FISCHER PLOTS CONSTRUCTED USING VARIABLE CYCLE DURATIONS AND THEIR ALTERNATIVE SEQUENCE STRATIGRAPHIC INTERPRETATION

Fischer Plot is effective in delineating cycle thickness stacking patterns in cyclic successions, which facilitate inter and intra-basinal stratigraphic correlation, sequence delineation, and interpretation of cyclic sedimentation. The plot commonly has a vertical axis as cycle thickness deviation from mean cycle thickness and a horizontal axis as cycle number, implying a constant subsidence rate and equal cycle duration. The thickening-and-thinning trends or waves have been interpreted as platform accommodation cycles and, because of the assumed constant subsidence, as approximate eustatic cycles. These interpretations assume that all shallowing-upward cycles shoaled to the sea level and that sedimentation rates were constant.

We constructed Fischer plots of two cyclic peritidal carbonate successions of the Lower Triassic Great Bank of Guizhou (GBG), South China, which are composed 84 and 68 peritidal carbonate cycles, respectively, using variable durations of individual cycles. Cycle durations were calculated using facies-dependent sedimentation rates obtained by gamma analysis of Kominz and Bond (1990). The subtidal facies is 5 times larger in sedimentation rate than the intertidal and supratidal facies and the cycle duration ranges from 3-93 ky. In comparison to the constant-cycle-duration plots, the variable-duration plots have more asymmetrical waves with a steeper thickening (up) limb and a gentler thinning (down) limb, suggesting that, assuming constant subsidence, accommodation increase and sea-level rise are faster and accommodation decrease and sea-level drop are slower. The number and amplitude of major waves remain the same although the shape of minor waves changed moderately.

The sea-level fluctuations during the Early Triassic greenhouse period were minimal and cyclic sedimentation was dominantly controlled by variations in subtidal carbonate productivity on the GBG. Thus, the waves on the Fischer plots may represent carbonate productivity cycles, not eustasy-controlled accommodation cycles. Commonly, a sequence boundary was interpreted at the deflection point on the thinning limb of the waves, assuming that the waves represent accommodation cycles. However, if the waves represent carbonate productivity cycles, the thinning would represent decreased or condensed carbonate sedimentation and the deflection points would represent the marine condensed sections. This interpretation is supported by the clustering of thin subtidal cycles around the deflection points on the thinning limbs and the restriction of exposed surfaces to thickening upward cycles. Last, the cycle thickness stacking patterns of the two GBG successions as depicted on the Fischer plots correlate well over a distance of 4 km, substantiating the physical stratigraphic correlation.