Antecedent Topography
Within a reef facies topographic
highs will be produced as shallow water corals grow faster than deeper water
forms and sediment accumulation will be reduced on higher platforms (Scoffin
et al., 1978). These topographic highs are also subject to increased water circulation,
which stimulates ooid formation, while the lows tend to be less energetic
where muds preferentially accumulate. The presence of antecedent topographic features will hence influence
the growth patterns and position of modern reef complexes (Figure 1).
Figure 1. Cross-section illustrating development of modern reefs upon pre-existing topographic highs of Pleistocene limestone (After James and Ginsburg (1979).
Currently, four classes of antecedent topography have been recognized (James and Macintyre, 1985):
1. Older Reefs: Includes the development of modern day reef structures atop older reefs as exposed platforms were submerged during the last rise in sea level. These modern complexes preferentially grew on the topographic residual of the earlier reefs. Examples of such features can be found at the Great Barrier Reef, and off the coasts of Florida, Belize, and Bermuda.
2. Erosional Terraces: Erosional terraces were produced throughout the Pleistocene when sea level low-stands allowed for wave erosion along many coastal zones. Reef organisms later colonized these etched terraces when the sea level rose (Goreau and Land, 1974).
3. Siliclastic/Volcanic Topographic Features: New reef growth has taken place on depositional features. These may be volcanic in origin or, as in the case of the reef structures in Belize, upon the river terraces of the Pleistocene.
4. Karstic Topography: Karst terrain occurs when sea level low-stands allowed for prolonged subaerial exposure. As illustrated in Figure 2, slightly acidic rainwater falls atop and dissolves the exposed carbonate shelf. Maximum dissolution occurs where runoff is most concentrated. Although the composition of the exposed carbonate shelf does influence the extent of karstification, the major control is climate. Karst is most common in carbonate terrains in humid regions of all kinds (temperate, tropical, alpine, polar), but is most commonly seen in warm, wet, tropical climates. These tropical climates are most susceptible to karst formation because although calcite and CO2 solubility both decrease with temperature, high temperatures generate greater CO2 production, which in turn offsets the diminution of CO2 solubility, thus increasing the acidity (H2CO3) of the rainwater which is delivered more frequently in humid regions. Karst development is, however, not always associated with subaerial carbonate exposure. Also associated with sea level low-stands is a progradation of terrigenous sediment. A blanketing of the exposed carbonates by siliclastic sediments will occur within the carbonate topographic lows, this limiting the rainwater dissolution effects to the exposed topographic highs (Figure 3).
Figure 2. Illustration of the evolution
of karstic topography from limestone platform exposure (drop in sea level)
to dissolution to occurrence of modern reef structures upon later transgression.
From Purdy (1974).
Figure 3.
Illustration of siliclastic deposition upon regression and it's influence
on karst formation, Northeast Australian Platform.
From Davies et al., (1989).
Proceed on to the carbonate platform question set.