I bet I'm not the only geologist who always wants to list "time machine" in the budget request for every grant proposal I write. Yes, we've got a lot of tools to sort out what's happened in the past, but wouldn't it be a lot easier if we could just go back and see for ourselves? So I love this month's theme for the reactivated Accretionary Wedge carnival.
I want to go back about 1.7 billion years, to see what on Earth was happening when my favorite local rock was being deposited.
This is the Vallecito Conglomerate. It's been metamorphosed, but its sedimentary features are still preserved. It's got big clasts, mostly of vein quartz and banded iron formation, and big trough cross-bedding. It's only found in a small but spectacular part of the Weminuche Wilderness, although there are similar rocks in other places from around the same time. And this one is at least a couple thousand feet thick.
There's been some recent research on it that piques my curiosity even more. Its deposition seems to have taken place right after a collisional episode known as the Yavapai Orogeny. (Its youngest detrital zircons are the same age as a series of granites that bracket the end of the oldest deformation in this area.) Then it was folded sometime between 1.7 and 1.4 billion years ago. One research group working in the area takes this as evidence for extension after the initial collision, and then another episode of shortening. (Imagine an accordion being pulled apart and then pushed together.) It's a reasonable explanation, though I've learned in other mountain belts that accordion models can oversimplify things. It's possible to get small, deep sedimentary basins in strike-slip settings, as well. I've been puzzling over how, exactly, to test a model like this - post-collisional extension should have other effects besides forming a sedimentary basin. (Unfortunately, everything's been heated again, several times, so some of the tools I would normally use won't work.)
But as much as the tectonics fascinates me, it's not the biggest puzzle in these rocks. 
The rocks lie on top of what was, at the time, new continental crust - metamorphosed arc volcanic rocks, intruded by granites. They presumably haven't traveled very far - most of the clasts are bigger than my fist. And yet the clasts are made up of minerals that survive incredible amounts of chemical weathering - quartz and hematite - and not much else. (Believe me, I've looked for good metamorphic minerals in them. Nothing, except at the top of the section, where the conglomerate grades into a metamorphosed quartz sandstone like in the photo on the left, and then, finally, a metamorphosed shale.) I had wondered where the iron formation came from - it isn't known in the local area, though apparently some exists elsewhere in the Southwest. But until I read a recent paper by Jamey Jones (who also did the detrital zircon work), I hadn't realized just how weird these rocks are.
They may be weird because they formed at a weird time in Earth's history. Banded iron formations are thought to be the geologic record of a major change in Earth's atmosphere, from air with a lot of methane and carbon dioxide to one containing free oxygen. (Read the Science News story for the full explanation, if you haven't heard it before - it's a good article.) The Vallecito Conglomerate is younger than most of the action - most BIFs (as we call them when we're feeling overly familiar) are older than 1.8 billion years. But maybe the atmospheric chemistry was strange 1.7 billion years ago, as well. Maybe there was something that speeded chemical weathering - carbonic acid, maybe, or something stronger. Jamey's possible explanations are intriguing, and I don't have any brilliant ideas about how to test them.
A time machine would be good.
Though maybe I would need acid-resistant field gear for the trip.
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Way cool! Next time I'm in the Durango area, I'd love to take a look. Sorry to ask you to do more work (I know how life with 5-yr old and busy job goes), but would it be possible for you post a google map with an outcrop location or two? Many thanks.
The zircon age tells you jack schmitt about the age of the BIF clasts. Since BIF, and chemical sediments in general, don't contain appreciable zircon, an older chemostratigraphic detrital source will be invisible in the zircon record.
If the heavy separates contained rutile, I'd use that to guess the vein quartz- they often form in similar metamorphic/ alteration environments. If you're lucky you can find exolved rutile in the qtz, but that generally has too little U to be useful.
And is "ca. 1680â1670 Ma" code for "We have an uncorrected 207/206 ratio, but properly dating xenotime is too hard"?
I'm not shelling out $25 to read it, but if you have specific methodological questions, quote and ask.
echo coconino - please post coords or roadside info so we amateur geologists can see this, too. many thanks
Noted and queued. As an aside, a metaconglomerate I've spent some time looking at on the north shore of Lake Huron is quite similar to what you're describing- mostly clasts of vein and other quartz, but occasional clasts of bright red chert and BIF. I was thinking it was Grenvillian in age (~1.1Ga), but I think it was deformed and cooked by that orogeney. So it's probably older.
Kim- what is the matrix of these conglomerates....Chert? I agree with the guess - that Jamey's paper makes - that these rocks appear sorted (in terms of chemical stability) due to diagenetic destruction of unstable matrix minerals. That could account for the apparent puzzle of a nearby source but a mineralogically sorted rock. I have seen similar early Proterozoic conglomerates in central India which have a complex diagenetic replacement history of the matrix, first by carbonate and later by chert.
I think your guess of higher carbonic acid is a good one, but it might have come into play in post depostional alteration.
Coconino & chas - I will try to put together another post with descriptions of locations to see cool stuff near these rocks sometime. In the meantime, the easiest place to see them is the Vallecito forest service campground. There are boulders of the conglomerate everywhere, and easily accessible outcrop at the trailhead to the Vallecito Creek trail. The trail goes through the conglomerate for quite a distance, and there are big, cross-bedded outcrops where the trail comes out into the open in the Vallecito gorge. It's beautiful, but remote - not a place to stop and ogle in the middle of a roadtrip.
LL - the paper was the first attempt to date this rock (and other similar rocks in the SW, except for ones with interbedded metarhyolites), so it raises a lot of questions. Given that relative dating limited the depositional age to "somewhere between 1.7 and 1.4 Ga," it's a great contribution! And it didn't try to address the source of the pebbles. In fact, I think that the question of "why is there so much quartz & BIF in these rocks, when the zircons appear to be mostly local sources?" may have come up at the end. (And I'm the one who is especially intrigued by the BIF, even though it creates lousy mineral assemblages to do metamorphic work.)
I wonder how far those pebbles could have been transported? Colorado's continental crust was juvenile at 1.8 Ga-ish, and the nearest Archean crust is in the Wyoming craton, on the other end of the state.
Lockwood - I believe the Baraboo Quartzite is about the same age. Don't know about rocks north of Lake Huron. (And sorry to have forgotten to comment on the call for posts with the url! I meant to do that!)
Suvrat - the matrix is quartz. Hard to say if it was chert originally - where I've looked at thin sections, the rock's been metamorphosed to andalusite grade. I know there's a paper about the Mazatzal quartzite (in Arizona, and one of the probably related rocks) that argues that it was altered during diagenesis. I'm curious whether younger conglomerates experience similar alteration - I haven't worked in metaconglomerates before, so I don't have experience knowing what survives. I think part of the puzzle is that the rocks appear to be first-cycle sediments and are coarse enough to have been transported a relatively short distance, but are chemically mature.
Kim: Thanks for the info. That's a nice trail, though I've only been there when there was actually snow and my young'un was still in diapers but refused carrying, so rocks were not really the focus of the day (or even a peripheral view, often). I look forward to going back when I get up that way for work. Thanks again.
No work subscription, sorry.
While rare, BIF's and cherts (many related to VMS deposits) can be found in the Yavapai rocks from Arizona to Colorado. There are a few very nice exposures of BIF near Phoenix and plenty of meta-chert north to the copper mine at Jerome, especially in the Mayer area, where they have been isoclinally folded into some really nice rootless folds. In Colorado, there are many small occurrences of meta-cherts related to VMS, but they don't seem as prevalent as their Arizona counterparts. I'm scratching my head to remember an in-situ BIF in Colorado.
The Vallecito Conglomerate is a great puzzle and I look forward to seeing the results of your study.