Structure, stratigraphy, and geochemistry of the Upper Ordovician Lawrence Harbour formation, Exploits Subzone, Newfoundland

Carter, James Edward (1998) Structure, stratigraphy, and geochemistry of the Upper Ordovician Lawrence Harbour formation, Exploits Subzone, Newfoundland. Masters thesis, Memorial University of Newfoundland.

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Abstract

The Upper Ordovician Lawrence Harbour Formation is a significant lithostratigraphic and biostratigraphic marker horizon found throughout the Exploits Subzone. In the Grand Falls-Windsor-Badger region the Lawrence Harbour Formation grades from graptolitic siliceous black shale, to dark grey silty shale, and light grey silty shale, from base to top respectively. It conformably overlies grey bioturbated chert and is itself conformably overlain by sandstone and siltstone turbidites of the Point Leamington Formation. The gradational coarsening upward characteristics and transition into overlying units suggest that the Lawrence Harbour Formation should be included within the Badger Group. -- Geochemical and lithological variation is documented vertically through the formation and appears to be related to increasing grainsize from base to top of the shales. The Lawrence Harbour Formation and inferred equivalents sampled from Notre Dame Bay to Bay D'Espoir. show no marked lateral variation in geochemistry. These trends support a coarsening upward sequence for the shales, and appear to support a model of deposition within a single interconnected basin for the Lawrence Harbour Formation. Geochemical signatures of the shales suggest a calc-alkaline source terrane, with deposition probably occurring in a back arc basin as hemipelagic distal turbidite sediments. -- Regional mapping has defined a grey bioturbated chert unit at least 10 to 15 metres thick that conformably underlies the Lawrence Harbour Formation. The upper gradational contact defined within the study area comprises about 2 metres of chert with interbeds of siliceous black shale that yields graptolites of the N. gracilis Zone. The chert unit has been used as a reliable marker horizon both in this study and in previous work, and as a result is herein elevated to formation status with the name "Northern Arm Formation". -- Extensive polyphase deformation of these Upper Ordovician units commonly produces complex regional outcrop patterns. Detailed outcrop-scale structural maps define reverse faults (D₃) that cross cut limbs of open to close folds (D₂). Regional penetrative cleavage is observed as axial planar to the F₂-folds. These structures were followed by a later stage of deformation that steepened the dirust faults, with a component of strike slip faulting with both sinistral and dextral sense along already formed D₃ thrust faults. Observations on a macroscopic scale define several fault-bound belts that strike northeast-southwest and cross cut southeast facing F₂ regional folds. In all, a total of six phases of deformation are recognized with D₀-D₄ being regional, and D₅ being identified as localised in extent. The D₂ and D₃ deformation events are likely due to the continued compression related to the Salinic and Acadian orogenies.

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