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ABSTRACT: Computer Models for Defining Eustatic Sea Level Fluctuations in Carbonate Rocks

J. F. Read, M. E. Elrick, D. A. Osleger

One- and two-dimensional computer models of carbonate sequences help define the amplitudes and periods of 20,000 years to 1 m.y. and 1-3 m.y. sea level fluctuations that have affected carbonate rocks. The models show that with low-amplitude 20-100 k.y. sea level fluctuations, tidal flats are likely to extend across the platform during short-term regressions, and vadose diagenesis is limited because sea level rarely drops far below the platform surface. With high-amplitude 20-100 k.y. sea level fluctuations, tidal flats are confined to coastal locations, very deep-water facies are overlain by shallow-water beds, and during regression sea level falls far below the platform, leaving karstic surfaces. The models also allow testing of random vs. Milankovitch-driven sea level c anges. The feasibility of cyclic sedimentation due to autocyclic processes under static sea level can be shown by the modeling to be less likely than Milankovitch climatic forcing for developing cyclic carbonate sequences. Models also help define relative dominance of 100 k.y. vs. 20 or 40 k.y. sea level oscillations. The presence of shallow-ramp vs. deep-ramp upward-shallowing cycles that are common on many platforms provides valuable constraints on the modeling in that the sea level fluctuations generating the shallow cycles also have to be able to generate the deeper ramp cycles. Sea level fluctuations of 1-3 m.y. are constrained by the modeling because overestimated amplitudes result in sequences and high-frequency cycles that are far too thick. Rate of long-term falls are constraine by the modeling because where fall rate exceeds driving subsidence, the outer platform becomes unconformable, whereas it remains conformable where fall rate is below driving subsidence rate. Quantitative modeling techniques thus provide the means of constraining amplitudes and frequencies of eustatic sea level fluctuations in ancient carbonate sequences.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990