Internal Geochemical Stratification of Bentonites (Altered Volcanic Ash Beds) and its Interpretation

Uuriti 26 proovi kahest suurest vulkaanilise tuha kihist, Kinnekulle ja BII Bentoniidist Kuressaare K-3 puuraugust Saaremaalt, eesmärgiga välja selgitada kihtide sisemine geokeemiline ning mineraloogiline heterogeensus. On kirjeldatud ja interpreteeritud märke materjali fraktsioneerumisest vulkaanituha transpordil ning ümbersettimisel merebasseinis ja tunnuseid elementide diageneetilisest ümberjaotumisest. Valdavaks autigeenseks mineraaliks Kinnekulle Bentoniidis on illiit-smektiit vähesema kaaliumpäevakivi lisandiga kihi äärtel. BII Bentoniit koosneb kloriit-smektiidist ja illiit-smektiidist. Magmaliste fenokristallide sanidiini ja biotiidi koostise stabiilsus mõlema kihi vertikaalläbilõikes tõendab, et mõlema bentoniidi lähtematerjal kuhjus ühest vulkaanipurskest. Sanidiini koostise stabiilsus (±0,5 mol %) mõlema bentoniidi läbilõikes kinnitab, et see on usaldusväärne kriteerium bentoniidikihtide identifitseerimiseks ja korrelatsiooniks. Mikroelementide jaotus osutab, et Zr, Ga, Rb, Nb, Ti ja Th püsisid immobiilsena vulkaanilise tuha muutumisel bentoniidiks, peegeldades hästi vulkaanilise tuha algset koostist. Siiski on märgata väikest Nb, Ti ja Y liikuvust kihtide kontaktil ümbriskivimiga. Märkimisväärsed S, Ca ja P akumulatsioonid kihtide kontaktidel ümbriskivimiga on tunnuseks, et vulkaanituha kuhjumine põhjustas märgatavaid hälbeid madalmere ning settelises keskkonnas.

Abstract

Volcanic ash beds altered into bentonites form perfect marker horizons for correlation of sedimentary sections, enable interpretation of tectonomagmatic environments in volcanic source areas and help construct a geological timescale using radiometric dating of well-preserved minerals. For a reliable interpretation, we need to know the within-bed compositional variability to compare it with between-bed variations. The aim of the study was to describe and interpret from the analysed data the geochemical and mineralogical heterogeneity of bentonite beds. To achieve these goals, a number of samples
were collected, with the aim of obtaining detailed distribution patterns of elements in the vertical sections of bentonites, and compositional variations of the thick bentonite beds under study. The acquired data were compared with compositions of host rocks and clay fractions of terrigenous material (see Paper II). The thesis concentrated on studying five thick (30 to 90 cm) bentonites, four from the Upper-Ordovician — two Kinnekulle Bentonites, the Sinsen Bentonite and the Pirgu (BII) Bentonite—, and one from Silurian Wenlock: the Grötlingbo Bentonite.

Compositional changes throughout the thick bentonites were studied, using XRF, XRD and SEM analysing techniques, which enabled to detect main chemical components, trace elements, mineral composition, amount and composition of biotite and sanidine micro-phenocrysts. Signs of ash-transport fractionation, redeposition of volcanic ash, also the differentiation from the terrigenous clay and diagenetic redistribution of the volcanic? material were described and interpreted.

The stability of sanidine and biotite phenocryst compositions throughout the vertical sections of bentonites indicates that all bentonites originated from a single eruption. The observed stable composition of pyroclastic sanidine in the vertical section of bentonites confirms the reliability of sanidine-based “fingerprinting” of the altered volcanic ash beds. Trace element distribution in bentonites and in host rocks indicates that Zr, Ga, Nb, Ti and Th were immobile during volcanic ash alteration and carry information from primary ash composition. Immobile trace elements, especially Zr, indicate fractionation of material during air-transport. Some redistribution of Nb and Ti, as well as Y, occurred near the contacts of bentonites with the host rock. Mobile elements (Mg, K, Si) indicate significant redistribution of materials during diagenesis. Considerable geochemical variations, including high sulphur content, near the upper and lower contacts of the Kinnekulle Bentonite, as well as elevated calcium and phosphorus concentrations in host rocks of both bentonites, suggest that the studied large ash-falls caused notable perturbations in shallow marine and early post-sedimentary environment.