Pyroclastic Sanidine in the Lower Palaeozoic Bentonites - A Tool for Regional Geological Correlations

Röntgendifraktomeetria- (XRD) ja röntgenfluorestsentsmeetoditega uuriti ~800 Ordoviitsiumi ja Siluri bentoniidiproovi. Põhiosa proovidest oli pärit Eesti–Läti–Leedu–Gotlandi regioonist, kus bentoniidid sisaldavad püroklastilist sanidiini. Savikatest bentoniitidest separeeritud terafraktsioonis on püroklastiline sanidiin sageli üheks põhikomponentidest. Sanidiini koostis määrati XRD meetodil 120 refleksi
asukoha täpse mõõtmisega. (Na,Ca)Si3O8 komponendi osa erinevate bentoniitide sanidiinis varieerub 20–58 mol%, see on konkreetse vulkaanipurske iseloomulik tunnus ja annab võimaluse bentoniitide rööbistamiseks. Sanidiini analüüside baasil koostati Telychi bentoniitide tabel, mis sisaldab üle 20 usaldusväärse korrelatsiooni Eestist Lätisse, lisaks mõned ka Leetu ja Gotlandile. Bentoniitide korreleerimine võimaldas jälgida ühele vulkaanipurskele vastava kihi koostise muutumist läbi erinevate settefaatsieste – madalaveelisest päevakivitufist sügavaveelise
kaoliniidirikkani, üleminekuvööndis domineerib illiit-smektiit. Bentoniitide koostise muutumisega kaasneb ka immobiilsete mikroelementide kontsentratsioonide suurenemine sügavaveelises faatsieses, mis on põhjustatud suuremast jääkrikastuse efektist seal. Bentoniitide abil saavutati olulisi täpsustusi läbilõigete korreleerimisel Aeronian/Telychian, Llandovery/Wenlock ja Wenlock/Ludlow piiride tasemel. Kirjeldatud meetodi kasutusvõimalused on siiski piiratud tuhapilvede regionaalse levikuga ja püroklastilise sanidiini olemasoluga bentoniitides. Seni on meetodit edukalt kasutatud Ida-Baltikumis ja Gotlandil. Sanidiini otsingud Bornholmi, Oslo piirkonna, Ukraina ja Siberi Ordoviitsiumi-Siluri bentoniitidest ei andnud positiivset tulemust.

Abstract

Around 800 samples of Ordovician and Silurian bentonites were studied by X-ray diffractometry (XRD) and X-ray fluorescence method. Major part of samples was collected from the Estonia–Latvia–Lithuania–Gotland (Sweden) region. In grain fractions separated from the clay-rich bentonites pyroclastic sanidine often occurs as one of the major components. Composition of the sanidine was analysed by the precise measurements of the 120 reflection. Content of (Na,Ca)Si3O8 in sanidine in different bentonites varies between 20 and 58 mol%. The composition of the sanidine
is typical for a particular eruption and enables to prove the correlations between sections. In the best studied Telychian Stage over 20 well characterized correlative bentonites were established in Estonia and Latvia. Some correlations were extended also to Lithuania and Gotland. Correlation of bentonites enabled to trace changes of bentonite composition through different facies – from feldspathic tuffs in shallow water facies through illite smectite domination in transition zone to the kaolinite-rich bentonites in deep-sea environments. Accordingly, concentrations of immobile elements also change being the highest in deep-sea facies due to the highest residual enrichment. Several significant refinements of stratigraphy were achieved at the level of the Aeronian/Telychian, Llandovery/Wenlock and Wenlock/Ludlow boundaries. Applicability of the method is restricted to the bentonites where sanidine is present and by the regional distribution of bentonites. Currently, the method is successfully
used in the East Baltic and Gotland. Sanidine was not found in bentonites from Bornholm, Oslo Region, Ukraine and Siberia.