Abstract

Autochthonous upper Middle Ordovician strata northwest of Logan's Line and northeast of Quebec City have been subdivided into six facies types. One or more facies type characterizes the revised formations of our previous report (Belt et al.). These facies were deposited in the following depositional environments: moderately shallow carbonate bank; deeper carbonate slope and foot of slope; submarine fan; and basin plain. The submarine fan facies (Beaupré and Saint-Irénée Formations) contain the only facies with appreciable sandstone. The source region of the sandstones (determined by petrography and paleocurrents) lay to the southeast of Logan's Line. This source was uplifted and eroded during the early phases of the Taconic Orogeny. Turbidity currents and debris flows brought sand into a foredeep trough that lay between the mobile Taconic Orogen and the more stable Canadian Shield. During Trenton time, a carbonate bank developed on the margin of the shield, northwest of the trough axis. Olistostromes, produced by bank-edge collapse, slid southeast into the trough and intercalated with the Saint-Irénée sandy fan lobes derived from the other side of the foredeep basin. Bottom currents, reworking the sand, flowed southwest along the axis of the trough. Later, after regional foundering of the carbonate bank, a larger (Beaupré Formation) submarine fan developed in the foredeep basin. Bottom currents continued reworking the sands down the trough to the southwest.The submarine fans found in this region never developed some of the facies associations commonly expected of suprafan lobes. The initial fan facies consists of lenticular coarse and pebbly sandstone and shale that are only rarely organized into coarsening-up successions. No definite feeder channel deposits are found in the Saint-Irénée Formation although three are recognized in the middle Beaupré Formation at the type section. The presence of these channels plus the geometry of all Beaupré facies and the paleocurrent divergence show that these facies are not disorganized base-of-slope or basin-plain deposits, but best fit a submarine fan model.

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