Modified fischer plots as graphical tools for evaluating thickness patterns in stratigraphic successions

Abstract

Fischer plots are a simple, objective graphical tool for illustrating thickness trends in cyclic successions and are particularly amenable for use in stratigraphy courses. Their utility in stratigraphic analysis is making its way into textbooks, and the large body of literature discussing the strengths and limitations of Fischer plots provides fertile ground for discussions about cyclic successions and concepts such as time-versus-thickness in undergraduate and graduate classes. In an effort to enhance the utility of Fischer plots and to reduce some of their inherent limitations, we suggest two modifications to the fundamental technique. First, we suggest that the cumulative departure from mean cycle thickness should be plotted against the individual measured cycle thicknesses rather than the unit-mean cycle thickness. This time-independent modification has the advantage of being directly comparable to measured thicknesses of the stratigraphic columns from which they were derived and also may, in some cases, enhance the sharpness of the waveform. The second suggested correction to the traditional Fischer plot is to decompact individual lithofacies prior to plotting. A correction for compaction is especially useful in successions that consist of heterogeneous facies that exhibit clear differences in compaction through time. In certain cases, decompacted Fischer plots reveal higher magnitude changes in thickness patterns that may not otherwise be recognizable in traditional or modified Fischer plots. In the spirit of 'the only way to understand it is to do it,' we provide a simple LISP subroutine for automatically drawing each type of Fischer plot under any Autocad environment.