The Lower Palaeozoic Rocks of Horton Quarry (Horton-in-Ribblesdale) – update from the Society Field Excursion July 2019
by Nick Riley MBE (YGS President) & John Knight (former YGS President)
Nick Riley and John Knight present some of the results from material investigated by members of the Society who attended the field excursion of 06 July 2019. The visit to the quarry was only possible with the active support of the quarry operator, Hanson Aggregates Ltd, which is gratefully acknowledged. The participation of the then Regional Geologist of the company, John Peate, who co-led this part of the excursion, is warmly thanked. Our objectives and scope of the visit were largely directed by the long term work of Jack Soper (co-leader but unable to attend on the day of the visit).
Horton Quarry has been in operation for many years as a producer of limestone from the Great Scar Limestone Group, resting unconformably on the folded Lower Palaeozoic succession which is known at outcrop around the southern slopes of Moughton Scar. In recent years the quarry has been permitted to open a lower cut through the base of the unconformity to extract a gritstone aggregate from underlying folded strata attributed to the Silurian Austwick Formation (McCabe 1972; Arthurton et al. 1988). This lower cut offers a unique perspective on the geometry of the sub-Carboniferous unconformity and its immediately overlying strata, and also a window on the structure and succession of the Austwick Formation. The excursion concentrated primarily on the lower cut and spent the morning inspecting the Silurian strata. Particular attention was devoted to a very large isolated block on the floor of the quarry, which displayed a characteristically light olive-green layer identified as a bentonite (altered ash-fall tuff), of which a number have been identified through the Silurian succession (Romano & Spears 1991; Merriman et al. 1995).
Immediately underlying the bentonite, a rich assemblage of monograptid graptolites could be observed, possibly a death assemblage caused by the volcanic ash-fall. There was wide-ranging discussion in the field, with plenty of photo-opportunities and a general assumption that there was a potentially exciting story to tell which would develop from further fieldwork in the quarry. Unfortunately this was not to be, as bad weather and flooding of the quarry in the latter part of 2019, followed by the restrictions of the COVID-19 pandemic, has meant that no further access to the quarry has since been possible.
However, a sample of the bentonite layer was collected during the visit and the opportunity was taken to submit this for radiometric dating (U-Pb CA-ID-TIMS) at the Earth, Ocean and Atmospheric Studies Department of the University of British Columbia, Vancouver. The presentation will review the record and robustness of radiometric dating of the bentonite layer, which fixes this horizon as of mid-Wenlock age, at the passage from Sheinwoodian to Homerian age.
The talk will aim to present an overview of the regional context of the Lower Palaeozoic succession and its structure, and review the available biostratigraphical and palaeoenvironmental context of the graptolite fauna. From the radiometric dating results, the succession can be firmly fixed within the Silurian time scale, which then suggests how the succession should fall within the biostratigraphical framework as defined by graptolite stratigraphy.