Wet-sediment deformation in the Upper Ordovician Point Leamington Formation: an active thrust-imbricate system during sedimentation, Notre Dame Bay, north-central Newfoundland

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Abstract
Summary: The Upper Ordovician (late Caradoc-Ashgill) Point Leamington Formation is essentially composed of fine-grained, thin-bedded turbidites that accumulated in a deep marine environment below wave-base. Immediately underlying, and apparently conformable with the formation, there are about 120 m of red, red-green and grey bioturbated cherts overlain by late Llandeilo-early Caradoc black shales—these argillaceous, ‘pelagic’, sediments conformably overlie about 800 m of mafic pillow lavas and flows of the Lawrence Head Volcanics (middle Exploits Group). The Point Leamington Formation, up to 2200 m thick, together with the mainly fine-grained, thin-bedded turbidities, contain locally abundant wet-sediment deformation in horizons up to tens of metres thick, although most of the thickest horizons appear to represent multiple events. Conglomerate-filled gullies, channels and canyons occur within the finer grained Point Leamington Formation facies-associations. The formation is overlain, gradationally, by the deep-water conglomeratic Goldson Formation, interpreted as submarine canyon deposits; with the Ordovician-Silurian boundary occurring towards the top of the Point Leamington Formation or towards the base of the Goldson Formation, based on the presence of lower Llandovery corals in limestone boulders within an olistostrome near the gradational boundary between these formations. Wet-sediment deformation occurs as: (i) coherent folded layers; (ii) semi-coherent folded layers; (iii) chaotic balled or brecciated sediments; (iv) boudinaged layers; (v) faulted layers, involving normal, reverse and thrust faults, and best seen on a micro- to meso-scopic scale. Clastic dykes, other liquefaction and fluidization structures, together with convolute lamination, also occur either isolated from, or in association with, the above listed wet-sediment deformation styles. The wet-sediment deformation generally appears to be related to gravity-controlled slope failure, in surface to near-surface mass failure or at unspecified depth of burial, on a margin with a regional south to south-eastward downslope dip, although some folds may be interpreted as thrust-related (tectonic) deformation in wet sediments. The overall stratigraphy, sedimentology and structure suggests Upper Ordovician to Lower Silurian deep marine sedimentation in small fault-bounded and fault-controlled basins, some of which contained relatively coarse-grained, small-diameter, submarine fans. Regional considerations are consistent with the succession having accumulated in a thrust imbricate system, active during sedimentation, with new evidence to indicate Ashgill-Llandovery volcanism and magmatism that can be related to initial subduction followed by possible crustal extension. Sometime during the Ashgill, subduction appears to have ceased as all the intervening oceanic crust was consumed and volcanic arc activity shut down. During late Ashgill-Llandovery time, thin continental crust of the ‘Gander’ terrane to the east then underplated the ‘Dunnage Zone’, probably leading to foreland basin development, analogous to the plate tectonic processes in the Banda Arc today. Thus, there was a reversal of subduction polarity from the eastwards subduction of pre-Caradoc times, such that the remnant backarc basin became, for a while, a forearc as the wide backarc or marginal basin floored, at least in part by oceanic crust, telescoped. Contemporaneous sinistral shear appears to have been important in the late Ordovician-Silurian, during which time unspecified ‘suspect’ terranes may have slipped in and out of the Dunnage Zone. The contemporaneous Ashgill-Llandovery bimodal igneous activity can be explained by phases of crustal transtension as re-entrants in opposing plates slid past each other, followed by phases of transpression to re-activate the thrust imbricate system.