Dewatering Structures, Biofilm Structures, Glacial Striae and Chatter Marks in Potsdam Sandstone near Newboro, Eastern Ontario

It has been a remarkably warm December for Eastern Ontario and we have yet to receive anything more than a light dusting of snow.  Yesterday, December 22nd, it was 8 degrees Celsius (46 degrees Fahrenheit), and  I couldn’t help but take the morning off work to do some Christmas shopping and to look at some outcrops. 

Below are photographs that I took yesterday of glacially polished  outcrops of Potsdam Sandstone a few kilometers north of Newboro, Ontario.    The sandstone has most recently been mapped as the Nepean Formation of the Potsdam Group by the Ontario Geological Survey and as the Covey Hill Formation of the Potsdam Group by the Geological Survey of Canada.

Dewatering Structures

The first three photographs show parts of three outcrops that are within about 270 feet (80 meters) of each other.  The first and the second photos show excellent examples of small dewatering structures.  The third photo is less convincing, but likely shows small dewatering structures.

     
In the second photo it is not clear what has weathered out of the surface layer to produce the pockmarked surface.   Many of the holes are rounded or peanut shaped.  One possible interpretation is that they represent gas bubbles trapped below a biomat.   Below are three more photographs of the pockmarked surface.   It appears that many of the bubbles have merged into chains or patches, resulting in  structure analogous to Kinneyia.

Biofilm Structures

The next two photographs provide a bedding parallel view of distorted laminations in quartz sandstone.  I believe the laminations to be biofilm structures in the quartz sandstone (rather than a distorted dewatering structure), where interlayered microbial mats provided cohesion during deformation.

It is possible that these distorted laminations are seismites.  The outcrops are about four kilometers south of the Rideau Lakes Fault and about the same distance from the soft-sediment deformation structures (seismites) mentioned in my October 22,  2015 blog posting.

Glacial Striae and Chatter Marks

Glacial striae and chatter marks were present on the surfaces of a number of the outcrops that I looked at.   In addition a few of the outcrops show prominent noses pointing in the same direction as the striae and chatter marks.   The next two photos provide examples of the glacial striae and chatter marks.

Possible Sandstone Dikes in Sandstone (or a Torn Microbial Mat)

Christopher Brett
Perth, Ontario