GROUNDWATER RESOURCE AND LANDSLIDE PROBLEMS ASSOCIATED WITH THE OCCURRENCE OF THE LATE EOCENE WAVERLY HEIGHTS FORMATION IN THE GREATER PORTLAND BASIN AREA

The name Waverly Heights formation (WHf) has been informally applied by Beeson and others (1989) to a sequence of late Eocene-age, oceanic island basalt flows, and associated sediments, in the Lake Oswego 7.5 minute quadrangle that crop out along the Willamette River in the Waverly Heights area. Correlative flows crop out along the Willamette River in the Petes Mountain water gap and have been penetrated by water wells in the West Linn area. Previously the WHf basalt flows had been identified and mapped by Trimble (1963) as Columbia River basalt (CRB) or Skamania Volcanics.

Individual WHf basalt flows are typically blocky to columnar jointed and have well developed flow tops. Exposed flow morphology and the lack of pillow complexes suggest that the upper portion of the WHf was emplaced as subaerial compound flows. Vesicles, as well as most cooling joints, are completely filled by secondary minerals. The top of the WHf is typically deeply weathered and marked by a +10 m-thick clay layer. While somewhat similar in appearance, WHf flows can be easily distinguished from younger CRB flows on the basis of lithologic and geochemical differences.

Unlike the younger CRB flows, WHf flows are typically a poor target for useable groundwater, both in quantity and quality. Due to the pervasive secondary mineralization of the WHf flows, yields from wells completed in the WHf typically range from 0 to 30 gpm and have a range of water quality issues (e.g., high iron, TDS). Well drillers believing they are in the CRB, when actually in the WHf, have drilled many hundreds of feet into the WHf in a futile attempt to find adequate quantities of groundwater. The physical character of the WHf is also a factor in landslides in the Sellwood-Riverdale-Tryon Creek area. Often the relatively thick clay developed on the top of WHf can act as a failure plane (e.g., west abutment of the Sellwood Bridge) allowing the movement of relatively large “blocks” of overlying CRB flows.