Relationships between clay mineralogy, hydrothermal metamorphism, and topography in a Western Cascades watershed, Oregon, USA

Abstract

This study investigates variation in clay mineralogy and its relation to hydrothermal metamorphism, hillslope processes, and topography in the western Cascade Mountains. The study area is the drainage basin of Dorena Lake, a medium-sized (686 km 2) watershed located near Cottage Grove, OR. The Bohemia Mining District is on the southeastern rim of the watershed in a hydrothermally metamorphosed region associated with a set of granodiorite plutons. To characterize large-scale patterns of clay mineral distribution within the watershed, suspended sediments were collected from 43 stream locations. Samples of several metamorphosed and unaffected volcanic and volcaniclastic rocks were collected to help clarify metamorphic reaction processes. One active earthflow was also sampled. X-ray diffraction methods were used to determine the mineralogy of the clay-sized (<2 μm) fraction of the samples. Clay mineralogy varies systematically across the watershed, and the three major stream tributaries carry sediment with distinct mineralogical signatures. Discrete minerals include kaolinite, smectite, chlorite, and illite. Interstratified kaolinite–smectite and chlorite-vermiculite (CV) are also present. The active earthflow and unmetamorphosed rock samples primarily contain smectite. In contrast, metamorphosed rock samples are composed of some combination of illite, interstratified illite–smectite, CV, and chlorite. Examination of clay mineral distribution reveals the effects of hydrothermal metamorphism in the mining district on clay mineralogy, hillslope processes, and landscape development. Compared with most of the watershed, the mining district has steeper slopes and higher elevations and lacks smectite almost entirely. Analyses of metamorphosed bedrock units indicate that smectite originally present in the rocks was converted to nonexpandable clay minerals during metamorphism. Induration of bedrock and loss of expandable clays resulted in thin soils and steep topography. Debris slides and flows are the dominant mass movement processes in this area; earthflows are noticeably absent, probably because thick, smectitic soils are lacking. Elsewhere in the watershed where smectite is abundant, slopes are more gentle; and both shallow and deep-seated mass movements are common. The clay mineralogy of bedrock and soils is thus integrally tied to the hillslope processes operating within the watershed and therefore affects the geomorphology of the landscape. High-relief, hydrothermally metamorphosed volcanic centers surrounded by weaker rocks in adjacent lowlands are common features in the western Cascade Mountains. These massifs were likely formed by differential erosion processes similar to those operating in the Dorena Lake watershed.