Pyrite in dolostone deposits near Perm-Triassic boundary : master's thesis

U radu su opisane karakteristike dolomita u dolomitnom litofacijesu profila Plavno nedaleko Knina. Dolomitni litofacijes predstavlja najstariju jedinicu donjega trijasa (grizbahij) što je dokazano prisutnošću konodontnih vrsta Hindeodus-Isarcicella. Karakteristike slijeda Plavno interpretirane su uvjetima taloženja prostrane epikontinentalne rampe. Dolomitni litofacijes odgovara najplićem, proksimalnom dijelu rampe. Prisutnost organizama u ovom litofacijesu (ostrakodi, foraminifere, školjke te ooida) ukazuje da je okoliš imao dovoljno kisika. U oksičnom okolišu teško je objasniti prisustvo pirita, naročito sitnog framboidnog pirita (20 μm i c) sitni framboid 20μm vezan je za ranodijagenetske procese u sedimentu uz samu granicu sediment-voda. Postanak pirita interpretira se lokalno prisutnim anaerobnim uvjetima nastalim uslijed razgradnje organske tvari pri čemu može biti potrošen sav kisik. Plitko ispod neposrednog kontakta sediment-voda (nekoliko centimetara) nema više dostupnog kisika što prisiljava bakterije da počnu trošiti kisik iz sulfatnog iona i nastaje sumporovodik (H2S). Tako su u sedimentu svega nekoliko centimetara ispod površine mogući anoksični uvjeti u kojima može nastati framboidni pirit dimenzija nekoliko desetaka mikrometara. Prisustvo sitnih framboida pirita 20 μm and c) small framboids (20 μm) occur within the sediment, near the sediment-water interface. Genesis of large framboids can be related to locally present anoxic conditions that occur at the sediment-water interface due to decay of organic matter when all oxygen is used in decay processes. Shallow in the sediment there is no oxygen present and therefore bacteria start to reduce sulfates. In this reaction H2S is produced. A few centimeters below the surface of the sediment, in the anoxic conditions a large framboids usually occur.The most intriguing is the presence of small pyrite framboids (< 20 μm). This small sized framboids nucleate in the anoxic conditions in the water column. Genesis of small framboids is interpreted according to the model proposed by KERSHAW (2015). There is a possibility that in the distal part of epeiric ramp, bellow fair-weather wave base, anoxic conditions were occasionally present in the water column. As the storms strongly influenced sedimentation in the Early Triassic Plavno section, it is possible to assume that small framboid pyrite particles from distal ramp were caught by storm currents and transported to the shallowest part of the environment and are preserved in the Dolostone lithofacies