CLARNO IGNIMBRITES          

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            Many welded tuff units (ignimbrites) are present within the Clarno Formation and one of widespread distribution occurs near Mitchell. An identifiable and traceable ignimbrite is significant because a single unit of this kind provides a key to one instant of geologic time wherever it can be found throughout the formation.  One such unit near Mitchell is here referred to as the Snowshoe Point Welded Tuff because I first encountered it at that locality in the southwest part of the Lawson Mountain quadrangle.  It had been included in the John Day Formation by Swanson (1969) but is clearly part of the Clarno Formation and has been Ar-Ar dated at 49 Ma.  I have traced it for 14.6 miles in a SW–NE direction across four 7.5 – minute quadrangles (see accompanying map).  

 ignimb map

 It is exposed in 21 isolated outcrops where it rests chiefly upon bedded tuffs but also upon mudflow deposits and andesite lavas.  At nearly all of these locations it is inclined to the west with other stratigraphic units. It might be traceable south and south-westward in the Ochoco highlands but exposures there are very poor. In some outcrops it is composed of two distinct cooling units, separated by only a few feet.

      

       This welded tuff was first described by H. E. Enlows (Oles and Enlows, 1971) from the best exposed locality west of Mitchell as a yellowish orange to a pale-gray rhyolite with sanidine phenocrysts, minor rock fragments, and without quartz or hornblende.  Pumice fragments have been flattened to devitrified glass.

 ignimbrite pumice

 

       One outcrop of this ignimbrite near Lawson Mountain, includes glass nodules formed around fragments of charcoal, preserving the visible wood grains for almost 50 million years. The wood was clearly not petrified; perhaps we should call it encinerated?

 

       I have found only one other Clarno ignimbrite in the Mitchell area. It is located in the NE Toney Butte quadrangle as a single outcrop (Tcaf) which is exposed near a broad field of landslide debris, probably obscuring other potential locations.

 

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