THE MOUNT AIX CALDERA: LITHOLOGIC AND CHEMICAL CHARACTERISTICS OF THE BUMPING RIVER TUFF

The Late Oligocene Mount Aix caldera is located along the Cascade arc, ~ 25 miles east of Mount Rainier in the southern Washington Cascade Range. Nestled in pre-Tertiary and early Oligocene deposits, previous work identified the caldera as a significant silicic vent complex and aided in the correlation and interpretation of the Mount Aix caldera as the source vent for the rhyolitic Bumping River Tuff (BRT).

The BRT is distributed over an area extending 6-7 km north and northeast of the caldera, and within the structure as intracaldera fill. The outflow tuff (625 m in thickness) is divided into four zones based on lithologic and stratigraphic characteristics. Zone 1 and 2 are massive, and display notable differences in degree of welding and color, and contain variable concentrations of crystal, pumice and lithic content. Zone 3 is lithologically diverse, marked by repetitious bedded surge deposits, lithic rich lenses, and large 3-5 m sized vitric blocks. Zone 4 is gray pumice tuff composed largely of fine vitric ash. The tuffs have similar major element variations, but differ by Zr concentration, delineating low-Zr (<160 ppm) and high-Zr (>160 ppm) groups.

These chemical differences indicate the BRT is composed of more than one source. The low-Zr group (zones 1 and 2, lower 3) shares chemical affinity with intracaldera tuff, inferring they are correlative with the products of the caldera forming eruption. Whereas the high Zr group (upper zone 3, zone 4) is not represented in the intracaldera tuff, and may have been eroded. Moreover, the high Zr group is composed of different lithologies; upper zone 3 samples consist of vitric blocks embedded in the tuff, and zone 4 is an ash cloud deposit. The vitric blocks are interpreted as glassy rhyolite flows originally deposited at the vent, but were subsequently broken up by strombolian and pelean eruptions and re-deposited by surge flows. Investigations in progress intend to determine the origin of the high Zr content and the vitric blocks.