Chapter 2: Description of Study Area

Highway 213, at the northern boundary of the study area, a 2.5 m diameter culvert was placed to reduce blockage and divert the creek under the highway.  There are several other spots along Highway 213 where culverts are needed to replace surface flow.  Culvert outlets are also located all along the southern and western boundaries where the current, high density development ends.  These culverts flow heavily during the winter months because they channel a lot of the runoff from the urbanized areas surrounding the canyon.

Oregon is considered a moist Mediterranean climate with a xeric moisture regime.  This type of climate results in an average temperature between 5°C to 17°C and a total average precipitation of 153 cm per year with the majority occurring during the winter months (Gerig, 1985).

Geology

The stratigraphy of the Oregon City area consists of generally flat-lying Miocene to Pleistocene basalts and fluvial sediments.  Bedrock in the area for engineering purposes includes the Columbia River Basalt Group (Grande Ronde and Wanapam Basalt) which is overlain by the Sandy River Mudstone interbedded within the Troutdale Formation which is topped in some places by the Boring Lavas or the Willamette Silts (Figure 2, Figure 3).  The Sandy River Mudstone/Troutdale Formation, the Boring Lavas, and the Willamette Silts are topped by a residual soil derived from the parent material.  The following description of the lithologies and residual soils is a composite of my field notes, well driller logs, bore hole logs, and information from Swanson (1986), Schlicker and Finlayson (1979), and Tolan and Beeson (1984) (appendix 1).

 Figure 2: Miocene to Pleistocene stratigraphy of Newell Creek Canyon (after Schlicker and Finlayson, 1979; Tolan and Beeson, 1984).

Figure 3: Geologic map of the study area displaying the contact between the Sandy River Mudstone/Troutdale Formation and the Boring Lavas where many of the landslides are located (after Schlicker and Finlayson, 1979; Oregon City, 1997; METRO, 1997).

The Columbia River Basalt Group

Within the study area the Miocene-age Columbia River basalt group does not crop out at the surface.  The Columbia River basalt is a series of lava flows or flood basalts ranging in thickness from 5 to 45 meters, with a total thickness of about 300 meters (Schlicker and Finlayson, 1979; Tolan and Beeson, 1984).  Thin, baked soil zones often separate the individual flows.  Prior to the deposition of younger formations on top of the Columbia River basalt, the Columbia River Basalt Group was gently folded and faulted resulting in topographic highs and lows (Schlicker and Finlayson, 1979).  In some of these topographic lows, such as Newell Creek Canyon, thick sedimentary units have been deposited on top of the Columbia River basalt, filling in the depression (Marvin Beeson, PSU Geology Department, Personal Communication, 1998).

Sandy River Mudstone and the Troutdale Formation

The Sandy River Mudstone and the Troutdale Formation unconformably overlie the Columbia River Basalt Group and are overlain by the Boring Lavas and in some places by the Willamette Silts.  These formations crop out in approximately 80% of the study area (Figure 3) and because they weather easily, they may form moderate (15°) to steep (60°+) slopes.  These two formations are the result of a complex fluvial system resulting in the interbedding of the two formations (Figure 2) (Schlicker and Finlayson, 1979; Tolan and Beeson, 1984).

The Miocene-Pliocene age Sandy River Mudstone consists of mudstone, siltstone, and fine to medium grained sand with some gravel lenses ranging in thickness of individual beds from 0.6 to 4.5 meters (Tolan and Beeson, 1984).  This formation is currently thought to be the same as the lower Troutdale Formation and represents the fine-grained, overbank facies of the ancestral Columbia River.  The fine grained facies tends to be an impermeable barrier to groundwater causing local high water tables and springs if they intercept the surface (Schlicker and Finlayson, 1979).  Moisture-sensitive clays, present in certain parts of the Sandy River Mudstone, expand with increasing moisture, decreasing its shear strength, and contract when drying (Schlicker and Finlayson, 1979).

The Miocene-Pliocene age Troutdale Formation is a fluvial deposit that is currently divided into two distinct lithologies: the upper and lower members (Swanson, 1986).  The lower member consists of paleo-Columbia River gravels and sands composed mainly of basaltic pebbles and cobbles and minor amounts of intrusive granite and metamorphic rock types of which quartzite is usually noted (Swanson, 1986).  These gravels will stand vertically for several tens of meters and are highly permeable (Schlicker and Finlayson, 1979).  In addition to some gravels, the upper member largely consists of finer grained sands, silts, and clays that are generally locally derived volcanic debris, altered to a clay, agglomerates and highly weathered basaltic gravels (Swanson, 1986).  The upper member, like the Sandy River Mudstone, also contains impermeable clay layers that are moisture sensitive (Schlicker and Finlayson, 1979).

The Boring Lavas

The Boring Lavas are basaltic lavas that overlie the Sandy River Mudstone and Troutdale Formation (Figure 2).  These Lavas are Pliocene-Pleistocene age basalts that are light gray in color and vary in thickness from 8 to over 150 meters.  They occur as blocky intracanyon flows, volcanic cones, and shield volcanoes, which result in deposits ranging from tuff breccias and agglomerates to lavas (Schlicker and Finlayson, 1979).  Where the flows are thin they are sometimes weathered to a red clay with scattered residual boulders (Schlicker and Finlayson, 1979).  In the study area, the Boring Lavas form the upper terrace with a relatively flat surface (0°-15° slopes) that covers the southwest, southern, and southeast boundaries.  Weathering of the Boring Lavas produces a 0.3 to 3 meter thick impermeable clay-rich soil over the bedrock which saturates quickly and forms ponds or heavy runoff though rivulets (Schlicker and Finlayson, 1979).

 Willamette Silt

The Willamette Silts overlie the Sandy River Mudstone/Troutdale Formation in a small northern section of the study area.  They are late Pleistocene terrace deposits made of unconsolidated beds with lenses of fine sands, silts, and clays (Schlicker and Finlayson, 1979).  These beds are commonly 10 to 15 cm beds of fine-grained material, but can be massive, without distinct stratification.  These sediments were deposited as a product of the Missoula Floods, occurring about 12,700 to 15,300 years B.P. (Waitt, 1985).

 Residual Soils

Most of the bedrock in Newell Creek Canyon is covered by a residual soil.  These soils are derived from the bedrock or parent material and external sources, such as glacial flooding.  Their properties are controlled by climate, topography, organisms, geology, and time (Birkeland, 1984).  The resulting soils are generally tan, silty clays when developed on the Sandy River Mudstone/Troutdale Formation and red, stiff clays with scattered basalt boulders when developed on the Boring Lavas.  The bedrock-soil contact is transitional and defined by a zone of weathered bedrock (Birkeland, 1984).  This contact is of great importance because it marks a change in strength of materials and as a result, many of the failure planes occur at this contact.  Thickness of these residual soils varies depending on topographic relief and changes from 0.2 meters thick on steep slopes to three meters thick on gentle to level slopes.

These residual soils are further divided into five soil series: Bornstedt (8), Helvetia (37), Jory (45), Woodburn (91), and Xerochrepts and Haploxerolls (92), which are subdivided into eight soil phases (Figure 4) (Gerig, 1985).  The soil series are based on similar profile characteristics while the phases differentiate slope steepness, thickness of soil profile, amount of erosion, and texture (Birkeland, 1984).

Table 1 displays some of the important properties of the residual soils found in the study area, such as the soil texture and permeability.  Table 2 displays some of the important engineering properties of the residual soils, such as the percent of clay, liquid limit, plasticity index, and other comments like soil strength.

In most of the steeper portions of the study area, the residual soils are colluvial, meaning they have been transported downslope from the place of origin.  These colluvial soils are a mixture of the Boring Lavas residual soils and Sandy River Mudstone/Troutdale Formation residual soils and are generally thinner than the alluvial soils but sometimes appear as 1000 m³ blocks indicating mass movement.

 Man-Made Fill

Some of the study area has been developed or is being developed and therefore has been or is being regraded and mantled with man-made fill (Figure 5).  Current development is taking place along the canyon wall where the important contacts between geologic bedrock and changes in soil series are located.  The properties and thickness of this man-made fill vary widely and are site specific.  Fill materials within the study area are generally composed of the local residual soil, basalt gravels and cobbles.  Also present in some of the fill used in the study area are bricks, organic debris, wood, concrete, and garbage.  The two main sites without much residual soil fill are the R & B Leasing Agency, located on Morton Road (west side of Highway 213), and the landfill on the Mountain View Cemetery, both of which were apparently not engineered, and as a result, settlement and landsliding have occurred at both sites.

Figure 4: Soil series map of the study area displaying the distribution between the Xerochrepts and Haploxerolls (92F) and Helvetia (37D) Series containing steeper slopes with thinner soils and the Bornstedt (8B, 8C), Jory (45B, 45C), and Woodburn (91B, 91C) Series with low to moderate slopes and thicker soils (after Gerig, 1985; after Oregon City, 1997; METRO, 1997).

Table 1: Residual soil properties examined through the soil series in the study area (after Gerig, 1985) (* Indicates no data available).

Table 2: Residual soils engineering properties examined through the soil series in the study area (after Gerig, 1985) (* Indicates no data available).

Figure 5: Location map of man-made fill, large cut-slopes, and current development within the study area (after Oregon City, 1997; METRO, 1997).

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