An upper Anisian microbial platform: the Latemar (Western Dolomites, Italy)

Field observations on the Latemar platform suggest a much higher microbialite amount compared to literature data. This is evident especially in the platform-break/upper slope zone, where microbial boundstone associated with calcareous sponges, Tubiphytes, botryoidal aragonite and radiaxial fibrous calcite clearly extend down to 250 m deep. A detailed facies analysis of all the portions of the platform allowed a subdivision of the Latemar isolated platform into four different facies belts: inner platform (up to 2.5 Km wide), outermost platform (ca. 200 m wide), margin (ca. 30 m wide) and slope (ca. 400 m wide). Each facies belt is composed by a different number of sedimentary facies, all representing subtidal environments with the only exception of two sedimentary facies of the inner platform. Point counting analysis was carried out on ca. 90 thin sections to quantify the different components. Components were grouped into five categories: skeletal grains, allomicrite, microbialite, cements and voids. Cements and microbialite are the most represented categories: they are, respectively, ca. 40% and ca. 37% for the whole platform. These results are similar to those reported for the lower Carnian Sella platform, and suggest that carbonate production at Latemar occurred in a M-Factory. Microbial carbonate production is higher on the slope and outermost platform, although microbialites are found in all facies belts. The presence of supratidal sedimentary facies restricted only to the inner platform and of a completely subtidal outermost platform suggest a new depositional model for the Latemar. The central portion is occupied by the flat platform interior sedimentary facies, cyclically subaerially exposed. Facies of the subtidal outermost platform surround the inner platform and display a progressive increase in depth toward the platform- break. Cyclical oscillations of water energy are testified. Oncoidal packstone, expressions of high energy, subtidal environments are present in the outermost platform when the Inner platform is exposed. On the contrary, when oncoids develop in the inner platform, clotted peloidal packstone-grainstone are deposited in the outermost platform. The margin is submerged and essentially constituted by cement and microbialite boundstone. The platform-break connects the margin to the slope, highly productive, microbialite-dominated in its middle-upper portion (0-250 m), not light-dependent, poor in platform top derived sediments, characterized by steep clinoforms (~ 35°). All the observations are in agreement with the slope shedding model observed in the Pennsylvanian microbial margin in Asturias (N Spain) and allow to interpret the Latemar as a flat-topped platform dominated by microbialites.