Form, genesis, and deformation of central California wave-cut platforms

Modern and ancient wave-cut platforms on Ben Lomond Mountain in central California are broadly similar in shape. They have a seaward slope composed of two segments: a steeper, slightly concave inshore segment, with gradients of generally 0.02 to 0.04 (20 to 40 m/km), and a flatter, planar offshore segment with gradients of 0.007 to 0.017 (7 to 17 m/km). The flattest inshore and offshore gradients measured were, respectively, 0.015 (15 m/km) and 0.005 (5 m/km), suggesting that these are close to minimum gradients for erosional platforms in central California. The inshore segments are generally 300 to 600 m wide and extend to a depth of 8 to 13 m. Platforms are widest in areas where soft sandstone crops out and where there has been least uplift.

Major storm waves now break in water 7 to 12 m deep. We conclude that inshore platform segments were associated with storm-wave surf zones and that offshore segments were associated with the zone of deep-water wave transformation. A gradient of 0.005 for the offshore segment would keep wave energy at the bottom constant (Zenkovich, 1967). A steeper gradient for the inshore segment would enable backwash undertow to counteract the strong onshore movement of surf, so that available coarse sediment could be moved laterally. Slopes less than the minimum would so dissipate wave energy in offshore areas that the surf zone would not be able to provide the needed longshore transport for coarse sediment, and beach progradation would result. Thus, platforms have a shape that allows efficient conversion of wave energy into erosion and longshore transport; their seaward gradient is not used for the downhill transport of sediment. Platform gradients decrease with time, at least until the minimum is achieved. Whether the offshore segments were eroded at their existing depths or were eroded by surf zones as sea level rose remains a matter of controversy.

Ben Lomond platforms have been uplifted and progressively tilted in a seaward direction, indicating that late Tertiary domical uplift has continued into Quaternary time. Uplift rates have ranged from 0.16 m/1,000 yr near Santa Cruz to 0.26 m/1,000 yr near Greyhound Rock. Tilts have varied from 0.001 (1 m/km) for the lowest prominent platform to 0.009 (9 m/km) for the highest platform (which may be as old as 10⁶ yr). Because of uplift, platforms must have been cut at times of eustatically high sea level.