THE KLAMATH PENEPLAIN: CENTENNIAL REVIEW OF J. S. DILLER'S CLASSIC EROSION SURFACE

One hundred years ago J. S. Diller (1902) published his classic work, 'Topographic Development of the Klamath Mountains', in which he proposed that a regional lowland erosion surface, the "Klamath peneplain", formed in the Klamath Mountains during the late Miocene. He further defined the "Bellspring peneplain" as having formed west of Klamath basement rocks, underlain by tilted Neogene sediments of the Coast Ranges, shortly following their deposition. This peneplain was thought to be practically continuous with the adjacent Klamath peneplain. The definition of several younger, more regionally restricted planation surfaces was complicated both by Diller's lack of both correct age determination of younger formations and an appreciation of neotectonics. W. P. Irwin's (1997) 'Preliminary map of selected post-Nevadan geologic features of the Klamath Mountains and adjacent areas…' (USGS Open file rpt. 97-465) clearly delineates the extent of the combined Klamath-Bellspring erosion surfaces. Aalto, Moley & Stone (Pacific Sect., SEPM book 75, 1995) and Aalto, Sharp and Renne (Canadian J. Earth Sci., v. 35, 1998) demonstrate an Idaho provenance for some late Neogene sands west of the Klamath Mountains and suggest that the western margin of Diller's Miocene peneplain was draped by a shelf sediment blanket prior to regional uplift of the Klamath block during the Pleistocene-Holocene. Thus Diller was correct inasmuch as: 1) a late Miocene Klamath peneplain did form in the Klamath province; 2) Neogene marine sediments accumulated on its western margin in response to a relative rise of sea level; and 3) ongoing post-Neogene land degradation accompanying uplift of both Klamath basement rock and sediment cover resulted in the formation of an erosion surface that approximates his combined Klamath/Bellspring peneplain. Diller remains a major player in the geomorphic interpretation of this tectonically active region.