Naslov
Paleoenvironmental and paleoclimatic changes during the deposition of Upper Jurassic bauxites and their immediate cover: case study of the Rovinj bauxite pit, Istria, Croatia

Autori
Perković, Ivor ; Durn, Goran ; Cvetko Tešović, Blanka ; Škapin, D. Srečo ; Matešić, Darko ; Vlahović, Igor ; Martinuš, Maja ; Mihovilović, Milan ; He, Tianchen ; Newton, J. Robert

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
Poster

Vrsta recenzije
Međunarodna recenzija

Ključne riječi
Rare earth elements ; Bauxites ; Transgression sequences ; Upper Jurassic ; Istria

Sažetak
The aim of this study was the reconstruction of paleoclimatic and paleoenvironmental changes which occurred during the deposition of Upper Jurassic bauxite, and their cover beds, which comprise a cyclic alternation of clays, limestones, and black pebble breccias. Clay mineralogy, bulk mineralogy, geochemistry and micromorphology of bauxites and clays was studied using XRPD, SEM-EDS, ICP-MS, XRF, AAS and petrography. The bauxite is mainly composed from haematite, boehmite and kaolinite with variable amounts of 14 Å clay, illite and titanium oxides. The composition of major oxides is consistent with the mineralogical composition and is uniform throughout the profile. The values of rare earth elements and variations in the trace element values displayed differences throughout the bauxite profile, distinguishing two bauxite sections, both capped by two clay illuviation rich horizons. The upper section has numerous erosional features and clastic bauxite beds, when coupled with its different trace elemental fingerprint indicates its deposition over the lower, primarily pelitomorphic section. This change is likely a consequence of a paleoclimatic shift towards a more seasonal climate during the formation of the younger part of the bauxite. This can be related to the oscillating transgression succeeding the bauxite formation, which is recorded as the cyclical beds covering the bauxite. The clays from the cover beds were studied in detail, and are composed from dioctahedral vermiculite, kaolinite, illite and a mixed-layer clay mineral (MLCM) consisting of illite and dioctahedral 14 Å phyllosilicate. Both the increase in illite and MLCM at the expense of kaolinite along the profile and the increase in Sr/Ba ratio were observed, indicating a change from a lacustrine towards a marine environment. This is also supported by the changes in the microfauna of the limestone beds and confirmed by the iron speciation data, which indicate a shift from euxinic towards equivocal conditions, supporting a transition from an initially closed lacustrine environment to a more open marine environment. Overall, the study of both bauxites and their cover, allowed a detailed reconstruction of a late Kimmeridgian transgression that followed a terrestrial phase in this part of the Adriatic carbonate platform.

Izvorni jezik
Engleski

Znanstvena područja
Geologija

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