ABSTRACT

Petrographic study of modern sand samples collected near Hopi Buttes volcanic field (northeastern Arizona) underscores the low preservation potential of mafic minerals and ultrabasic volcanic detritus even in an arid, intraplate tectonic setting. The Miocene-Pliocene Hopi Buttes volcanic field has been interpreted as a maar-diatreme and scoria cone field in which phreatomagmatic eruptions took place in a playa lake basin. The tephra, necks, dikes, flows, and maar crater fill are now sources of sediment found in local ephemeral streams. Once the volcanic field is eroded away, the main record of its existence will be as a provenance component in the sand it is now shedding. The locally derived volcanic components include monocrystalline pyroxene, glass shards, scoria, and a narrow range of ultrabasic volcanic rock fragments. Thin-section point counts (450 grains) on four sieved sand fractions from each of 7 sites downstream (0, 2, 4, 8, 12, 21, and 31 km) from one margin of the volcanic field provided the following results.

(1) The proportion of volcanogenic to total sand grains decreases from an initial value of about 40% at a rate of about 0.8% per kilometer downstream from the source rocks over the first 30 km of stream length.

(2) Dilution by sand eroded from other local source rocks, rather than physical grain-size reduction or chemical dissolution, drives the diminution of volcanogenic grain proportions.

Detritus sourced from the Hopi Buttes volcanic field has a very low preservation/recognition potential, despite the arid climate and stable tectonic setting far removed from a plate margin. It follows that maar-diatreme volcanism is generally underrepresented in the sedimentary record.