Correlation and Timing of Platform-Margin Megabreccia Deposition, Cow Head and Related Groups, Western Newfoundland: ABSTRACT

Abstract

The allochthonous Cow Head Group of Cambrian-Ordovician age is a well-exposed but laterally discontinuous succession of continental-margin deposits. Spectacular megabreccias that punctuate the succession at discrete levels can now be correlated throughout the proximal Cow Head region to the more distal Humber Arm area, and tentatively into the metamorphic terrane of the Fleur-de-Lys, far to the east. The oldest breccia is of latest Early Cambrian age (Irishtown Formation). It is composed of shallow-water limestone and sandstone (Labrador Group) as well as Precambrian basement clasts in a quartz-sand matrix. Deposition coincides with progradation of supermature quartz arenites out to and over the edge of the shelf margin (Hawke Bay Formation). The main pulses of megabreccia deposition after the Early Cambrian produced carbonate-rich units. They occurred in the medial Middle Cambrian, near the transition between Cambrian and Ordovician time, and at the transition between the Early and Middle Ordovician. These breccias contain a mixture of clasts, predominantly from the shelf margin and upper part of the slope. The youngest carbonate megabreccia, lying above the Middle Ordovician Table Head Formation, is autochthonous; it records a massive rearrangement of the continental margin during ophiolite obduction. Normal platform-margin sediments in this succession, periplatform ooze (ribbon limestone), carbonate turbidites, breccia deposits (of ribbon-limestone clasts and some platform-margin clasts), and shales coincide with flooding of the platform. Megabreccias coincide with major changes in shelf sedimentation or regression, now apparent as formation boundaries. End_of_Article - Last_Page 474------------