Sunday, August 17, 2014

Geo 730: August 15, Day 592: Plants, Old and New

The new plants are pretty obvious, though browning in the early August heat. The old "plant" is not so obvious, but easy to spot once you know what to look for. See the black splotch in the bedrock, occupying the central portion of the photo? That's a large block of what seems to be permineralized charcoal. About a month ago, I posted a microscopic view of this material. It's weird stuff. The carbon in the charcoal is preserved, unmodified, but all the pore space has been filled with quartz. As a result, the charcoal exposed at the surface will smudge your fingers, but the rock as a whole is hard enough to scratch steel.

Unfortunately, I only have two photos of this terrific outcrop. I think there are two major issues: I've done this trip close to fifty times, so it's all deeply familiar to me. I simply don't think about the necessity of having illustrations for people who *haven't* been here. Also, this is a long, tiring day, and often when I'm tired, I'm just operating on autopilot... I'm not even thinking abut thinking. At any rate, I'll just describe it verbally. At the base of the exposure is lahar deposits, based on the angular nature of the clasts, their volcaniclastic composition, and their extremely poor sorting. There may be a second lahar over the oldest exposed, but the outcrop is weathered enough now that I can't see the possible contact I thought I had identified many years ago. Over that is a clearly water-transported and sorted sandstone, which is a ledge-forming bed about a foot thick. Rarely, you can find permineralized, horizontally oriented, woody fossils, which I take to be roots. Above the sandstone is a thin layer of siltstone, which, if and when you can find it, has abundant plant fossils: leaves that look like alder, some sort of coniferous needles, sticks and twigs, and (very) rarely, seeds, cones and catkins. On top of the silty layer is a ruddy layer that I think is a paleosol, a few inches thick. Above all that, finally, is the rubbly layer containing the charcoal. The cobbles are poorly sorted, but seem better rounded. I'm not sure how to interpret the rock, but I'm suspicious I'm seeing sphereoidal weathering, not transport-related rounding. The blocks of charcoal look as if they're entire tree trunks, and their size- up to several feet in diameter- is consistent with that. For more photos, see Dana's posts, a geologic riddle, and with a better overview of the outcrop, the follow-up answer.

Photo unaltered. August 5, 2012. FlashEarth Location.

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