Hopi Buttes, Arizona
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In northeastern Arizona, USA, largely within the Navajo Nation, the roots, vent structures, sedimentary crater infillings, and ejecta rims of an extensive maar-diatreme volcanic field are magnificently exposed at various depths of erosion in the Hopi Buttes volcanic field. The field is one of few in the world in which an entire suite of maar crater deposits, tephra aprons and underlying diatremes is exposed. This allows features of root zones, lower diatremes and upper diatremes to be assessed in light of a known ejecta stratigraphy that indicates the order of eruption. Root zones comprise irregular nephelenitic dikes complexly intermingled with tephra consisting of fragmented country rock and sediment with variable proportions of juvenile sideromelane fragments (now palagonitized).
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They originate by a combination of subsurface brecciation during dike intrusion and continued intrusion into the base of the maar-diatreme vent during explosive eruption. Lower diatremes are sub-cylindrical zones of well-mixed tephra that comprises juvenile fragments and fragmented country rock in varying proportions. Bedding is not preserved except within subsided blocks, and the dominant fabric apparent is a "pipe in pipe" structure suggesting multiple pulses of tephra passing vertically through the lower diatreme. Upper diatreme rocks are considered to be those formed by effectively subaerial eruptions taking place within open craters.
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They consist of scoria, spatter, and minor lava, yet lie at levels below the ground surface; they are constructional volcanoes that initiated within maar craters but then typically filled and grew beyond the crater to form positive topographic features. Where eruptions ceased without such infilling of open craters, maar lakes initially developed in which accumulated a variety of lacustrine sedimentary deposits. Many craters initially held isolated, groundwater-fed lakes. Ephemeral streams extending across the playa entered some of the lake-filled craters, and built coarse-grained Gilbert-type deltas and subaqueous fans along the margins of these craters. The small, coarse-grained fans and deltas have many features in common with much larger coarse-grained deltaic and fan-deltaic deposits. However, the local production of coarse-grained volcanic sediment, low gradients in the local stream catchment, steep subaqueous relief, and the small size of the receiving "basins" resulted in a unique combination of features. Cone-shaped subaqueous fans initially formed at the mouths of incised feeder streams. The fans are small accumulations of steeply-dipping gravelly tephra that consist almost entirely of overlapping lobes constructed by density-modified grain flows. Gravelly Gilbert-type tephra deltas formed in brimfull craters fed by a freely-migrating feeder stream. They are concave lakeward, mimicking the underlying crater-wall topography. Complex deltaic geometries are defined by topset strata that steeply onlap tall foreset beds. They suggest that feeding-stream floods caused rapid and comparatively large variations in lake level within the small-volume crater lakes. Bed-specific carbonate alteration is common, and probably resulted from both influx of detrital carbonate across the playa and alteration of tephra beds by carbonate-saturated lakewaters during between-flood periods of high net evaporation.Results of my work in Hopi Buttes are accessible in:White, J. D. L. (1989) Basic elements of maar-crater deposits in the Hopi Buttes volcanic field, northeastern Arizona., J. Geol., 97, 117-125.White, J. D. L. (1990) Depositional architecture of a maar-pitted playa, Hopi Buttes volcanic field, norheastern Arizona, U.S.A., Sediment. Geol., 67, 55-84.White, J. D. L. (1991) The depositional record of small, monogenetic volcanoes within terrestrial basins. In: Sedimentation in Volcanic Settings (Ed. by R. V. Fisher and G. A. Smith), Spec. Pub. Soc. econ. Paleont. Miner., 45, 155-171.White, J. D. L. (1991) Maar-diatreme phreatomagmatism at Hopi Buttes, Navajo Nation (Arizona), USA. , Bull. Volcanol.,53, 239-258.White, J. D. L. (1992) Pliocene subaqueous fans and Gilbert-type deltas in maar crater lakes, Hopi Buttes, Navajo Nation (Arizona), USA., Sedimentology,39, 931-946.White, J. D. L. (1996) Impure coolants and interaction dynamics of phreatomagmatic eruptions., J. volcanol. geotherm. Res., 74, 155-170.