Saturday, November 9, 2013

Geo 365: Nov. 9, Day 313: Greywacke

A boulder near the parking area at Cape Blanco. I guess the things that jump out most strongly to me are the veins and patches of secondary calcite, simply because they're so high-contrast in comparison to the rest of the rock
Looking at a crop of the above photo, the nature of the rock itself is more apparent. It is (according to the nomenclature I learned, which may now be dated) a lithic greywacke- poorly sorted and texturally immature: the clasts are mostly angular to poorly rounded. Despite my statement a few days ago that this is Cretaceous in age, it is in fact Jurassic Otter Point Formation, according to this (2 MB) PDF of an Ore Bin issue from 1975 (See near the top of the second page of text). "Melanges" were, I suspect, only beginning to be comprehended at the time of that publication, but by the time I was finishing up my first year of geology in 1981, they were tentatively explained to us younglings as likely representative of old, exhumed, eroded and exposed subduction zones. That idea seems to have been thoroughly accepted. Indeed, if you look at the sorts of lithologies that can be found in the Otter Point Formation, nothing else comes close to making sense. Tagged with "Accretionry Wedge," because it amuses me... this may be the first time I've used the term in its literal sense,as opposed to the Geoblogosphere's carnival.

Photo unmodified. May 7, 2013. FlashEarth location.

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