Globally, approximately 10% of all carbonate production occurs within shallow marine settings. Between both deep and shallow water environments 60% of the total carbonate accumulation is performed by reefs. Organism production rates vary depending on specific individuals, but provided ambient conditions are ideal (i.e. salinity, temperature, water depth, etc.), mean productivity rates allow for 1.5-4.5 kg CaCO3 yr-1 (0.5-1.5 m ky-1) (Schlager, 1981).

Calculating the amount of sediment produced by modern carbonate secreting organisms, and factoring in observed effects of position and depth on growth determines these accumulation rates. Caution must be taken, however, as lateral progradation of reef sediments can appear as vertical accretion within the sedimentary record (Hardie et al., 1991).

Not only is carbonate accumulation dependent on ambient water conditions but also location across a platform.Lagoonal and back reef regions have typically recorded accumulation rates between 0.2-0.4 m ky -1 (Florida Keys, Boescence, 1989). These accumulation rates are lower than mean platform accumulation rates due to lower nutrient inputs and possible high salinities and temperatures. Likewise, deeper ramp accumulation rates also reflect less than ideal growth conditions with an average of 0.1-0.2 m ky-1 (Boescence, 1989). To the contrary, accumulation along the reef front is bolstered by high nutrient delivery and subsequently growth rates can be as high as 6 m ky-1 (Boescence, 1989).

Modern examples of carbonate accumulation quantification stem from the Bahama region, and the Great Barrier Reef. Accumulation rates for the Florida Reef Tract are unusually high over 3-6 ky. Maximum thickness over this interval is 14 m, indicating accumulation rates were 2.3 m ky-1. Modern mean productivity rates allow for accumulation of 0.5-1.5 m ky-1.

Ancient examples of carbonate accumulation rates are low compared to the modern mean. From the Bahama Platform only 4km of carbonate was deposited from the Upper Cretaceous (100 Ma ago). This indicates a net accumulation rate of only 0.04 m ky-1. Another example can be found in western Texas with an accumulation rate of 0.05 m ky-1.

Initially the determined 2x orders of magnitude differential between ancient and modern warm water accumulation rates appears disconcerting. Upon further examination, however, this apparent discrepancy in accumulation rates can be explained by the fact that ancient carbonate sections represent only a small portion of geologic time. Breaks found within these sedimentary sections thus may be representative of long periods of time within which active deposition and erosion (via sea level fluctuations and subaerial exposure) could have distorted calculated accumulation values (Wilkinson et al., 1991).

Index to carbonate shelf sediments

Proceed on to the carbonate platform question set.