Monday, September 29, 2014

Geo 730: September 29, Day 637: Unexpected Contact

I had noticed this cobble conglomerate before, but I had not, before this trip, consciously thought about its implications with respect to the overlying contact with the basalt breccia. (Lens cap right of center, diameter is 52 mm)

Why do geologists spend an inordinate amount of time fussing over contacts? From outside, it might appear as if we're simply obsessed with rocks- and to an extent, there some truth to that. But it's not just the rocks themselves, it's what they tell us. First, and foremost, we want to winkle out little tidbits of earth's history; we want to know how the planet has changed and evolved through time. Rocks tell us a great deal about what the conditions were like when they formed. Contacts tell us about how conditions changed. Change is really at the heart of what we're interested about. So fussing over contacts is central to what we do.

Here, we see a contact between a cobbly basalt conglomerate- representative of an environment very much like the cobbly beach behind me- and an overlying basaltic breccia. The latter rock probably represents the front of an advancing flow into the ocean, so likely a combination of flow and phreatic (referring to steam explosions) breccias.

Now why is this of interest to me? I would be unsurprised to see cobbles on top of basaltic breccia- indeed, that's mostly what you see along this beach. However, I didn't expect to see basalt cobbles underneath breccia. That implies that there was already basalt exposed nearby, which in turn suggests that Yaquina Head may consist of two or more flows of Columbia River Basalts, separated by enough time to erode the earlier one, and round those clasts. This wouldn't necessarily take very long, maybe a decade to a century, but it does seem unlikely to have originated from a single eruptive episode.

Contacts are fun, especially when they take you by surprise!

Panorama stitched in HugIn. July 15, 2014. FlashEarth Location.

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