SOME OF THE TYPES OF MICROFOSSILS AND NANNOFOSSILS FOUND IN THE DORSET MESOZOIC SEDIMENTS.

BY KEITH W. ABINERI, UK.

42 West Borough, Wimborne, Dorset BH21 1NQ, UK
Tel. 01202 885547


 

Introduction.

The numbers of genera and species of microfossils and nannofossils, which have been detected in the Mesozoic sediments along the Dorset coast, are varied and complex. Furthermore classification can be very difficult. The sheer numbers of individual microorganisms on 1 cm
2 of the area of each peel shows that many of these sediments are largely composed of actual biogenic material. These include crinoidal limestone, Kimmeridgian shales, clays and limestone bands, Purbeck micrite1 and clay beds, Cretaceous chalk and marl2 layers etc.

Of course individual examination of separated microfossils and nannofossils can be undertaken by crushing, sieving, decanting and treatment with various chemicals, of the original rocks. These procedures are certainly very useful for the classification of many microfossils. However the cellulose lacquer peel technique, discussed here, gives an overall picture of the structure of the various sediments. This applies also to cellulose acetate peels and accurately prepared thin sections.

The six accompanied illustrations, Figures (1) to (6), show a further selection of cellulose lacquer rock peels. These relate to some different types of microfossils and nannofossils. However (1) shows an image of an Upper Chalk foraminifera on an unstained lacquer peel, using darkfield illumination with XPL. This method of lighting records the fine structure of the chamber walls.

The variety of microfossils and nannofossils.

There is little doubt that the microfossils and nannofossils, present in rocks and sediments, are the largest part of the geological record, in terms of variety and numbers of individual past forms of life. Their study leads to increasing understanding of the Earth's history, past climatic changes, past geographical and geological developments (from plate tectonics and ocean floor spreading), the dating of rock strata, the search for valuable mineral deposits (petroleum exploration) and also a large part of the history of life on our planet.

Furthermore sedimentary rocks contain many microscopic fragments of larger plants and animals, which are worthy of our attention and help to build a general picture of ancient environments (e.g. Mesozoic conifer forests).

Those amongst us who are privileged to observe Nature with an optical microscope, or an electron microscope, can become fully aware of the exquisite complexity of our surroundings. This applies to the study of micropalaeontology, in particular, because the active use of the imagination is required, as well as natural science and accurate observation to build pictures of the remote past ages. Obviously there are many problems and uncertainties to challenge our ideas and theories, and encourage new thinking. This is an age of developing natural science with the aid of rapidly advancing technology and new methods of processing information.

It is hoped that the following illustrations and notes will be of interest to those who wish to observe some of these microscopic objects and features with their own microscopes. The preparation of the cellulose lacquer rock peels does require some practical skill, which can be acquired by "trial and error" methods. To obtain the best results the softer layered rocks should be sampled. The type of peel can be assessed usually with XPL to show the material which has been removed from the rock surface. Likewise XPL can indicate which images on the peel are replicas, and produce no optical figures.


Various types of nannofossils and microfossils. i.e. Figures (1) to (6).

Figure (1). This is a special dark field image of a foraminifera from the Cretaceous, Upper Chalk, Actinocamax Quadratus Zone. Map Reference : SY.851.802. West of Arish Mell on the cliff, Dorset coast. Objective N.A. 0.65. Total field area of the image = circa 170 microns x 140 microns.

This image shows darkfield illumination combined with XPL, with no replica images. The oblique section is circa 163 microns in maximum diameter. The walls of the interior chambers show considerable detail under this mode of illumination. The species is believed to be Globotruncana. Unstained Upper Chalk peels generally will give these results with dark-field illumination and XPL, when applied to foraminifera images.

Figure (1) required an exposure of about five seconds with the ASA 100 film. Visual observations of the fine structure of the interior chamber walls need a period of "dark adaptation" by the eye.

Figure (2). Here we have an intact Coccolithophore3 nannofossil, buried in kerogen. This image was derived from an unstained cellulose lacquer peel, from the Jurassic, Kimmeridge Clay, Maple Ledge Shales. Map Reference : SY.908.790. East of Gaulter Gap, Kimmeridge Bay, Dorset coast. Oil-immersion objective N.A. = 1.25. Total field area of the image = circa 30 microns X 20 microns.

Figure (2) is an example of a removed thin layer type of image. The real diameter of the Coccolithophore is estimated at 11 microns. The surface coccoliths are circa 4.2 microns in diameter. Clearly electron microscope images are needed for closely detailed examination. The species here is believed to be Ellipsagelosphaera brittanica4. Similar coccolithophore images have been identified on stained cellulose lacquer peels, but these are usually less well defined than on the unstained peels. However, most surprising, some of the stained coccolithophores (and coccoliths) show a deep blue colour due to Ferroan Calcite composition, whereas many others are pale pink in colour suggesting very pure calcite composition. This differential staining of coccolithophores and coccoliths requires confirmation.

Figure (2) was obtained with brightfield illumination using one polarizing plate above the objective.

Figure (3). Here we have images of Dinoflagellate5 cysts from a stained cellulose lacquer peel. The highly magnified picture was derived from the Jurassic, Upper Kimmeridge Clay, Freshwater Steps Stone Band. Map Reference : SY.943.772. Freshwater Steps, west of Hounstout Cliff, Dorset coast. Oil-immersion objective N.A. = 1.25. The total field area of the image = circa 100 microns X 70 microns.

Mainly the images of these microfossils on the cellulose lacquer peels are buried in the kerogen and/or encrusted with coccoliths or coccolithophores. The best images are obtained for classification by the techniques of Palynology6, which involves the removal of all the mineral contents of the sediment, using concentrated hydrofluoric acid7. This technique is used also for the isolation of pollen grains and plant spores. Slides of the organic material only are prepared in a Canada-balsam mounting, as a strew of microfossils for microscopic examination. An important paper by N.S. Ioannides, G. N. Stravrinos and C. Downie l9768 based on the Kimmeridgian microplankton from Clavell's Hard, Dorset coast, classified some sixty species of Dinoflagellate Cysts and came to some important conclusions about the origins of the kerogen in the Kimmeridge Oil Shale9.

Figure (3) was obtained with brightfield illumination and PPL. The complete diameter of these Dinoflagellate cysts range from circa 20 to 50 microns.

Figure (4). Here we have a bed of broken Ostracod10 carapace sections on a stained lacquer peel from the Upper Jurassic, Middle Purbeck, Lulworth bed 107. Map Reference : SZ.036.784. Durleston Bay, nr. Swanage, Dorset coast. Objective N.A. = 0.10. The total field area of the image = circa 2340 microns x 1920 microns.

Ostracods are microscopic Crustacean arthropods11 of great geological interest and are frequently studied because of their widespread occurrence in various sediments. As arthropods, living Ostracods are complex multicellular animals with a digestive system, a central nervous system and developed genital organs. The bodies appear unsegmented and are contained in a calcitic carapace consisting of two valves. The microfossils are generally found as carapaces only.

In Figure (4) the prominent Ostracod carapace has a maximum diameter of about 813 microns. There are also fragmented carapaces present, as well as a fragment of conifer wood, consisting of removed fusain and replica type imagery. The red colour on the peel is due to stained Micrite. The darker colour may be due to Marl or clay. Lulworth bed 107 in Durleston Bay lies just below the well-known "Cinder Bed", which marks a temporary marine incursion, and is the accepted boundary between the Jurassic and Cretaceous periods. The "Cinder Bed" shows masses of marine Oyster-rich sediment but the beds below it show numerous freshwater fauna, including Ostracods.

Figure
(4) was obtained with brightfield illumination and one polarizing plate above the objective.

Figure (5). Here we have the image of a single Ostracod carapace section on a stained cellulose lacquer peel derived from the Upper Jurassic, Middle Purbeck, Lulworth bed 102. Map Reference : SZ.036.784. Durleston Bay, nr. Swanage, Dorset coast. Objective N.A. = 0.25. The total field area of the image = circa 1350 microns X 900 microns.

This Cypridea Ostracod12 carapace section has a main axial diameter of about 1210 microns. This lacustrine and brackish water type of Ostrocod is widely distributed in the Purbeck freshwater beds. The Cytheracea Ostracods13 are found in marine environments. The red stained structures on Figure (5) indicate calcite and micrite composition. Marl and clay areas are also shown. The peel was examined with brightfield illumination and one polarizing plate above the objective. Some fusain can be seen on this image.

Figure (6). Upper Chalk sediments contain numbers of microfossils, nannofossils, calcisphereres, foraminifera, fragments of larger life forms etc. However here we have an unidentified microfossil with a flask or bottle-like appearance on a very detailed stained cellulose lacquer peel. This picture is from the Cretaceous, Upper Chalk, Actinocamax Quadratus Zone. Map Reference : SY.851.802. West of Arish Mell on the cliff, Dorset coast. Objective N.A. = 0.65. Total field area of the image = circa 250 microns X 170 microns.

The differential staining of the minute nannofossils on this cellulose lacquer peel are accentuated by the brightfield PPL illumination. Apart from the flask or bottle-like microfossil image, Figure (6) shows calcispheres and numerous nannofossils, as well as probable sponge fragments. This unique flask-shaped object either represents an unknown species (perhaps a type of Calpionellid14) or may represent the cell fission of an unkown microorganism.

Notes and References.

1. Micrite : micro-crystalline calcite (grain size less than 10 microns).

2. Marl : a calcareous mudstone.

3. Coccolithophores : coccospheres, a variety of marine algal Phytoplankton made of coccolith calcareous plates.

4. Ellipsagelosphaera brittanica : a species of coccolithophore which occurs in the Kimmeridge Clay mainly as dispersed coccoliths.

5. Dinoflagellate cysts : a family of organic microfossil plant cysts best separated by palynological techniques by treating the rock with concentrated hydrofluoric acid and some concentrated nitric acid. Marine Phytoplankton.

6. Palynology : The separation technique used in 5 above, and also for the isolation of pollen grains and other land based plant spores.

7. Hydrofluoric Acid a powerful mineral acid which will dissolve all silicate and other mineral rocks.

8. N. S. Ioannides, G.N. Stavrinos and C. Downie, 1976. "Kimmeridgian microplankton from Clavell's Hard, Dorset, England." This important paper reports and describes some sixty species of Dinoflagellate Cysts from the Kimmeridge Blackstone (oil shale). However from the high numbers of Terrigenous palynomorphs (land based pollen and plant spores) in the oil shale layers, the authors deduced that much of the Kerogen was derived from land widespread swamp floras, due to a slight rise in sea level, and flooding into the Kimmeridgian sea. A close correlation was observed between the quantity of Kerogen and the proportion of terrestrial palynomorphs.

9. Kirmmeridge Oil Shale : Variable amounts of these deposits, rich in Kerogen, point to fluctuations in rainfall, flooding and sea levels. Kimmeridge Clay seems to be one of the source rocks for North Sea Petroleum.

10. Ostracod Carapaces : These occur in vast numbers in the Purbeck beds. Marine, brackish and freshwater species of these Crustaceans have occurred in geological times.

11. Arthropods : Animals with Jointed legs. A large phylum including classes Crustacea, Arachnida, Insecta, Myriapoda and Trilobita.

12. Cypridea Ostracod : A species of ostracod found in Lacustrine and brackish water environments.

13. Cytheracea Ostracod : A species of ostracod found in marine water environments.

14. Calpionellid : A flask-shaped microfossil.

Editor's note: Some of the quality of the author's original 35mm slides is lost in the scanned and compressed web images. Comments to the author are welcomed, who can be contacted at the above address or comments can be passed on via the Micscape Editor, see contact on magazine index.

Other articles in this series can be accessed in the Micscape on-line library by typing in the author's surname 'Abineri' in the Library search engine (link below).

 

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