Friday, December 24, 2010

Paul Karrow

Department of Earth and Environmental Sciences, University of Waterloo

Marl is a soft, light-coloured (white to pale gray or) mud-like sediment, seen in shallow waters of some small lakes or ponds, or under swamp lands, where it typically is covered by black organic sediment (Figure 1).

Typical sequence of deposits where marl is found.It usually contains an abundance of visible mollusc shells (snails, clams) as well as a host of other fossils best seen under a microscope (ostracodes, diatoms, insects).  The term marl has been applied to a variety of sediments, including marine sediments, and calcareous till or varved clay because they effervesce (“fizz”) vigorously when a drop of acid is applied.  However, today the term is normally limited to freshwater sediment.

Marl forms through the combined action of algae (e.g. Chara) and other aquatic plants and animals living in hard water streams and springs, rich in dissolved calcium carbonate.  Such conditions are associated with bedrock such as limestone or dolostone, or Precambrian marble, and with calcareous drift derived from them (Figure 2).  
Distribution of Paleozoic basins with lime-rich bedrock and resulting calcareous glacial deposits on the Precambrian Shield.
Large areas of the Precambrian Shield are covered with calcareous drift derived by glacial erosion of carbonate bedrock under the Hudson Bay Lowland and spread outward onto the Shield.  Such widespread occurrence of calcareous drift, even on large areas of the Precambrian Shield, has led to problems in radiocarbon dating because of the incorporation of old Paleozoic carbon in lake sediments and associated aquatic plants.  This has resulted in radiocarbon dates which may be too old by several thousand years (the “old carbon problem” or “the limestone effect”).  To avoid this problem, careful selection of terrestrial plant material must be made for dating.  Unfortunately, hundreds of erroneous ages have been published, distorting the history of ice retreat and vegetative change over postglacial time.
 
In the late 19thand early 20th centuries, marl was a valued mineral deposit used for cement manufactur (Guillet 1969).  In 1907, 13 cement plants in southern Ontario produced portland cement from marl, and interest continued long after its use was superseded by limestone quarrying. Guillet (1969) described a variety of uses for marl and reported on 107 Ontario deposits.  Provincial, state, and federal organizations provide information on deposits elsewhere, and marl deposits are known across Canada.
 
Wilder Lake.The marl deposits at Wilder Lake, Grey County, Ontario, southeast of Owen Sound are a typical example (Figures 3 and 4).  Marl deposits ranged up to a depth of a few meters, with about 85% CaC03 content. The National Portland Cement Company operated a cement plant in Durham from 1903 to 1919, with marl transported to the plant from the lake by a railway spur.  Newspapers of the time reported claims that the Durham plant was the world’s largest and that marl reserves would allow production of 5000 barrels of cement a day for 200 years.  However, by 1908, the marl supply at Wilder Lake was exhausted and other nearby deposits continued the production for a few more years.   In 1906, production was 260,000 barrels of cement and 150 men were employed.  
View of Wilder Lake today.A study by Vreeken (1981) of 50 marl deposits in eastern Ontario obtained radiocarbon dates indicating marl deposition began as early as 11,000 years ago and mostly ceased over a wide span of time, from 10,000 to 2,000 years ago. Deposition continues at some sites today, however.  Marl sites tend to go through a cycle, with deposition in local depressions on glacial deposits, beginning with lacustrine clay, followed by marl, then by terrestrialization as vegetation spreads into the shallow water and the marsh is succeeded by swamp, then forest (Figure 1).  Many factors contribute to this progression, including warming climates after deglaciation, which fostered changing patterns of vegetation from tundra, through boreal coniferous forest, to the deciduous forest we have today.
 
Marl deposits have been subjected to various kinds of scientific study.  They provide good study sites for recovery of a variety of remains of fossil plants and animals to allow analysis of changing environments in postglacial time.  One of the most-studied marl sites (now covered by buildings) is the Gage Avenue site in Kitchener, Ontario, where contained plants (pollen and macrofossils) and insects were described by Schwert et al. (1985) and molluscs by Yang et al. (2001).  The marl there was about 4 m thick and yielded 176 types of insect fossils, 36 types of pollen grains, 49 types of plant macrofossils (seeds, needles, etc.) and 25 types of molluscs.  As mentioned above, some of the radiocarbon dates from this site were affected by the “old carbon problem” (e.g. 17,200 years and 20,000 year ages were determined near the base of the marl), yielding dates that are thought to be more than 4000 years too old.
 
Probably the most notable kinds of fossils found in marl deposits are the bones of large extinct mammals such as mastodons and mammoths, and even giant beaver!  These discoveries are accidental, as farmers dig ditches to drain low wet areas, or excavate to create stock ponds.  The bones are often found in the upper part of marl deposits, which would be soft, mushy ground where the animals may have wandered in and got into trouble.  More than a hundred such sites have been reported in southern Ontario alone.  The most famous of these is the Highgate mastodon, southwest of London, Ontario.  See the history of this site on-line through Google and in a popular book from the Earth Sciences Museum, University of Waterloo (Russell et al., 2010).
 

Acknowledgements

Thanks to the Ontario Geological Survey for permission to use the photo of the workings in Wilder Lake and to Tom Arnott for information on the Wilder Lake and the present view of the lake.
 

References

Guillet, G.R., 1969.  Marl in Ontario. Ontario Department of Mines, Industrial Mineral Report 28, 137p.
 
Russell, P., Hoganson, J., Karrow, P., and Motz, J., 2010.  A mastodon in a biscuit box.  Earth Sciences Museum, University of Waterloo, 20p.
 
Schwert, D.P., Anderson, T.W., Morgan, A., Morgan, A.V., and Karrow, P.F., 1985.  Changes in late Quaternary vegetation and insect communities in southwestern Ontario.  Quaternary Research 23, 205.226.
 
Vreeken, W.J., 1981.  Distribution and chronology of freshwater marls between Kingston and Belleville, Ontario.  Canadian Journal of Earth Sciences 18, 1228.1239.
 
Yang, J., Karrow, P.F., and Mackie, G.L., 2001.  Paleoecological analysis of molluscan assemblages in two marl deposits in the Waterloo region, southwestern Ontario, Canada.  Journal of Paleoliminology 25, 313.328.
 
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