Pliocene subaqueous fans and Gilbert-type deltas in maar crater lakes, Hopi Buttes, Navajo Nation (Arizona), USA



Maar craters of the Mio-Pliocene Hopi Buttes volcanic field of Arizona formed within a broad playa system, and accumulated a variety of lacustrine sedimentary deposits. Many craters initially held isolated, groundwater-fed lakes. Ephemeral streams crossing 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 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.