ON THE USE OF CYCLE THICKNESS DIAGRAMS AS RECORDS OF LONG-TERM SEALEVEL CHANGE DURING ACCUMULATION OF CARBONATE SEQUENCES

Analysis of variation in changing rates of accommodation space creation, as represented by stratigraphic thickness, has become an integral part of ongoing investigations into the origin of ancient, meter-scale, upward-shoaling, cyclic marine carbonates. Groupings of thicker than average and thinner than average cycles within long stratigraphic sections have been thought to represent the influence of long-term eustatic sealevel variation on rates of accommodation space creation under conditions of constant basin subsidence. Statistical analysis demonstrates that a large percentage of early Paleozoic cyclic carbonate sequences contain thickness distributions indistinguishable from those anticipated for random associations of cycles, and that only a few have characteristics commensurate with long-term changes in rate of accommodation space creation. Moreover, in those cases where some extraneous control is suggested, computational considerations readily demonstrate that change in subsidence rate and/or differential compaction could easily give rise to similar stratigraphic associations of cycle thickness. Even if it is accepted that long-term sealevel change has occurred during the accumulation of cyclic carbonates, significant time gaps and difficulty in recognizing and documenting thin cycles results in considerable uncertainty when attempting to use cycle thickness diagrams as records of long-term sealevel change.