Termination of carbonate slope progradation: an example from the Carnian of the Dolomites, Northern Italy

Abstract

Abstract The demise of carbonate platforms during Carnian time is a widespread phenomenon of the Western Tethys, but the controlling factors are not well-understood. The Dolomites of the Southern Alps (Northern Italy) are well-known for their large-scale Ladinian and Carnian prograding carbonate platforms. Platform growth stopped during the Early Carnian and both platforms and basinal areas were subsequently covered by mixed carbonate–siliciclastic sediments (Raibl Group). We present a carbonate platform, whose slope progradation came to a halt prior to draping by the siliciclastic sediments. Judging from the height of the clinoforms a platform-to-basin relief of at least 100 m existed during the final stage of progradation. This paleo-slope was colonized by coral patches draped by skeletal packstones to microbial bindstones, forming typically lens-shaped mounds on the steep slope. The formation of mounds on the platform flank was only possible, because the downslope transport of detritus from the platform and/or upper slope was stopped. The sedimentary evolution in the adjacent basin indicates a poorly oxygenated environment with abundant brackish water ostracods of low diversity. Based on the inherited platform-to-basin relief and the lithofacies of the basin fill a local sea-level drop in the order of 60–80 m is postulated. With rising sea level the accommodation space between former basin and platform became filled by shallow-marine, mixed carbonate–siliciclastic sediments. The abundance of epibenthic suspension feeders of the basin fill such as megalodonts suggests increased nutrient supply. The termination of slope progradation was a multiphased process triggered by a stagnation of the regional subsidence, a reduction of water circulation, followed by a sea level-drop, the influx of freshwater and the input of siliciclastics. Fluctuating salinity caused intermittently a biotically stressed environment.