Episode 373. A walk to Branham Lakes

Upper Branham LakeToday’s episode will be a little different from what you are used to. I’m going to try to give some of the story of the Precambrian here in southwestern Montana, but I’ll do it in the context of a little hike I did yesterday to the Branham Lakes in the Tobacco Root Mountains. So there will be some of the usual narration, but also some snippets that I recorded while I was on the walk, which are not included in the script below. You can expect some huffing and puffing. See also this blog post by Pat Munday. probably hypersthene (Mg Fe silicate)When I was learning the geology of this region back in 1969, the Precambrian rocks of the Tobacco Root Mountains were considered to be Archean, older than 2.5 billion years. They were (and are) the northwestern-most corner of the Wyoming Craton, one of the ancient, fundamental building blocks of North America that we talked about last year. And the Wyoming Craton is definitely Archean in age. At least most of it is. More recent analyses of age dates in southwestern Montana gave rise to another interpretation, by Tekla Harms and her colleagues a few years ago, that the zone through the Tobacco Roots, Highland Mountains south of Butte, and Ruby Range east of Dillon, Montana, represents the old margin of the craton, where a pile of sedimentary rocks formed – possibly during Archean time, but if it was then, it wasn’t long before the 2.5-billion-year cutoff date for the Archean. The sediments might have been early Proterozoic, called Paleoproterozoic. In any case, Harms and colleagues interpret age dates in some of these rocks at about 1.75 to 1.9 billion years to represent the collision between the northwestern corner of the Wyoming Province and another terrane, now mostly in the subsurface of central Montana. There isn’t much doubt that such a collision happened, but there remain questions as to whether the Precambrian metamorphic rocks of southwestern Montana were already there, Archean, or if they were sedimentary rocks that got caught up in that collision and metamorphosed a few hundred million years after they were deposited. Geologic Map of part of the Tobacco Root Mountains. Reds and oranges are igneous rocks of the Tobacco Root Batholith, about 75 million years old. Grays are Precambrian rocks, about 1700 to 2500 million years old. Both maps from Vuke et al., 2014, Geologic Map of the Bozeman quad, Montana Bureau of Mines and Geology Open-file map 648. Black box in lower left corner is enlarged below. Oranges (Khto) are Tobacco Root Batholith, grays are Precambrian. X=Paleoproterozoic, about 1.7 to 1.9 billion years old; A = Archean, over 2.5 billion. XA means we aren't really sure. qfg = quartzofeldspathic gneiss, ah = amphibolite and hornblende gneiss. Xsp = Spuhler Peak formation. Branham lakes are blue. There isn’t much doubt that the metamorphic rocks there were originally mostly sedimentary rocks, sandstones, shales, siltstones, maybe even a few limestones, and that they were intruded by some igneous rocks like basalt, all before they were metamorphosed. We can infer what these protoliths, the original rocks, were, from the chemistry and mineralogy of the rocks today. So it’s a question that doesn’t matter too much, although it has big implications for the detailed story of this part of the world – when were sediments laid down, when were they metamorphosed. That in turn has implications for the structural and tectonic history, and understanding THAT helps us explore for mineral resources and understand things like earthquake fault distributions. I’m not going to solve the question by walking up to the Branham Lakes. This beautiful location is about 9 miles or so up Mill Creek, east from Sheridan, Montana. Most of the major valleys on the flanks of the mountains of southwest Montana held glaciers during the most recent glacial period that ended about 12,000 years ago or so. Kyanite, Aluminum SilicateSediments like silts and muds usually contain plent