Abstract

Four main facies have been identified in the East Bay area of Minnitaki Lake. The first consists of coarse arkose and arkosic conglomerate. Individual beds are commonly graded but lack other sedimentary structures. Plagioclase makes up 30 to 40 percent of the rock, and quartz about 6 percent. The second consists of graded graywackes interbedded with slates. Quartz here reaches about 20 percent, plagioclase only 15 percent, and rock fragments 10 to 15 percent. The third facies consists of graded conglomerates with abundant very well-rounded granite boulders up to 40 cm in diameter, and the fourth facies is a black slate with very thin (1 to 5 cm) graded siltstones. The slate is the background sediment of the basin, and is interbedded with the arkoses, arkosic conglomerates, graywackes, and granite-bearing conglomerates.

Mapping and measuring has revealed an almost unbroken stratigraphic column 800 m thick. Arkoses and arkosic conglomerates are restricted to the basal 150 m, and above this level there is a gradual increase in the ratio of quartz to feldspar.

The graywackes are interpreted as turbidites, and the graded conglomerates as a very proximal resedimented facies. There is no evidence of shallow, agitated water sediments. Transport of the arkosic conglomerates was westward along the present tectonic strike, and the source area, a quartz-porphyry stock, has been identified at the eastern end of the basin. The quartz-rich graywackes and granite-bearing conglomerates indicate a granite or granite-gneiss source area, devoid of greenstone cover, because greenstone debris is very rare in the sediments. The gradual increase in quartz during basin filling suggests progressive uplift of the terrain bordering the trough, with steep shorelines and no development of marginal facies.

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