TEPHROCHRONOLOGY OF FAULTED, FOSSIL-BEARING, HOLOCENE TO PLIOCENE SEDIMENTS NEAR SUMMER LAKE, OREGON

Lacustrine sediments preserved near Summer Lake, which occupies a portion of the northwestern sub-basin of Pluvial Lake Chewaucan, contain an exceptional long-term record of Cascade arc pyroclastic volcanism and therefore provide a key reference locality for tephrochronology. Summer Lake is located 120 km east of the main locus of Cascade arc volcanism and about 55 km east of the line of rear arc volcanoes. Multiple studies at Summer Lake have also examined significant paleoclimate, paleomagnetic, and paleoseismic records. The vast majority of this work has been conducted on Pleistocene lacustrine sediments from outcrop and cores in the northwestern and western parts of the basin.

Since 2013, we have been studying additional exposures of tephra-bearing lacustrine sediments at multiple locations in the southeastern portion of the basin. These exposures include much older sediments, absolute lake-level indicators, tufa mounds, multiple fault traces & scarps, at least one site rich in fish bones, and numerous tephra beds. We have been chemically fingerprinting the tephra beds for identification and correlation using major-element analyses of glass shards conducted on the electron microprobe at Concord University.

The fossil fish site exposes about 8 m of sediment, mainly on the upthrown (east) side of a north-south trending fault which can be traced along the west side of the exposure. The uplifted block yielded 8 rhyolitic tephras and 4 mafic to intermediate tephras. The most prominent rhyolitic tephra is 30 cm thick and contains pumice up to 3 cm. In spring 2014, plagioclase from this bed was submitted for Ar-Ar dating. Fish bones are abundant, and identification of one species (Ken Gobalet, written communication 2005) suggests a late Pliocene age for the uplifted sediments. The downthrown side of the fault yielded one rhyolitic tephra, identified as the ~25 ka Trego Hot Springs bed. The fault also offsets post-glacial dune sand which contains the ~7600 BP Mazama tephra, confirming a Holocene age for the most recent ground-rupturing earthquake.

Additional stream gully exposures reveal a series of unconformities which likely result from the interplay of lake level and faulting. Tephra beds and tufa mounds (samples submitted for U-Th dating) provide age control for the lake-level history recorded at these locations.