Tectonic control on sedimentary evolution of three North Atlantic borderland Mesozoic basins1

Abstract

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.

The evolution of the Porcupine and Rockall basins, offshore Ireland: the geological template for carbonate mound development

“…They might also be anticipated throughout much of the Rockall Basin (Shannon and Naylor 1998). In the Porcupine Basin, the succession comprises a braided fluvial sandy facies, interpreted by Sinclair et al (1994) as the product of an onset warp phase of tectonism prior to major rifting. The Upper Jurassic strata reflect deposition of basin-edge sandy to conglomeratic alluvial fans, meandering fluvial sandy and silty sediments and deep-marine submarine sandy fans in a series of syn-rift sub-basins (Croker and Shannon 1987;MacDonald et al 1987).…”

An overview of the Upper Palaeozoic–Mesozoic stratigraphy of the NE Atlantic region

“…the onset of rifting). Rifting reached its nadir in Late Jurassic times with the formation of tilted fault blocks and major facies variations (Shannon 1991;Sinclair et al 1994). In the central and southern Porcupine Basin, and in the southern Rockall Basin, crustal stretching was extreme (hyperextension) in Late Jurassic time, resulting in significant differential subsidence towards the basin centres.…”

The Magellan mound province in the Porcupine Basin

“…This underfilled sedimentary basin, of Upper Palaeozoic to Recent age, formed through a series of multi-directional rifts of Permo-Triassic, Upper Jurassic to Early Cretaceous and mid-Cretaceous age (e.g. Sinclair et al 1994), interspersed with periods of pronounced thermal subsidence, sedimentation and igneous activity. Locally more than 9 km of Mesozoic and Tertiary sediments are present, which gradually onlap the basin margins (Moore and Shannon 1992).…”