Misuse of Fischer plots as sea-level curves

Abstract

Fischer plots are graphic representations of cyclic carbonate deposits showing cumulative departure from average cycle thickness plotted against cycle number and corrected for assumed subsidence during each cycle. Observed cycle thicknesses in excess of subsidence have been interpreted to represent depositional accommodation space formed by eustatic sea-level rise. However, implicit in this interpretation is the assumption that preserved cycle thickness is a proxy for accommodation. To test this assumption, a survey of carbonate-sediment accumulation patterns (i.e., cycle thicknesses) developing during a single transgressive 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., subsidence was minimal). This survey demonstrates that Holocene sediment (cycle) thickness and accommodation are uncorrelated (r 2 = 0.03). Consequently, Fischer plots constructed by using Holocene cycle thicknesses are poor representations of the Holocene transgression. In extreme examples, Holocene Fischer plots would be interpreted to show relative sea-level fall during the Holocene on Great Bahama Bank because Holocene 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 insensitive to broad variations in stratigraphic completeness, cycle duration, or subsidence. Because cycle thickness apparently is uncorrelated with accommodation and the gross form of Fischer plots is relatively invariant, it seems prudent to reevaluate the practice of interpreting Fischer plots as sea-level curves per se in the analysis of ancient carbonate cycles.