The study area lies in southwest Douglas County 5 kilometers south from Camas Valley and is accessible by state highway 42. The purpose of the study was to map the geology at a 1:31,250 scale, determine the stratigraphy, study the petrology of the formations, and determine the provenance within a tectonic setting.

The Clackamas River drainage within the western Cascade Range is approximately aligned with a northwest trending lineation defined by the Portland Hills and the Brothers Fault zone. This area is occupied by an extensive Columbia River Basalt sequence that is deeply incised by the Clackamas River and its tributaries. Two major basalt units of the Yakima Basalt Subgroup, including the Grande Ronde Basalt and the Frenchman Springs Member of the Wanapurn Basalt, are distinguishable in a 515 meter to 550 meter accumulation. Of particular interest is the presence of five trending right-lateral strike-slip faults is consistent with a stress model of north-south compression and east-west extension.

This work on Marquam Hill area in Portland, a relatively undeveloped urban hillside area, is a pilot study in which environmental factors are evaluated quantitatively in order to delineate limitations on development. The study was undertaken at the request of and in cooperation with the City of Portland Planning Commission and with the State of Oregon Department of Geology and Mineral Industries. Factors considered include various aspects of the land, vegetation and attitudes of inhabitants. Findings are not intended to satisfy need for individual site studies by qualified experts but should show where that expertise is needed. Field data were collected on site by observations, borings, personal interviews and by geophysical surveys; laboratory tests were made on soil samples; studies were compiled of topographic maps and aerial photographs; and pertinent data from available previous works were incorporated. Data were integrated into a series of environmental factor maps including ground slope, bedrock and soil, bedrock structure, soil thickness, ground stability, hydrology, and vegetation. Land use constraints and environmental limitations were defined using u.s. Department of Agriculture, Soil Conservation Service, soil suitability and limitations criteria as a guide. Mapped geologic and environmental constraints were overlayed with the mapped soil criteria to identify areas by limitations. An interpretive map and chart show that most of the study area is moderately to severly limited for most land uses. Some small areas can be considered as slightly limited, but the majority of these are already developed. The constraints are so severe that most of the area should be left in open space. The major recommendation is that any development must be carefully controlled, utilizing stringent grading codes (such as Chapter 70 of the Uniform Building Code) and professional expertise to assure the safety and environmental compatibility of the site. It is suggested that planned unit development, in the form of clustered or low- to medium-rise structures would optimize design for natural conditions, siting locations, and residential density least affecting the surrounding, naturally vegetated, hazardous slopes.

The Portland Hills fault forms a strong northwest trending lineament along the east side of the Tualatin Mountains. An en echelon lineament follows North Scappoose Creek, Alder Creek, and the Clatskanie River along the same trend, through Columbia County, Oregon. The possibility that this lineament follows a fault or fault zone was investigated in this study. Geophysical methods were used, with seismic refraction, magnetic and gravity lines run perpendicular to the lineament. The seismic refraction models indicate the near surface basalt is broken in many places, with 15 - 30 meters (50 - 100 feet) vertical displacement, down to the west, at Bunker Hill along the Alder Creek fault. Gravity models required a faulted zone approximately two kilometers wide across the lineament. The proposed fault zone is more clearly defined in the south, becoming more diffuse and branching in the northern part of the study area. The Bouguer gravity values from this study distort the -40 milligal contour farther to the northwest than is shown on the Complete Bouguer Gravity Anomaly Map of Oregon {Berg and Thiruvathukal, 1967b). The existence of sharp topographic features and the geophysical evidence indicate fault activity along the zone.

The Breitenbush Hot Springs area lies on the boundary of folded middle to late Tertiary Western Cascade rocks and younger High Cascade rocks. Within the mapped area the Western Cascade rocks are represented by four formations. The Detroit Beds, a sequence of interstratified tuffaceous sandstone, mudflow breccia, and tuff, is overlain unconformably by the Breitenbush Tuff. The Breitenbush Tuff consists of three units of welded pumice-rich crystal-vitric ash-flow tuffs interbedded with tuffaceous sedimentary rocks. The Outerson Formation unconformably overlies the Breitenbush Tuff and consists primarily of basaltic lava and breccia. The Outerson Formation includes three localized members: a basal, glassy, aphanitic basalt, the Lake Leone Sediments, and the Outerson Tuff. The Outerson Formation is cut by a number of feeder dikes and plugs and is unconformably overlain by the Cheat Creek Sediments, composed of volcanic sedimentary rocks and a distinctive basaltic tuff. The Western Cascade formations total more than 1660 m {5500 ft) of strata and range from Oligocene to Pliocene in age. The High Cascade rocks are represented by two formations: the Triangulation Peak Volcanics of basalt and andesite lava and breccia, lying unconformably atop the Cheat Creek Sediments; and unconformably beneath the Collowash Volcanics, a series of thin basaltic lava flows and breccias. The Western and High Cascade rocks are covered extensively by surficial deposits, primarily glacial drift. The High Cascade formations are at least 840 m (2800 ft) thick, ranging in age from Pliocene to Pliestocene. The Western Cascade rocks have been folded and faulted in the Breitenbush Hot Springs area, and form the eastern limb of the north-trending Breitenbush Anticline. The folded rocks and the erosional unconformities between the rock units probably represent two local episodes of orogeny: one in early to middle Miocene and another in late Pliocene to Pleistocene time. The Outerson Formation represents a depositional sequence between the periods of uplift and deformation. Faulting accompanied the orogenic sequences. The primary volcanic landforms in the area have been destroyed by erosion but skeletal remains of High Cascade volcanoes are recognized. Stream erosion and glaciation are responsible for the present landforms. Breitenbush Hot Springs occurs, in part, along basaltic dikes which channel the water through impermeable Breitenbush Tuff. The dikes are believed to be associated with the Outerson basalts. The Hot Springs discharge upwards at 3400 l/min. (900 gpm) of water at temperatures up to 92°C (198°F).

Marsh paleoseismological studies were conducted in four bays (Necanicum, Nestucca, Siletz, and Yaquina) along the northern Oregon coast and compared with completed studies in two other bays (Netarts and Alsea). Coseismically buried peats were identified in all bays, based on 1) abrupt contacts, decreases in organic content, increases in sand content, increases in beach sand, and changes in diatom assemblages, all from the peat to the overlying sediments, 2) distinct sandy layers and key plant macrofossils, such as Triglochin, above the buried peat, and 3) widespread correlation of the buried peats within the bay. The stratigraphy and the ages and depths of the top six coseismically buried peats were compared between bays. The following similarities were noted: 1) All bays recorded five burial events in the top 2.6 meters within the last 2200 years. 2) Six burial events were recorded in six bays in the top 3.0 meters, except Alsea Bay (3.3 m), and all six events occurred within the last 2600 years except Yaquina (2780 years). 3) The depth to the top of each buried peat in the bays is consistent, falling within discrete ranges, except for the top two events at Yaquina. 4) Distinct sandy layers (tsunami-deposited) are present over the topmost buried peat in all bays except Yaquina and over the 4th in all bays except Yaquina and Nestucca. 5) Distinct tsunami-deposited sandy layers are absent over the third buried peat in Netarts, Nestucca, Siletz, Alsea, and possibly Yaquina, but present at Necanicum. The evidence strongly suggests synchroneity of coseismic events between the Necanicum River and Alsea Bay (a distance of 175 km), with the exception of the 2nd and 6th event. The 6th coseismic event would be synchronous between Alsea and Netarts, a distance of 105 km. The support for synchroneity of the 2nd event is weak. Synchroneity of coseismic burial events on the northern Oregon coast would argue for paleomagnitudes of at least 8.1 Mw, given a minimum rupture width of 50 km and a rupture length of 105 km. The paleomagnitudes were determined using the moment magnitude equation, Mw = 2/3 IOg10 Mo - 10.7 where Mo = shear modulus x rupture area x seismic slip. The seismic slip is estimated from a minimum recurrence interval of 300 years and a minimum convergence rate of 3.5 cm/yr.Marsh paleoseismological studies were conducted in four bays (Necanicum, Nestucca, Siletz, and Yaquina) along the northern Oregon coast and compared with completed studies in two other bays (Netarts and Alsea). Coseismically buried peats were identified in all bays, based on 1) abrupt contacts, decreases in organic content, increases in sand content, increases in beach sand, and changes in diatom assemblages, all from the peat to the overlying sediments, 2) distinct sandy layers and key plant macrofossils, such as Triglochin, above the buried peat, and 3) widespread correlation of the buried peats within the bay. The stratigraphy and the ages and depths of the top six coseismically buried peats were compared between bays. The following similarities were noted: 1) All bays recorded five burial events in the top 2.6 meters within the last 2200 years. 2) Six burial events were recorded in six bays in the top 3.0 meters, except Alsea Bay (3.3 m), and all six events occurred within the last 2600 years except Yaquina (2780 years). 3) The depth to the top of each buried peat in the bays is consistent, falling within discrete ranges, except for the top two events at Yaquina. 4) Distinct sandy layers (tsunami-deposited) are present over the topmost buried peat in all bays except Yaquina and over the 4th in all bays except Yaquina and Nestucca. 5) Distinct tsunami-deposited sandy layers are absent over the third buried peat in Netarts, Nestucca, Siletz, Alsea, and possibly Yaquina, but present at Necanicum. The evidence strongly suggests synchroneity of coseismic events between the Necanicum River and Alsea Bay (a distance of 175 km), with the exception of the 2nd and 6th event. The 6th coseismic event would be synchronous between Alsea and Netarts, a distance of 105 km. The support for synchroneity of the 2nd event is weak. Synchroneity of coseismic burial events on the northern Oregon coast would argue for paleomagnitudes of at least 8.1 Mw, given a minimum rupture width of 50 km and a rupture length of 105 km. The paleomagnitudes were determined using the moment magnitude equation, Mw = 2/3 IOg10 Mo - 10.7 where Mo = shear modulus x rupture area x seismic slip. The seismic slip is estimated from a minimum recurrence interval of 300 years and a minimum convergence rate of 3.5 cm/yr.

The Tualatin Valley is a well defined elliptical basin centered at Hillsboro, with a major axis trending roughly N65°W. The valley is bordered on the northeast by the Tualatin Mountains (Portland Hills) which are a faulted, northwest-trending asymmetrical anticline. Topographic and geophysical evidence have defined the Portland Hills fault, which occurs along the northeast side of the Tualatin Mountains. The possibility that a fault or fault zone occurs along the southwest side of the Tualatin Mountains was investigated in this study.

A geologic study of the Parrett Mountain area, located twenty miles (32 kms) south-southwest of Portland, Oregon, was initiated by the Oregon Water Resources Department. The main goal was to create a stratigraphic and structural model of Parrett Mountain in order to better understand the local basalt aquifers present there. Previous geologic studies of the area revealed the mountain to be composed of Columbia River basalt. Field mapping and hand lithologic and geochemical analyses allowed the basalt to be subdivided into eleven basalt flows. These flows are as follows: (from oldest to youngest) the Wapshilla Ridge (WpR) , the Ortley-Grouse Creek (undifferentiated) (OGC), the Umtanum (U), the Winter Water (WW), and the Sentinel Bluffs (SB) basalt units of the Grande Ronde Basalt and the Ginkgo (Gk) flow of the Frenchman Springs Member of the Wanapum Basalt. All the basalt flows were found to be laterally extensive throughout the entire area creating a stacked pancake-layered structure. Thickness variations in the Wapshilla Ridge, Ortley- Grouse Creek and Ginkgo basalt flows reflect paleotopographic relief present during the emplacement of the basalt flows. Sedimentary interbeds appear to be highly localized, occurring sporadically throughout the entire area stratigraphic column. Faults were identified based on geologic map compilation, cross section analysis, and topographic linear features. They trend N-S, E-W, NE and NW and are believed to dissect the entire basalt column. All faults have been designated with a normal sense of displacement, except those known to be thrust or r~verse. Cross-cutting relationships suggest the NS-trending faults are the youngest and the NWtrending faults are the oldest. The faulting created twenty-four separate basalt blocks, each represented by a distinct strike and dip. Flowtop morphology, stratigraphic layering, and the local geologic structures influence local groundwater systems. All flow boundaries yield water to at least one well on the mountain. The most commonly used aquifers are the U/OGC boundary, interflow zones within the OGC, the OGC/WpR boundary and permeable zones within the WpR. Faulting is believed to promote recharge of the groundwater systems by increasing the vertical infiltration of percolating precipitation through the highly fractured fault zone.

The Upper Eocene Cowlitz Formation is exposed in surface outcrops southwest of the town of Vernonia, in Columbia County, Oregon. The Cowlitz Formation also occurs in the subsurface of the Mist gas field where its Clark and Wilson (C and W) sandstone member (informal) acts as a natural gas reservoir, and its upper Cowlitz mudstone member (informal) acts as a cap rock. Surface exposures and continuous core were studied in order to determine Cowlitz Formation stratigraphy, and its depositional environment. Fresh core samples were also studied petrographically, and with a scanning electron microscope, in order to determine the effects of diagenesis in the gas producing C and W sandstone member.

Growing concern over earthquakes in the Pacific Northwest has prompted the mapping and location of near surface faults in the Portland area, Oregon. Visible evidence of faults is poor, requiring the use of geophysical methods to assist in mapping and defining structures in the basin. Gravity maps and models may help in addressing this problem.