Governing Board

Information from well logs, geophysical logs, excavations, and outcrops near Blue Lake County Park in eastern Multnomah County, Oregon, indicates that the stratigraphy and geologic structure are more complex than previously described. These data suggest that the gravel deposits that form the Troutdale gravel aquifer are post-Troutdale Formation in age and that the contact separating the two rock units is an angular unconformity. Structure contours on the Troutdale Formation show a large dome centered just south of Blue Lake. The Troutdale Formation was severely eroded along the northeast flank of the dome, presumably during Pleistocene catastrophic flooding events. Gravels of the Troutdale gravel aquifer overlie a portion of this eroded surface and thicken significantly toward the south and west. Well logs and natural gamma logs from the Blue Lake area suggest that the northeast portion of the dome was cut by a northwest-trending fault with vertical displacement of up to 500 ft. Erosion has removed much of the upper portion of the Troutdale Formation north of the fault trace. The absence of mappable siltstone units in the Troutdale Formation indicates lateral discontinuities that may have been caused by channel incision. These incisions, together with changes in sediment size and source areas, attest to the dynamic depositional environment of the east Portland Basin. INTRODUCTION Several investigations have been completed in the east Portland Basin since Trimble (1963) mapped the geology of east Multnomah County and the City of Portland studied the area in 1977 and 1978 for a potential well field. These investigations focused on the hydrogeology of the east Portland area, with most work concentrated southeast of Blue Lake County Park. This paper assimilates new geologic data into a model, accounting for the relatively complex stratigraphic and structural relationships displayed in the Troutdale Formation within the study area (Figure I). Geologic cross sections, structure-contour maps, and isopach maps were constructed on the basis of the interpretation of geologic logs, driller reports, and natural gamma logs when available. Well locations used in this paper are shown in Figure 2. In addition to well logs, outcrops of the Troutdale Formation were examined in the field, and a review of pertinent literature was incorporated into the development of the model. Well logs provided data for geologic interpretations, because outcrops of the Troutdale Formation are rare in the study area. REVIEW OF PREVIOUS INVESTIGATIONS Previous work can be divided into geologic and hydrogeologic investigations. Geologic investigations Trimble (1963) mapped the geology of east Multnomah County and incorporated most of the previous work into a comprehensive investigation of the area. He described the Troutdale Formation as consisting primarily of vitric sandstone and conglomerate with a deeply weathered surface. Trimble separated and renamed the lower Troutdale Formation the Sandy River Mudstone, basing this separation on differences in lithology and genesis between the Sandy River Mudstone and the Troutdale Fonnation. Mundorff (1964) complemented Trimble's work by mapping the geology and hydrogeology OREGON GEOLOGY, VOLUME 55, NUMBER 3, MAY 1993 OREGON VICINITY MAP