Abstract
Abstract. The geochemical conditions conducive to dolomite formation in shallow evaporitic environments along the Triassic Tethyan margin are still poorly understood. Large parts of the Triassic dolomites in the Austroalpine and the southern Alpine realm are affected by late diagenetic or hydrothermal overprinting, but recent studies from the Carnian Travenanzes Formation (southern Alps) provide evidence of primary dolomite. Here a petrographic and geochemical study of 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 homogeneous beds, millimetre-scale laminated beds, and nodules associated with palaeosols. The dolomite is nearly stoichiometric with slightly attenuated ordering 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 composition, 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 being exposed to seasonally wet conditions with significant siliciclastic input and the inhibition of significant lateral groundwater flow by impermeable clay deposits. Thus, the ancient Tethyan margin was different from modern analogues of primary dolomite formation.
Highlights
The formation of dolomite (CaMg(CO3)2) under Earth surface conditions in modern and ancient environments is still a major unsolved problem in sedimentary geology
Stable isotope and fluid inclusion data often indicate that massive dolomites formed due to the replacement of precursor calcium carbonate during burial diagenesis, i.e. at higher temperatures and under conditions decoupled from the ancient depositional environment
We suggest that the composition and origin of ionic solutions conducive to primary dolomite formation, from either continental water or seawater, are an indication of separation between the two palaeogeographic domains
Summary
The formation of dolomite (CaMg(CO3)2) under Earth surface conditions in modern and ancient environments is still a major unsolved problem in sedimentary geology. The precipitation of dolomite under laboratory conditions has been difficult (see Land, 1998), and the factors that may have influenced dolomite formation throughout Earth history remain poorly constrained. Van Tuyl (1914) discussed several competing theories for dolomite formation, one of which was the chemical theory, whereby dolomite is a primary precipitate, forming as the result of prevailing conditions within the depositional environment. Stable isotope and fluid inclusion data often indicate that massive dolomites formed due to the replacement of precursor calcium carbonate during burial diagenesis, i.e. at higher temperatures and under conditions decoupled from the ancient depositional environment.
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