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Date of Award

2-1979

Degree Type

Thesis

Degree Name

Doctor of Philosophy (PhD)

Department

Geology

Supervisor

Professor R.G. Walker

Abstract

The Archean Manitou Group occurs as a northeast-southwest trending arcuate belt in the central part of the Wabigoon greenstone belt, northwestern Ontario. The Manitou Group is mainly conglomerate, sandstone, tuff, tuff-breccia, and argillite, with minor lavas (somewhat alkaline), and iron formation. Mapping has established four formations: the Cane Lake, Sunshine Lake, Uphill Lake, and Mosher Bay. The Uphill Lake includes two members: the Surprise Lake and Rush Bay.

The Manitou Group lies upon a thick sequence of pillowed and massive basalts with minor volcanogenic sandstones and argillites. They formed a large submarine platform upon which the pyroclastic and sedimentary units of the Manitou Group were constructed.

The Cane Lake Formation is dominated by thick, massive, heterolithic tuff-breccias deposited as lahars and ignimbrites, with minor air-fall tuffs. They form a subaerial volcanic pile built up by explosive volcanic activity and modified by very minor aqueous reworking.

The Sunshine Lake Formation partly intertongues with, and partly overlies the Cane Lake formation. It is composed of intermediate, somewhat alkalic, massive subaerial lava flows.

The Uphill Lake Formation overlies the Sunshine Lake Formation and intertongues with the Cane Lake Formation. Near the Cane Lake Formation, the Uphill Lake Formation consists of massive, coarse pyroclastic breccias identical to the Cane Lake Formation, but away from the contact (north-eastward), it grades into more thinly bedded volcanogenic conglomerate and sandstone with rare cross-bedding. This portion of the formation has been interpreted as an alluvial fan composed of reworked material derived from the volcanic cone of the Cane Lake Formation. Further east, the Uphill Lake Formation is not exclusively volcanogenic but also contains granite, iron formation, chert, and quartz clasts, and displays abundant large-scale (up to 1 m) cross-bedding in both conglomerates and sandstones.

This eastern part of the formation represents the deposits of a braided fluvial system.

The Surprise Lake Member occurs near the base of the Uphill Lake Formation and is a small, thin unit of laminted siltstone and argillite considered to represent a lacustrine deposit.

The Rush Bay Member is a highly variable group of siltstones, argillites, volcanogenic and quartzose sandstones, conglomerates, tuffs, and tuff-breccias occupying the top of the Uphill Lake Formation. From its stratigraphic position, the member could be interpreted as a coastal or shallow marine deposit, but, unfortunately, it is entirely without any diagnostic features.

The Mosher Bay Formation is an assemblage of argillites, sandstones, conglomerates, and minor iron formation of the Resedimented Association at the top of the Manitou Group. The sandstones are classical turbidites, and the conglomerates are confined to lenses. The lenses probably represent channels cut into the finer sediments, on a series of submarine fans. The presence of these deep-marine conglomerates and the terrestrial conglomerates of the Uphill Lake Formation at Manitou negates the notion that conglomerates define basin margins.

Paleoflow data for the Cane Lake and Uphill Lake Formations indicate that a single pyroclastic cone to the west shed volcanic debris, whereas in the east a braided stream deposited material from a different source.

There is wide variability among the Archean greenstone belts described to date. However, they commonly have 1) a mafic volcanic base upon which a felsic volcanic pile is built, 2) Non-marine Association sedimentary rocks followed stratigraphically by the Resedimented Association, 3) a lack of identifiable shallow-marine or coastal deposits, 4) terrestrial sedimentary environments indicative of high relief, and 5) sialic debris introduced early. Increased sialic contents stratigraphically upwards appear to be related more to diminishing volcanism than increased exposure of plutons. These suggest that Canadian Archean landmasses were small, had high relief and steep shorelines, were composed of mixed volcanic and plutonic rocks, and were tectonically active. This contrasts sharply with the stable continents of the South African Shield.

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