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