Geomorphology of Beach Ridges and Holocene Terraces on Kamchatka: A Complex Interplay of Tectonics, Volcanism and Coastal Processes

Despite decades of analysis, the origin of beach ridges remains debatable, and coastal morphotectonics an open field. On the eastern coast of Kamchatka, along about 700 km of coastline (northwest Pacific and southwest Bering Sea), we have measured accumulative Holocene coastal profiles spanning a range of conditions including variable tectonics (co-seismic and longer term), wave climate, sea ice, and sediment supply (including fluvial, debris-flow, eolian, longshore transport). We can constrain the age of many of these coastal features (beach ridges, terraces) using well-mapped and dated Holocene marker tephra-typically present at time intervals of 500-1000 yrs (or less) over several thousand years. Here are some generalizations for Kamchatka's east coast: 1) Beach ridge complexes are typical of coastlines that are relatively stable tectonically (little or no uplift, mild subsidence); coastlines undergoing uplift at rates on the order of 10-100 cm/kyr exhibit terraces, rather than beach ridges. Subsiding coastlines are erosional. 2) Some beach ridges are clearly correlative with co-seismic subsidence (local sea-level rise) and erosion, building up and then stranding a beach ridge during the interseismic cycle. Other beach ridge complexes are well-developed in areas where co-seismic deformation is unlikely. 3) Beach ridge recurrence intervals range from decades (Uka site in the Bering Sea) to hundreds of years (e.g. south Kamchatka). Especially in narrower embayments, beach ridge development in maximized toward the center of the bay. 4) Along the Kamchatka coast, beach ridge height is highly variable, and in some cases tied to eolian processes, or age duration. Beach ridge height on any one profile is also variable. If the shoreline is subsiding, not prograding, or eroding, the first beach ridge may be two times higher than the ridges landward.