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The Mark at Eugene January 6, 2026 Seismic Hazard Study 2 Project No.: 2251077 Eugene, Oregon Landmark Construction, LLC The building site is located within the Willamette Valley, near its southern extent. The Willamette Valley is a broad, north-south-trending basin separating the Coast Range to the west from the Cascade Range to the east. In the early Eocene (±55 million years ago), the present location of the Willamette Valley was part of a broad continental shelf extending west from the Western Cascades beyond the present coastline (Orr and Orr, 1999). Basement rock underlying most of the north-central portion of the Valley includes the Siletz River Volcanics (early to middle Eocene, ±50 to 58 million years old), which erupted as part of a submarine oceanic island-arc (Bela, 1979; Yeats et al., 1996). The thickness of the basement volcanic rock is unknown; however, it is estimated to be ±3 to 4 miles thick (Yeats et al., 1996). The island-arc collided with, and was accreted to, the western margin of the converging North American plate near the end of the early Eocene. Volcanism subsided and a forearc basin was created and infilled to the south with marine sediments of the Eugene Formation and terrestrial sedimentary and volcanic deposits of the Fisher Formation and Little Butte Volcanics throughout the late Eocene and Oligocene (Orr and Orr, 1999; Wiley, 2008). These sediments typically overlie, but are also interbedded with, basalt and volcanics of the Siletz River Volcanics and younger Tertiary volcanics in the Eugene area. After emerging from a gradually shallowing ocean, the marine sediments and volcanic formations were covered by the terrestrial Columbia River Basalt (CRB). The CRB poured through the Columbia Gorge from northeastern Oregon and southeastern Washington and spread as far south as Salem, Oregon (±17 to 10 million years ago, middle to late Miocene) (Tolan et al., 2000). Uplift and folding of the Coast Range and the Western Cascades during the late Miocene formed the trough-like configuration of the Willamette Valley (Orr and Orr, 1999; O'Connor et al., 2001; Wiley, 2008; McClaughry et al., 2010). Following the formation of the Willamette Valley, thick layers of Pliocene gravel filled the Southern Valley (McClaughry et al., 2010). The deposits were then incised by the Willamette River, forming alluvial terraces. In the Pleistocene (±1.6 million to 10,000 years ago), the Central and Southern Valley was refilled with fan-delta gravel, originating from the melting glaciers in the Cascade Range. The Willamette and McKenzie Rivers in the Eugene area incised deeply into the fan-delta deposits during the Quaternary and deposited recent alluvium adjacent to the riverbanks and major tributaries which continues into the present (Madin and Murray, 2006). Also, during the Pleistocene (over 15,000 years ago), catastrophic flood deposits mantled the Willamette Valley floor as far south as Eugene (Hampton, 1972; Yeats et al., 1996; O'Connor et al., 2001; McClaughry et al., 2010). These deposits originated from a series of glacial-outburst floods that periodically drained Glacial Lake Missoula in western Montana (Allen et al., 2009).