Multiple episodes of extensional tectonism dominated the formation of Mesozoic fault‐bounded basins on the Grand Banks of Newfoundland, the Irish Continental Shelf and the central North Sea. A range of structural and stratigraphic responses in the Jeanne d'Arc, Porcupine and Moray Firth basins support widespread synchronous tectonic controls on sedimentation during one of these episodes, the Late Cimmerian. Rifting was preceded by a phase of related tectonism during which subsidence rates began to vary across broad areas but without significant fault block rotation. This Late Cimmerian ‘onset warp’ pattern of subsidence is considered to have been essential in the establishment of restricted anoxic basins from latest Oxfordian through Kimmeridgian (sensu gallico) time and the development of one prolific layer of organic‐rich source rocks. The most prominent and widely recognized structural/lithostratigraphic response to Late Cimmerian rifting was the deposition of sediment wedges. Tithonian to early Valanginian strata generally thicken‐ into northerly trending faults in the Jeanne d'Arc and Porcupine basins, indicating that extensional stress was orientated WNW‐ESE across a very broad area. The misalignment of this regional Late Cimmerian extensional stress with local inherited structural fabric may be responsible for transpressional uplift of individual fault blocks in the Outer Moray Firth basin. Sedimentological responses to Late Cimmerian rifting were varied, though a common lithofacies stacking pattern is recognized. Variably thick conglomerates and/or sandstones were widely deposited at the start of rift deformation, while palaeoenvironments ranged from alluvial and braid plain to submarine fan even within individual basins. The relatively coarse basal sediments fine upwards into a second layer of commonly organic‐rich shales and mark The widest variations in palaeoenvironments and sediment thicknesses occurred during the last phase of Late Cimmerian rift tectonism, though all three basins show evidence of decreasing water depths, increasing oxygen levels and increasing grain size. This lithofacies stacking pattern of relatively coarse to fine to coarse (reservoir/source/reservoir) and the development of bounding unconformities are largely attributable to progressive changes in rift‐controlled subsidence. Rift basin subsidence rates are interpreted to increase from a low at initiation of faulting to a mid‐rift peak, followed by slowing subsidence to the end of extension. A number of counteracting crustal mechanisms that may account for progressive variations in rift‐induced subsidence are considered.
Seismic, wireline log and core data from three geographically separate basins in the North Atlantic provide an understanding of the regional dynamics of the basin system, and quantify the sedimentary geometry response to a composite rift episode. The response to the Late Cimmerian rift episode in the Jeanne d’Arc, Porcupine and Moray Firth Basins is broadly comparable, with an initial episode of clastic reservoir facies deposition associated with active growth faulting. This was followed by a mid-rift period where subsidence outstripped sedimentation, resulting in organic-rich source rocks and culminating in a mappable maximum flooding surface. During the final phase of the rift episode, the decrease in subsidence rate resulted in the return of reservoir-quality clastics. The reservoir-scale response to the initial rifting is illustrated in cored sequences from the Jeanne d’Arc Basin. These indicate the development of axial and lateral sandy and conglomeratic braided fluvial and alluvial fan systems, with periods of muddy lacustrine deposition. Pedogenic surfaces offer much potential for detailed correlation and the analysis of sandbody geometry. The onset of anoxic conditions conducive to source rock deposition during the mid-rift phase of Late Cimmerian tectonism, and the preferential development of reservoir facies at the base and top of syn-rift sediment fill, are characteristics of considerable importance to exploration of the North Atlantic borderland rift basins.
Variations in structural architecture indicate that a ninety degree rotation of extensional stresses occurred between multiple Mesozoic rift phases, resulting in local structural inversion in the Jeanne d'Arc Basin on the Grand Banks of Newfoundland, offshore eastern Canada.NW-SE-orientated extension caused dip-slip movement on NE-SW-trending en echelon faults during the Late Triassic to earliest Jurassic rift episode. These en echelon faults appear to have been separated by tilted basement relay ramps in accommodation zones rather than by cross-strike transfer faults.NE-SW-orientated extension during the mid-Aptian to late Albian rift episode resulted in oblique-slip reactivation and linkage of the earlier normal faults. Transpressional stresses were generated by oblique-slip motion at restraining bends in Jeanne d'Arc Basin faults. Structural responses include: 'pop-up' blocks between upward-diverging faults; reverse faults separating overlapping 'shingles' of strata; forced folds in strata overlying deep-seated faults; and wrench-related folds trending oblique to through-going transfer fault zones. Salt and shale layers are seen to act as horizons of fault detachment, modifying the transmission of tectonic stress and strain from the basement into the sedimentary cover. The transpressional mechanism and direction of strike-slip motion are confirmed through recognition of associated transtensional structures located along fault strike at nearby releasing bends.Transpressional structures in the Jeanne d'Arc Basin are recognizable over a wide range of scales from relatively large-scale reverse faults visible on seismic data to small-scale reverse faults seen in core.
Analysis of well and seismic data indicates that both the Jeanne d’Arc Basin on the Grand Banks of Newfoundland and the Porcupine Basin on the continental shelf off western Ireland were undergoing extension characterized by subsidence and rotation of fault blocks during mid-Aptian to late Albian times. Stratigraphic sequences in these conjugate basins developed in response to synchronous rift tectonism. The base of each syn-rift sequence is defined by an angular unconformity which formed in response to regional uplift. This mid-Aptian rift-onset unconformity is buried beneath a depositional sequence characterized by retrogradational coastal sediments overlain by progradational coastal sediments in each basin. However, the lower retrogradational deposits dominate the syn-rift sequence in the Jeanne d’Arc Basin, while the upper progradational deposits dominate the syn-rift sequence in the northern Porcupine Basin. The relative dominance of sediments deposited along either transgressive or regressive coastlines was mainly determined by the local interplay of variable rates of rift-induced subsidence and sediment input. The initiation of seafloor spreading between the Flemish Cap/Orphan Knoll and Goban Spur/Porcupine Bank continental margins near the end of Albian time resulted in the establishment of regional thermal subsidence, rapid coastal transgression and fully marine deposition above a locally-developed sequence-bounding unconformity.
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