Diatremes and Uranium Deposits in the Hopi Buttes, Arizona*
Published:January 01, 1962
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E. M . Shoemaker, C. H. Roach, F. M. Byers, Jr., 1962. "Diatremes and Uranium Deposits in the Hopi Buttes, Arizona", Petrologic Studies, A. E. J. Engel, Harold L. James, B. F. Leonard
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The Hopi Buttes diatremes erupted in a shallow lake that was filled in as volcanic activity progressed in Pliocene time. At the end of the period of volcanism the landscape included low lava domes capping some of the diatremes, a few flows, and numerous craters of the maar type, surrounded by rims of volcanic debris with gentle exterior slopes that merged with the surrounding plain. Erosion, probably chiefly during early and middle Pleistocene, has exposed the diatremes in varying stages of denudation.
The diatremes are funnel-shaped volcanic vents that range from a few hundred feet to about 2 miles across where they cut the top of the lake beds. They are filled with limburgite tuff and tuff breccia, fine-grained clastic and carbonate rocks of nonvolcanic origin, agglomerate, intrusive and extrusive monchiquite, and blocks of sedimentary rocks derived from the vent walls. In most of the vents several hundred to a few thousand feet of lacustrine and fluvial crater deposits were laid down after an initial explosive phase of volcanic activity. The structural and sedimentary evolution of many of the diatremes culminated in the quiet up welling of lava.
The initial opening of the diatremes is at tributed to the rapidunm ixing of gasfrom magma ascending through the crust along many separate fissures. Fractures were propagated to the surface hydraulically, and decompression waves were then propagated in to the ascending magma and fissure walls as the gas was drained away. Enlargement of the channels of gas flow is attributed chiefly to spalling. The funnel-shaped orifices of the diatremes, which formed the craters at the surface, were in many cases later enlarged by slumping and collapse of the crater walls during subsidence of the materials filling the vent. The most important causes of the subsidence are probably stoping and assimilation of the porous vent debris by the underlying magma and displacement of the magma.
Numerous low-grade deposits of uranium occur in carbonate rocks within the diatremes, and small deposits of higher grade occur locally in the fine-grained nonvolcanic clastic rocks, in the tuffs, and in fragments of sedimentary rocks derived from the walls of the vents and in the vent walls. Widespread radioactivity in the carbonate rocks suggests that the uranium may be at least in part syngenetic, but the highest concentrations of uranium, in the clastic rocks, are related to structures that would have influenced the flow of solutions and are probably epigenetic.
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The 24 papers in this volume, written in honor of A.F. Buddington, cover a wide range of topics and geographic areas. H.H. Hesss History of Ocean Basins perhaps the most famous paper in the volume, introduces the concept of seafloor spreading.