RHYOLITE IGNIMBRITE BOULDERS AND COBBLES IN THE MIDDLE JURASSIC CARMEL FORMATION OF UTAH AND ARIZONA: AGE, COMPOSITION, TRANSPORT, AND STRATIGRAPHIC SETTING

A stratigraphic layer containing rhyolite cobbles and boulders in the Carmel Formation of southern Utah represents a singular, unusual event in the otherwise low-energy sedimentation of this formation. These volcanic cobbles have been previously studied by Chapman (1987, 1989, 1993), Marzolf (1990), and Blakey and Parnell (1995). Our work is an update on these earlier studies. A single-crystal ⁴⁰Ar/³⁹Ar age of 171.73 ± 0.19 Ma obtained from sanidine in one of the clasts and a zircon U-Pb age obtained on a fallout tuff from the sediments surrounding the clasts (163.9 ± ~3.3 Ma) show that the clasts are about 8 m.y. older. The volcanic clasts are poorly-welded rhyolite ignimbrites that may have been deposited as much as 200 km from the eruptive center, perhaps along pre-existing valleys. The tuff deposits likely remained in place for several million years during which time they were subjected to weathering, alteration, and perhaps topographic inversion, creating mesas capped with tuff underlain by soft Middle Jurassic silts and muds. Triggered by unusual rainfall or earthquakes, debris flows carried the clasts a few 10s of kilometers from their outcrops to the depositional site. Earlier work has proposed that the Middle Jurassic arc was a low-standing, arc-graben. If this was the case, then the tectonic setting was likely similar to the modern Central American arc in the vicinity of Nicaragua where tuffs erupted from a low-standing arc have been deposited onto an adjacent highland and have then been eroded by streams flowing to the east onto a fluvial plain that is not too distant from the sea.