Superimposed Platform Carbonate Cycles: Eustatic Response of an Aggradational Carbonate Buildup, Middle Triassic of the Dolomites

Robert K. Goldhammer

The Middle Triassic Latemar carbonate buildup of the Dolomites (Italy) records high-frequency stratigraphic cyclicity (fourth and fifth order) superimposed on low-frequency (third order) cyclicity. The Latemar platform contains four facies stacked vertically: (1) lower platform facies (LPF), 250 m of subtidal grainstones with abundant early marine diagenesis, noncyclic, subaerial exposure horizons every 10-15 m; (2) lower cyclic facies (LCF), 90 m of meter-scale cycles each composed of subtidal deposits overlain by a vadose diagenetic cap; (3) tepee facies (TF), 120 m of meter-scale cycles periodically interrupted by tepee zones depicting extended subaerial exposure; (4) upper cyclic facies (UCF), 210 m of meter-scale cycles similar to the LCF. High-frequency cyclicity is represented by at least 500 meter-scale cycles (average 0.65 m/cycle) occurring within the LCF, TF, and UCF, each of which records subaerial exposure on a 10⁴ year scale (approximately 20,000 years average). Five meter-scale cycles (fifth order) are grouped into asymmetric megacycles (fourth order). This 5:1 ratio of the superimposed high-frequency orders and the asymmetry of the 10⁵ year fluctuation are predicted by a Milankovitch glacio-eustatic mechanism. Lower frequency cyclicity (third order) is expressed by the Latemar facies succession (LPF ^rarr LCF ^rarr TF), which records progressive increase in platform subaerial exposure and decrease in cycle thickness, reflecting a progressive decline in relative subsidence rate. The LPF ^rarr LCF ^rarr TF facies succes ion depicts a third-order eustatic sea level fluctuation (10 m.y. period, 60 m amplitude).

AAPG Search and Discovery Article #91030©1988 AAPG Annual Convention, Houston, Texas, 20-23 March 1988.