Andesitic tuff-breccia showing unusual structural relationships is exposed throughout at least 100 square miles within two river valleys in the Absaroka Range of northwest Wyoming, a dissected mass of essentially horizontal volcanic deposits. The tuff-breccia occurs as sheetlike bodies within an otherwise dominantly detrital volcanic sequence. At least one, and perhaps all, of these sheets merges with thick masses of pyroclastic material. They display both intrusive and apparently gradational relationships to the detrital beds.

The tuff-breccia has the composition of pyroxene andesite but contains much foreign material, largely blocks of dacitic detrital volcanic rock, some of which are probably several miles from their source. Pronounced hardening (silicification ?) and greenish-gray coloration characterize dacitic detrital rock adjacent to the tuff-breccia.

Most of the tuff-breccia was probably erupted from a northwest-trending series of large vents. The structural relationships can be best explained by post-depositional movement of several sheets of tuff-breccia originally deposited at the surface and buried by detrital sediments. The tuff-breccia seems to have been injected as mud at relatively shallow depth into the detrital sequence, which was poorly consolidated.

How this post-depositional emplacement took place is not determined. Neither primary injection nor tectonic activity alone seems adequate to explain the unusual structural relationships; more probably, gravity was the main control, possibly aided by local doming.

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