Page 43 - Carbonate Sedimentology and Sequence Stratigraphy

34 WOLFGANG SCHLAGER

Sea cliffs ly, cliffs in carbonate rocks retreat particularly fast and the
Sea cliffs are not restricted to carbonate rocks but they are adjacent abrasion platforms are nearly devoid of debris from
particularly common in limestones and dolomites. Sea cliffs rockfall.
form by wave erosion in the uppermost subtidal and the in- The spectacular and common appearance of cliffs on the
tertidal zones. In carbonates, intertidal bioerosion adds to modern carbonate shores constrasts sharply with the scant
the power of wave erosion by undercutting the cliff and re- evidence reported from the distant past. Fig. 2.30 summa-
moving the boulders on the abrasion platform. Consequent- rizes personal observations from modern carbonate shores

300
theoretical denudation rates
A) homogeneous, hard rock B) ± homogeneous, soft rock
250

200

denudation (mm ky -1 ) 150 C) inhomogeneous rock,

seaward dip of profile depends on orientation of bedding
100

50

500 1000 1500 2000 2500 3000
-1
precipitation - evapotransporation (mm y )
Fig. 2.29.— Denudation rates of karst surfaces. Lines represent Fig. 2.30.— Shape of the cliff profile as a function of anatomy
rates calculated with the model of White (1984) that assumes net of the rock and dip of the bedding. Based on field observations of
precipitation, CO 2 content of the meteoric water, and temperature B.W. Fouke and the author.
to be the three most important controls. Some observed rates
plotted with mean and standard deviations fit the model fairly well.
After Dreybrodt (1988), modified.

Fig. 2.31.— Influence of bedding on cliff profile – an example from Exuma Islands, Bahamas. Rocks are Pleistocene eolianites
that consist of harder and softer layers. Where bedding is flat, a near-vertical cliff develops; the varying hardness of the layers
only shows in the rugged outline of the cliff. In the lower part, where bedding dips seaward, the weathering profile follows the
bedding. Photo courtesy of D.F. McNeill.