Page 40 - Carbonate Sedimentology and Sequence Stratigraphy

CHAPTER 2: PRINCIPLES OF CARBONATE PRODUCTION 31

Bioerosion
Erosion by organisms is particularly intensive in lime- intertidal zone where it is rapidly bioeroded (Fig. 2.24). Bio-
stones and dolomites because they are highly soluble and erosion on hard substrates in the shallow subtidal zone are
consist of relatively soft minerals that are easily abraded. lower. In-situ experiments with limestone slabs indicate
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Furthermore, most carbonate skeletons contain abundant weight losses on the order of 150 – 500 g m y (Vogel
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food in the form of organic matter between the carbonate et al. 1996). Assuming a density of 2.5 g cm for the lime-
minerals and this is an added incentive for organisms to at- stone samples, this translates into a surface erosion rate of
tack the grains. 60 – 200 µ/y sustained over about a year. This is several or-
The intensity of bioerosion varies enormously. It seems to ders of magnitude less than the rate of carbonate production
be most intensive in the intertidal zone of the tropics where in shallow tropical environments (Fig. 2.19). Bioerosion in-
notches cut by bioerosion advance at rates of 2500 µ/y av- creases with the nutrient content of the ambient water (e.g.
3
eraged over 10 y (Neumann and Hearty, 1996). The peak Hallock, 1988). Consequently, eutrophication of reefs inten-
of bioerosion in the intertidal zone is largely responsible for sifies bioerosion of the reef rock thus decreasing the growth
the formation of the spectacular limestone cliffs because the potential. In cool-water environments, bioerosion may ex-
rapidly advancing notch undercuts the cliff like a saw; from ceed the rate of cementation. The result is an overall de-
time to time the cliff collapses, putting more material in the structive diagenetic environment.

Fig. 2.24.— The cliffs of a Pacific island illustrate several principles of carbonate erosion. Erosion is most intensive in the intertidal
zone where bioerosion and wave abrasion combine to undercut the island rocks, producing near-vertical sea cliffs. Intertidal erosion
is also responsible for the water-covered terrace in the foreground and the overhanging beach at the foot of the cliff. Note uplifted
fossil intertidal zone higher in the cliff. Karst erosion (by carbonate-dissolving rainwater and plants) creates the more gentle landforms
above the cliff and modifies the upper parts of the cliff. In the shallow-marine environment, carbonate erosion is normally outpaced
by precipitation. After Menard (1986).