Pirit u naslagama dolomita uz granicu Perm-Trijas

master's thesis

• Title: Pirit u naslagama dolomita uz granicu Perm-Trijas : diplomski rad
• Title (english): Pyrite in dolostone deposits near Perm-Triassic boundary : master's thesis
• Author: Ljubica Filipović
• Mentor: Dunja Aljinović (mentor)
• Committee member: Dunja Aljinović
• Committee member: Uroš Barudžija
• Committee member: Marija Horvat
• Granter: University of Zagreb; Faculty of Mining, Geology and Petroleum Engineering; (Department of Mineralogy, Petrology and Mineral Resources); Zagreb
• Defense date and country: 2015-12-11, Croatia
• Scientific / art field, discipline and subdiscipline: NATURAL SCIENCES; Geology; Mineralogy and Petrology

Abstract

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). Uočene su tri vrste pirita: a) pirit kao ispuna ljušturica ostrakoda, b) krupni framboid >20 μm i c) sitni framboid < 20 μm. Pirit prisutan kao ispuna ljušturica ostrakoda svjedoči o lokalno prisutnim reduktivnim uvjetima nastalim truljenjem organske tvari u unutrašnjosti ljušturice. U takvim se uvjetima može pretpostaviti redukcija željeza, nastajanje sumporovodika i konačno postanak pirita. Krupni framboidni pirit >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 < 20μm je najinteresantnija pojava u istraženim uzorcima. Framboidni pirit ovih dimenzija veže se za proces nukleacije u stupcu vode pri anoksičnim uvjetima u samoj vodi. Pri interpretaciji je primijenjen model kako ga predlaže KERSHAW (2015). Moguće je da su u uvjetima taloženja u distalnom dijelu epikontinentalne rampe ispod osnovice valova lijepa vremena, povremeno egzistirali anoksični uvjeti u kojima je bila moguća nukleacija pirita u stupcu vode. Uz pretpostavljeno djelovanje oluja u tom prostoru, sitni framboidi pirita su također mogli biti zahvaćeni i transportirani u najplići, proksimalni dio epikontinentalne rampe, te su očuvani u dolomitnom litofacijesu.

Abstract (english)

Abstract: Significant petrographic characteristics of dolomites from Dolostone lithofacies at the Plavno succession (locality near Knin) have been investigated. Dolostone lithofacies represents the oldest Early Triassic sedimentary rocks, deposited immediately after Permian-Triassic mass extinction. According to the presence of conodont species Hindeodus-Isarcicella this interval has been determined as Griesbachian. Sedimentary characteristics of Plavno sections were interpreted as deposited on a broad epeiric ramp. Dolostone lithofacies represents deposits of the most proximal and the shallowest part of the ramp. Fossil remains found in this lithofacies point to the oxygenated environment. Therefore it is difficult to interpret presence of pyrite in this lithofacies. It is especially true for the small pyrite framboids (< 20 μm). Pyrite occurs in the dolomite in following forms: a)pyrite within ostracod shells , b) large framboids >20 μm and c) small framboids (< 20μm). Pyrite that occurred within ostracod shells points to locally present anoxic conditions produced by decay of organic matter after dying of organisms. Therefore, the origin of pyrite is local and diagenetic. Large 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.

• Keywords: pirit; framboidi; granica Perm-Trijas; epeirička rampa u donjem trijasu; Dinaridi; Hrvatska
• Keywords (english): pyrite; framboids; Permian-Triassic boundary; Early Triassic epeiric ramp; Dinarides; Croatia
• Language: croatian
• URN:NBN: urn:nbn:hr:169:859440
• Study programme: Title: Geology; Study programme type: university; Study level: graduate; Academic / professional title: magistar/magistra geologije (magistar/magistra geologije)
• Type of resource: Text
• File origin: Born digital
• Access conditions: Open access
• Created on: 2016-01-12 13:06:12