The southwest portion of the Dunnage Mélange and its relationships to nearby groups

Hibbard, James P. (1976) The southwest portion of the Dunnage Mélange and its relationships to nearby groups. Masters thesis, Memorial University of Newfoundland.

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Abstract

The Dunnage Mélange is typified by slumped blocks of clastic sediment and mafic volcanics set in a dark shale matrix. It outcrops for 40 km. southwestward from its type area in Dildo Run, with a maximum outcrop width of 13 km. Previous work has been confined to the Dildo Run area, where the interpretation of stratigraphic relationships of the Dunnage Mélange and surrounding units has been controversial. The present study focusses on the southwest portion of the mélange in an attempt to resolve this controversy as well as to investigate the character and extent of this portion of the mélange. -- The southwest portion of the mélange overlies and interdigitates with the gabbro-infested, Ordovician New Bay Formation. The mélange contains gabbro blocks that are confined to the area proximal to this transition zone. Similarly, in the northwest portion, the largest mafic volcanic blocks define a wide zone that is correlative westwards with the Lawrence Head Volcanics of the Exploits Group. Along its northwest border, the Dunnage is locally conformable with Caradocian or later, shale and greywacke units, though in most places this contact is fault-modified. These relationships imply that the Dunnage is an easterly chaotic equivalent to part of the Exploits Group. -- The chaotic nature of the mélange can be attributed primarily to discontinuous, dominantly extensional, soft rock deformation of strata due to massive slumping. This possibly occurred as a single progressive event and involved rocks in various stages of lithification. Forces responsible for soft rock deformation existed from New Bay depositional time until the Lower Silurian, intensifying prior to the Caradocian, following which they decreased and terminated. The chronology of later penetrative events is uncertain, though most hard rock structures are presumed to be Devonian, as flat lying Carboniferous strata occur elsewhere in the Exploits Zone. -- Regional relationships indicate that the Dunnage Mélange occupied a basin on the southeast flank of a Lower Ordovician island arc complex. Previous workers have viewed this fact to indicate that mélange formation was related to an active, west dipping, subduction zone. The present study reveals that no valid criteria exist for interpreting the mélange terrane to have occupied an active trench. Rather, the tectonic position of the mélange and its apparent synchroneity with other Newfoundland mélanges suggest it was a massive slump during widespread Taconic events, though the direct cause of slumping remains ambiguous.

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