TEPHROCHRONOLOGY OF SEDIMENT CORES FROM PAULINA MARSH IN THE GREATER FORT ROCK BASIN, NORTHERN GREAT BASIN, OREGON

Oregon’s northern Great Basin is known for its early archaeological records and dynamic geologic history. A lack of high-resolution paleoenvironmental records from lowland contexts in this region prompted us to conduct sediment coring in the greater Fort Rock Basin, locus of the Connley Caves and Fort Rock Cave sites among others. We collected 5 cores from Paulina Marsh, near the town of Silver Lake to investigate environmental conditions and provide context for the area’s archaeology during the Holocene and latest Pleistocene.

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.