Lorenz Keim

Automicrite facies on steep slopes (Triassic, Dolomites, Italy)

SummaryThe Triassic platforms of the Dolomite Alps of northern Italy are famous for their well-preserved platform flanks. We report on the discovery of extensive automicrite on these platform slopes. Automicrite stands for autochthonous micritic carbonate, formed by in-situ precipitation that was mediated by organisms. The automicrite occurs on the 400 m high, 25°–35° dipping slopes of the Sella platform that were pervasively dolomitized. Textures include peloidal aggregates (thrombolitic microfabric), irregular spar-filled cavities as well as biodetritus and are in many samples clearly visible despite the dolomitization. The peloids may occur as loose particles floating in lighter-colored cement or form a loosely connected framework. The preservation of automicrite textures along with the nearly unaltered slope geometry of the platform lead to several conclusions on the development of automicrite on such steep and high platform slopes.1)Automicrite formed patches and layers from the platform top down to over 200 m on the flank and constitutes about 25% of the volume of slope sediment.2)Automicrite alternates with layers of skeletal and lithoclastic rubble and sand.3)Despite the extensive occurrence of automicrite, no mud mounds developed on the steep slopes. The autochthonous carbonate only forms layers and cushions that do not significantly alter the planar clinoforms.4)On the slope, layers of automicrite slid, became fragmented and turned into breccias that dominate the lower slope. The undolomitized, distal ends of these breccias contain the Cipit boulders extensively described in the past.

Quantifying the geometry and sediment fabric of linear slopes: examples from the Tertiary of Italy (Southern Alps and Gargano Promontory)

The geometries and sediment characteristics of two well-exposed submarine slope successions of ancient carbonate platforms were studied to analyse the relationship between inclinations of linear slopes and sediment fabric. The two examples are an Oligocene platform-to-slope succession exposed on the Brione Mountain in the Southern Alps, and an Eocene slope-tobasin succession exposed on the Saraceno Mountain in the Gargano Promontory. The planar slope of the Brione Mountain outcrop is inclined up to 20–25j and consists of fine to coarse sand sized, skeletal packstones and grainstones, and that of the Saraceno Mountain outcrop has a modal angle of 15–22.5j, and comprises very fine to coarse sand sized, poorly washed skeletal packstones, grainstones, and rudstones. The good agreement between the inclinations of the linear slope profiles and angles of repose published in the literature implies that these slopes are at or very close to the angle of repose. The general rule that in systems built to the angle of repose, slope angle is directly correlated to sediment fabric, can be applied to linear slopes. D 2002 Elsevier Science B.V. All rights reserved.