Identification of High Frequency Eustatic Records Using Stacking Pattern Analysis in Peritidal Carbonate Successions

Paul A. Dunn

One and two-dimensional forward modeling of carbonate deposition were used to test the relative impacts of sea level and subsidence changes on peritidal carbonate parasequence (depositional cycle) stacking patterns. If high frequency episodes of tectonic uplift were ruled out, then the effects of high frequency eustasy were distinctly different from those of variable subsidence rates. Model runs which used smoothed-random ("red noise") sea level oscillations and constant subsidence yielded parasequence stacking patterns which were dominated by asymmetric, thinning upward bundles. These parasequence sets formed due to "over-aggradation" of the platform during anomalously high amplitude or prolonged flooding events which allowed the platform top to aggrade above its equilib ium position within the envelope of high frequency eustasy. Subsequent parasequences were deprived of space until subsidence returned the platform top back to an equilibrium zone. Variable (but always downward directed) subsidence driven by the same smoothed-random function together with static sea level did not produce the thinning upward bundles. In the absence of frequent episodes of uplift, subsidence lacks the zero-crossing character of sea level which permits "over-aggradation" and formation of thinning upwards parasequence sets.

In exposures of ancient, shallow water carbonates it may be possible to document the role of eustasy through stacking pattern analysis in which observations at the subfacies level are assembled into a succession of parasequences (commonly shallowing upwards depositional cycles). The ordering of parasequence thicknesses, subfacies compositions, and diagenetic features can then be used to interpret the relative importance of the primary controls on platform growth. Stacking patterns observed in a Triassic succession of peritidal carbonates from the Dolomites Mountains of NE Italy which contains numerous thinning upwards parasequence sets can be interpreted as the record of short term accommodation minima triggered by pulses of "over-aggradation."

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995