Growth of carbonate platforms controlled by salt tectonics (Northern Calcareous Alps, Austria)

The Mid Triassic section of the Northern Calcareous Alps (NCA) is dominated by carbonate platforms, which grew diachronously on the Neo-Tethys shelf beginning in the Middle Anisian and ending in Lower Carnian times. The platforms grew isolated in previous deeper marine settings with high growth rates reaching 1.5 to 2 mm per year. The concept of self-controlled growth of carbonate systems on salt changes the understanding of Mid-Triassic NCA sedimentology. Conceptual models of the carbonate platform growth were done based on field observations, construction of cross-sections and subsidence analysis of selected carbonate mini-basins. To satisfy the observed boundary conditions of platforms growth in respect of timing, water depth and basin evolution, fast accumulation rates have to be assumed best represented by salt deflation and down-building of carbonate minibasins. A feedback loop of carbonate growth (creating a load gradient) and subsidence by salt evacuation initiates once the pre-kinematic layer reaches the sea level and the first layer of carbonate is produced. An initial phase of fast carbonate aggradation ends once the salt below the platform is fully evacuated and the minibasin is primary welded.

To further analyse and quantify boundary conditions necessary for the observed carbonate mini basin evolution, a series of thermo-mechanical numerical experiments were conducted. The density and rheological parameters for rock salt applied in the experiments were mainly gathered from observations and mechanical experiments on salt from salt mines and from an exploration well by OMV in the Vienna Basin. The numerical simulations essentially support the concept of down-building carbonate platforms. Self-controlled growth of carbonate systems on salt allows a completely new perspective to understand Mid-Triassic NCA carbonate platforms and their boundary conditions, such as the accumulation of thick carbonates (>1.5 km) without basement faulting, the isolated growth of platforms, or the transition of aggradational to progradational growth.