Since the rate of carbonate production is depth dependent, both sea level and accumulation space represent critical boundaries for consideration. Directly related to these variables are three end members:
Exposure: This is associated with periods of low sea level when the carbonate platform is subaerial and subject to erosion and diagenesis.
Drowning: Rapid may submerge the carbonate factory below the photic zone and subsequently shut down the carbonate factory. The regression must exceed the rate of carbonate accumulation for this to occur. Such drowning events may create unconformities from siliclastic deposition. Platform drowning may also occur from episodic rapid subsidence induced by tectonism. If the depth to which drowning occurs is less than extreme, sediment accumulation may allow the seafloor to rise into the photic zone and again begin carbonate production. While reefs may keep up with sea level rise due to high accumulation rates, shallow tidal carbonates, with lower growth rates, may be drowned more easily thus forming a rimmed platform (Kendall and Schlager, 1981).Flooding: This may be seen as an intermediary between the previous two extremes. It refers to a steady state situation between subsidence (i.e. space for accumulation), accumulation, and sea level (Schlager, 1981).The following diagrams illustrate the variations of response of carbonate platforms given changes in sea level.
Drowning:As indicated above, this is associated with a relative rise in sea level that is too rapid to allow for carbonate vertical accretion. The result of this situation is an inactive carbonate factory as water depth has effectively exceeded maximum tolerances for carbonate production (Example: Cretaceous time and left diagnostic features that are recognizable both in seismic section (i.e. "growing unconformities" of Schlager, (1989)) and outcrop (i.e. abrupt facies changes from shallow to deep-water sediments).Back Step:Similar to drowning, this stage too is associated with a rise in sea level. The rate of this transgression, however, is not so rapid as to exceed vertical growth of the carbonate mound. In response to the change in base level the carbonate platform re-establishes itself in a more landward position. At this stage in the depositional cycle this may be linked to an early transgressive systems tract.Catch Up:This event is initiated by a short, rapid rise in sea level immediately followed by a slower rise. Upon the first rise, carbonate accumulation slows as the increase in water depth stunts, but does not shut off the carbonate factory. As the rate of sea level rise slows the carbonate platform effectively builds vertically and if this accumulation rate is greater than the rate of sea level rise, the carbonate factory will become increasingly efficient. Ultimately, the carbonate platform will accumulate and "catch up" to the rising sea level until depths for optimum productivity are established. Under these high productivity and accumulation rates vertical accommodation space may become limiting and lateral progradation may associate. This event is also associated with a transgressive systems tract and thick, shallowing upward subtidal sequences. It may be capped by prograding tidal flat deposits.Keep Up:As indicated by the term, this event is characterized by a relative rise in sea level whose rate does not exceed that of carbonate accumulation. As a result of this balance between sea level rise and vertical carbonate accretion accommodation space is continually created at the same rate it is filled. This steady state situation creates a uniform depositional sequence with subtidally dominated sequences (See also transgressive systems tract).Seaward progradation:Here carbonate production exceeds the rate of relative sea level rise and subsequently little space for vertical accommodation limits vertical accretion. Growth, therefore, is directed seaward as the platform accretes. Depositionally, this period is associated with a highstand systems tract.Fillup and Spillout:This event is similar to platform progradation, except here the excess in carbonate production over relative sea level rise is directed landward. This shoreward shift in carbonate production promotes tidal flat progradation over the platform which effectively reduces the size of the carbonate factory by lowering the width of the platform itself. Depositionally, this period is also associated with a highstand systems tract (Kendall and Schlager, 1981).
Index to carbonate shelf sediments