Moose River Basin: geology and mineral potential - Geology Ontario

Moose River Basin
west and by outcrops of the Precambrian Shield to the east (see Figure 2.1). The
depositional environment of Cretaceous sediments differed in the two basins on
either side of the Grand Rapids Arch. The differences are accentuated by the
predominance of lignite deposits in the Onakawana area. Preservation of lig
nite requires a reducing environment such as that obtained in a closed-in area
with stagnant ground-water conditions and lack of ventilation. Such conditions
apparently prevailed at Onakawana where the fireclays associated with lignite
beds are of a dark grey colour. An association of quartz sand and kaolinitic clay
seems to be lacking at Onakawana although some clays become sandier in a
westward direction approaching the Grand Rapids Arch (Trusler et al. 1974).
In the area southwest of the Grand Rapids Arch, thick deposits of white
quartz sand and kaolinitic clay alternate with beds of vari-coloured fireclay in
cluding bright red, buff, white, purple and grey clays, and occasional seams of
lignite. The deposits discordantly overlap subcrops of Middle and Upper Devo
nian Formations and rest directly on Precambrian rocks in the escarpment
area. In one location along the Mattagami River south of the escarpment a "...
highly kaolinized syenite gneiss occupies the bottom of a ravine..." (Cross 1920,
p.17). Deeply weathered crystalline rocks south of the escarpment have pro
vided the source of kaolinitic clay and quartz sand of the Moose River Basin. In
the deltaic environment north of the escarpment, fast flowing currents depos
ited coarse, pebbly sands in riverbeds while clays were carried into adjacent la
goons or oxbow lakes at times of flooding.
At the time of deposition the lateral distribution of Mesozoic sedimentary
rocks was governed by the prevailing river system. Traces of this system have
no doubt been preserved in the topography of the Canadian Shield south of the
Precambrian escarpment. Unravelling of the drainage history of this area re
quires a detailed geomorphologic study. The results may show, for example,
whether or not the broad indentation in the 700-foot topographic contour be
tween the Missinaibi and Mattagami rivers indicates that previously a major
river took the approximate course of the present Opasatika River. Such inform
ation would enchance the intelligent planning of bore hole locations in the
Moose River Basin.
Drill information obtained along the Missinaibi and Mattagami Rivers has
shown that, in places, clay beds are remarkably flat lying and persistent over
distances of 1.5 km to 2.5 km at least. In lagoons of this size the character of the
clay deposits will have been determined partly by the screening effect of vege
tation and will vary accordingly from edge to center of the lagoon. Vegetation
may also have affected subsequent alteration of the clay. Plant root systems
tend to withdraw alkalis while humic acids react with soil components, wash
ing away the fluxing elements to produce refractory clay. One theory holds the
former process responsible for development of a particularly refractory clay, re
ferred to as underclay, found under coal measures. Thick sections of fireclay
were reported from a location on Coal River, between the Precambrian escarp
ment and the Missinaibi River, and from an area on the Mattagami River 9.5 to
11 km north of the escarpment (Montgomery and Watson 1929, p.82). On Coal
River at least 6 m of black, brown and grey clays were reported. In the Matta
gami River section 28 m of similar clays were found between depths of 9 m to 37
m. In both areas, some lignite occurs in association with black clays.
Fireclay deposits of this magnitude, basically kaolinitic clays with a
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