Hopi Buttes volcanic field, Hopi Buttes Mining District, Navajo County, Arizona, USAi

Regional Level Types
Hopi Buttes volcanic field	- not defined -
Hopi Buttes Mining District	Mining District
Navajo County	County
Arizona	State
USA	Country

The Hopi Buttes dominate the landscape north of Holbrook, Arizona, commonly rising to heights of 180 meters above the surrounding countryside. The buttes are underlain by individual diatremes, or in some cases by a complex of diatremes. Some sediment-filled diatremes also crop out as inconspicuous low hills (some may also be buried beneath the alluvium). The diatremes erupted into the late Miocene-early Pliocene Hopi Lake. No region in the world is known to contain a greater density of diatremes than the Hopi Buttes, where more than 300 diatremes occur within about 2500 km2. The diatremes of the Hopi Buttes are somewhat unique; they, along with few others, most notably the Miocene diatremes of the Schwabian Alb, formed maars in which lacustrine sediments accumulated. These lacustrine sediments were the hosts for syngenetic uranium mineralization. The funnel-shaped vents are filled with limburgite tuff and tuff breccia, agglomerate, monchiquite dikes, necks, and flows, fine-grained clastic and carbonate rocks, and blocks of sedimentary rocks, especially the Wingate Sandstone, derived from the vent walls.

The erosional exposure of the deposits varies with those in the eastern portion exhibiting the shallowly eroded maar deposits and those in the western portion the more deeply eroded feeder diatremes. The maars result from explosive interaction of the hot diatreme material with the groundwater system and result in a mixture of volcanic tuff material and sediments of the Miocene–Pliocene lacustrine sediments of the Bidahochi Formation. In the western portion of the field the buttes consist of the feeder diatremes of monchiquite and nepheline syenite magmas.

Most of the volcanic activity occurred between 8.5 and 6 million years ago, with the most recent dated at 4.2 million years ago.

The late Miocene-Pliocene Hopi Buttes volcanic field contains at least 300 maar and diatreme features in an area of 1800 km2. Phreatomagmatic explosions involved the interactions of monchiquitic to nephelinitic magma with groundwater, lake water, or liquefied sediments from the underlying Bidahochi Formation. These maars produced craters within the underlying strata that then increased in size due to subsidence of unstable crater walls. The field relations between marginal deposits, maar crater deposits, and country rock both inside and outside of the crater link deformation of sediments with processes occurring during and due to an eruption.

At least 23 vents are exposed within approximately 51 km2 of the First Flat Mesa area in the north-central Hopi Buttes. Locally these vents provide well-preserved exposures of marginal deposits, maar crater deposits, and country rock. These units provide critical geologic constraints regarding the vertical and lateral facies changes that occur proximal to the vent. Ten volcanic facies, three limestone facies, and one marl facies have been described and provide spatial and temporal information about processes and interactions that occur within and adjacent to the vent during a phreatomagmatic eruption. A majority of these facies suggest magma interaction with water-saturated sediment which created explosive eruptions that produced juvenile lapilli, blocks and bombs, and clasts of country rock that were recycled within the vent or dispersed away from the vent by base surges or fallout. These consist of massive lapilli tuff to moderately bedded lapilli tuff that typically contains blocks and bombs up to 2-3 meters. Where vents occur within close proximity, saturated sediments filling one crater may break the wall between them and slump from one vent into the neighboring vent as it is erupting. Some eruptions began phreatomagmatically, used up the available water, and became magmatic, producing scoria deposits that filled in the craters. Crater-lake facies suggest that most of the material removed by the eruption was deposited outside of the crater, allowing water to fill the crater and deposit limestone. Crater-lake facies are not found in vents where scoria deposits are found, although occasionally lapilli and ash are found within the limestone, from a nearby vent eruption and depositing volcanic material into the lake. Facies descriptions and interpretations provided information used to produce an eruptive model for the vents within the study area.