Did a volcano kill these graptolites…….?
Written by John Knight
Update from the Society Field Excursion to Horton-in-Ribblesdale July 2019 – and an appeal
It’s been over a year since the Society field excursion to Horton-in-Ribblesdale (06 July 2019). On the day some 27 members and friends visited Horton Quarry, at the invitation of Horton Aggregates, and benefited from some splendid weather to visit the lower levels of the quarry, which were working a sandstone/gritstone succession of the Silurian Windermere Supergroup (Austwick Formation). An earlier reconnaissance visit by the leaders (Jack Soper, John Knight, Nick Riley) had found a large block (approximately 2 m x 1.5 m x 1.5 m) which had been left, undoubtedly for further breakage, on the floor of the quarry (NGR SD 79939 72089) and which on inspection displayed spectacular features on the upper surface. This block was one of the highlights of the Society visit to the quarry.
The block displays a light olive-green crenulated bedding surface; the orientation of the crenulations clearly reflects the pervasive cleavage. The olive green colour was clearly a thin (<10 mm) finer-grained bed corresponding in appearance to other horizons identified in the Windermere Supergroup as bentonites, or ash-fall tuffs (see photo 1). Where the bentonite had lifted or been chipped free there was an exceptional assemblage of monograptid graptolites immediately below the bentonite horizon (photos 2, 3). It was suggested by Nick Riley, in the field, that we were looking at a death assemblage triggered by the volcanic ashfall into the water column.
Photo 1
Photo 2
Photo 3
Photo 4
The quarry management recognised our comments on the significance of the block and were entirely supportive of the idea of recovering the whole piece (no small engineering task) and moving it to a point where it would be accessible for safe inspection by members of the public. Hanson Aggregates has for some years maintained one of the few accessible outcrops of the Ingletonian succession (presumed lower Palaeozoics in greenschist facies) near the entrance to Horton Quarry, and it was suggested that this would be a suitable area in which to preserve the block. Council members at the time suggested that this Society would be willing to fund an appropriate structure to shelter the block and provide explanatory signage, a project commitment which Council has since confirmed.
However, the best laid plans are commonly delayed. Since the field trip, during the autumn/winter months the quarry has on at least one occasion been flooded to considerable depth and temporarily inaccessible over a period. The quarry management has subsequently informed us of the impact of COVID-19 restrictions which have imposed changes on quarry operational practice; there can be no access other than for quarry employees until restrictions ease, at earliest in the latter part of the year.
In another development, a sample of the bentonite was submitted for assessment as suitable for radiometric dating to the Earth, Ocean and Atmospheric Sciences (EOAS) Department of the University of British Columbia, Vancouver, Canada. In February, EOAS were able to report “The Horton Quarry bentonite yielded very good quality zircon that appears as a single magmatic population. --we have selected more than enough grains for CA-TIMS work, and they will be annealed over the weekend.” In other words there are good prospects, albeit by no means certain, that a reliable radiometric date can be obtained. Unfortunately, EOAS has also been affected by lengthy shut-down due to COVID-19 restrictions. We had hoped to be able to post some preliminary results in time for this Circular, but latest information from EOAS suggests results will only be available from late August.
It will be apparent to readers that this block and its contents offers a fascinating and potentially important story through the association of a relatively rich assemblage of apparently well preserved and identifiable monograptids in close association with a bentonite horizon which we can hope will yield a reliable radiometric date. The sedimentary relations of the bentonite with the underlying graptolite-bearing sediments also present scope for interpretation (photo 4).
The Appeal
The only available photos are those taken by attendees on the field excursion. The attached Photos 1-3 were taken by Dr John Giles of Hull who has kindly made them available and they are reproduced here with permission. Clearly we will be unable to obtain fresh photographs for some time. So we invite those who attended the field excursion and who feel that they have suitable photographs of the location, the block and any detail of the graptolites, to submit them to the writer (on behalf of all the team leaders). If we can use them in any means of publishing or documenting this exceptional find, all such images will be fully acknowledged.
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