ONE SEA LEVEL RISE - TWO CYCLES: IMPLICATIONS FOR ANALYSIS OF ANCIENT STACKED CARBONATE TIDAL FLAT CYCLES

Lawrence Hardie and his students were trendsetters in the description and analysis of shallowing-upwards cycles in carbonate rocks. One widely-recognized problem in the numerical analysis of stacked successions of carbonate cycles is the possibility of incomplete records (e.g. so-called “missed beats”). The ~ 2 m thick Holocene sediments preserved beneath Cotton Key, a mangrove-covered intertidal-supratidal island in Florida Bay, present the opposite problem. Cores of the sediments beneath Cotton Key reveal two cyanobacterially-laminated muds (Hardie’s “laminite cap”) separated by thin-bedded pond sediments. Although these sediments accumulated in intertidal to supratidal settings, during one continuous sea level rise, it seems likely that in ancient rocks this one cycle could be logged as two separate cycles. This is especially likely because snails and bivalves are preserved in the thin-bedded pond sediments and careful analysis would be required to differentiate these faunal elements from the more open marine fauna of the mud banks and lake sediments. This double cycle probably reflects minor variations in elevation of the top of the island during sea level rise and island aggradation. It is difficult to judge how important such double cycles are beneath the keys of Florida Bay. Coring reveals that the lower laminite extends a few hundreds of meters beneath Cotton Key. Enos and Perkins (1979) show a number of other examples of this double cycle phenomenon beneath other keys. Clearly one or two such occurrences in the analysis of a section comprising one hundred or more cycles would not cause errors in spectral analysis. However, numerical experiments show that if just 5 to 10% of cycles in a succession of Lower Paleozoic carbonate cycles were in fact misidentified double cycles (similar to that from Cotton Key), spectral analysis of this succession could be in error. Finally, I would argue that the lower cycle preserved beneath Cotton Key is a bona-fide example of a modern carbonate shallowing-upward autocycle: a beast heretofore only found in thought experiments and computer simulations.