Hull Geological Society
Mike Horne FGS
This is unfinished work that has not been edited or peer reviewed by the Society.
The geochemistry of some Marl Bands in the Yorkshire Chalk
A lecture presented to the Yorkshire Geological Society in 1995
By Mike Horne*
>text only - diagrams to be inserted<
[note – this is the lecture version reporting the highlights of this research; a
“scientific paper” may be published later.]
I am an amateur geologist. I love studying geology, but the more I study I find
that I don’t find answers, just more questions. Geology is a bit like a big
jigsaw puzzle and most of the pieces are missing, but because of that it is
challenge to try to piece it back together.
[The Chalk is a fine grained, very pure limestone that was deposited in the Late
Cretaceous. If you dissolve a piece of Chalk in dilute hydrochloric acid there
will be a residue of about 1-2% by weight of clay and quartz. In Yorkshire the
exposed sequence is about 420m thick. Between some beds of Chalk there are bands
of flint and bands of calcareous clay, known as marls. <link to Yorkshire Chalk
Website> Can the geochemistry of the marls indicate the origin of the
non-carbonate material in the Chalk?]
To fully understand the Chalk we have to start with the stratigraphy: measuring
the thicknesses of the marl bands and flints and the distances between them. We
then define the sequence and we can name or number them.
I thought it would be nice if there was some way to “finger print” the marls: to
easily identify a particular marl and find where it fits into the sequence. With
this in mind I have taken up offers to look at the chemistry of the marls.
Bryn Jones of Newcastle University analysed some of the major elements [like
sodium, calcium, potassium..] but there was not much variation in the results.
Then Dick Middleton of the University of Hull offered to run some samples
through the XRF. So we tried some and the results looked interesting. So I then
collected samples from every 3 or 4 metres (when exposed) and Dick analysed them
for me. We also ran some comparisons from the Speeton Clay. The measurements are
of untreated whole rock all on the same machine.
There was variation in some of the elements –
Rubidium – was generally higher in samples from the Flamborough Formation.
Strontium showed no particular pattern
Zinc, copper, nickel and, to a lesser extent, lead showed a trend of increasing
through the Turonian, decreasing in the Coniacian, relatively low in the
Santonian and Campanian but increasing in higher beds.
Yttrium exhibited the same trend but was more exaggerated.
Niobium and Zirconium have the same trend but more pronounced.
The Basal Marl, Grey Bed, Black Band Member seem to be enriched in copper, lead
and nickel. These are at non sequences: times of erosion, non-deposition and
sometimes anoxia as if these elements are being concentrated by the events.
By plotting some elements against each other using Dick Middleton’s MICA
computer program some trends emerge – Y/Rb and Nb/Rb show nice patterns.
There seems to be three groupings within the marls –
·
Detrital marls
·
Volcanic marls
·
Concentrated deposits
The geochemistry of the Middleton Marl seems to be very similar to the Black
Band. The marl is very dark and has a diminished microfauna, though not as
dramatically as the Black Band (s.l.)
Conclusions –
·
Yes it is possible to identify 3 groups of marl band by their geochemistry, but
it may not be possible to “fingerprint” all the individual marls.
·
The majority of the marls probably represent clays that have washed or been
blown into the Chalk sea [just as red Sahara sands sometimes end up on our cars
when parked in the street]. [The Chalk contains about 2% acid insoluble material
such as clay and quartz. These may become more concentrated through solution of
the calcium carbonate in the sediment.]
·
Some marls have higher concentrations of zirconium, niobium and yttrium, for
example the Ulceby, Melton Ross, North Ormsby and Deepdale Marls. Could these
have originally layers of volcanic ash [the Atlantic was staring to open]?
·
These marls coincide stratigraphically with the maximum development of flint –
is this just a coincidence or was there an input of silica from volcanic
activity or did the ash falls have a catalytic effect?
·
The Middleton Marl exhibits dark patches and has a similar geochemistry to the
anoxic parts of the Black Band Member – is this significant? [Is this related to
Oceanic Anoxic Event number3 that occurs near the Coniacian-Santonian boundary?]
Post Script –
At the same meeting Dave Wray presented a lecture about the rare earth elements
of the mid Turonian to mid Coniacian Chalk (from the Chalk Hill Marls up to the
Ulceby Marl) marl bands of eastern England, and concluded that the following
marls are volcanic in origin – Barton Marl no. 1 (lowest of 4), Melton Ross
Marl, Deepdale Lower Marl, North Ormsby Marl, Ulceby Marl and Little Weighton
Marl no. 2 (middle of 3). The Little Weighton no. 2 Marl was not sampled in my
work.
Wray
D S & C J Wood 1998. Distinction between detrital and volcanogenic clay-rich
beds in Turonian-Coniacian chalks of eastern England.
Proceedings of the Yorkshire Geological
Society 52, 95-105.
Additional information in separate files–
Acknowledgements – I thank Christine Clive, Richard Middleton and Shigeru Suzuki for their help and
advice.
* Contact details - Mike Horne, Honorary Fellow, Department of Geography, Geology and Environment, University of Hull, Cottingham Road Hull, HU6 7RX
Copyright Mike Horne 1995
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