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Stratigraphic and Structural Relationships in the Foreland Basin and Humber Arm Allochthon on Port au Port Peninsula, western Newfoundland Open Access


Other title
Type of item
Degree grantor
University of Alberta
Author or creator
Lacombe, Ryan A
Supervisor and department
Waldron, John (Earth and Atmospheric Sciences)
Examining committee member and department
Waldron, John (Earth and Atmospheric Sciences)
Johnston, Stephen (Earth and Atmospheric Sciences)
Harris, Nicholas (Earth and Atmospheric Sciences)
Department of Earth and Atmospheric Sciences

Date accepted
Graduation date
2017-11:Fall 2017
Master of Science
Degree level
Port au Port Peninsula, in western Newfoundland, sits at the western edge of the Appalachian orogen. Middle Ordovician foreland basin strata deposited on the Laurentian margin are primarily derived from, and overridden by, Cambrian to Ordovician deep-water rocks previously mapped as mélange and assigned to the Humber Arm Allochthon. Abrupt thickness changes in the foreland basin are associated with high-angle faults and increased accommodation space in the hanging wall of extensional faults. Foreland basin strata are found between two packages of Humber Arm Allochthon on Port au Port Peninsula; they unconformably overlie the lower, allochthonous, West Bay Thrust Sheet and are overlain in turn by the upper, allochthonous, Lourdes Thrust Sheet. The West Bay Thrust Sheet was emplaced during a period of Taconian extension as an overextended and thinned wedge emplaced rapidly during the Middle Ordovician. Down-dropped along normal faults, the West Bay Thrust Sheet was covered by foreland basin strata; further Acadian (Devonian) deformation duplicated the Humber Arm Allochthon on Port au Port Peninsula to fill a tectonic wedge within the foreland basin. Taconian, extensional faults were reactivated; map offsets and slickenlines suggest two phases of reactivation which are related to Acadian inversion and Carboniferous strike-slip. The Humber Arm Allochthon comprises the Cooks Brook, Middle Arm Point and Eagle Island formations, and is structurally highly disrupted. Outcrop-scale disruption is assigned a value from 0-V with I-IV constituting broken formation and V constituting mélange. More coherent, folded outcrops of allochthonous rocks show three fold generations; early tight to isoclinal folds are overprinted by two later deformation events which we link to Acadian (Devonian) orogenesis and inversion, and subsequent Carboniferous strike-slip along high-angle faults. Mélange, representing the highest disruption mapped in the area, commonly contains igneous blocks from a variety of sources which we link to olistostromal processes at the early Taconian deformation front. Measured blocks within mélange range from thin-section scale (0.5 mm) to 150 cm. Mélange at outcrop scale shows, on average, 24% blocks to 76% scaly shale. Oriented thin sections collected from mélange show both clockwise and counterclockwise oblique-shear fractures as well as hydrocarbons. Autobrecciation, dewatering structures and sandstone dykes imply high fluid pressures. We suggest that the allochthon underwent coaxial extension within an overall compressional setting as a result of high fluid pressure; due, in part, to the expulsion of hydrocarbons from allochthonous source rocks within the wedge. This resulted in rapid forward movement (and thinning) of the Taconian wedge.
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