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 creatio n, as represented by stratigraphic thickness, has become an integral p art 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 sect ions have been thought to represent the influence of long-term eustati c sealevel variation on rates of accommodation space creation under co nditions of constant basin subsidence. Statistical analysis demonstrat es that a large percentage of early Paleozoic cyclic carbonate sequenc es contain thickness distributions indistinguishable from those antici pated for random associations of cycles, and that only a few have char acteristics commensurate with long-term changes in rate of accommodati on space creation. Moreover, in those cases where some extraneous cont rol is suggested, computational considerations readily demonstrate tha t change in subsidence rate and/or differential compaction could easil y give rise to similar stratigraphic associations of cycle thickness. Even if it is accepted that long-term sealevel change has occurred dur ing the accumulation of cyclic carbonates, significant time gaps and d ifficulty in recognizing and documenting thin cycles results in consid erable uncertainty when attempting to use cycle thickness diagrams as records of long-term sealevel change.