This is an extract from the Black Country Geological Society newsletter 209, October 2011 entitled Geobabble.
Stratigraphy has always been a very important area in the study of geology. Within any geographical area, if you are looking at the geology you need to understand the sequence of the rocks, and visualise it as a column, with the oldest beds at the bottom. Geologists soon get used to writing down these sequences starting at the bottom of the page and working upwards. Traditionally stratigraphy was taught to budding geologists by starting with the oldest rocks in Britain and working upwards to the youngest, so in the Silurian, you would start with the Llandoverian and work through the Wenlockian, into the Ludlovian and so on. The traditional way of finding where you are in the sequence is to look at the fossils, as they will change with time as life evolved and so distinct horizons were identified with fossil zones. Staying with the Silurian, my ‘British Palaeozoic Fossils’, dated 1975 has 23 graptolite zones, mostly Monograptus.
But what about the Silurian of the Black Country? Graptolites did not survive in the high energy or shallow environments that produced the Much Wenlock Limestone Formation, and although experts could tell you where you might be in the sequence using other fossils, graptolite biostratigraphy is not a secure indicator of relative time. So we must turn to other methods that have been developed through looking at bentonites in the succession. These are clays produced by the weathering of volcanic ash and they are very common in our local Silurian. If each bentonite is analysed chemically it is found that no two are identical; each has its own distinctive ‘fingerprint’. These volcanoes were mostly from a subduction zone and so varied in magma composition with time, and sequences and changes in the chemical composition can be recognised. Another feature of these bentonites is that they contain crystals of zircon, a mineral that is basically Zirconium Silicate (ZrSiO4), but will give an accurate absolute radiometric age when put through the complex and expensive procedures required. (See ‘The Science and Mysteries of the Wren’s Nest NNR’, pp.6-7 above. Ed.)
These techniques are all very well, but what can be seen without resorting to complex chemical procedures? Well, you can use your traditional biostratigraphical methods, but using microfossils. Some of these techniques are being used at Dudley Museum and Art Gallery by Graham Worton and his Geoteam, a group of young geology students who volunteer to do this research. You simply collect your sample of bentonite from a known horizon, dry and sieve it into its various size fractions and then look at it under a binocular microscope and identify the microfossils. It sounds easy but I assure you that it is time consuming and requires a great deal of skill, particularly when you appreciate that these microfossils are measured in microns. The table illustrates the local succession with the absolute age of the Much Wenlock Limestone Formation, only 1 m.y. long. The Geoteam are looking at 17 bentonites, mostly in the Much Wenlock Limestone and Coalbrookdale Formations, but stretching right up to the top of the Pridoli. This cutting edge research gives us another stratigraphic method to help in the understanding of our Black Country rocks.
Written and illustrated by Bill Groves