Carbonate platform slopes of the Alpine Triassic and the Neogene - a comparison.

Angle and curvature of terrestrial slopes depend on the grainsize of the slope-forming material and this principle, with slight modifications, also applies to the slopes of carbonate platforms. The Triassic platforms of the Dolomites area of the Southern Alps and occasionally in the Northern Calcareous Alps, are characterized by straight, steep (>30 degrees) slopes that plunge to 400-800 m water depth. In agreement with the above principle, the Triassic slopes consist of sand, rubble and lenses of micritic carbonate precipitated in situ under the influence of microbes. Neogene platforms of the Bahamas and the Maldives, as well as smaller platforms in the Caribbean and off eastern Australia show vertical cliffs and steep talus cones only in the upper 150-200 m, i.e. in the approximate range of sea-level fluctuations. The main parts of the slopes are gentle (<5 degrees) and abound with evidence of creep and slumping. The sediment of these lower slopes is predominantly mud, in particular clay-size aragonite shed from the platforms. The contrast between Triassic and Neogene platform slopes is surprising at first, because the basic depositional environments on the platforms show very similar zonation of tidal flats, lagoons and wave-resistant organic framework at the platform margin. We propose that the difference lies in the way the fine-grained (= micritic) carbonate is formed. In the Neogene, fine-grained carbonate is either precipitated in the water column or stored as loose accumulation on the platform, easily re-suspended during storms. Both processes generate frequent suspensions of carbonate fines on the platform that may be carried off into the basin. In the Triassic, strong circumstantial evidence indicates that most precipitation occurred on the sea floor as crusts and pellets in microbial mats. This material was carried off the platform as debris of hard pellets and intraclasts. The platform slopes reveal the difference between the mud-shedding Neogene and debris-shedding Triassic platforms.