Petrology and potential tectonic significance of a K-bentonite in a Taconian shale basin (eastern Ontario, Canada), northern Appalachians

Abstract

A 6 cm thick K-bentonite, herein defined as the Russell Bed, occurs in an Upper Ordovician deep-basin shale succession in eastern Ontario, Canada, forming part of the distal Taconic foreland in eastern North America. The bed lies within the pygmaeus graptolite Biozone, which is about 451 to 452 Ma in age. Although some bentonites are reported from this interval in eastern North America, we are reporting the first set of compositional data for a bentonite of this age. Gamma-log correlation identifies a potential minimum distribution area of < 2 × 10 5 km 2 for the K-bentonite, covering part of southern Quebec, New York State and eastern Ontario. The deposit coincides with the first influx of distal turbdites into this shale basin, associated with Taconic flysch, and simultaneous abrupt ventilation of the once anoxic deep-water basin, which had formed initially after foundering of the Upper Ordovician carbonate platform. Concurrent intrabasinal extinction of several graptolite species suggests that change in sedimentation, palaeoceanography and volcanism were linked to a regional external process. Compositionally, the bentonite is distinct from the older Ordovician platform deposits in eastern North America. The deposit contains abundant titaniferous phlogopite with 1.6 % BaO, fluoroapatite with 2.5 % F, and dynamically shaped glass spherules now altered to clay. The spherules and clay matrix constitute 45 % of the bed and, compositionally, define an illite (> 90 %)smectite (I/S) structure with about 7.5 % K 2 O %. Age-dating by Ar-Ar analysis of the phlogopite crystals yielded a younger than expected (440-445 Ma) age. This difference, along with evidence of localized chloritization of phlogopite, likely reflects known post-Ordovician hydrothermal activity within the basin. On the basis of several geochemical proxies, the magmatic source of the Russell K-bentonite falls within the trachyandesite field and was Ba-enriched. Comparison of geochemistry and mineralogy with older, Middle to Late Ordovician and younger Early Silurian K-bentonites within the Taconic orogen along eastern Laurentia and Baltica reveals that the potential source magma for the Russell Bed was more mafic, more alkaline, and less fractionated than sources typical of older (platform) bentonites. Instead, it is more similar to the younger Llandovery bentonites of Scandinavia and Scotland. It remains uncertain if it signals local or regional compositional change in volcanic source in the northern Appalachians.

Late Ordovician (Chatfieldian) catastrophic volcanism and abrupt carbonate platform-interior subsidence: A tectonic link across a Taconian foreland basin (Quebec Embayment) due to inherited crustal weakness

“…An R3 ISII illitic clay structure without smectite usually forms above burial temperatures of 150-180 °C (Bühmann, 1992). A minor (5%) presence of smectite in the Verulam bentonite, and slightly greater (10%) percentages in the younger bentonite in the overlying Billings Formation, sampled from the same Russell core (Sharma et al, 2005), support lower burial temperatures. This is consistent with minimum burial temperatures of 110-130 °C according to conodont alteration indices (CAI = ~3) for local Upper Ordovician limestone (Legall et al, 1981).…”

“…Relict structures of this source are preserved in some bentonites from the equivalent and younger basin-hosted bentonites in the Taconian (Berkley and Baird, 2002;Sharma et al, 2005). The mostly illitic composition of the Verulam bentonite is predicted by the mineralogical transformation of glass→smectite→illite→ ± chlorite (Chamley, 1989;Eberl, 1993) anticipated by the burial history of the Ottawa Embayment (Legall et al, 1981).…”

Levantamentos Gamaespectrométricos em Granitos Diferenciados. I: Revisão da Metodologia e do Comportamento Geoquímico dos Elementos K, Th e U

“…A geração de illita ou de argilas I/S, tanto em áreas continentais como marinhas, com a retenção de K, ocorre de várias maneiras, em especial durante a diagênese e transformação de sedimentos clásticos: a. por geração de K-bentonitas, originadas por alteração de camadas piroclásticas constituídas por vidros vulcânicos, rochas estas muito frequentes em várias regiões da América do Sul e do Norte; por este processo, aparece então a chamada illita neoformada (e.g., Grathoff et al, 2001;Sharma et al, 2005); b. por infl uência da atuação diagenética de salmouras intersticiais sobre argilominerais acumulados (e.g., Michael et al, 2003;Worden et al, 2006;Lonnee e Machel, 2006;Arostegui et al, 2006), que comprovadamente podem atuar durante dezenas de milhões de anos em bacias antigas (Worden et al, 2006), e que pode afetar até a caolinita (e.g., Arostegui et al, 2006;Martin-Martin et al, 2006), formando-se então o tipo de illita gerado por transformação; c. argilas I/S formadas em solos e transportadas e acumuladas em bacias continentais e marinhas, eventualmente com recristalização durante a etapa de diagênese, aparecendo então a illita ou argila I/S herdada ou detrítica; d. em bacias marinhas, por illitização de outros argilominerais (e.g., esmectitas), formando o mineral então por vezes mais de 70 -80% dos sedimentos pelíticos (folhelhos, mudstones e limolitos etc. ; Grathoff et al, 2001;Elliott e Haynes, 2002;Lindgren et al, 2002;Bottomley e Clark, 2004;Sharma et al, 2005;Martin-Martin et al, 2006).…”

“…; Grathoff et al, 2001;Elliott e Haynes, 2002;Lindgren et al, 2002;Bottomley e Clark, 2004;Sharma et al, 2005;Martin-Martin et al, 2006). Merriman (2005) discute o ciclo dos argilominerais, caracterizando a série de reações de esmectitas com (Ca,K,Na) primeiramente para argilas I/S, e a seguir para illitas e, em sequência, muscovitas (no metamorfi smo subsequente de baixo grau), em contraposição ao ciclo de transformação das esmectitas com (Ca, Mg), trioctaédricas, que passam para tipos Cl/S (chlorite/smectite, cloritas interestratifi cadas com esmectitas) e fi nalmente para cloritas.…”

“…Estes estudos foram completados com levantamentos terrestres em regiões do CGC (gamaespectrometria, magnetometria e medição de susceptibilidade magnética, e estudos gravimétricos em escala crustal), com resultados parciais apresentados recentemente (Shukowsky et al, 2006;Ferreira et al, 2001Ferreira et al, , 2006b.…”