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Mike Horne FGS

Unfinished Works

This is unfinished work that has not been edited or peer reviewed by the Society.

"The marl bands of the northern Chalk and their origin".

 by Mike Horne

Abstract :

 130 samples of marl bands were taken at semi-regular intervals throughout the Yorkshire Chalk (about 400m thick) and were analysed with an XRF spectrometer . Some of the Marls in the Turonian and Coniacian Chalks contained significantly higher than average quantities of Zirconium, Yttrium and Niobium, indicating that these may have a volcanic origin. These marls are : the Ulceby, Lower Deepdale, Melton Ross, North Ormsby and Upper Deepdale marls ( with over 600 ppm Zr). Other marls showed moderately high levels : the Rowley marl  no. 4 and Barton Marls nos. 1, 2 and 4 ( with 200 to 350 ppm Zr).

Marls from the Cenomanian, Santonian and lower Campanian did not  have such high levels.

The marls from the Flamborough Formation (Santonian and lower Campanian) tended to exhibit lower values and less variability for Lead, Nickel, Copper and Zinc and higher values for Rubidium, when compared with the marls from the younger chalks. Tests on the Ulceby, Grasby, Melton Ross and some other marls, and Black Band materials showed fairly consistent values across the region.

  

Introduction :

                The Chalk is a pure white bioclastic limestone. In Yorkshire it is about 400 metres thick and to most people it all looks very much the same - a white rock which, if you look closely, contains white fossils., some flint bands and some clay bands.

                The band have been used to correlate the stratigraphy in the region and have been named by Chris Wood (Wood  & Smith 1978, Gaunt et al 1992 ) and Felix Whitham ( 1991 & 1993 ), though some of the flints may be diachronous. The marls bands have also been used by Chris Wood (e.g. Gaunt et al. 1992) for correlation with other regions, e.g. East Anglia, southern England and Germany. The stratigraphy in this paper is based on personal observations and using the names already published, plus some new ones.

                The Upper Cretaceous Chalk contains layers of calcareous clay,  known as marl bands. In the Northern Chalk, and sometimes in the Southern Chalk, these are very distinctive, with abrupt bases and tops. There is usually no bioturbation of the marl into the underlying chalk, or of the overlying chalk into the marl.

                Macrofossils are sometimes visible in the field in the marl bands. They often contain patches of iron oxide minerals, and occasionally iron sulfide minerals (which oxidise on weathering). Green coloured glauconite can sometimes be observed in the field.

                As the marls are soft, they are often eroded, especially at coastal exposure, forming a negative feature in a cliff or quarry face. They are very prone to colonisation by plants, and in abandoned quarries or on some cliffs, the marls are often picked out by a line of green vegetation.

                For a considerable time there has been discussion about the origin of the marl bands, whether they are the result of solution of the chalk naturally enhancing the percentage of clay minerals, the formation of new minerals during diagenesis or are representatives of tuff layers. Jeans (1968) studied the clay  mineralogy and concluded that the montmorillonite in the Cenomanian chalks was neo-formational and detirtal. Chris Wood  (Gaunt et al. 1992) suggested that the "buttery" marls were argillated ash falls. 

                I have often wondered about this and thought it would be useful for stratigraphy if the marl bands had a microfossil or geochemical “fingerprint”,  so that we could place small exposures in their correct stratigraphical level. A few years ago Richard Middleton in the Department of Geography at the University of Hull, offered me the opportunity to run some samples through an x-ray fluorescence spectrometer (XRF). The first few samples showed an interesting variation in results, so more samples were tested to see if a pattern emerged. Unfortunately the XRF machine was scrapped before the project could be completed.

 

Methods.

                Samples of major marl bands were collected, attempting to cover  the complete succession of the chalk and also test regional  variation.  The samples were dried and then whole samples were  powdered in a ball mill. They were not treated in any other way, so that the chalk content in the samples varied. The powder was  then pressed into pellets and analysed in a Philips PW1410 X- Ray  Fluorescence Spectrometer. Results for the following elements  were obtained for most of the samples : Cu, Zn, Ni, Pb, Sr, Rb, Y, Nb and Zr. All results are given as parts per million (ppm).

                A list of the sample numbers and localities is given in table 1 and the results are given in table 2.

 Regional Comparisons.

 In order to test whether particular marl bands have a distinctive fingerprint, samples of the same marl horizon from different localities were analysed. If these showed great variation it would show that the geochemistry of the marls is not a reliable stratigraphic tool.

 The Khaki Marl ( part of the Black Band Member ).

This occurs above an erosion surface and below a black carbon  rich clay. It contains well preserved fish teeth, a varied  microfauna and occasionally nests of  'terebratulids' and very rarely the  belemnite Actinocamax plenus.

          NEH5      EL3       GD21      KNA2

   Sr     442         380          312         347

   Rb      31           32            35           38

   Y       30           32             53           37

   Nb       4             8               5             6

   Cu      38           41            36            40

   Zn      81           41            72            51

   Ni      59           66             69           67

   Pb      28           20             16           23

 

 

The Ulceby Marl.

This marl occurs towards the top of the Sternotaxis plana zone,  approximately 91m from the base of the Chalk on the composite log. It is commonly more than 4cm thick and is notable for the well preserved Bourgeticrinus  that can be found in it.

 

          UL1B      EN11      KP2B      NL1B

   Sr     444          341         507          329

   Rb      20           18           14            20

   Y        88           84           52            94

   Nb      28           50           17            82

   Zr     455          811          77          997

   Cu      31           42           20            39

   Zn      94         182           54          182

   Ni       25           76           21           86

   Pb       20           22          17            30

 

Comparison of  marls with other materials

 

          SPE65    EP2       BEV9     KIM       C9B       D7       Average Marl

   Sr     477        808         831        293         228       654         523

   Rb      35            3             9        152         130         10           42

   Y       50             7             9          22           25       108           45

   Nb       9             1             1          16           28       205           19

   Zr      81           18           37        123         159       908        177

   Cu      11           15          13                                                    26

   Zn      55           20          24                                                     64

   Ni      56           16           13                                                     52

   Pb      22             4             5                                                     12

 

Sample is Red Chalk from Speeton , EP2 Coniacian Chalk and BEV9 Santonian (flint free) Chalk. Sample KIM is Kimmeridge Clay, C9B a phosphate rich clay and D7 a bentonite clay all from Speeton.

Unusual results :

Samples MEL23 and GD22 are the black clay from the Black Band Member, a carbon rich  clay containing fish scales and a depleted microfauna.  This black clay is thought to represent an anoxic event (OAE2) when oxygen deficient water flooded onto the shelf killing most of the benthos (ref ....). Sample SF51 is the basal Cenomanian marl from South Ferriby and probably represents a condensation/transgression event. These produced a set of results which are ......  low in ... but rich in ..... When    and    are plotted against       and     (see fig      ) these form a distinctive group. Interestingly the samples of the Middleton Marl plot in a similar position on that graph. Could this also be a sign of anoxia or a some condensation of the sequence ?

In the field dark grey patches can be found in the Middleton Marl which as reminiscent of the black clay in the Black Band Member.

New stratigraphic names :

the Arras Flint

Arras Flint., A carious tabular flint between the Enthorpe Marls and the Kiplingcotes Marls near the top of the Turonian Chalk. The top is flat and the base is undulating. Type Section is Arras Hill Quarry (RIGS)

Marsupites Marl. A thick indurated marl to be seen S W of Danes Dyke, which contains numerous plates of the crinoid Marsupites

 Conclusions :

 

References :

Gaunt  G D, T P Fletcher & C J Wood, 1992. Geology of the country around Kingston Upon Hull and Brigg. British Geological Survey Memoir.

Jeans C V 1978. The origin of the montmorillonite of the European chalk with special reference to the Lower Chalk of England. Clay Minerals 7, 311-329.

Whitham F 1991. The Stratigraphy of the Upper Cretaceous Ferriby, Welton and Burnham Formations  north of the Humber, north-east England. Proceedings of the Yorkshire Geological Society   48,227-254.

Whitham F 1993. The stratigraphy of the Upper Cretaceous Flamborough Chalk Formation north of the Humber, north-east England. Proceedings of the Yorkshire Geological Society  49,235-258.

Wood C J & E G Smith 1978. Lithostratigraphical classification of  the Chalk in North Yorkshire, Humberside and Lincolnshire. Proceedings of the Yorkshire Geological Society 42, 263-288.

 

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