Preliminary Report on Gowganda Mining Division District of Nipissing Ontario

by W. H. Collins

Upper Conglomerate.

The greywacke-slate-quartzite series passes conformably upward into a conglomerate differing little from that at the base. Indeed, where conglomerate outcrops are small and isolated a distinction between the two cannot be made with certainty.

Arkose.

In addition to the above there is an arkose forming at least two well defined areas, whose relations with the rest of the Huronian are in some doubt. This rock is of distinctive appearance, resembling at first glance an ordinary granite, but on closer inspection it is seen to consist of clastic materials such as would result from the disintegration of a granite. Occasional conglomerate streaks in which pebbles of quartz and greenstone are recognizable, demonstrate its bedded character, but ordinarily it is massive looking. Part of the large island in the middle of Duncan lake and the shore to the south are of this formation. It is much more widespread on Obushkong and Gowganda lakes, the bedding being unusually well shown on the large island in the latter body of water.

From its apparent relationship in either of these localities it might be taken to be a member of the Lower Huronian series just described, and equivalent to the basal conglomerate. In the Obushkong area it is probably underlain by Laurentian, for it lies nearly horizontally, and Laurentian is known to occur a short distance east of the lake. In the neighbourhood of Lake Timiskaming it is said to grade imperceptibly into granite, and is believed to be derived from the latter by detrition in situ. Arkose, apparently identical with that under consideration, occurs in the Cobalt district, and is thought to lie unconformably with the greywacke, and for this reason is classified as Middle Huronian. In these pages it is given no definite position in the formational succession, and as there is no field evidence of its Middle Huronian position, is not differentiated from the lower series.

Structural Features and Disturbances.

The Huronian has been subjected to no very severe disturbance, judging by its present condition and attitude. Frequently the strata lie almost horizontally, as for instance at the south end of Firth lake, and in the vicinity of Lake Lehmann, but over the most of the region they rest at inclinations as high as 30°, this attitude remaining constant over extensive areas, and developing a characteristic topographical feature. From Pigeon lake eastward the dip is uniformly to the east at angles ranging between 15 and 30 degrees and the beds overlap one another after the fashion of slates on a roof, the resultant topographical expression of which is a succession of north and south ridges with gentle eastern slopes, while the western sides form escarpments. This condition appears constant over all the Huronian east of a line midway between Pigeon and Duncan lakes. To the extreme north-west, however, a westerly dip was observed; the strata forming the 550 foot hill west of Duncan lake are also either horizontal or dip gently to the west. A confident statement cannot be made until further work has been performed, but the condition just outlined suggests a large anticlinal structure whose arch lies a little west of Duncan lake. However, there seems reason to believe that the structure is more complicated than would result from simple arching and erosion. If the present overlapping system represents the original succession of strata a total thickness of over two miles would be necessary, and there would be not two but several conglomerate horizons, which is improbable. A satisfactory solution is hindered by the general fact that planes of possible dislocation occur in low ground and are obscured by swamp or water, but it seems evident that tilting was accompanied by lateral or vertical displacements.

Certain abrupt disturbances of the general uniform attitude suggest differential movements. Along the West branch below Wapus creek the dip and strike of the shale and greywacke are constant, until where the river’s course changes to due east. Along the shores of this stretch the rocks are mostly hidden by swamp, but where they do outcrop they are standing vertically or dipping steeply to the south, and the strike corresponds with the course of the stream; that is, their positions are at right angles to those farther south. Disturbances of this sort are known, due to the contiguity of igneous intrusions, but at this point no such intrusive is known. It will also be seen from the map that exactly in the same line the East branch makes a similar abrupt change in course. Whether a line of low relief is continuous across the interval between the two streams at this point is not easily determined; however, as a possible explanation of the conditions stated, faulting along this line is suggested. A similar abrupt change from conditions of approximate horizontality to a dip S. 60 W., < 80° and strike of S. 30° E. was noted on the west side of Firth lake.

Relations to other Formations.

The intrusion of the quartz diabase into the Huronian was gently accomplished at most points, and the beds of the latter, both above and below the intrusives, are inclined only a few degrees more steeply than in localities where no diabase can be found. Evidence of intrusion, however, is common, and at some points the Huronian next to the diabase has suffered local but intense physical change. Near the middle of the east shore of Firth lake a rocky islet only a few square yards in extent consists of coarse diabase and conglomerate in intimate contact, little tongues of the former being protruded into the sedimentary rock and peripherally chilled. Most remarkable, however, is the change in condition of the conglomerate; a few chains away on the main shore it is of ordinary character, but on the islet the pebbles lie within a fairly well foliated schist, standing vertically and striking about east and west. An identical condition exists at the south end of a little pond lying 40 chains west of Mosher lake. The vertical foliation of the conglomerate suggests the neighbouring diabase intrusion to have been by vertical ascension rather than lateral spread, so that these points may represent portions of vents through which the diabase magma ascended, and for that reason are more affected than where sills have been quietly injected.