Namurian bentonites in the Pennine Basin, UK – origin and magmatic affinities

Abstract

Nine Namurian clay bands retrieved from boreholes in the northern part of the Pennine Basin are, on the basis of their petrography, mineralogy and geochemistry, shown to be volcanic in origin and are therefore bentonites. The bentonites, which have a fragmental texture, are normally graded and show rare preservation of shard textures, representing vitric tuff deposits that have been altered subsequently to clay‐dominated horizons. Crystals are a minor component of the bentonites, but biotite, in particular, is concentrated at the base of the beds. A clay mineral assemblage of mixed‐layer illite–smectite with subordinate kaolinite identifies most of the samples as K‐bentonites, but kaolinite dominates two samples that can be classed as tonsteins. Temporal variation of salinity within the depositional basin is suggested to explain these different clay assemblages. The major element geochemistry of the bentonites reflects their clay mineralogy and the compositions of diagenetic minerals present, the latter including pyrite, carbonates and hydroxyapatite. Enrichment of the bentonites in some trace elements (including Ba, Sr, Pb, Cu and Ni) can be related to the presence of the diagenetic minerals, but the extent to which the elements are added from external sources as opposed to being redistributed within the ash is unclear. Immobile trace element systematics suggest a rhyodacite/dacite composition for the original ash and derivation from the collision of plates, this being supported by evidence provided by the rare earth elements (REE) in one group of samples. However, in another group of samples, variations in REE concentrations may be caused by mobility of these elements during alteration. The chemistry of the Namurian bentonites contrasts markedly with that of the local Carboniferous volcanics but is comparable, in some respects, with one group of Westphalian tonsteins, although the latter are more rhyolitic in character. It is suggested that the Namurian bentonites and the Westphalian tonsteins of acid affinities originated from volcanic activity associated with a destructive plate margin in the Variscan externides and that the observed compositional trend may reflect magma evolution possibly related to the progressive east–west closure.

Geochemistry and origin of Carboniferous (Mississippian; Viséan) bentonites in the Namur-Dinant Basin, Belgium: evidence for a Variscan volcanic source

“…Christidis, 1998;Spears et al, 1999). Diagenetic modification of the Viséan NDB bentonites is suggested by the high Al concentrations (20 -30% Al 2 O 3 ) in many of the samples, which exceed what could reasonably be expected to occur in unaltered volcanic rocks, and probably originates from residual enrichment.…”

“…The transitional nature of the change in K 2 O/Al 2 O 3 , MgO/Al 2 O 3 and clay mineralogy between bentonites W6 and W8 may record a short-term change in ash composition. Conversely, fluctuations in depositional environment linked to ocean connectivity have been argued to control the clay mineralogy of Namurian (Serpukhovian) bentonites within the Pennine Basin (Spears et al, 1999;Spears, 2006). Whilst such variations in depositional environment were not detected in the field for the limestone succession between bentonites W6 to W8, such environmental changes cannot be entirely excluded.…”

Source to sink: A review of three decades of progress in the understanding of volcaniclastic processes, deposits, and hazards

“…Smith, 1988) as well as providing reservoir rocks (Paredes et al, 2007). Widespread tephra beds (K-bentonites or tonsteins) are useful for regional correlation, for example in European coal measures (Spears et al, 1999), while distinctive volcaniclastic seismic facies can aid interpretations of basin stratigraphy and reservoir characterization. The reservoir quality of clastic sediments is governed by the grain-size distribution and diagenetic effects.…”

Characterization and correlation of Upper Jurassic (Oxfordian) bentonite deposits in the Paris Basin and the Subalpine Basin, France

“…5A) were considered as probable bentonites because of their pale colour (pale grey, ochre to orange; Fig. 5B–D) and because they are softer and more plastic than the enclosing marls, as observed for most bentonite deposits elsewhere (Teale & Spears, 1986; Merriman & Roberts, 1990; Kolata et al ., 1996; Spears et al ., 1999; Jeans et al ., 2000). In addition, these clay layers are picked out by vegetation because the smectite sheets absorb more water than other clay minerals and provide an environment that encourages more abundant plant growth (Fig.…”