Naslov
Glauconite formation in a palaeosol as an indicator of the incipient sea-level rise: case study of the Zlatni rt, Istria, Croatia

Autori
Perković, Ivor ; Durn, Goran ; Cvetko Tešović, Blanka ; Škapin, D. Srečo ; Matešić, Darko ; Vlahović, Igor ; Martinuš, Maja

Vrsta, podvrsta i kategorija rada
Sažeci sa skupova, sažetak, znanstveni

ISBN
978-953-6907-79-3

Skup
36th International Meeting of Sedimentology

Mjesto i datum
Dubrovnik, Hrvatska, 12.06.2023. - 16.06.2023

Vrsta sudjelovanja
Predavanje

Vrsta recenzije
Međunarodna recenzija

Ključne riječi
Glauconite ; Bacterial clay authigenesis ; Palaeosols ; Upper Jurassic ; Istria

Sažetak
Glauconite is usually found as a replacement of bioclasts and faecal pellets through neoformation and replacement of iron rich smectite in deepwater environments on the shelf-slope break. In the last two decades the formation of glauconite in shallow water environments was progressively more recognized and utilized in palaeoenvironmental reconstructions. The Lower Kimmeridgian to Late Tithonian Zlatni rat (ZR) clay is also one of the examples of glauconite formation in shallow water environment and a rare example of glauconite formation from a palaeosol. The ZR clay occurrence represents a decimetre thick horizon of grey clay embedding the black pebbles. The clay is also present as infills of the karstified channels and fissures in the karstified bedrock, where glauconite is found in the contact zone between the clay and the carbonate bedrock. The clay is primarily composed from mixed-layered illite– smectite, 2M1 illite, kaolinite, vermiculite, pyrite, marcasite and titanium oxides. The clay itself can be identified as a palaeosol, with abundant vertic features, which formed in contact with the marine environment, indicated by the high Sr/Ba ratios and enrichment of heavy rare earth elements and a slight negative cerium anomaly. The presence of glauconite was confirmed by both SEM- EDS, XRPD and FTIR. The glauconite formed mainly through the fixation of potassium and iron into illite and illite–smectite, but there is also strong evidence of its neoformation through bacterially mediated dissolution of present phyllosilicates. The source of iron was most likely terrigenous, as there is evidence for a ferralitic input through the presence of kaolinite in the ZR clay and contemporaneous ferralitic soils in the vicinity. Glauconite is also present in two forms: a dark green, more reduced form and a light green, more oxidized form. Both types of glauconite commonly display an alternation with pyrite in veins, which indicates the oscillations in the redox potential during glauconite formation. This probably happened as a consequence of variations in water column depth, which can be taken as evidence of an oscillating transgression. The final drowning of the ZR clay is recorded with the precipitation of coarse euhedral pyrite, during which the deposition of lagoonal Upper Tithonian Kirmenjak limestones had started.

Izvorni jezik
Engleski

Znanstvena područja
Geologija

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