Burrows or Not Burrows?

Adolf  Seilacher (1925 – 2014) was a German palaeontologist who made major contributions to the study of trace fossils.  He is credited with advancing the concept that trace fossil assemblages are far from random, that the range of associated trace fossils is constrained by the environment of the trace-making organisms, and that the sedimentary environment  at its time of deposition can be deduced by noting the fossils that are in association with one another.  He taught at both the University of Tübingen and Yale University, wrote numerous papers, and authored a text book entitled Trace Fossil Analysis that I find helpful.   While his text book is designed to be used as a course book in conjunction with real material (including representative specimens and plaster casts), the book is so informative and well written that one cannot help but learn something every time it is opened.  There is however one comment in the book with which I disagree, namely Professor Seilacher’s introductory sentence to Plate 58 dealing with Synsedimentary Structures.   He states:

“The repetitive patterns of ordinary depositional structures, such as ripple marks in sand and sun cracks in mud, are too familiar from modern environments to be mistaken for fossils in the fossil record.”   

(Adolf  Seilacher, 2007, Trace Fossil Analysis,  Springer;  226 pages at page 166)

That statement is just not true:  I have no problem making those mistakes.  While there are many specimens that are obvious ripple marks or obvious sun cracks (what others call mud cracks or desiccation cracks), there are specimens where it is not clear whether one is looking at a trace fossil.

Below I provide a few problematic examples from the Potsdam Group sandstones and March Formation sandstone.  I periodically look at outcrops of  Potsdam Group sandstone and March formation sandstone in Lanark County and south of Perth down towards Kingston.   The outcrops are composed of any or all of (oldest to youngest):

(A) pink to brick red to burnt umber to greyish purple to almost  black (mostly) aeolian and (minor) fluvial sandstones – the Hannawa Falls Member of the Covey Hill Formation of the Potsdam Group;
(B)  white to buff to grey-green  fluvial and alluvial sandstones  and conglomerate (with minor aeolian sandstone) – the Chippewa Bay member of the Covey Hill Formation of the Potsdam Group; 
(C)  white to buff  marginal and shallow marine sandstones – the Nepean Formation of the Potsdam Group;
(D) white to buff to grey to black to green to maroon sandstones and siltstones - the March Formation

Below are photographs of specimens of those rocks.

Specimen 1.   Greyish purple sandstone, definitely the Hannawa Falls Member of the Covey Hill Formation of the Potsdam Group

When I first looked at the sample I assumed that I was looking at desiccation cracks, because the linear features on the surface intersect at almost right angles.  However if you look closely you will see (a) that the linear features are tubes that cross over one another, and (b) that the tubes pinch and swell.  This suggests that the linear features are backfill burrows. 

While the sample appears to be light grey, the rock is actually a darker grey purple colour.  I took the photograph near noon on a bright sunny day, and the sunlight washed out the photograph.

Sanford and Arnott 2010 report that the Hannawa Falls Member is divisible into two units:
- a lower unit of probable fluvial origin that is composed of brick-red shale, pink to maroon sandstone and quartz pebble to cobble conglomerate
- a dominant upper unit of eolian origin composed of red to pink quartz arenite containing a basal quartz-cobble conglomerate
Both units outcrop at this location.   It is not clear whether Specimen 1 is a sample from the lower or upper unit.

Hagadorn,  Collette  and  Belt  2011 discuss the rock and trace fossils in the Hannawa Falls Member in upstate New York, reporting that they examined mainly eolian beds but also found a minor subaqueous facies which they interpret as flooding in coastal dunes.   I assume that they were looking at the upper unit of the Hannawa Falls Member.  Based on trilobites found in underlying and overlying formations, they assign deposition of the Hannawa Falls Member in upper New York State to from early to mid-middle Cambrian time, which I assume to be the age of comparable rocks in Ontario.  They found Arenicolites U-shaped burrows in the subaqueous facies and Protichnites trackways, Diplichnites trackways, and Diplopodichnus trackways in the eolian beds. 

Specimen  2.   Greyish purple sandstone,  the Hannawa Falls Member of the Covey Hill Formation of the Potsdam Group

This is a polygonal pattern.   Many polygonal patterns are sun cracks (desiccation cracks).   Some polygonal patterns are trace fossils.  This one looks more like burrowing than a desiccation pattern.

Specimen 3.    Loose specimen of sandstone at quarry, either  Chippewa Bay member or Nepean Formation

To me this looked like either really bad burrows or poor desiccation cracks.  I passed this photograph to a geologist who has worked on these rocks and his comment was that the specimen “might include some burrows but much of the surface is reminiscent of the blocky texture produced by microfaulting of a water-stabilized sand dune surface.”  Not something that I can identify.

Specimen 4  - March Formation sandstone

When I saw this specimen I thought ‘mud cracks’ – the term that I had been using since high school.  I showed the specimen to two geologists and they told me that it couldn’t be a mud cracks because sand doesn’t shrink that much and it must be a  microbial mat shrinkage feature.  I sent the photo to a geologist who has written extensively on microbial mats and he told unless I could find the top slab and underlying slab (in order to ensure that no mud was present) I couldn’t identify this as microbial mat shrinkage feature.

Specimen 5.   Same sandstone as produced the Protichnites trackways mentioned in my blog posting from July 9, 2013 - Likely the Nepean Formation of the Potsdam Group

I have been assuming that this photograph shows ripple marks.   However, there is a chance that it could be the burrowing trace fossil Climactichnites youngi as the bars in the photograph, particularly the J-hooks at the end of the bars, compare favourably with the bar bifurcations  in Figure 5.3 in Getty and Hagadorn, 2008, and with Figure 43 in Yochelson and Fedonkin, 1993,
My suggestion notwithstanding, the ripple marks in the Potsdam can be quite variable, and the photograph is more likely to record ripples than to be a trace fossil.       

Christopher Brett
Perth, Ontario           


References and Suggestions for Further Reading:

Bruce V. Sanford and Robert W.C. Arnott, 2010
Stratigraphic and structural framework of the Potsdam Group in eastern Ontario, western Quebec, and northern New York State.  Geological Survey of Canada, Bulletin 597, 85 pages
publications.gc.ca/collections/collection_2010/nrcan/M42-597-eng.pdf

David G. Lowe,  Robert W. C. Arnott, and Bruce V. Sanford, 2013, 
Before the Great North American Carbonate Bank: A Complex Cambrian-Lower Ordovician Transgressive History Recorded in Siliciclastic Strata of the Potsdam Group, Southeast Laurentia
 Adapted from extended abstract prepared in conjunction with oral presentation at AAPG Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19 -22, 2013
http://www.searchanddiscovery.com/pdfz/documents/2013/50859lowe/ndx_lowe.pdf.html   

Ellis L. Yochelson and Mikhail A. Fedonkin, 1993,    
Paleobiology of Climactichnites, an Enigmatic Late Cambrian Fossil
Smithsonian Contributions to Paleobiology • Number 74 
 Smithsonian Institution Press ,Washington, D.C. 1993
http://www.sil.si.edu/smithsoniancontributions/Paleobiology/pdf_lo/SCtP-0074.pdf

Patrick Ryan Getty  and J. Whitey Hagadorn, 2008,
Reinterpretation of Climactichnites Logan 1860 to Include Subsurface Burrows, and Erection of Musculopodus for Resting Traces of the Trailmaker
Journal of Paleontology 82(6):1161-1172. 
http://dx.doi.org/10.1666/08-004.1

J. Whitey Hagadorn, Joseph H. Collette  and Edward S. Belt,  2011,
Eolian-aquatic deposits and faunas of the middle Cambrian Potsdam Group: Palaios, v. 26, p. 314-334.