Journal of Conference Abstracts

Ribbon-Like Dolomites in Main Dolomite Deposits in the District of Barnowko - Mostno - Buszewo Oil Deposit (West Poland)

Anna Depowska (andep@pgnig.com.pl)

ul. Jagiellonska 76, Warsaw, Poland

Ribbon-like dolomites in the Main Dolomite deposits profile in the district of Barnowko-Mostno-Buszewo oil deposit, occur mostly in three facies: (1) supra and intertidal ooid-intraclast pack/grainstone with grain aggregates, (2) ostracode-bivalve-gastropod-microbial wacke/grainstone, (3) thrombolite mounds with ostracods and rarely sponges. In supra and intertidal deposits the ribbon-like dolomites create a lacely rim on the grains, as well as they are present in the inter and rarely intragranular space. Generally elongated ribbon-like crystals diverge from a common point and create a spherulitic structure. The average dimension of spherulitic dolomites is about 50µm. In the ostracode-bivalve-gastropod-microbial wacke/grainstone and in the thrombolite mounds, the ribbon like dolomites create structures similar to those dumbbell ones. Their average dimension is approximately 100 µm. They also create spherulitic dolomites.

The ribbon-like dolomite crystals show sweeping extinction under cross-polarized light and dull-red luminescence under cathodoluminescence. It is possible to trace conversion from ribbon-like dolomite crystals into baroque dolomite aggregates or even the dumbbell structures are embedded in them. Microspheres (about 0.3 µm) are observed in spherulitic ribbon-like dolmites in their proximal part (regards to the point of divergance) due to the scanning electron-microscope analysis. In distal part, the ribbon-like dolomites create euhedral crystal faces.

The ribbon-like dolomites often accompany anhydrites. Specially in the mounds they precipitate in fenestral space on the top of the anhydrite crystals, which in their turn, precipitate more or less vertically. Usually the anhydrite crystals are better developed in the bottom of the pore space. Often dolomite ribbon-like crystals are embedded in anhydrite. In the mounds it is possible to observe an unusual structures of fibrous anhydrite in cavernal space. Crystals diverge from a common point and create a broom-like structures best developed on the bottom of the cavernal space. Sometimes broom-like anhydrite structures diverge from a common section and create rafts. The ribbon-like dolomites are visible between the fibrous crystals, being concordantly elongated with anhydrite ones. Fibrous calcite precipitate occasionally in the mounds. In this case, calcite enclosed ribbon-like dolomite and anhydrite, the anhydrite crystals being corroded. Usually anhydrite and dolomite accompany pyrite, too. Pyrite is often embedded in the central part of the anhydrite crystals.

Dumbbell pattern of dolomite crystal aggregates and microspheres allow to assume the bacterially mediated origin of ribbon-like dolomites. Irregular fractures in the mounds and co-occurrance of ribbon-like dolomites and broom-like anhydrite structures in the cavernal spaces suggest, that they are related to groundwater springs with enhanced salinity and temperature.

132