Fossils and Geology of Lanark County, Ontario: September 2013

A year ago I collected a number of specimens of fossils, and photographs of fossils, from a quarry in Lanark County about a five minute drive north of Perth. The quarry is in lot 10, concession IV of Drummond Township. The quarry falls in an area that has been mapped as the March Formation.   It is a section of flat lying beds of sedimentary rock (primarily sandstone) that in an earlier posting I described as the Drummond Sequence as it falls in an area that was surveyed as Drummond Township.   The Drummond Sequence is bounded by faults that separate it on the west from Nepean Formation (Potsdam Group) sandstones, on the southeast from March Formation, on the northeast from Nepean Formation sandstones, on the north from Precambrian Shield and on the northwest from Precambrian Shield. It has an irregular shape and covers over 100 square kilometers. It can be viewed on Ontario Geological Survey Maps P2724 and P 2725 that can be downloaded from:

P2724 - Paleozoic geology, Perth area, southern Ontario
http://www.geologyontario.mndmf.gov.on.ca/mndmfiles/pub/data/imaging/P2724/P2724.pdf

P2725 - Paleozoic geology, Carleton Place area, southern Ontario
http://www.geologyontario.mndmf.gov.on.ca/mndmfiles/pub/data/imaging/P2725/P2725.pdf

(The March Formation is unit “3" on the maps, while the Nepean Formation is unit “2".)

The March Formation is generally considered to be Lower Ordovician in age and is equivalent to the Theresa Formation in Quebec and New York State. As originally envisaged by Sir William Logan of the Geological Survey of Canada it was the passage or transition beds between the underlying Potsdam Sandstone and the overlying Calciferous formation.   In 1937, 1938 & 1946 Alice E. Wilson of the Geological Survey of Canada renamed the Potsdam Sandstone as the Nepean Sandstone (overlooking the older portions of the Potsdam), renamed Logan’s passage beds as the March Formation, and renamed the Calciferous as the Oxford Formation.

Wilson (1937 & 1938), described the March Formation unit as being composed of "thick beds of interstratified grey sandstones with a calcareous cement and sandy blue-grey dolomites, both weathering a rusty brown".   Her 1946 description was “The March formation consists of alternating grey sandstone and sandy dolomite or blue-grey dolomite, all weathering dark rusty brown. ... The formation represents a transition from the Nepean sandstone to the Oxford Dolomite, the sand content being most evident at the base..” She stated that “The lower contact [with the Nepean sandstone] is placed arbitrarily at the lowest dolomitic layer.”

In the quarry where I was collecting things are more complicated. Anything goes. While the predominant rock is sandstone, I’ve found dolostone, mudstone, siltstone, minor shale, and a bit of limestone. The sandstones, mudstones and siltstones can be just about any colour (reddish, brown, ochre, black, grey, tan, etc.).   There are many examples of mud cracks and ripple marks, and possible microbial mat structures.

[2019: I have retracted a number of paragraphs from the original post. I did this because I failed to recognize that I had found Ediacaran fossils.    The discs shown under number 7 below are Aspidella. Under number 4, the sixth and seventh photos show Ediacaran fronds.   Most of the photos show Ediacaran fossils or microbial mat textures. See my March, 2019 postings for a better explanation.
At most only the top layers of the quarry are March Formation. ]

The Fossils

Below I’ve provided the photographs. I had hoped to go through and look up the scientific names for the fossils. As that hasn’t happened over the past year, I’ve decided to put a selection of photographs on my blog to stimulate interest in the Drummond Sequence..   I’ve grouped the fossils in a way that makes sense to me.

1. Bedding Parallel Burrows in a bed close to the top of this section of the quarry.
Sam_0180 and 182

2. Simple cylindrical, curving burrows roughly parallel to bedding plane
Sam_0037

3. Larger, rougher, cylindrical, curving burrows roughly parallel to bedding plane
Sam_0525

4 Branching, overlapping, possibly interpenetrating, roughly parallel to bedding plane
P820760, Sam_0016, Sam_0023, Sam_00 31, Sam_00 34, Sam_0102, Sam_0140