The Evolution of Guffey Butte Tuff Cone Complex, Western Snake River Plain, Idaho

Guffey Butte is a basaltic maar and tuff cone complex that was formed about one million years ago near the southwestern margin of the western Snake River Plain. Its evolution was characterized by two distinctly similar, yet non-synchronous, sequences of events. The result of these events is a single complex volcanic structure comprised of deposits from two separate vent systems. The stratigraphy in both parts of the volcano reflect one or more drying out cycles in which the amount of external water that interacted with the magma was gradually reduced. The two vent complexes differ from one another in the chemical compositions of their magmas, the nature of their pyroclastic deposits, and the overall morphology of their cones and craters. The earlier of the Guffey Butte vent systems (GBV1) produced a maar-tuff ring characterized by thick deposits of planar and dune bedded pyroclastic surge deposits. Juvenile ash in GBV1 deposits is commonly vesicular (Type 2 ash shards of Woheltz, 1983). GBV1 basalts are slightly more mafic than those in the younger complex; however, they also appear to have assimilated slightly greater amounts of shallow crustal rocks. The average composition of plagioclase phenocrysts in GBV1 basalts is about AnS7. The younger Guffey Butte vent system (GBV2) produced a tuff cone composed largely of massive, unbedded tuffs containing at least two units of welded spatter. Juvenile ash in GBV2 deposits consists mainly of blocky glass shards (Type 1 shards of Woheltz, 1983). GBV2 basalts are slightly more differentiated than those of GBV1 and plagioclase phenocrysts have average compositions of An47. The relative ages to the two vent systems were determined in part by the fact that GBV2 pyroclastic deposits contain accidental fragments having the ash morphology characteristics and chemical and mineralogical compositions of GBV1 basalts. In contrast, no material typical of GBV2 was found in the deposits produced by the GBV1 vent. The two vent systems at Guffey Butte may have produced different types of structures and deposits because the physical condition of the stratigraphy through which they erupted was different. The pre-Guffey Butte stratigraphy consists of fluvial-lacustrine sediments and a few basaltic lava flows, at least one of which is up to 33 m thick. The GBV1 magma may have become stalled beneath one of these flows, resulting in a buildup of pressure and a large highly energetic explosion. The maar-tuff cone morphology, dominance of surge deposits, vesicular shards, and crustal assimilation is consistent with this scenario. The GBV2 magmas encountered no significant barrier so they erupted less explosively, producing a tuff cone. The Type 1 shards and lower level of contamination associated with these basalts suggests a more rapid rise and greater access to ground water. The evolution of Guffey Butte therefore supports models for hydrovolcanism that emphasize the nature of the stratigraphy through which magmas rise as well as the relative mixing proportions of water and magma