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Biostratigraphy.
Most of the early studies
of the Chalk of the Northern Province have concentrated on the biostratigraphy,
see
figure.
Early works by Young and
Bird and J Phillips in the 1820s carried very little detail. There were
several detailed publications in the 1870s. Barrois (1876, quoted in Blake
1878) made a brief visit to the area and divided the Chalk into 8 biozones.
Blake (1878) proposed 6 biozones. There were other papers published in
subsequent years giving details of exposures and comment on the Chalk by
the Rev. E Maule Cole, the Mortimer brothers and G W Lamplugh. The Mortimer
collection was purchased by Hull Museums in about 1919.
In the early 1900s, Jukes Brown published
memoirs for the Geological Survey and Rowe (1904) added the description
of the Yorkshire coastal exposures to a series of papers for the Geologists'
Association.
In 1929 Ennis published a
short paper on the zonation of the Yorkshire Chalk, turning Rowe's Uintacrinus
Band into a zone. In the 1930's D W Toyne started a project to visit all
the inland exposures, and assign them to biozones. Unfortunately his early
death meant that he did not complete the project (Horne 1989), but his
field notebook was passed to the young Ted and Willy Wright, who continued
the work until the start of the Second World War. They published the incomplete
results in 1942, fearing that they might not survive the War. In this they
adopted Rowe's suggestion of using Hagenowia rostrata as a local
zonal fossil, and proposing a subzone of Discoscaphites binodus for the
highest beds. They did not take the local research any further after the
War.
Wilson's zonation in 1948
follows most of these changes. Neale 1974 split the Hagenowia rostrata
zone into a flinty and flinty part, in the same way as Rowe split his Micraster
cor-anguinum zone. Chris Wood, after working on the Hull and Brigg maps,
introduced a further zone of Sciponiceras gracile in the 1980 B.G.S. Regional
Guide.
This zonation, is broadly
followed here, for convenience; though with the knowledge that the whole
biozonation is not scientifically defined. All of the biozones in common
use are actually assemblage biozones and none are defined by a bioevent
at the base. It is not clear how authors have decided when where they should
mark the base of the zones, but they do manage to measure them nonetheless.
The 'rostrata' biozone is still named after a fossil that has been wrongly
identified, and is only common in part of the zone. This zone is also commonly
divided into two parts on a lithological basis, that is probably diachronous!
Lithostratigraphy
Traditionally the Chalk in
the Northern Province has been divided into three units, derived from the
divisions of the chalk of the Southern Province: the Lower (or Grey) Chalk,
Middle Chalk and Upper Chalk (e.g. Jukes Brown 1903 4). This is why Rowe
did not include the Cenomanian Chalk in his 1904 paper about the White
Chalk.
G W Lamplugh published measured
sections of the coastal exposures around Flamborough Head, in 1895. Although
these tended to group beds together, they fit well with recent work. This
was good lithostratigraphy and if his work had been continued many of the
problems with the Yorkshire Chalk would have been solved years ago.
But Rowe placed such importance
in his publications on the use of fossils to study the Chalk, that no lithostratigraphy
was done for over 60 years (Gale & Cleevley 1989). Toyne (MS notebook)
and the Wright brothers (1942) followed Rowe's lead and only recorded the
fossils to be found in the inland quarries they visited. Many of these
were found loose, and Willy Wright saw no need to record the horizons from
which they came (verbal comm.). Most of the pits they visited have now
been filled in.
Chris Wood mapped the Chalk
of the area for the British Geological Survey in the 1970's, leading to
the publication of the Hull and Brigg sheets, and the memoir (Gaunt, Fletcher
& Wood 1992). His work was the first study of the lithostratigraphy,
with bed by bed measuring and logging since Lamplugh. He divided the Chalk
into four Formations: the Ferriby Chalk Formation, the Welton Chalk Formation,
Burnham Chalk Formation and the Flamborough Chalk Formation. He also started
a new trend in the study of the Northern Province Chalk by naming important
flint and marl bands (Wood and Smith 1978, Gaunt, Fletcher & Wood 1992).Much
of Wood's work is based on the exposures in Lincolnshire, and there were
gaps in the Burnham Formation to be filled in and no detail for the Flamborough
Formation was given (Wood & Smith 1978).
Felix Whitham, an amateur
geologist and Treasurer of the Hull Geological Society, had also realised
the importance of measuring exposures and bed by bed collecting of fossils.
He had started visiting quarries in East Yorkshire and published some of
the results as field handouts for field meetings and as short papers in
Humberside Geologist. This work continued as part of the Hull Geological
Society Centenary Chalk Project and led to the publication of two papers
on the stratigraphy (Whitham 1991 & 1993). These named further marker
horizons, indicated observed macro fossil ranges and gave the first detailed
description of the Flamborough Formation, since Lamplugh's work nearly
one hundred years earlier.
More recently, Simon Mitchell
has published details of the sequence at Speeton.
Thickness of the Chalk.
From the times of early research, geologists have been trying to give a thickness for the Yorkshire Chalk:
Barrois 1876
637 ft.[ca. 193m]
Blake 1878
790 ft.[240m]
Jukes Brown 1903-4
1270 ft.[385m]
Rowe 1904
1013 ft.[307m] ('White Chalk' only)
Ennis 1929
1416 ft.[429m]
Wilson 1948
1432 ft.[434m]
Neale 1974
426.78m
Wood 1980
529m
This paper
over 430m
The earlier figures were based
on estimates or measurements of the bio zones, with all problems involved
due to lack of scientific definition of the base of the zones. Neale's
amazingly precise figure to the nearest centimetre is probably the metrication
of previous author's measurements.
Wood based his figure on
the lithostratigraphy, but realised that there was a lack of exposure in
the Burnham Formation, and probably included borehole information from
Holderness, as the youngest beds are not exposed.
The figure used here is based
on the coastal exposures of the Flamborough Formation and inland exposures
of younger beds. There is still a gap in exposure at the top of the Burnham
Formation and between the cliff exposure at Sewerby and the higher beds
of the Discoscaphites biozone. Also, I believe that the inland exposures
represent a thinner sequence than the coastal exposures, due to the effects
of the Market Weighton Structure'
The real answer to the question
"How thick is the Yorkshire Chalk?" is "It depends on where you measure
it!" There is regional variation in the thickness of the beds and the erosion
of the top of the Chalk is different for each outcrop or subcrop. Without
borehole information one has to piece together a composite log based on
the correlation of different exposures, and accuracy of these measurements
will depend on the criteria used when recording individual beds.
Bibliography of the Yorkshire Chalk
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