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

Abstract

Modern and ancient wave-cut platforms on Ben Lomond Mountain in central California [USA] are broadly similar in shape. They have a seaward slope composed of 2 segments: a steeper, slightly concave inshore segment, with gradients of generally 0.02-0.04 (20-40 m/km), and a flatter, planar offshore segment with gradients of 0.007-0.017 (7-17 m/km). The flattest inshore and offshore gradients measured were, respectively, 0.015 (15 m/km), suggesting that these are close to minimum gradients for erosinal platforms in central California. The inshore segments are generally 300-600 m wide and extend to a depth of 8-13 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 7-12 m deep. Inshore platform segments were associated with storm-wave surf zones and 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. A steeper gradient for the inshore segment would enable backwash undertow to counteract the strong onshore movement of surf, so that available course 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 course sediment, and beach progradation would result. 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 undetermined. Ben Lomond platforms were uplifted and progressively tilted in a seaward direction, indicating that late Tertiary domical uplift continued into Quaternary time. Uplift rates ranged from 0.16 m/1000 yr near Santa Cruz to 0.26 m/1000 yr near Greyhound Rock. Tilts varied from 0.001 (1 m/km) for the lowest prominent platform to 0.009 (9 m/km) for the highest platform possibly as old as 106 yr). Because of uplift, platforms must have been cut at times of eustatically high sea level.