Abstract
Distal tephra beds provide important records of pyroclastic volcanism that enhance our overall understanding of eruptive frequencies, magnitudes, compositions, and hazards. Some beds also serve as widespread chronostratigraphic markers. Lacustrine sediments near Summer Lake, Oregon (United States), record numerous eruptions of Cascade arc sources over a period exceeding 2.5 m.y. Late Pleistocene sediments exposed in outcrop have yielded 88 visible tephra beds, including many beds not previously documented. Of these beds, 44 are characterized by rhyolitic glass, 40 contain predominantly basaltic or intermediate glass, and 4 are strongly heterogeneous in composition. Only 23 have been correlated to deposits outside of the Summer Lake basin. The remaining 65 beds provide a record of Cascade arc volcanism that is as yet unique to Summer Lake. Age-depth relations are well constrained for the upper 6 m of section, but are less certain in the lower 12.4 m. Tephra correlations and an overall age model suggest the following: bed B1 originates from an eruption of Mount Mazama (Crater Lake) ca. 20 ka. Beds I and W likely originate from eruptions of Mount St. Helens ca. 80 and 190 ka. A 7-cm-thick tephra bed correlated to Shevlin Park Tuff probably dates to ca. 198 ka. Tephra correlated to the Antelope Well tuff from Medicine Lake volcano dates to ca. 215 ka. Bed NN, at the base of the section, has an estimated age of at least 240–250 ka and probably originated from Newberry Volcano. Overall, this record significantly refines the Pleistocene tephrostratigraphic framework for western North America.
Highlights
Much of what we know about the Cascade volcanic arc has been determined through studies of lava flows and proximal tephra deposits (e.g., Bacon and Lanphere, 2006; Hildreth, 2007)
The Summer Lake outcrop and core records represent one of the most complete and detailed tephrostratigraphic records of Pleistocene volcanism in western North America. This sequence serves as a key reference locality that puts together in a single stratigraphic context multiple beds that are known from many different locations
It serves a critical role in refining the Pleistocene tephrostratigraphic framework for western North America
Summary
Much of what we know about the Cascade volcanic arc (western North America) has been determined through studies of lava flows and proximal tephra deposits (e.g., Bacon and Lanphere, 2006; Hildreth, 2007). Our study, which focuses primarily on the outcrop tephrostratigraphy in the Ana River canyon, was initiated on the basis of: (1) the presence of many tephra beds of undetermined glass composition, (2) the suggested presence of additional tephra beds, (3) the apparent lack of suitable correlatives to large pyroclastic deposits such as the Shevlin Park Tuff, and (4) uncertainties in the stratigraphic relations between outcrop locations. To this end, the C, E, and F outcrop sites were reexcavated Sampled in detail for tephra (Table 1 and Supplemental Tables S1–S31)
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