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Slope Facies of High-Relief, Steep-Slope Triassic Carbonate Platforms (Esino Limestone, Southern Alps, Italy): Sedimentary Processes and Marine Cementation

Berra, Fabrizio *1; Jadoul, Previous HitFlavioTop 1; Brack, Peter 2; Pessina, Riccardo 1
(1) Department of Earth Sciences, University of Milano, Milano, Italy.
(2) Departement Erdwissenschaften / focusTerra, ETH Zurich, Zurich, Switzerland.

Slope facies in carbonate platforms are characterized by a wide range of textural and sedimentologic features resulting from different types of carbonate producers, source area and depositional processes. Steep slopes are common in high-relief platforms, where they were produced by the dismantling and re-deposition of materials from the slope and reef portions and by carbonate production on the slope itself.

The greenhouse Late Anisian-Carnian high-relief (up to >500 m of platform top to basin relief) carbonate platforms of the Central Southern Alps (Italy) are characterized by prograding steep (25-35° on average) slope facies generally consisting of massive looking, coarse-grained, clast-supported bodies. Volumetrically the slope facies represents the prevailing facies of these carbonate systems. Prograding breccias are organized in m-thick clinostratified, clast-supported layers consisting of boulder to cobbles derived from the erosion of reef-upper slope, as documented by the facies of the clast. Size distribution and internal organization of the slope facies (vertical fining-upward trend and decrease in grain size basinward) indicate the deposition of breccia layers by mass-flow phenomena due to the episodic collapse of portions of the reef-upper slope , separated by periods of carbonate production on the slope (automicrite). The absence of fine-grained sediments indicates a good selection of the clastic carbonate material (original porosity is around 45% on average), whereas crusts of marine isopachous cements indicate a rapid and pervasive cementation, which rapidly reduced the porosity to 4-6% . The nature of the deposits and the interfingering of slope breccias with basinal calciturbidites suggests a different source for the slope and basinal deposits.

The sedimentological analysis of the slope facies of the Late Anisian-Early Carnian platforms of the Central Southern Alps records the sedimentary and early diagenetic processes that resulted in the deposition of huge volumes of massive coarse-grained slope facies, which were characterized by rapid marine cementation. Cement precipitation was thus responsible for 1) the rapid lithification of the reef and upper slope facies (i.e. the source of the clasts of the slope breccias) and 2) the rapid occlusion of the pore space of the slope facies, responsible for the stabilization of the slope itself.

 

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California