Massive Burial Dolomitization: The Jurassic Vajont Oolite of Northeastern Italy

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The origin of massive replacement dolomite remains one of the major unresolved problems of sedimentology. Presently favored interpretations for the origin of massive dolomites have centered on "early" low temperature processes involving surface or near- surface waters, and alternative processes such as "late" burial dolomitization have been overlooked by all but a few workers. The recent development of theoretical and computer models of groundwater flow systems in sedimentary basins makes the time ripe for a new look at burial dolomitization. Fluid flow is the one new factor of major significance that needs to be added to our attempts to resolve the dolomite problem. This is a study of the dolomite of the Jurassic Vajont Oolite of the Venetian Alps in northern Italy where preliminary field and geochemical work indicates that dolomitization occurred at elevated temperatures under burial conditions. The approach will be to integrate fieldwork, petrography, isotope and fluid inclusion data, and hydrologic modeling. The Vajont Oolite offers several special features that make such a study worthwhile: (1) it is massively but not completely dolomitized so that dolomitization fronts that give direct clues to the subsurface pathways of the dolomitizing fluids can be mapped (see Newton, Hardie and Phillips, 1990); (2) it consists of shallow water ooids that were resedimented in deep water by slope processes before alteration, thus shallow water dolomitization processes are ruled out; (3) the ooids and their intergranular cements are well preserved in the undolomitized parts of the Vajont sot hat it is possible to document in detail their pre-dolomitization petrography and isotope geochemistry; thus, the changes produced by dolomitization can be identified and measured; (4) exposures of partly but massively dolomitized Vajont in roadcuts reveal that dolomitization fluid pathways were controlled by fracture and fault planes of Late Tertiary age (Alpine orogeny), indication unequivocally that the dolomitization was of "late" burial origin. The results of the study could shed new light on the role of deeply circulated subsurface fluids in the origin of massive dolomites, particularly those that act as reservoirs to oil, gas, and hydrothermal ores.