Geo 730: Jan. 24, Day 389: Galice Slate

So far this year, I've been focused heavily on the igneous component of the Josephine Ophiolite. I'm pretty sure I read recently that the term "ophiolite" has been restricted to strictly the igneous sequence, with which I have no problem. However, as soon as the fresh sea floor is exposed- typically pillow basalt or breccia- it's subject to sedimentation. The sources of such sediment are numerous: near the ridge itself, metallic sulfides from black smokers can be significant. Accumulation of siliceous tests, for example from radiolaria and diatoms, in the absence of terrigenous sediments, can accumulate surprising thicknesses.

Image Credit: College of The Redwoods

In the case of the Josephine Ophiolite though, as a back-arc basin, it started receiving terrigenous (land-erosion-derived/terrestrial) sediment very soon after the sea floor was formed. In the above schematic, this sediment is labeled "pelagic sequence," and is named the Galice Formation. During deformation and compression, presumably associated with the accretion of the western Jurassic belt to North America, the Galice was metamorphosed to slate. This outcrop is at a little park just south of the major tunnel on 199.

BTW, the issue with the index I mentioned yesterday has been fixed, and based on several test clicks, last year's links are now working correctly.

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