Odd. Even for the Potsdam

Below are a number of photographs of slabs of sandstone from Burgess Ward, Tay Valley Township, Lanark County, Ontario.   The area has been consistently mapped as Potsdam Group sandstone, which is from Middle to Upper Cambrian in age. 

The sandstone at this location exhibits two facies: a wind blown facies evidenced by very fine cross bedding, and an aquatic facies evidenced by burrows (including  U-shaped burrows that are Diplocraterion, and single burrows perpendicular to the bedding plane).  The outcrop at this location is a drainage ditch, close to which are a number of piles of sandstone.   Every time I visit I find something interesting. 

First Slab: Photographs Sam_0499, 0500, 0501

These photos are likely of the sole of the slab, as it appears from photo 501 that the sand filled in a depression.  Whether the depression was caused by burrowing or a vortex (caused by wind or water) is not clear.  It is unfortunate that half of the depression is missing as one needs the missing half to tell whether the depression is a spiral.

Second Slab
Both the top and bottom of this slab are interesting.  The top can be determined because of the V-shaped burrowing on one side.

Thin Brown Lines on Top of Second Slab: Photographs Sam_0471 and 0472

While these could easily be recent staining along roots, or staining cause by water seeping along fine cracks, I’ll have to pass this by an ichnologist and paleontologist  before I rule it out as being a fossil.  It would be particularly interesting if it were an algae from the Middle to Upper Cambrian.

[Added September 15, 2022:]   Another possibility is that this is a fossil graptolite.  See the graptolites in Figure 3 in the following paper:

Geyer, G., Landing, E., Meier, S. et al. , 2022, Oldest known West Gondwanan graptolite: Ovetograptus? sp. (lower Agdzian/lowest Wuliuan; basal Middle Cambrian) of the Franconian Forest, Germany, and review of pre-Furongian graptolithoids. PalZ (2022). 

https://doi.org/10.1007/s12542-022-00627-5

Sole of Second Slab
White polygonal  pattern within troughs in ripple marks: Photographs Sam_0507 and 0487



 

A number of the polygons are hexagons, but many are not.   The sides of the polygons are about 5 mm in length and about 2 mm thick.    Normally one would think that a polygonal pattern found on the bedding plane of a sedimentary rock is made by the infilling of desiccation cracks.   However, there are two reasons why the polygonal shapes in the photos  may not be desiccation cracks.  First,  this rock is sandstone and sand on drying doesn’t contract by the amount required to form the polygons shown in the photos.   Second, the sides of the polygons are  ellipsoidal to  cylindrical in cross-section and resemble burrows.

Is this  a trace fossil somewhat like Paleodictyon?  Possibly.  Paleodictyon consists of a network of hexagonal meshes and has been found in rocks ranging in age from the Cambrian to recent times.   Photographs of Paleodictyon can be found on Wikipedia  and by simply searching the word in Google images   (See, for example:  http://en.wikipedia.org/wiki/Paleodictyon
 http://hexnet.org/content/paleodictyon  )   The polygonal pattern on the slab that I have found resembles a number of the examples of trace fossils that others have identified as Paleodictyon.
   
Another possibility for the polygonal  patterns is that they are evidence of microbial mat shrinkage cracks.

Crystals on Sole of Second Slab: Photo Sam_0506 (and above two photos)

These crystals are interesting.   They are not calcite, as they don’t effervesce in dilute acid.  From the colour and luster they appear to be gypsum, but the crystal system appears to be wrong for gypsum.   I might have to beg a favour and pass a sample to a friend with access to a scanning electron microprobe.

Third Slab: Microbial Mat Shrinkage Cracks

The next photo, Sam_0510, shows three crescent shapes in the troughs of ripple marks in sandstone.   I believe these curved shapes  to be evidence of microbial mat shrinkage cracks (see my blog posting from November 5, 2012).

Fourth Slab: Burrowing in Ripple Marks: Sam_0475

I believe this to be burrowing as the hand specimen shows one sandstone tube going under the other.  (I admit that the photo could be clearer.)

Fifth Slab: Diplocraterion: Sam_0464

I’m including this photo, not because the trace fossil is odd (it’s fairly common), but because I found a sample that shows a good cross-section of three Diplocraterion traces.  (The top of the slab is on the right hand side of the photo.)

Christopher Brett
Perth, Ontario