Category: 2020 Events

2020-01-18 Jeff Tepper — Accretion of the Olympics and Birth of the Cascades

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From the Accretion of the Olympics to the Birth of the Cascades: The Eocene Transformation of Washington Geology

About the lecture

Dr. Jeffrey Tepper

 Dr. Jeffrey Tepper showed how Port Townsend lies between two mountain belts with very different origins, the oceanic Olympic Mountains to the west and the continental Cascade Range to the east. But despite their contrasting traits, the histories of these two chains are intimately connected. He explored how the arrival of the Siletzia terrane about 52 million years ago led to a profound geological transformation of the Pacific Northwest. Over a 10 million year interval in the mid-Eocene (~54-44 Ma), volcanic activity swept across the region from as far east as Montana, extensional stresses triggered faulting and formation of sedimentary basins, the Olympics were established, and volcanism in the Cascades was initiated. All of these phenomena can be linked to the accretion of Siletzia, which caused the Farallon Plate that had been subducting beneath Washington to slow, roll back, and ultimately break off. The evidence of these dramatic events is recorded in rocks we see at the surface as well as in geophysical anomalies deep in the mantle. This event was sponsored by Quimper Geological Society, an Earth Science outreach group of Jefferson Land Trust.

About the speaker

Jeff Tepper was on faculty of the Geology Department at the University of Puget Sound, he began his emeritus status in 2021. He earned his PhD from UW, where he studied the origins of granite in the North Cascades.  At UPS he teaches classes in mineralogy, petrology, geochemistry, GIS, and geomythology. Most of his research projects focus on the magmatic and tectonic history of the Pacific Northwest (including Tamanous Rock), or on the geochemistry of water and sediment from Tacoma-area lakes. Visit Jeff’s website for more information:  https://www.jeffreytepper.com/

2020-11-14 Pat Pringle — Buried and Submerged Forests of the Pacific Northwest

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How much we know about the Pacific Northwest’s post-Ice Age geologic events has grown rapidly in recent decades. The discovery and study of buried and submerged forests-Victims of past catastrophes, has opened the door to new understanding. Testing techniques such as Carbon 14 wiggle matching and dendrochronology (tree-ring dating) now allow more precise dating of the geological past as well. These capabilities are revolutionizing our understanding of the history, behavior, and effects of past volcanic eruptions, regional earthquakes, major landslides, and (to a limited extent) regional climate history and fluctuations.

We will take a snapshot tour of some of the many locations of buried and submerged forests around Washington State and other Pacific Northwest locations to observe trees that were submerged by postglacial tectonic forces, or killed by a flow of hot volcanic fragments (pyroclastic surge) or buried in landslides, or drowned in lakes dammed by landslides. On our tour we’ll get a picture of the scale of some of the dramatic postglacial landscape changes, such as the episodic disturbances caused by volcanism- changes that the pre-Euro-American settlement peoples experienced and adapted to, and that early explorers, like Lewis and Clark, described vividly in their journals.

We’ll see how samples of “subfossil” trees are collected and prepared for tree ring analysis, and learn about what tree rings in these “time capsules” can tell us. Some of the scenic areas we will visit include the Washington Coast, the Olympic and Cascade Mountains, the Puget Lowland, and the Columbia Gorge, with a special focus on the Bonneville landslide, and Mount Rainier and Mount Hood volcanoes (rescheduled from Oct. 26, 2019).

Pat Pringle is Professor Emeritus of Earth Sciences at Centralia College, Washington. He taught at Centralia College from 2005-2017 and was a Research Geologist at DNR Geological Survey of Washington from 1990-2005 and at US Geological Survey Cascades Volcano Observatory from 1982-1990. Pat studies volcanoes, earthquakes, landslides, and debris flows using radiocarbon and tree-ring analysis to establish the history of past geologic events. He is the author of books on the roadside geology of Mounts St. Helens and Rainier as well as many published papers and reports; the Mount Rainier book won the Geoscience Information Society’s “Best Guidebook Award” for 2009, presented at the Geological Society of America’s Annual Meeting that year.