Early-marine diagenetic dolomitisation of a giant Upper Triassic carbonate platform

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Regionally extensive (100s of km) and thick (1000s of m) carbonate platforms that have undergone homogeneous, early-diagenetic marine porewater dolomitisation typify passive margins and epeiric seas over periods of many Myr. The marine-diagenetic transformation of very large volumes of sedimentary and biogenic CaCO3 to (calcian) dolomite is one of the most intriguing processes in carbonate research. Replacive, early-diagenetic dolostones are present as mosaics of fine-crystalline, fabric-preserving `dolomicrite’ differing significantly from later burial dolomite. The research proposed here focuses on the outstanding case example of the Norian `Dolomia Principale´ in the Alpine domain, specifically the Lombardy Basin, that dwarfs most other early-diagenetic dolomitised platforms of the Phanerozoic. Several features set the Dolomia Principale apart: The early-diagenetic dolomitisation homogeneously affects all inner platform, platform-margin and slope carbonates, while intraplatform basin deposits are calcitic. Dolomitisation discriminates this platform from the underlying (Carnian) and overlying (Rhaetian) ones, characterised by partially dolomitised peritidal inner-platform limestone facies. The complex bathymetry (inner platform versus intra-platform basins) allows studying patterns in the dolomitisation mechanism (reflux, sabkha, revised mixing zone, microbially-induced dolomitisation etc.). The processes involved likely interacted highly complexly, but we argue that they are deterministic. We will apply detailed field and core sedimentology and petrography using samples from key locations in the inner platform, the margin, microbial buildups, slope breccia (all dolomitised) and basinal limestones. Samples will be studied through a multiproxy isotope-geochemical and elemental approach involving all major elements of CaMgCO3. As documented in our previous work, the combination of detailed sedimentology, petrography, cement stratigraphy and geochemistry has a significant potential to assess these samples’ diagenetic pathways and dolomitisation mechanisms. Towards the end of year two and during the third project year, stratigraphic forward and reactive transport modelling (in cooperation with project partners) will place the data and their interpretation in a wider context beyond this study object. The project is ambitious (yet feasible). It aims to systematically and mechanistically assess the process of early marine-diagenetic dolomitisation in the context of a regionally and topographically complex, widespread platform system that recorded marine replacement dolomitisation over time scales of 15 Myr. We suggest that data and science resulting from this project can serve as a template for future studies in this research field.