CLARNO FORMATION VOLCANIC MUDFLOWS

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       In some parts of the Clarno Formation mudflows are even more abundant than lavas, probably because great quantities of volcanic ash fell upon steep volcanic surfaces under subtropical rainfall.  Rivers of mud carried fragments of lava flows, limbs and tree trunks, and even large bodies of solid rock that had fallen from canyon walls.

 coarse mudflow

 Some mudflows even transported house-sized bodies of underlying Cretaceous conglomerate, supported by boulders of lava the size of automobiles.

 very large clastboulder in mudflow

 On the flanks of Keyes Mountain, east of Mitchell, successive mudflows accumulated to a total thickness in excess of 1500 feet.

       In tracing a given mudflow unit from source area to distal positions, the proportion of fine ash changes from support of the large fragments,

to a dominant constituent, and is finally reduced to a massive tuff containing only a few fragments of broken lava.

 unusually large clastdominant matrixfine grained mf

  The distal tuffs are sometimes weathered to such an extent that spheroidal alteration provides the false appearance of large rounded clasts:  

 altered mf

       Exposed cross-sections of some mudflows present an appearance of graded bedding which is more likely to have been produced by successive, superimposed mudflows.

 mf not graded

       As mudflows advanced over marshes or grasslands, large fragments were often left behind and thin ash beds were deposited. Conditions changed so often that mudflow deposits usually contain a mixture of coarse and fine layers.

 layered mf

  Small plant stems such as equisetum and ferns were frequently bent over and preserved in the ash as evidence of flow direction.   Even large trunks and limbs of the jungle trees were incorporated and eventually became petrified when silica from alteration of the ash replaced organic tissues.  As the ash became increasingly altered, secondary crystals of zeolite minerals (especially stilbite) and calcium carbonates (chiefly calcite) were produced in sufficient size and quality to be prized by collectors.

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