SEIZE THE DAY: OPPORTUNISTIC UNDERGRADUATE RESEARCH OF VOLCANICLASTICS COLLECTED DURING A FIELD TRIP TO CRATER LAKE NATIONAL PARK, OREGON

A student research opportunity arose during a field trip to the US Pacific northwest at a site along OR230 in the Rogue River valley due west of Crater Lake National Park, Oregon. A fresh roadcut offered a chance to describe characteristics and depositional structures of 2 volcaniclastic units, collect samples for microscopic and macroscopic examination of clasts for insights to deposition, write a funding proposal for C¹⁴ age determinations, and to hypothesize about modes of emplacement for the strata and rates of post-eruption incision of the Rogue River. Unit 1, at the base of the roadcut, was identified as an ignimbrite based on field and lab observations and the literature (Bacon and Wright, 2017). Unit 1 lacks bedding, extends below the road, possibly to the river, consists of a mixture of fine-ash to bomb-size clasts, including glass shards, lithic fragments, and pumice, and large (25-100 cm) chunks of charred wood. Unit 2 overlies Unit 1 and is similar in clast composition but is distinguished by an erosional contact with Unit 1, north to south trending cross beds, graded-bedding, and lenses of coarse clasts up to a meter thick. While volcanic surge deposits have been reported near this location (Bacon and Wright, 2017), characteristics of clasts and depositional structures within Unit 2 strongly resemble those associated with fluvial deposits. Both units have C¹⁴-dates of 7,700 ± 30 yr BP, consistent with origins from the climactic Mount Mazama eruption. Volcaniclastics exposed in mass-wasting scars and cutbanks along the east side of the Rogue River valley, coupled with topographic profiles across the valley, indicate infilling by volcanics from the current valley floor to at least road level, a 68 m elevation difference. Assuming Unit 2 represents streams immediately reforming and reworking volcanic fill, a simple calculation yields an 8.8 mm/yr average incision rate.