Platform/Foreslope Facies and Buildup Geometry Resulting from Short-Term and Long-Term Eustatic Sea Level Fluctuations: Latemar Buildup (Middle Triassic), Dolomites, Northern Italy

Mark T. Harris, Robert Goldhammer

Superimposed short-term and long-term eustatic sea level fluctuations directly controlled Latemar platform stratigraphy and indirectly influenced the deeper water facies and overall buildup geometry. Deeper water facies, the foreslope and toe of slope, are a function of platform submergence (highstand shedding) and subaerial exposure (lowstand lithification and erosion) and thus only indirectly reflect eustatic fluctuations.

The Latemar consists of a platform core (3-4 km wide, 700 m thick) with a narrow margin, flanked by foreslope (30-35° dips), toe of slope, and basin deposits. The shallowing-upward platform sequence records a long-term (about 10 m.y.) eustatic sea level oscillation with an amplitude of about 150 m. The lower 250 m marks an initial catch-up phase (subtidal carbonates); the upper 450 m marks the sequential keep-up phase (meter-scale cyclic carbonates). These cycles record platform submergence and exposure caused by short-term (10⁴-10⁵ years) Milankovitch eustatic oscillations superimposed on the long-term trend.

Platform submergence and exposure conditions result in contrasting foreslope deposits. During highstands, platform-derived sands bypass the foreslope, accumulating as toe-of-slope graded beds and basin turbidites. During lowstands, sand supply ceases, producing basin hardgrounds. Foreslope megabreccias contain margin-derived boundstone clasts, with only minor platform-derived sands (highstands) and lithified clasts (lowstands).

As the platform margin/foreslope contact is nearly vertical, a progressively increasing volume of foreslope megabreccia was needed to maintain the depositional geometry. This coincides with the most commonly exposed platform interval, suggesting that platform exposure determines buildup flank geometry by controlling megabreccia clast production.

AAPG Search and Discovery Article #91038©1987 AAPG Annual Convention, Los Angeles, California, June 7-10, 1987.