USING QUALITATIVE CLAST, SAND, AND SOIL DESCRIPTIONS TO INVESTIGATE TERTIARY GRAVELS OF THE KLAMATH PENEPLAIN EROSIONAL SURFACE IN HUMBOLDT COUNTY, NORTHWESTERN CALIFORNIA

Southern Cascadia marks the tectonic change between the northern San Andreas Fault System, and the subduction of the Juan-de-Fuca and Gorda plates beneath North America. Southern Cascadia (defined here from the Mendocino Triple Junction to the Oregon-California border), is a region of complex Quaternary deformation, high rates of erosion, and vertical uplift. The “Klamath Peneplain”, named in 1902 by J.S. Diller, is a low relief, heavily dissected, relict surface that is visible as gently sloping ridges preserved along the coast of Southern Cascadia, generally between elevations of 610 and 1,520 m.a.s.l.. The erosional age of the Klamath Peneplain and the depositional age of preserved fluvial gravels and marine sediments that onlap them is constrained in one locality near Crescent City, CA, to late Miocene. These age constraints are based on a diatom species assemblage (Stone, 1992) and saprolitic paleosols thought to be late Miocene in age (Aalto et al, 2006).

In this study, we first remap the Klamath Peneplain surface using a surface classification model in ArcMap, setting a slope criterion of 4 degrees. We find that the surface mapped previously using USGS topographic data (i.e. Irwin, 1997) differs slightly from our interpretation, particularly the highest elevated regions that are the most dissected. Secondly, we describe the fluvial gravels within a sandy soil matrix that are well exposed along a road cut on the Klamath Peneplain surface, locally known as Bald Hills, elevated to 762 m.a.s.l.. At this exposure, an 8-10m thick section of fluvial gravels unconformably overlies the Jurassic Franciscan Formation. We describe and classify soil type as well as clast size, imbrication, composition, and degree of rounding of fluvial gravels, that will enable us to place this depositional unit in a broader stratigraphic and depositional context, compared to other known locations. Because the Klamath Peneplain represents a time period of extensive erosional beveling and deposition from fluvial sources, its age and origin are crucial in understanding paleogeographic development of what is now Southern Cascadia.