Burrows on Burrows
Ordovician Thalassinoides in the Grand Rapids Uplands, Manitoba
The concept of trace fossils is probably somewhat foreign to most people. People may not think often about preserved evidence of ancient biological activity, but many are quite familiar with certain sorts of tracks and traces. If you live in southern Manitoba or have visited buildings such as the Canadian Parliament Buildings in Ottawa, you will have seen the trace fossils that give the Ordovician Tyndall Stone (Red River Formation, Selkirk, Member) its characteristic mottled or “tapestry stone” appearance. These mottles are actually the branched boxworks of preserved burrows, often assigned to the ichnogenus Thalassinoides.
The beautiful mottling of Tyndall Stone demonstrates the existence of tremendous numbers of organisms burrowing into an ancient seafloor. Although the Thalassinoides in this stone is impressive, it is certainly far from its only occurrence in the rocks of Manitoba. Revisiting sites in the Grand Rapids Uplands last week, I had the chance to photograph stone at one locality where it contains burrows even more abundant than those seen anywhere in Tyndall Stone. Not only are they abundant, but their preservation is such that the surrounding stone weathers away in places, leaving a near three-dimensional meshwork of burrows visible on the rock’s surface!
This remarkable exposure, some 500 kilometres north of Winnipeg, is part of the Gunton Member of the Stony Mountain Formation (Upper Ordovician, upper Katian, about 445-447 million years old). It is a bit younger than Tyndall Stone, but still within the same period of geological time. The rock is exposed on broad open surfaces that had been scraped smooth by the Ice Age glaciers, and more recently “re-cleaned” by bulldozers during development of the hydro and highway rights-of-way. In the years since that last human work was done, they have been gently washed by the rain, and somewhat less gently weathered by the cold season’s frosts.
When we first visited this place in about 2004, many of the bedding plane surfaces had already been broken up and had begun to turn into a plain of gravel. We feared that the beautiful burrows would soon be lost to science, and returning this year I was afraid that there would be no bedrock left to observe.
The rock is, however, tougher than that. Right now some of the surfaces are at the optimal stage of weathering: the slightly softer dolostone around and between many of the burrows has been removed, leaving the burrows standing in beautiful three-dimensional relief. The weathering has progressed so far in some instances that you can see down between burrows to those in the layer beneath. Sadly though, some surfaces have progressed past this point, and are spalling off to the extent that they are better road fill than they are evidence of ancient life.
But what teeming life do these immensely abundant burrows provide evidence of, anyway? The truth is that we don’t really know. Thalassinoides-type burrows in younger rocks were probably most often made by crustaceans such as thalassinid shrimp. These sorts of crustaceans are, however, not known as far back as the Ordovician, and it has been suggested that Ordovician burrows may have been made by asaphid trilobites. And maybe they were made by sea anemones or polychaete worms? There are still so many questions about the world for us to answer!
To read more on topics related to this post, you might want to look at some of the following:
Cherns, L., Wheeley, J.R., and Karis, L. 2006. Tunneling trilobites: habitual infaunalism in an Ordovician carbonate seafloor. Geology, 34: 657-660.
Coniglio, M., 1999. Manitoba’s Tyndall Stone. Wat on Earth: Waterloo University Earth Sciences Newsletter, Spring 1999, p. 15-18.
Gingras, M.K., Pemberton, S.G., Muehlenbachs, K., and Machel, H. 2004. Conceptual models for burrow-related, selective dolomitization with textural and isotopic evidence from the Tyndall Stone, Canada. Geobiology, 2: 21-30.
Kendall, A.C. 1977. Origin of dolomite mottling in Ordovician limestones from Saskatchewan and Manitoba. Bulletin of Canadian Petroleum Geology, 25: 480-504.
Young, G.A., Elias, R.J., Wong, S., and Dobrzanski, E.P. 2008. Upper Ordovician Rocks and Fossils in Southern Manitoba. Canadian Paleontology Conference, Field Trip Guidebook No. 13, CPC-2008 Winnipeg, The Manitoba Museum, 19-21 September 2008, 97 p.
© Graham Young, 2012