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.