MAAR MARGINS IN THE HOPI BUTTES VOLCANIC FIELD, NAVAJO NATION, ARIZONA: FIELD MAPPING YIELDS INTERPRETATION OF MAAR VOLCANO PROXIMAL FACIES

The late Miocene-Pliocene Hopi Buttes volcanic field contains >300 maar and diatreme features in an area of 1800 km². Phreatomagmatic explosions involved the interactions of monchiquitic/nephelinitic magma with groundwater, lake water, or liquefied sediments from the underlying Bidahochi Fm. 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. Eight facies form associations that 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 m. Where vents occur within close proximity, saturated sediments 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 lapilli and ash are found within the limestone, likely from a nearby vent erupting and depositing volcanic material into the lake.