Abstract

Boulder-bearing argillite that contains a varied assemblage of both native and exotic sedimentary and volcanic clasts crops out over a large area in southeastern Notre Dame Bay, Newfoundland. It occupies a consistent stratigraphic position throughout this area and is the oldest unit exposed in sequence; it lies beneath Caradocian graptolite-bearing slates. Middle Cambrian trilobites occur in limestone associated with lenticular masses of volcanic rocks that may be allochthonous. The entire unit is thousands of meters thick and is believed to be primarily a sedimentary deposit that has been secondarily remobilized into a chaotic mélange.

Larger fragments within the argillite consist of graywacke boulders and blocks of lava as much as hundreds of meters long; they float suspended in a matrix of pebbly mudstone. Smaller clasts range from granules of quartz and lithic fragments to cobbles of intrusive and extrusive rocks. The matrix is black, scaly, and pyritiferous with contorted green silty interbeds, commonly all homogenized into a chaotic paste devoid of primary structure.

Various lines of evidence suggest polygenetic origin of chaos. Stratigraphic continuity, sedimentary origins of clasts, soft sediment deformation, and brittle rock deformation are all documented as having been important. Structural, stratigraphic, and sedimentological relations are like those in the argille scagliose of the Apennines and suggest similar environments and mechanisms.

This chaotic mélange records Early Ordovician tectonic movements at the northern extremity of the Appalachian belt. These movements may have heralded the onset of the Taconian Orogeny. Coeval movements in western Ireland were of an almost identical nature.

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