Wednesday, January 29, 2014

Geo 730: Jan. 29, Day 394: What's On The Slate

A final shot of the Galice Formation slate along Route 199, just southwest of the major tunnel on that route.

I've spent this month so far on the Josephine Ophiolite, describing the typical sequence of rocks exposed in such environments. From the bottom, these include a cumulate ultramafic peridotite of mantle affinity, a less mafic cumulate gabbro representing the base of oceanic crust, several kilometers of sheeted dikes, consisting of dikes intruding dikes intruding dikes, to the point where there is no longer any "original" host rock left, simply younger dikes intruding older dikes. Over those lie a few hundred meters of pillow basalt and other volcanic rock, which in turn is typically buried by a variable thickness of sediment of one form or another. It's my current understanding that the overlying sediment (metamorphosed in the case of the rocks above) are not considered to be a part of the "ophiolite" proper- that is, only the igneous components are considered as part of the ophiolite. That being said, the sedimentary cover is an inevitable follow-up to that sequence: I see no way that it could be avoided. I'm not saying that it "should" be considered a part of the ophiolite, just that it seems to me as if such sediment will always follow the creation of a new segment of oceanic crust.

While the sequence as presented so far is disjointed, that's unavoidable. The Josephine is cut by several faults with major vertical offset, so the traveler hops and skips up and down the sequence as they move in an east-west direction. The only part of the above sequence that we didn't stop at was one or more representing the pillow basalt/sea floor volcanics. We did see highly altered peridotite at Patrick Creek, but we've not yet seen a good example of relatively fresh ultramafics. Starting tomorrow, I'm going to skip the rest of March 8, and zip forward to later in the day of March 9, when we looked at some really nice remnants of fresh peridotite at an oddball nickle mine, farther north in the Josephine.

Photo unmodified. May 8, 2013. FlashEarth Location. Indexed.

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