TEPHROCHRONOLOGY OF SEDIMENT CORES FROM PAULINA MARSH IN THE GREATER FORT ROCK BASIN, NORTHERN GREAT BASIN, OREGON
Fieldwork in 2018 and 2019 resulted in 3 cores extracted using a modified Livingston piston corer and 2 using a Geoprobe 7822DT. Three cores were obtained from the southwest marsh: 2.47 m PAUL-2A with basal radiocarbon age of 9000±25 14C BP, 6 m PAUL-2B with an age of 9270±90 14C BP at 4.07 m but undated below that; and the undated PAUL-C. The 1.02 m PAUL-6 core obtained from the central marsh has a basal radiocarbon age of 6690±30 14C BP. The 10.67 m PAUL-7 is from the northern marsh, approximately 2 km from Connley Caves. PAUL-7 was not datable by radiocarbon but does contain tephra.
Six samples were collected from core PAUL-7 for tephra analysis, 3 from discrete tephra deposits and 3 from tephra-bearing sediments. Samples 4-3, 4-4, 4-5, and 4-6 are from a 22 cm interval between 5.04 and 5.26 m. These yielded abundant ash-sized grains. 4-3 at 5.06 m and 4-4 at 5.12 m yielded two glass shard chemistries, a major population closely matching the ~46 ka Mt. St. Helens Cy tephra and a minor population closely matching the slightly older Paulina Creek (Olema) tephra of Newberry volcano. Sample 4-5 at 5.23 m yielded subequal amounts of both chemistries. Sample 4-6 at 5.26 m yielded primarily the Newberry chemistry with a sub-population of Mt. St. Helens shards. Sample 6-2 at 8.97 m yielded lapilli-size grains. The glass is slightly heterogeneous with a range of 71.5 to 75.0 wt% SiO2, and the source eruption is as yet unidentified. Sample 7-1 at 9.04 m yielded a mixture of redeposited tephra grains from unidentified sources. Four chemistries are represented by two or more grains, and several others are represented by only a single shard. Together the tephra results indicate that the PAUL-7 core extends much further back in time and therefore contains a substantially older paleoenvironmental record than the other cores.