Bentonites from the Horton Formation (Upper Silurian) of Ribblesdale, Yorkshire

Abstract
SUMMARY: Previously unrecorded K-bentonites are identified from the lower part of the Horton Formation (Ludlow) in Horton in Ribblesdale. The thickest (14 cm) of these is unusual in that it contains a distinctive internal stratigraphy which allows unambiguous correlation with sequences on both limbs of the Studrigg-Studfold Syncline. A further feature of the bed is that a lower nodular part preserves shards and vesicles. Illite (1M polytype) is the major component of the bentonite. Illitisation of a precursor smectite took place during diagenesis, which approached the anchizone stage, with a percentage expandability of less than 5%. Quartz, albite, chlorite and muscovite (2M 1 , polytype) are also present; these are the detrital minerals that dominate the associated siltstones. The siltstones appear to have attained a higher diagenetic level than the bentonite because they incorporate detritus derived from higher grade source rocks. Immobile trace elements in the bentonite indicate a rhyodacite/dacite composition, possibly a high-K lava type within the arc lava field. Although carbonate nodules are present in the bentonite, diagenetic stages in the chemical evolution are not preserved.