The HGS Centenary Chalk Project
Stratigraphic methods, biases and problems
Members of the project group measured exposures in the field, firstly concentrating on inland quarries and then tackling the coastal exposures. Beds were measured to the nearest centimetre and where the thickness of a bed varied the thickest extent was recorded. This of course may lead to an exaggeration of the overall total thickness of the Chalk. the occurrence of fossils in situ was recorded but not normally to species level; we were mostly working on fragmentary remains.
Sources of bias
It was realises that there are a number of sources of bias such as -
the experience of the geologist in recording the stratigraphy: for example decisions have to be made in the field about what constitutes a "bedding plane".
the experience of the geologist in finding and identifying Chalk fossils: it does take some practice to be able to see white fossils in a white rock and be able to identify the fossils when they are broken
the nature of the fossil record itself - aragonitic ammonites and gastropods are not normally fossilised in the Chalk; large organisms have less chance of being fossilised whole.
the nature of the exposure - some exposures are weathered which means that samples collected for microfossil, nanofossil and geochemistry may be contaminated. The weathering at coastal exposures tends to make fossils stand out but at inland exposures can obscure them. It was also noticed that the Chalk in the top couple of meters of an exposure was thinly bedded (probably due to frost shattering) whereas in borehole chalk putty tends to be found in the top couple of metres)
the size of the exposure - some records such as the base of Nafferton Quarry and Enthorpe Cutting are based on trenches cut into the scree whilst we were able to observe the same beds between Green Stacks and South Landing for a walking distance of over half a kilometre, that is why a wider range of fossils was recorded.
being too close - by paing too close attention to the beds in the rock face you can lose the big picture
There are some gaps in the sequence -
the stratigraphy of the Red Chalk Formation is highly variable at exposures; it is not the same at either side of the tiny Rifle Butts SSSI and Gordon Ostler showed that there was significant variation at Middlegate Quarry.
The Ferriby Formation is also quite variable
Though the Black Band Member at Middlegate Quarry seems to be commonly accepted as the "type section" (probably because there was good access to clean exposures (until quarrying techniques were changed to make it unsafe) it is not really typical
there is a major gap in exposure between the highest inland Santonian exposure (Middleton Quarry) and the lowest flinty Chalk on the coast at Selwicks Bay
Faults at South Landing and Danes Dyke do not make correlation easy. Lynden Enery solved the problem at South Landing by informally identifying two sets of bed as "dry stone walling" and "bricks". Donald Beveridge noticed that we had logged beds repeatedly at Danes Dyke due to landslides when he looked at the cliff form the low tide line.
It is far from clear how the inland exposures in the binodosus zone can be correlated and the White Hill Quarry (commonly assumed to expose the highest Yorkshire Chalk) was filled before World War II.
There is no inland exposure of the Rowe Formation though it does occur in "rafts" in the Holderness Boulder Clays.
we don't have ammonites which are the Mesozoic biostratigraphy's fossil of choice
several of the zone fossils are rare in Yorkshire and others are difficult to identify.
It has been pointed out several times the biozones have not been scientifically defined apart from linking them to the lithostratigraphy with very little evidence
the Yorkshire Chalk is hard which means that samples of marl have to be used for micropalaeontology. Ostracods are rare. Many of the foraminifer faunas are benthic and difficult to identify at the species level required for stratigraphic use.
The nanofossils from marl bands seem not to agree with the accepted macrofossils zones of previous authors.
Mike Horne FGS
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