Clay mineralogy, chemistry, and diagenesis of Late Devonian K-bentonite occurrences in northwestern Turkey

Abstract

The products of explosive eruptions in the form of volcanic ash (tephra), after being transported for long distances by wind, are settled and altered to bentonites (smectite-rich volcanogenic clay rocks) in early diagenesis. In late diagenesis, these bentonites are transformed into K-bentonites by chemical modification and K-fixation with progressive illitization and then finally into metabentonites by low-grade metamorphism (Fortey et al., 1996). During diagenesis and very low-grade metamorphism, due to potassium enrichment, smectite transforms to mixed-layer illite-smectite (I-S) and then illite in K-bentonites (Weaver, 1953;Nadeau et al., 1985;Merriman and Roberts, 1990).The discovery of K-bentonites dates back as early as the 1920s. Since then, their geologic importance has been revealed and proven by ongoing interdisciplinary research. Nelson (1921Nelson ( , 1922 originally denoted the pyroclastic nature of some rocks in the Paleozoic rock system in the eastern part of the United States. Following Nelson's original remarks, Allen (1932), based on the crescent-shaped shard structures and the presence of sanidine, apatite, and euhedral zircon crystals, evidenced the volcanic origin of those deposits. Rosenkrans (1934) and Kay (1944aKay ( , 1944b were the first to use bentonites for stratigraphic correlations. The significance of K-bentonites as widespread marker beds has been recognized recently since they are also datable using fission track and U/Pb dating of zircons, K/Ar, and Ar/Ar of amphibole, biotite, and sanidine (Kolata et al., 1996).Worldwide research activities on K-bentonites have diversified aims such as discovering localities and studying their mineralogical, petrographic, and

Geological features and geochemical characteristics of Late Devonian–Early Carboniferous K-bentonites from northwestern Turkey

“…3e,f ). A green clay bed, 30 cm thick, bounded by limestone layers, was sampled (O-17A) The limestone samples (O-18A and O-18B) yielded the same Late Devonian foraminiferal fossil assemblage as the K-bentonite-bearing succession in the upper part of the Yılanlı Formation cropping out at Gavurpınarı Quarry near Bartın, as studied previously (Günal-Türkmenoglu et al, 2015).…”

“…spp. (Günal-Türkmenoglu et al, 2015). Most are known widely, in many regions, and range stratigraphically from Devonian to Early Carboniferous (Dil, 1976;Vachard, 1991Vachard, , 1994Vachard et al, 1994;Racki & Sobon-Podgorska, 1993;Kalvoda, 2001;Sabirov, 2004;Mamet & Preat, 2009;Özkan & Vachard, 2015) except for the age-diagnostic taxon, the multichambered foraminifer Eogeinitzina which is represented by only two transverse sections and three doubtful specimens (Fig.…”

“…Although extensive research has been performed worldwide on K-bentonites of volcanic origin, their first discovery in Turkey was published recently (Türkmenoglu, 2001;Türkmenoglu et al, 2009;Günal-Türkmenoglu et al, 2015). In the Zonguldak Terrane (ZT) in NW Turkey (Fig.…”