Geologic map of the Tillamook Highlands, northwest Oregon Coast Range (Tillamook, Nehalem, Enright, Timber, Fairdale, and Blaine 15 minute quadrangles)

Abstract

The geology was originally compiled at 1:48,000 scale on a composite base made from 24 seven and a half minute quadrangles. The geology was subsequently scanned into ARCINFO, a proprietary geographic information system, to produce the digital map. The geology was composited with a scanned topographic base derived from the old 15 minute base maps to expedite Open File publication.A major goal of our investigation was to establish the basic stratigraphy and structure necessary to guide energy, mineral, hydrologic, and geologic hazard investigations in northwest Oregon. The map area is centered on the Eocene Tillamook Volcanics and related rocks which form the core of the Coast Range uplift, and includes Tertiary marine strata ranging from Eocene to Miocene age on its eastern and western flanks (Figure 1). Most of the Eocene volcanic sequence consists of thick accumulations of submarine and subaerial basalt interbedded with deep water marine sedimentary rocks. At present, hydrocarbon exploration is concentrated in the shallow marine sequence which overlies the Tillamook Volcanics. About 30 km north of the map area is Mist, Oregon's only producing gas field. At Mist, the producing zones are in the shallow marine sandstones of the Eocene Cowlitz Formation above the Tillamook Volcanics. In the map area, repetitive basalt sequences dominate the stratigraphy, but arkosic sandstone that locally interfingers with Tillamook Volcanics to the northeast could be a potential reservoir at depth. Oil shales that are locally exposed in the Yamhill Formation beneath the Tillamook Volcanics may indicate petroleum source rock potential in the region.Also of interest in the Tillamook Highlands are local areas of mineralization that are concentrated in previously unrecognized silicic flows and intrusions of the Tillamook Volcanics (see map). Semi-quantitative emission spectroscopic analyses of small quartzsphalerite-pyrite veins in silicic flow breccias and fault zones indicate the presence of zinc, arsenic, silver, and gold (Table 1).Another important goal of this study was to determine the tectonic environment of the Eocene Tillamook Volcanics, which comprise a large part of the northern Oregon Coast Range. These basalt flows interfinger with marine strata and are chemically and GEOLOGIC SUMMARYThe Tillamook map area crosses a broad, northeast-plunging structural arch in Tertiary volcanic and sedimentary strata that form the northern Oregon Coast Range (Figure 1). The core of the uplift consists of Eocene basalt and interbedded marine strata which were previously correlated with the Siletz River Volcanics of the central Oregon Coast Range (Wells and Peck, 1961). In this study, the Eocene volcanics have been divided into five units, and a distinction is made between the lower Eocene Siletz River Volcanics and the overlying Tillamook Volcanics of late middle Eocene age. Marine mudstone and sandstone are interbedded with all of the volcanic units and comprise most of the late Eocene to Miocene stratigraphic section which forms the flank...

Slope Failure Detection through Multi-temporal Lidar Data and Geotechnical Soils Analysis of the Deep-Seated Madrone Landslide, Coast Range, Oregon

“…There are a number of mapped faults near the site and an inferred fault is mapped parallel to the unnamed creek at the landslide toe (Figure 2-4). This inferred fault ends perpendicular to an inferred and uncertain fault running parallel to Panther Creek (Wells et al, 1994). The Yamhill River Fault zone is located about 4 km north of the site (Wells et al, 1994;Wells et al, 1983).…”

“…The site is located east of the crest of the Oregon Coast Range, which is a northeast plunging anticline of uplifted Eocene to Miocene sedimentary and volcanic rocks along the western edge of Oregon (Wells et al, 1994 Geologic structure in the area is primarily a broad, north-northeast trending arc dissected by northwest and northeast trending faults. There are a number of mapped faults near the site and an inferred fault is mapped parallel to the unnamed creek at the landslide toe (Figure 2-4).…”

“…Sample collection for dry unit weight was completed at four locations as shown on Figure 3-3. environments and can be difficult to distinguish in the field. Adjoining geologic map boundaries to the south of the Madrone Landslide from Wells et al (1994) and Yeats et al (1996) show some discrepancies in geologic mapping. At the map boundary, Wells et al (1994) maps Yamhill Formation to the north while Yeats maps the unit as the Nestucca Formation to the south.…”

“…Volcanics but intrude rocks as young as Yamhill Formation, suggesting a minimum age of about 43 Ma (Wells et al, 1983). Weathering of the Yamhill Formation where diabase sills are present makes it difficult to identify from older geologic units (Wells et al, 1994).…”

“…Pacific madrone (Arbutus menziesiiis) a broadleaved evergreen tree and a member of the heath family (Ericaceae). It is distinguished by its smooth trunk, orange-red deciduous bark, white flowers, and red berries (Niemiec et al, 1995 (Wells et al, 1994) of the vicinity of the Madrone Landslide. The site is located on the Yamhill Formation (Ty), and near units of diabase (Tibd), Siletz River VolcanicsBasalt lapilli breccia unit (Tsbr), landslides (Qls), Fluvial deposits (Qf), and older fluvial deposits (Qt).…”