In situ formation of welded tuff-like textures in the carapace of a voluminous silicic lava flow,Owyhee County,SW Idaho |
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| Authors: | C R Manley |
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| Institution: | (1) Department of Geology, Box 871404, Arizona State University, Tempe, Arizona 85287–1404, USA, US |
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| Abstract: | The Badlands rhyolite, on the Owyhee Plateau of southwestern Idaho, can be demonstrated to be a large lava flow on the basis
of its geometry of large and small flow lobes, its well-exposed near-vent features, and its response to pre-existing topography.
However, samples of the dense upper vitrophyre of the unit reveal a range of annealed fragmental textures, including material which closely resembles the
compressed, welded glass shards which are characteristic of ignimbrites. Formation of these tuff-like textures involved processes
probably common to emplacement of most silicic lava flow units. Decompression upon extrusion causes inflation of pumice at
the surface of the lava flow; some of this pumice is subsequently comminuted, producing loose bubble-wall shards, bits of
pumice, chips of dense glass, and fragments of phenocrysts. This debris sifts down around loose blocks and into open fractures
deeper in the flow, where it can be reheated, compressed, and annealed to varying degrees. The end result is a dense vitrophyre
layer (beneath the true upper, non-welded carapace breccia) which can be extremely texturally heterogeneous, with areas of
flow-foliated lava occurring very near lava which in many aspects looks like welded ignimbrite, complete with flattened pumices.
Identical textures in other silicic units have been cited by previous workers as evidence that those units erupted as pyroclastic
flows which then underwent sufficient rheomorphism to create a flow-foliated rock which otherwise appears to be lava. The
textures described herein indicate that lava flows can come to mimic rheomorphic ignimbrites, at least at scales ranging from
thin sections to outcrops. Voluminous silicic units with scattered fragmental textures, but with otherwise lava-like features,
are probably true effusive lava flows.
Received: January 30, 1995 / Accepted: January 22, 1996 |
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| Keywords: | Silicic lava flows Rhyolite Rheomorphic ignimbrites Textures Welded shards Idaho Owyhee Plateau |
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