Gregory J. Retallack* and Joshua J. Roering

Dept. of Geological Sciences, University of Oregon, Eugene, Oregon 97403, USA

Abstract

Despite evidence that weathering plays a role in forming intertidal, estuarine, and fluvial rock platforms, many publications uncritically refer to “wave-cut” or “stream-cut” rock platforms. The alternative explanations of chemical weathering and physical water erosion of rock platforms have been debated since 1839. Our new approach to this long-standing problem introduces data from fluvial as well as coastal rock platforms and documents the saprock-bedrock division of weathering profiles. We report local field evidence for a dominant role of differential weathering of sedimentary beds in an intertidal rock platform at Sunset Bay and a riverside platform near Days Creek, both in southwest Oregon, USA. Despite continuing tectonic uplift, both rock platforms remain within the zone of modern water table and intertidal fluctuation. Above this zone of frequent wet/dry cycling, rock is weakened by subaerial weathering, as documented by decreased rock hardness from the application of a Schmidt hammer, high slaking quantified by wetting-drying experiments, Munsell color indication of elevated oxidation of fine fractures, and peripheral loosening of calcareous fossils. At these sites, the shape and elevation of rock platforms in coastal and fluvial settings appear to reflect differences in strength between bedrock and saprock within the zone of water table fluctuation. In our study sites, waves and floods do not “cut” rock but remove clasts already weakened by weathering to expose the local water table as a bedrock surface. Other local agents of rock removal include salt weathering, eolian abrasion, landslides, debris flows, and fungal, plant, and molluscan bioerosion.

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Manuscript received 17 Nov. 2011; accepted 8 Feb. 2012.

DOI: 10.1130/GSATG144A.1