A Phanerozoic Time Chart for Canada

INTRODUCTION When Nick Eyles and I compiled our book Canada Rocks (Eyles and Miall 2007) no nationwide synthesis of the geology of Canada had been written since the Geological Survey of Canada’s Economic Geology Report #1 (Douglas 1970). The Geological Society of America’s Decade of North American Geology (DNAG), spanning the 1980s, generated a suite of regional volumes covering the entire continent, including Canada. The Canadian portion of the project was supervised by the Geological Survey of Canada and resulted in four regional volumes, plus a volume on part of the Canadian Shield and one on the Quaternary record. However, these were written for specialists and either did not include regional syntheses or incorporated highly technical summaries directed primarily at a professional audience. In our planning for Canada Rocks we debated at length whether to adopt a ‘time-slice’ approach for the narrative or a ‘regional’ approach. The Canadian Shield is itself a distinctive ‘region’ of Canada, and was therefore treated as such (Chapter 4), but the description of its geology followed a sequential approach based on the age of the various terranes and orogenies. This worked well, in part because many of the Precambrian terranes are quite geographically limited in area or distribution, and therefore the description of each unit served as a miniregional summary as well. For the Phanerozoic the decision was more problematic. A timeslice approach works well for a description of the evolution of life and for the broader features of global geology, and we used this arrangement for the summary chapter (Chapter 3), which provides a concise history of the geology of Canada from the Archean to the present day. For the Phanerozoic history of Canada, we decided that a ‘regional’ approach to the geology would work better. We predicted that most of our readers would be interested primarily in the region of Canada in which they lived, and that regional chapters for each region would therefore immediately satisfy their major interest. A regional approach allows for a continuous narrative through the various tectonic and other episodes for a given area, gradually building the geological framework for the landscape that exists at the present day. We used this approach for the Phanerozoic development of the four major regions of the country in four separate chapters (the Craton, Eastern Canada, Arctic Canada and Western Canada). The one disadvantage of this approach is that, without considerable repetition, it is difficult to discuss regional histories in the context of the continental and global setting, and almost completely eliminates comparisons between the various regions. One of the most interesting aspects of Canadian geology is the plate tectonic history, in which events on one margin of the continent are matched by contemporaneous, but different, types of event on another margin. This kind of information tends to get lost in a regional approach. The time-slice approach can, by contrast, keep a focus on continental and global events, but at the expense of a very fragmentary treatment of regional geology. A description of the geology and geological history of any given area might require dipping into four or five different chapters. For the non-specialist reader, we judged that this would be confusing. The accompanying chart (Fig. 1) has been developed primarily so that the regional geologies of the different parts of Canada may be related to each other in terms of the time sequence of the major events.