COMPARISON OF OLDER AND NEWER GEOLOGICAL MAPS OF PRECAMBRIAN TERRAINS: AN EXAMPLE FROM THE HURONIAN SUPERGROUP IN PORTER TOWNSHIP, NEAR SUDBURY, ONTARIO

Improvements in geological mapping of Precambrian terrains are illustrated by 2 similar-scale, Ontario Geological Survey maps of Porter Twp, made ~50 years apart. Porter Twp, located 60 km W of Sudbury, is underlain predominantly by Huronian Sgp metasedimentary rocks (2.4-2.2 Ga) located N of the Murray fault. The area north of the Murray fault zone was generally thought to record a relatively simple structural history related to the distal effects of the ~1.835 Ga Penokean orogeny. The first map (Ginn 1961) is lithological, and was produced prior to standardization of the Huronian Sgp stratigraphy in 1969. Nonetheless, it records an abundance of structural data, and a major ENE fault system in southern Porter Twp. The second map (Easton 2006) is tectonostratigraphic, and led to 2 important advances: 1) recognition of 2 periods of folding resulting in a dome and basin interference pattern. Although this pattern is recognizable on the older map, its significance was not realized at the time. Nipissing gabbro is folded by F1 in the lowermost part of the stratigraphy, but is also emplaced along fractures related to F2 axial planes. This suggests either multiple periods of emplacement, or syn-folding gabbro emplacement. 2) At least 5 major fault sets are present, 4 of which are post-folding. The earliest faults are N-trending, and juxtapose 2.65 Ga Archean granitic basement against Huronian strata. E-NE faults juxtapose Huronian strata against basement, but are post- F1 with vertical and lateral movement. They may be associated with a set of N- to NE, dominantly normal faults, which may have an older thrust component. Most significant in terms of map pattern, are E- to ENE normal faults, across which major changes in stratigraphic level occur. Any thrust component to these faults has been obscured by subsequent vertical movement, and the presence of extensive zones of Sudbury breccia. The Sudbury breccias suggest that the ENE fault set developed at 1.85 Ga, the time of the Sudbury impact. Finally, significant vertical displacement, occurred along a set of NW faults, which are also the loci of Sudbury swarm diabase dikes (~1.24 Ga).

Ginn, R.M. 1961. Geology of Porter Township; Ontario Department of Mines, Map 2011, scale 1:12 000.

Easton, R.M. 2006. Geology of Porter and Vernon townships; Ontario Geological Survey, Map P.2845, scale 1:20 000.