Abstract

The Arenigian Tourelle Formation is dominated by thick, coarse, graded sandstone layers. These poorly-sorted sands were deposited on submarine fans which formed in response to uplift during initiation of the Taconic orogeny. The base of the formation consists of interbedded red and green shales, and rippled dolomitic siltstones, and is injected in spectacular fashion by fine to coarse sandstone dikes and sills. Sills are the dominant type of injection and may exceed 3 metres in thickness. Depth of burial during injection was on the order of 20 to 60 metres, with sand mobilization being the result of liquefaction and perhaps the thixotropic behavior of a primary clay matrix. The source of injected sand in the Cap Ste-Anne section, which was studied in detail, is inferred to have been a sand-filled submarine channel to the southwest of the present outcrop area. Sand was injected laterally along bedding planes and upward from the base to the top of the sequence. Liquefaction was facilitated by sediment loading and compaction under a clay seal, and was triggered by (1) slumps, which are present in the section, and/or (2) earthquakes associated with contemporary orogenesis. Clastic injection did not occur at higher levels in the Tourelle Formation for want of an impermeable clay seal capable of preventing escape of excess pore fluids during compaction.

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