MISUSE OF FISCHER PLOTS AS SEA-LEVEL CURVES

Fischer plots are graphic representations of cyclic carbonate deposits showing cumulative departure from average cycle thickness plotted aga inst cycle number and corrected for assumed subsidence during each cyc le. Observed cycle thicknesses in excess of subsidence have been inter preted to represent depositional accommodation space formed by eustati c sea-level rise. However, implicit in this interpretation is the assu mption that preserved cycle thickness is a proxy for accommodation. To test this assumption, a survey of carbonate-sediment accumulation pat terns (i.e., cycle thicknesses) developing during a single transgressi ve event (the Holocene postglacial sea-level rise) was conducted on a shallow carbonate platform (Great Bahama Bank) where accommodation was created primarily by sea-level rise over the platform margin (i.e., s ubsidence was minimal). This survey demonstrates that Holocene sedimen t (cycle) thickness and accommodation are uncorrelated (r(2) = 0.03). Consequently, Fischer plots constructed by using Holocene cycle thickn esses are poor representations of the Holocene transgression. In extre me examples, Holocene Fischer plots would be interpreted to show relat ive sea-level fall during the Holocene on Great Bahama Bank because Ho locene subsidence currently exceeds sediment thickness. In addition, a simple sensitivity test shows that eustatic sea-level interpretations based on interbasinal correlation of Fischer plots are equivocal. The gross form of Fischer plots appears so overly robust as to be insensi tive to broad variations in stratigraphic completeness, cycle duration , or subsidence. Because cycle thickness apparently is uncorrelated wi th accommodation and the gross form of Fischer plots is relatively inv ariant, it seems prudent to reevaluate the practice of interpreting Fi scher plots as sea-level curves per se in the analysis of ancient carb onate cycles.