewgreen" -the guy you went to College with in Missoula and consistently spent hungover days eating Pizza Pipeline with. That said, Hubski user steve is good people and you really ought to join a Hubski meetup. Interesting people. Hope all is well out in Denver. Don't be a stranger
ha yeah, at first I thought it was you, then he asked a question and it was obviously not you. I hope all is well. Denver is good, all is well, in the middle of major house renovations which are stressful, but nothing to complain about.
Denver is a great city, it's been a few years since I've been there but my wife and I used to go every year -good food town. We live in Chapel Hill these days, if you ever find yourself in NC, let me know. Good luck with the renovations!
Thanks so much for that. It makes it that much more interesting to me to know something about it. I also learned that there is a Keweenaw Fault while I was Googling, not that it was involved. I don't think it's sandstone, as there aren't noticeable grains. It was found among sandstone, but it's much harder. Is there a way that I could tell if it were shale or mudstone? What would be the effect? I have HCl available. I spend a bit of time picking through the rocks on that shore every year, and I have never found one like it. I always keep an eye out. I've found a few agates, but never another one of these. I have found Petoskey's before in the LP.1) The angular (very angular) dark red to brown clasts, probably shale to slate or mudstone, to maybe sandstone if the grains are large enough
Off-white to light yellow matrix. The white matrix looks to be quartz which is a common form of silica – common in almost every rock. The other option is calcite, but calcite weathers and is usually softer, so soft that large rocks with a pure calcite matrix are far less common than quartz. Without scratching the rock, or the ability to drop dilute amounts or hydrochloric acid on it, I cannot tell if its quartz or calcite, but quarts seems the most reasonable.
I have no idea if this is a common local rock and if it has a local name – like the Petoskey stone as an example.
1) Sandstone and shale or mudstone often times have the similar composition - being dominantly quartz grains, but sandstone grains you can usually see, plus they are usually more beige to tan, orange or pink, not dark red/brown like this - think of Florida beach sand, that's clean sand waiting to be sandstone in a few million years. Shale/Mudstone is very very fine-grained quartz and other grains, mostly clay, that you cannot always see with the naked eye. The fact that you cannot see any little grains, leans me towards shale/mudstone or slate - which it also could be and is common in these age of rocks in the area. Slate is nothing more than a mudstone/shale that has gone through low-grade metamorphism. All this means is temperature and or pressure, beyond the normal burial and geothermal gradient has altered it by removing even more water and aligning the mineral grains. It becomes very hard and any grains are hard to see - pool tables have a slate underneath the felt. 2) Calite (Ca) is composed of calcium and calcium does not like to be alone, it always wants to be paired with something and its usually carbonate (CO3) in marine sedimentary environments. Together they form calcium carbonate CaCO3 and this is very basic - it's chalk, or limestone - the main ingredient in Tums or any antacid. Hydrocloric acid, is an acid obviously and when you combine an acid and a base, you get a reaction. If you were to drop 10% HCl on this rock and it began to fizz and stink a little, that is 100% diagnostic that it is calcium carbonate, the gas being released in the fizzing is carbon dioxide. Quartz is silica, made of silicon and silicon is very, very stable, it is extremely happy with oxygen as SiO2, it has no reason to do anything. This is why it is so common in rocks. It's extremely abundant and very stable. The chemical reaction is like the volcano that kids make in science classes with vinegar + baking soda, vinegar is acetic acid and baking soda is sodium bicarbonate, identical reaction from similar acid/base reaction. Sadly vinegar and calcium carbonate do not create the same reaction. It is a much slower reaction that takes days. You could take the rock and drop it a bowl of vinegar and in a few days it will dissolve if it's calcium carbonate, but then you'd be destroying it and that's not the point, a drop of HCl is harmless.
Seeing as it is quartz, then this is most likely a fault, or hydrothermal breccia. Quartz is extremely common so there are a few explanations, but here’s one that's reasonable given location. Quartz is soluble in water at high temperatures, like other elements and minerals, and water moves through everything in nature – even rocks. Think of table salt, when you boil water and add salt, the salt crystals dissolved faster in hot water. Upper Michigan has mining – copper and other metals – all related to the increased heat flow, therefore heated water from the Keweenawan Rift system, so that probably supplied the heat flow, which expelled hot water that allowed the mobilization of the quartz. As water moves over and over again over millions of years it will eventually cool or become super-saturated, which then precipitates the quartz, which then fills in cracks and broken fissures, encasing whatever rocks or fragments of rocks that are near – sometimes called veins. All this happened well below the surface, later it was exposed, eroded into a big chunk that got transported via water, broken up, rounded and smoothed along the way into this rock.