A Few Trace Fossils in Potsdam Group Sandstones of Eastern Ontario

The Potsdam Group sandstones and conglomerates generally form the basal layer of Paleozoic rocks overlying the Precambrian Shield in Eastern Ontario. The sandstones are thought to be middle to upper Cambrian in age, are generally barren of body fossils (they yield the occasional Brachiopod), but contain abundant trace fossils. The most common trace fossils found in the Potsdam Group sandstones in Lanark County are worm burrows, including burrows perpendicular to the bedding plane (with Diplocraterion and Skolithos being the most common that I've found) and burrows parallel to the bedding plane. (My posting on October 1, 2012 is a field trip to eight outcrops near Perth, where seven of the outcrops are Potsdam Group sandstones that exhibit mostly Diplocraterion but also  possible Arenicolites burrows.) The most famous trace fossils to come out of Lanark County are Climactichnites trails and Protichnites trackways. However, only one quarry in Lanark County has yielded those fossils.

The Potsdam Group sandstones outcrop south of Perth down towards Brockville and down towards Kingston. Those sandstones also exhibit trace fossils, with the sandstones near Sunbury yielding museum quality specimens of Protichnites trackways and Climactichnites trails.

This past weekend I headed south to take photographs of sandstone outcrops, noted some interesting trace fossils, and went back yesterday to collect a slab of rock. Below are a few of my photos of slabs of rock found south of Lanark County.

The first two photos, Sam_0382 and Sam_0383  show a ridge that I suspect is a surface trail beside ripple marks,  where the ridge is similar to the pushed up  lateral ridge for a Climactichnites trail. (The ridge is a surface feature.)

The next three photos, Sam_0354 , Sam_0356  and Sam_0361, are from the same slab .   These show a trace fossil that is a trackway that  extends across the slab .  It is arguably the trace fossil Diplopodichnus, which is a trackway consisting of a pair of longitudinally continuous lateral furrows.

The next two photos, Sam_0358 and Sam_0359, were taken of a different slab of rock, and show two curved grooves. If a reader can provide a name to attach to this trace, please let me know.  My best guess: a bilobate trace, which consists of an  meandering trench bounded by a ridge of excavated sediment on either side.

The last  photo, Sam_0379, shows a narrow burrow parallel to the bedding surface of a different slab  where the burrow cuts across ripple marks.  This burrow is more like a cylinder or tube.

The above photos are of loose slabs of rocks at an outlier   that was mapped as part of the Covey Hill Formation of the Potsdam Group in 2010 by B.V. Sanford and R.W.C. Arnott. (Stratigraphic and structural framework of the Potsdam Group in eastern Ontario, western Quebec, and northern New York State; Sanford, B. V. and Arnott, R. W. C. (2010); Geological Survey of Canada, Bulletin 597, 2010)


Christopher Brett
Perth, Ontario

(July 12th Addendum:   Since posting the above I have been contacted by a friend who is studying the Potsdam Group, who has told me that in addition to the Chippewa Bay member of the Covey Hill Formation  this outlier also exhibits a younger facies similar to the Nepean Formation near Ottawa, with an angular erosional unconformity between the two.   Accordingly, I may have been wrong in assigning the specimens that I collected to the Covey Hill Formation. ]