Precipitation of dolomite from seawater on a Carnian coastal plain (Dolomites, northern Italy): evidence from carbonate petrography and Sr-isotopes

Abstract. The geochemical conditions conducive to dolomite formation in shallow evaporitic environments along the Triassic Tethyan margin are still poorly understood. Most of the Triassic dolomites in the Austroalpine and the South Alpine realm are affected by late diagenetic or hydrothermal overprinting, but recent studies from the Carnian Travenanzes Formation (South Alpine) provide evidence of primary dolomite. Here a petrographic and geochemical study of the dolomites intercalated in a 100-m-thick Carnian sequence of distal alluvial plain deposits is presented to gain better insight into the conditions and processes of dolomite formation. The dolomites occur as 10- to 50-cm-thick homogenous beds, mm-scale laminated beds and nodules associated with palaeosols. The dolomite is nearly stoichiometric with slightly attenuated c-reflections. Sedimentary structures indicate that the initial primary dolomite or precursor phase consisted largely of unlithified mud. Strontium isotope ratios (87Sr / 86Sr) of homogeneous and laminated dolomites reflect Triassic seawater, suggesting precipitation in evaporating seawater in a coastal ephemeral lake or sabkha system. However, the setting differed from modern sabkha or coastal ephemeral lake systems by seasonally wet conditions with a significant siliciclatic input and inhibition of significant lateral groundwater flow through impermeable clay deposits, thus representing a non-actualistic system in which dolomite formed along the ancient Tethyan margin.