More Evidence of Microbial Mats in Potsdam Sandstone near Newboro, Eastern Ontario

Winter has arrived in Eastern Ontario.  Eight centimeters of snow fell last night and more is forecast for today.   I’m glad that on December 26th I took the opportunity offered by the absence of snow  to revisit the area around Newboro and to look at outcrops near Westport.  I’d wanted to check for glacial meltwater eroded outcrops (and found some) and wanted to look again for further evidence of microbial mats.  Below I report on three outcrops  near Newboro.   The first two outcrops are mapped as Potsdam Group sandstone.  All of the outcrops are about  four kilometers south of the Rideau Lakes Fault. 

The silver ruler in the following photographs is one meter (39 inches long).  The blue ruler is 12 inches (30 cm) long.

First: An Outcrop Revisited

This is one of the outcrops from my last blog posting, the one mentioned under the subheading
‘Biofilm Structures’ where I provided  two photographs showing  a bedding parallel view of distorted laminations in quartz sandstone.    Below are photographs of additional distorted sandstone laminations visible at this outcrop.  The structures are difficult to envisage in sandstone unless the beds were bound by microbial mats.   The first photograph  may show a mat roll up structure.

Second:  Distorted Layers in Flat Lying Beds of Potsdam Sandstone

Below are photographs showing a distorted bed that is up to about 12 inches (30 cm) thick contained  within a sequence of flat lying beds of sandstone.

This distorted bed contains broken,  folded and crinkled layers, plus layers that appear to have been thrust over or under other layers.    I believe that this distorted bed represents pre-lithification  distortion of  biomats and stromatolitic layers in the sandstone, probably caused by seismic activity along the Rideau Lakes Fault.    Seismic activity caused rupturing and folding of the unconsolidated sediment, but the biofilms preserved the laminations in the strata. 

The mechanism that I have proposed to account for the  30 cm wide distorted bed near Newboro is analogous to the model proposed by Donaldson and Chiarenzelli (2004) to account  for the meter thick convoluted layer in the Nepean sandstone outcrops in Kanata, Ontario, photographs of which were included in my  November 4,  2015 blog posting.    Both outcrops display soft-sediment deformation   in  sandstone, where seismic activity is the likely cause of the deformation.  
   

Third:  A Badly Weathered Outcrop

I’ve included this outcrop because it has weathered in such a manner that it looks like the outcrop in Kanata, in a field off the Old Quarry Trail, photographs of which were included in my  November 4,  2015 blog posting.   It is missing the convoluted folds shown at the outcrop in Kanata, but otherwise the pattern of weathering is the same.  Both the outcrop in Kanata and this outcrop show repetitive, weathered out, thin layers in sandstone.   Below are photographs of the side view and the top view of the outcrop near Newboro.

I did not have my rock pick or sledgehammer with me on the 26th, but pulled a piece off the third outcrop, took it home, and cracked it with a small sledge.  It’s sandstone, badly weathered sandstone, but sandstone.

Christopher Brett
Perth, Ontario
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Addendum (January 5, 2016):         

In the comment below Howard Allen of Calgary has suggested that the second outcrop may represent evaporite accumulation, subsequent solution and collapse.  Others have reported on evaporites in Potsdam sandstone, and have suggested that they are responsible for observed structures in the sandstone. 

Wolf and Dalrymple (1984) mention that “At one locality near Phillipsville, a zone of disrupted laminae caps the burrowed beds.  The nature of the laminae suggests collapse into small cavities. .... The disrupted laminae which cap the cycle at the Phillipsville locality may represent the collapse of overlying sand into cavities formed by the dissolution of evaporites, such as gypsum or halite (B.W. Selleck, Colgate University, personal communication, 1983).” 

Wolf and Dalrymple (1985) mention that “the tops of cycles are disrupted by soft-sediment deformation, due either to the leaching of evaporites or to (earthquake induced?) liquifaction. ....Evaporite moulds in one outcrop of this facies near Gananoque... give evidence of elevated salinities, a finding that is compatible with an evaporite-solution origin of the soft-sediment deformation features.”

Donaldson and Hilowle (2002) report for an outcrop of quartz arenites of the Nepean Formation in Kanata that they observed “evaporite pseudomorphs, including silica-replaced rosettes of barite and/or gypsum” and “a distinctive unit of synsedimentary breccia inferred to have formed in response to dissolution of a layer of bedded evaporites”.

Donaldson and Chiarenzelli (2004) suggest,  for one of the outcrops of Potsdam sandstone (Nepean Formation)  at Kanata, that “a few beds that stand out as glassy markers display abrupt truncations (Figure 6), suggesting that they may have been penecontemporaneously cemented, and then locally disrupted by erosional undercutting of unconsolidated substrate, localized upwards pressure associated with dewatering, or dissolution of intercalated evaporites.”    

Sanford and Arnott (2010) reported that the  “Imperial Oil Ltd., Laggan No.1 borehole encountered numerous gypsum interbeds throughout the upper half of the Nepean Formation.”  In addition they noted that “Fairly extensive brecciation in the GSC Lebreton No.1 borehole, and to a lesser extent in the GSC Russell No.1 borehole, might also suggest the initial presence of minor halite in those areas, with subsequent dissolution and collapse.”  Further, “The widespread paucity of normal marine fossils, except for local occurrences of trace fossils, represents evidence of elevated salinity throughout the Ottawa embayment.   The occurrence of stromatolites, which can survive and even thrive in hypersaline conditions, is also a good indicator of evaporitic conditions.”

(Added: September 28, 2016)
David Lowe (2015, 2016), in his work on the Potsdam strata has recognized six siliciclastic paleoenvironments: (a) braided fluvial, (b) ephemeral fluvial, (c) aeolian erg, (d) coastal sabkha, (e) tide-dominated marine and (f) open-coast tidal flat.   Wikipedia mentions that “Sabkhas are supratidal, forming along arid coastlines and are characterized by evaporite-carbonate deposits with some siliciclastics. Sabkhas form subaerial, prograding and shoaling-upward sequences .”   The sabkha facies of the Potsdam Group are found in sandstone that  in Ontario we call the Nepean Formation, in Quebec they  call the Cairnside and in New York State they call the Keeseville.