POTENTIAL FOR A THREE-MILLION-YEAR RECORD OF LAKE LEVEL, CLIMATE, AND VOLCANISM FROM SUMMER LAKE, OREGON, USA

The Summer Lake sub-basin of pluvial lake Chewaucan has been a locus of research for decades, yielding important records of environmental conditions and climate in the northwestern Great Basin for the late Pleistocene. Lithostratigraphic, ostracode, pollen, and geochemical indicators obtained from cores and outcrop provide evidence for vegetation changes in the surrounding uplands, hydrologic variations within the basin, and local paleotemperatures. Additional information is available from sediment magnetism, gastropods, fish bones, and tephras. The numerous tephra beds preserved together in a single stratigraphic context make Summer Lake a key reference locality for tephrochronology in western North America and simultaneously provide a unique record of Cascade arc volcanism. The tephras provide age control for the sediments and also constrain the timing of earthquakes in this tectonically active basin, including the age of Holocene surface rupture.

The most continuous and highest resolution records have been obtained from cores near the depocenter where paleomagnetic and paleoclimate studies have achieved multi-decadal resolution from about 20 to 50 ka. Longer, but lower resolution records going back several hundred ka have been obtained closer to the basin’s northern periphery.

More recent work in the southeastern part of the basin has described an 8 m thick, fault-uplifted outcrop section containing 12 tephra layers, including a 30 cm thick bed with pumice up to 3 cm in longest diameter. Plagioclase from this bed has yielded a ⁴⁰Ar/³⁹Ar age of 3.1 to 3.2 Ma. This suggests that the well-documented records of climate and volcanism known for the late Pleistocene extend back much further in time, into at least the late Pliocene. Furthermore, a gravity survey conducted in the 1970s suggests that the basin may contain as much as 2 km of sedimentary fill. Therefore it should be possible to obtain a long record with millenial scale or better resolution by deeply coring the Summer Lake basin.