Page 37 - Carbonate Sedimentology and Sequence Stratigraphy

28 WOLFGANG SCHLAGER

As in siliciclastic systems, mechanical erosion may have Chemical erosion
a profound effect on sediment accumulations in carbonate
environments. In addition, carbonates may be modified by At present, the surface waters of the ocean are nearly ev-
chemical dissolution and bioerosion (Fig. 2.24). The differ- erywhere saturated with respect to calcite. Dissolution in
ent types of erosion may act in consort. Furthermore, sea- shoal-water carbonate settings is restricted to the most solu-
floor lithification and mechanical erosion are often coupled ble mineral phases, i.e. magnesian calcites with very high
because lithification is a relatively slow process that operates magnesium contents (>18 mol%) and aragonite grains of
most effectively where currents sweep the seafloor. very small size (1 micron or less). In the deep sea, how-
ever, sea-floor dissolution is intensive. There is no fixed
depth level that marks the onset of dissolution. Currently,
Mechanical erosion
the tropical Atlantic is undersaturated with respect to cal-
cite below about 4 km; the eqivalent level in the Pacific lies
Mechanical erosion is distributed very unevenly over car- at approximately 1 km. The analogous levels for aragonite
bonate depositional environments. It is most intensive at are 1-3 km for the tropical Atlantic and 0.2 km for the trop-
platform margins where the construction of reefs and lithi- ical Pacific. The difference between the two ocean basins is
fied sand shoals drastically alters the equilibrium profile be- the result of deep-sea circulation and basin-basin fractiona-
tween the sediment and the wave energy in the water col- tion (p. 3). We may assume that similar ranges in dissolution
umn. Siliciclastic systems adjust easily to the ocean’s energy levels occurred in the geologic past. Carbonate dissolution
regime and develop shelf breaks on average around 100 m need not be restricted to the deep sea. There is a tendency
water depth. In contrast, reefs or carbonate sand shoals can to develop a shallower dissolution maximum in the thermo-
build to sea level at the same position. In fact, modern coral cline, between about 0.1 and 1.5 km in the water column that
reefs are able to withstand all but the seas and swells of may affect epeiric seas and intracratonic basins.
ocean-facing coasts in the trade wind belt – the most ener- Terrestrial dissolution of limestones or dolomites is con-
getic wave regime in the tropics. It is obvious that mechani- veniently summarized under the term karst . The dominant
cal erosion is intensive in these settings and constant frame- factor in karst erosion is dissolution by rain water and as-
building and cementation are required to repair the damage. sociated goundwater. Mechanical erosion plays a secondary
It should be noted that sea-floor lithification is common and role. Dissolution occurs at the surface and within the car-
geologically coeval with deposition. This greatly reduces bonate rocks, using matrix pores and fractures as avenues.
the rate of mechanical erosion and makes piles of carbonate A zone of intensive internal dissolution is around the water
sediment more resistant to lateral displacement. table and in the mixing zone of fresh water and sea water
Drowned platforms in open-ocean settings are affected (Moore, 2001, p. 210). Karst erosion goes hand in hand with
by particularly intensive mechanical erosion. (Figs 2.25, cementation and is normally described as diagenetic alter-
2.26, 2.27). Like submarine volcanoes, oceanic platforms ation if it occurs within a carbonate formation.
disturb the normally sluggish oceanic tidal waves by in- The rates of karst erosion increase with net precipitation
ducing eddies that circle the platform. Velocities in these and CO 2 content of the water, but with decreasing temper-
“topography-trapped waves” may become an order of mag- ature. The rates of surface lowering (“surface denudation”)
nitude higher than the orbital velocity of the original tidal are particularly important for sedimentologists as they in-
wave (Fig. 2.26), leading to erosion and sediment redistribu- fluence the morphology of the subsequent phase of marine
tion. On living platforms, this erosion is restricted to the rim deposition. Surface denudation is often less dramatic than
and upper slope. On drowned platforms, however, the cur- intuitively assumed. Fig. 2.28 shows a depositional topogra-
rents may sweep freely across the top. As a result, the edges phy of sand bars and channels that survived 120 ky of karst
of drowned oceanic platforms are often bare and the pelagic erosion in a humid tropical setting. Rates of karst surface de-
sediment cap on the inner platform is thin, lens-shaped and nudation are low compared to rates of carbonate deposition.
replete with hiatuses. Hiatuses in this setting easily span Fig. 2.29 presents rates of surface denudation as well as rates
tens of millions of years (Figs 2.25, 2.27). The interplay calculated with an empirical formula including the princi-
of mechanical erosion and sea-floor lithification on current- pal parameters mentioned above (Dreybrodt, 1988). Rates
swept, drowned platforms creates very irregular morphol- −1 1
determined at time scales of 10 –10 yby chemicaltech-
ogy that may resemble subaerial karst. niques are in the range of 10-100 µ/y (Fig. 2.29). Rates de-
Interplay of mechanical erosion and lithification is also termined from the Neogene and Quaternary geologic record
observed on the flanks of modern carbonate platforms. Con- for time scales of 10 –10 y and reported by Purdy and Win-
5
3
tour currents and turbidity currents are the eroding agents terer (2001) are similar or higher. The absence of a scaling
and the high amount of metastable minerals shed from the trend analogous to the trend for sedimentation rates (Fig.
platform factory drives sea-floor lithification in areas that 2.19) is somewhat surprising but the datasets are very small.
are swept clean by the currents (Schlager and James, 1978). In any case, the rates of karst denudation are lower than
As on seamounts, bizarre micromorphology reminiscent of the carbonate sedimentation rates reported for similar time
karst or desert weathering is the result. scales.