The photographs show two specimens that were found about a five minute drive directly north of Perth in Drummond Township, Lanark County in an area that is mapped as March Formation, which is generally thought to be Early Ordovician in age. The March Formation (the Theresa Formation in New York State and in the Province of Quebec) is younger than the underlying Nepean Formation sandstones. The Nepean Formation (Keeseville Formation in New York State; Cairnside Formation in Quebec) sandstones form the upper part of the Potsdam Group in Eastern Ontario.
Others have reported microbial mat structures in the underlying Potsdam Group sandstones of the Ottawa Embayment. A specimen showing circular and sinuous cracks confined to troughs in wave ripples was reported in Potsdam Sandstone from Canada in a paper read before the Geological Society of London in 1890 by Sir J. William Dawson, one of Canada’s famous geologists. Below is a photograph of the drawing from his paper.
"Fig. 14 shows a rippled surface in Potsdam Sandstone with marks of worms or molluscs, arranged in the hollows of ripples. The marks are simple trails, of that curious circular or chain-like form sometimes observed, and seem to have been made by animals creeping in the furrows between the ridges of the ripples in the ripple-marks."
[Dawson (1890), On Burrows and Tracks of Invertebrate Animals in Palaeozoic Rocks, and other Markings, Vol. 46, The Quarterly Journal of the Geological Society of London, 595 at 610-611.]
While almost everyone would now agree that Dawson was wrong, he made the above observation approximately 100 years before the feature was commonly accepted as resulting from the shrinkage of a microbial mat. Today I can easily identify the structure as a mat shrinkage structure because I spent part of this past winter reading a number of papers on microbial mat features preserved in sandstones and this is one of the most common textures pictured in the articles. I would not have made that identification without the benefit of those papers.
It is interesting to look at the literature to see how others interpreted these structures. Similar structures were found in the middle of the last century in sedimentary rocks of the Precambrian Shield and attracted varied interpretations. The following is a sampling of these interpretations.
Wheeler and Quinlan (1951) reported on sinuous traces lying in the troughs on the bedding surfaces of ripple marked quartzites in Precambrian rocks in Idaho and Montana, and identified the sinuous traces as mud cracks. They rejected an earlier identification of similar structures in Huronian quartzites from Montana as "trails" or "burrows".
A slab with vermiform markings was found near Sault St. Marie in Precambrian (Upper Huronian) arkosic sandstone. These were interpreted by Frarey, Ginsburg and McLaren (1963) as bodies analogous to the tubes of modern annelids (i.e., worms). They considered and ruled out an origin by desiccation crack filling.
In 1965 similar corrugated specimens were found in Precambrian (Upper Huronian) arkosic rocks east of Flack Lake near Elliot Lake, Ontario. Hofmann (1967) described them as questionably organic, but did not rule out an inorganic origin (the possibility that they were mudcrack fillings, injection or crystal growth was considered). He assigned them the names Rhysonetron lahtii and Rhysonetron byei.
Grant M. Young in articles published in 1967 and 1969 in the Canadian Journal of Earth Sciences reported on similar structures in Huronian Rocks near Elliot Lake, Ontario. He described the beds as cherty quartzites and described the structures as crescent ridges and sinuous ridges in ripple troughs. In his 1967 article he found the origin difficult to explain in terms of inorganic processes, and favoured an organic origin, with some of the structures being probably the casts of vermiform organisms. In his 1969 article he reported that new evidence indicated that the corrugated vermiform structures were formed by the infilling of shrinkage cracks in fine grained sediments, that the cracks were not dessication cracks caused by subaerial exposure, and that the structures were probably formed in sediments containing water.
J. Allan Donaldson (1967) observed modern structures along the margins of ponds. He found "tunnel-like ridges in relatively flat algal mats... [where] the ‘tunnels’ appear as distinct linear, curved, and sinuous ridges that commonly branch and typically taper and disappear over short distances..." He proposed that "at least some of the Huronian vermiform structures may be related to algae rather than to metazoans" and noted that "a muddy environment is not essential for algal growth" and that "algal mats may completely decay subsequent to burial, leaving only the structures they served to create as a record of their former presence."
Hofmann (1971) in his publication Precambrian Fossils, Pseudofossils and Problematica in Canada, Geological Survey of Canada Bulletin 189, reported on a new specimen from the Precambrian rocks near Flack Lake which showed "that the corrugated spindles (Rhysonetron lahtii ) are arranged in a distinct shrinkage crack pattern, and that a biogenic origin can no longer be considered." However, he rejected a mud crack hypothesis (in part because of the paucity of mud), concluding that the specimens "make it evident that the rhysonetron stucture is a Manchuriophycus-type pattern that has undergone unusual diagenetic modification... that involve[d] the reduction, if not elimination, of the pelitic layer, possibly by solution under considerable pressure." He concluded that "Rhysonetron is a sedimentary-diagenetic structure, resulting from shrinkage crack filling, modified by compaction and injection processes, and ... accompanied by almost total removal of the pelitic layer."
Fast forward to the present. Bosch and Eriksson (2008) report on vermiform structures in ripple troughs in 2.1 billion year old sandstones near Pretoria, South Africa. They describe them as "connected or disconnected, curved spindles or rods along the troughs of the ripple marks, resembling worm burrows. These casts form very shallow moulds, up to 2mm deep, on the bedding surfaces and protrude up to 3mm above the bedding surfaces. In plan they curve, branch, taper and may also be longitudinal, and sometimes they overlap." In their analysis of these vermiform curved markings they follow Donaldson (1967), concluding that the sinuous forms found in the South African specimens equate to "a special form of microbial shrinkage crack, normally developed within the thicker mats that occur within the troughs between ripples...".
It is interesting that there have been many theories for these sinusoidal and curved lines in sandstone. As they resemble burrows, it is not surprising that they have been mistaken for burrows. As they resemble mud cracks, it is not surprising that they have been mistaken for mud cracks. However, the curved lines in sandstone resemble curved and sinuous ridges formed on modern microbial mats, and are best explained as a microbial mat shrinkage feature. The solution to their origin is an example of a basic concept of geology that the natural laws and process that operate now have operated in the past. The present is the key to the past.
Added November 12th: Below I’ve provided two photographs of specimen that shows a similar texture on a bed surface that does not show ripple marks. Here the spindles or rods curve and taper, and sometimes overlap.
Chris Brett
Perth
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