Monday, November 25, 2013

Geo 365: Nov. 25, Day 329: Not Geopetal

I've mentioned or alluded to "geopetal" structures a number of times in this series (2/14, 2/15, 2/16, 2/28, 3/1, 3/2, 3/8), structures or features in a rock that provide an indication of which direction was stratigraphic "up." In the context of sedimentary rocks, this tells you which beds are younger and which are older. It may not seem like a big deal, but it's the difference between telling the story accurately or backwards, and is of enormous importance. At first glance, this might appear to be an example. It would be easy to assume this represents a suddenly decreasing flow regime, from high-energy conglomerate to lower-energy sandstone. The problem is that, at least at a cursory look, one can't tell with any confidence that it wasn't the opposite situation, representing a sudden increase in energy. In the end, this is an appealing cobble, but it doesn't tell me a whole lot more than "there was an abrupt change in energy in this depositional environment." Of course, that's just looking at texture. Had I looked more carefully at the clast compositions, I could say much more about their source. But time, which the earth has in nearly limitless amounts, is limited for us poor humans, and this day was rushing by much too quickly.

Photo unmodified. May 7, 2013. FlashEarth location.

1 comment:

mickle said...

Enjoyable post and nice article