FRAGMENTS OF MESOZOIC CONIFER WOOD FOUND IN THE KIMMERIDGE CLAY
AND PURBECK SEDIMENTS AND RELATED TOPICS.
BY KEITH W. ABINERI, UK.
West Borough, Wimborne, Dorset BH21 1NQ, UK
Tel. 01202 885547
During the latter parts of the Jurassic Period, i.e. in Kimmeridgian, Portlandian and Purbeckian times, the North Atlantic was beginning to open up between North America and Europe. In Kimmeridgian times this appears to have resulted in the accumulation of large quantities of land swamp vegetation in the Kimmeridge marine deposits. This seems to have occurred under fluctuating conditions due to sea level changes, or changes in the land rainfall and levels of the water-tables. This suggests low lying forest land on both continental areas. The decaying vegetation could take the form of Kerogen, a variety of plant products, including pollen and plant spores, and fragments of wood fusain and pyrites. These would be deposited on a shallow sea floor together with marine nannofossils and microfossils in enormous numbers. Periodic anaerobic conditions would occur, either by virtue of the allochthonous1 decaying land organic material, or the periodic occurrence of marine algal "blooms" from the abundance of nannofossils or microfossils, or effects from both processes.
It is possible that the earliest phases in the splitting of Laurussia (the Canadian shield, North Europe, Scandinavia and European Russia) occurred at the beginning of the Triassic Period. During the Jurassic Period the North Atlantic seems to have expanded from a narrow sea channel to more open seas in the Cretaceous Period. This process of ocean floor spreading continued throughout the Tertiary and Quaternary Periods until the present time. It is believed now that equally momentous changes in the climate, environment and forms of life took place on this vast time scale. Today we are obliged to consider other massive influences on the evolution of life, which have occurred throughout geological time. These could have included enormous volcanic activity, impacts by meteorites, asteroids or comets and mass extinctions of many different species. Of course the global volcanic processes would be related to plate tectonics or mantle "hot spot" activity.
Here we are concerned with more gradual changes in the geological and biological environments, which can be studied under the microscope, from the fine structure of sediments. This refers especially to minute fragments of Mesozoic conifer wood found in the Kimmeridge Clay and Purbeck beds, as well as related features.
In l983 Dr Jane E. Francis published a very valuable and informative paper on the Lower Purbeck (Upper Jurassic) Fossil Forest of Dorset. The title of this paper was "The Dominant Conifer of the Jurassic Purbeck Formation, England."2 Dr Francis identified this dominant conifer tree, for the first time, as the species Protocupressinoxylon purbeckensis3. Full information was given on the shape, size and structure of these trees, the fine structure of the wood and the characteristics of the pollen. Remains were found in the form of fusain and silicified wood. Perhaps most interesting to what follows here were the minute structures of the tracheid "bordered pits"4, which were typical of these conifer trees. These were first found by the present author below the Purbeck beds in the Kimmeridge Clay using the cellulose lacquer technique during l987. Dr Francis confirmed that these "bordered pit" conifer wood structures from the Kimmeridge Clay beds showed similarity to those for Protocupressinoxylon purbeckensis, which had been studied by her in the Purbeck beds. Later these microscopic structures were found on cellulose lacquer peels from the Lower Purbeck.
A further very interesting paper on the Mesozoic conifers was published by Dr J. E. Francis in l984, under the title "The Seasonal Environment of the Purbeck (Upper Jurassic) Fossil Forests."5
Furthermore, since the Kimmeridge Clay is believed to be the source rock for North Sea petroleum, there appears to be an analogy with the much more ancient formation of coal from the great fern forests of the Carboniferous Period. The Kerogen from the Mesozoic forest swamps of the Jurassic Period, under the action of heat and pressure would form liquid petroleum. From the following illustrations (Figures (1) to (6),) it would seem that many of these forest trees were Mesozoic conifers.
"Bordered Pit" Tracheid structures from the Kimmeridge Clay and Purbeck Sediments i.e. Figures (1) to (4).
|Figure (1). Fragments of Mesozoic conifer wood with damaged bordered pit structures 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 105 microns X 80 microns.|
Figure (1) shows also unstained coccoliths embedded in kerogen and is mainly a removed thin layer type of image. The fusain has been peeled from the rock with the kerogen. It is part of a stained cellulose lacquer rock peel. Many lacquer peels were prepared from the shales in this location and, on average, 50 to 100 minute wood fragments were found on 1 cm² of each peel. In areas very rich in coccolithic limestone the numbers of wood fragments was much lower.
Figure (1) was obtained with brightfield illumination.
|Figure (2). Here we have a relatively intact but isolated bordered pit structure from a Mesozoic conifer tree. This minute object was found again on a stained 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 90 microns X 60 microns.|
Figure (2) shows also unstained coccoliths embedded in kerogen and is mainly a removed thin layer type of image. The fusain ring components have been peeled from the rock with the kerogen. The maximum outer diameter of these rings averages circa 13 microns.
Figure (2) was obtained with brightfield illumination.
|Figure (3). A further relatively intact but isolated bordered pit tracheid structure from a conifer tree, comparable with Protocupressinoxylon purbeckensis, was found on a stained peel, from the Jurassic, Kimmeridge Clay, White Stone Band, Dark Shale Layer Map Reference SY.936.776. West of Freshwater Steps, Dorset coast. Oil-immersion objective N.A. = 1.25. Total field area of the image = circa 60 microns X 45 microns.|
Figure (3) shows once more unstained coccoliths embedded in the kerogen layer. These objects are golden yellow in appearance and generally not focussed because of the thickness of the kerogen layer, and the small depth of focus with the oil-immersion objective. This cellulose lacquer peel is clearly a removed thin layer type, the kerogen, various minute pieces of fusain and coccoliths having been stripped from the rock surface. Numbers of other minute wood fragments were found on this single 1 cm² peel, some of which showed damaged bordered pit tracheid structures.
Throughout the Kimmeridge Clay sediments, bordered pit structures similar to those associated with Mesozoic Conifer Protocupressinoxylon purbeckensis have been found on cellulose lacquer peels. The numbers vary considerably, and the darker shale layers contain the most examples.
Figure (3) was obtained with brightfield illumination. The maximum outer diameters of the fusain rings on Figure (3) are circa from 14 to 15 microns.
|Figure (4). This picture of a conifer bordered pit tracheid structure was derived from the Jurassic, Lower Purbeck, Cypris Freestones, Micrite and Clay Layer. Map Reference : SY.972.785. Swanworth Quarry6, West of Swanage, Dorset coast. Oil-immersion objective N.A. = 1.25. Total field area on the image = circa 120 microns X 85 microns.|
A large part of the bordered pit structure here is a replica image, but the red stained micrite is a removed layer of rock material. A small layer of fusain has also been removed from the rock surface. Small broken coccoliths in the micrite may be due to a lagoon environment, which was believed to exist in Purbeck times. The maximum outer diameters of the pits on Figure (4) are circa from 15 to 16 microns. The size and shape of these minute features agree with the original findings of Dr J.E. Francis.
Figure (4) was obtained with brightfield illumination used on the stained cellulose lacquer peel and shows an obvious advantage of some replica images.
Protective Resin (Amber) associated with the Purbeck Mesozoic Conifers.
|Figure (5). A microscopic flake of resin peeled from a minute wood fragment derived from the Jurassic, Lower Purbeck, Cypris Freestones, Micrite Ostracod bed. Map Reference : SY.972.785. Swanworth Quarry, West of Swanage, Dorset coast. Oil-immersion objective N.A. = 1.25. Total field area of the image = circa 90 microns X 65 microns.|
The background of Figure (5) consists of red stained micrite, unstained silica grains and some small particles of pyrites or fusain. There are also some small damaged coccoliths suggesting a lagoon environment. The size of the resin flake is about 18.5 microns X 37 microns. Numbers of similar resin flakes have been found in these Purbeck sediments, derived from coated wood fragments. It is assumed that this resin (amber7) was extruded by the conifer wood for protection against insect attack. (See also Figure (6).). This predatory activity would have been especially severe at a time before the emergence of flowering plants at the later part of the Cretaceous Period.8
Figure (5) was obtained with brightfield illumination used on a stained cellulose lacquer peel.
A Mesozoic Conifer Resin Duct.9
|Figure (6). A longitudinal section of a Mesozoic Conifer Resin Duct from the Jurassic, Lower Purbeck, Cypris Freestones, Micrite Insect bed, Map Reference : SY.972.785. Swanworth Quarry, West of Swanage, Dorset coast. Oil-immersion objective N.A. = 1.25. Total field area of the image = circa 115 microns X 60 microns.|
The main image in Figure (6) is a replica type, which shows the longitudinal section of the duct, with globules of resin, in a complex tube type structure. A similar image of a conifer resin duct, also with the appearance of a replica image, had been found previously on an unstained peel from the Kimmeridge Clay, Rope Lake Head Stone band. Map Reference SY.926.775. To have found these two minute longitudinal sections both showing the resin globules was a lucky chance, and tended to confirm the predatory activity of insects towards these ancient conifers. Furthermore many insect fossils have been found in the Purbeck Micrite beds.
The resin globules shown on Figure (6) have linear dimensions which vary from < 2 microns to about 13 microns. Figure (6) was derived from a stained cellulose lacquer rock peel using brightfield illumination with one polarizing plate above the objective. The resin flakes found in the Purbeck beds were far too small to contain complete insects, similar to those found in the "Baltic Amber" from the Tertiary10. Likewise in the Kimmeridge Clay beds it has not been possible to determine the proportion of kerogen which might have been derived from amber resin.
Notes and References.
1. Allochthonous : The term applied to the material forming sediments which have been transported to the site of deposition.
2. Dr Jane E. Francis 1983. "The Dominant Conifer of the Jurassic Purbeck Formation, England." Palaeontology, Vol. 26, part 2, 1983, pp. 277-294, pls. 38-41. This is a very valuable and informative paper on the Lower Purbeck Fossil Forests of Dorset and gives a full account of the dominant conifer tree, including the shape, size, structure, the fine structure of the wood and the form of the Classopollis conifer pollen.
3. Protocupressinoxylon purbeckensis : Systematic species name by Dr J. E. Francis l983 for the dominant Mesozoic conifer.
4. Bordered Pit tracheid structures : These characteristic microscopic features were found among the wood fusain fragments in the Kimmeridge Clay and the Purbeck beds on cellulose lacquer rock peels. These were a strong indication of similar Mesozoic conifer trees. Often the minute structures were damaged or completely separated as individual fusain rings. They are pits in the walls of the elongated xylem tracheid cells adapted for conduction of water, as part of the transpiration mechanism. The shape, size and arrangement of the pits vary with different species of Mesozoic conifers. The pits can be separated from other organic residues by Palynology techniques.
5. Dr J. E. Francis 1984. "The Seasonal Environment of the Purbeck (Upper Jurassic) Fossil Forests.". Palaeogeography; Palaeoclimatology; Palaeoecology; 48 (1984); 285-307. Elsevier Science Publishers B.V. Amsterdam. Here Dr J.E. Francis discusses the association of evaporites and fossil forest vegetation, representing well developed gymnosperm forests, which grew on the borders of the shallow, hypersaline Purbeck lagoon which covered southern England during the late Jurassic. The trees were adapted to growing in a semi-arid environment, which approximated to a Mediterranean type of climate, with warm wet winters when the trees were able to grow but with hot, arid summers suitable for the formation of evaporites. Dr J. E. Francis is a very experienced research scientist in the area of fossil forests and past climates. She has worked extensively for the British Antarctic Geological Survey and in the far North of Canada and elsewhere.
6. In the late 1980's it was reported that fossil insects had been seen in the Cypris Freestone beds being cleared from above the Portland Stone being quarried at Swanworth. Local amateur geologists were invited to examine some of these freshly exposed beds, through the Dorset Open University Geological Society Group, in co-operation with Dorchester Museum. Numbers of insect fossils were found, but the present author found also many small fragments of wood. Figures (4),(5) and (6) are based on peels prepared at this time.
7. Amber : This fossil resin was extruded by conifer trees to protect them from vigorous attack by insects. Well-known examples of complete fossil insects and other related arthropods have been found embedded in pieces of Tertiary amber. In 1988 a Cretaceous bee was found in amber from New Jersey.
8. Insects had existed on land at least 320 million years ago in the Carboniferous fern forests, but in the late Cretaceous period insect evolution accelerated because of the emergence of flowering plants. Nectar became an important food and gave rise to "symbiosis " between flowering plants and insects. This appears to have led to the emergence of advanced social insects (ants, bees etc).
9. These appear to be similar to contemporary conifer resin ducts. Some amber has been found in the Lower Purbeck beds using Palynology techniques. Although it is suspected that some of the transparent kerogen in the Kimmeridge Clay may be derived from amber, this is difficult to prove.
10. A Special note on Fossil Insects.
Some of the best examples of fossil insects have been investigated by Dr Ed Jarzembowski of the Booth Museum of Natural History, Brighton, Sussex, UK. These are mainly from the Wealden in southern England, where they are embedded in siltstone beds. The quality of preservation is outstanding and they include a large variety of species.
Editor's notes: 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.
The previous three 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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