Separation of late cimmerian rift and post-rift megasequences:comparison of the jeanne d'Arc Basin, Grand Banks and the outer moray firth, North Sea

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Late Cimmerian tectonism has long been recognized as providing a dominant control on the styles of structuring and sedimentation, as well as hydrocarbon-trapping mechanisms, in basins marginal to the North Atlantic Ocean. There is less agreement, however, on the timing and structural/sedimentologic responses to the end of this rift period. The boundary between the Late Cimmerian rift and post-rift megasequences is defined in the Jeanne d'Arc Basin on the Grand Banks of Newfoundland by a mid-Valanginian unconformity and its correlative conformity recognized as separating the Hibernia and Whiterose Formations. Correlations of wells and seismic data from the Claymore and Scapa Fields of the Outer Moray Firth in the North Sea indicate that the megasequence-bounding “Base Cretaceous” unconformity, present across numerous structural highs of the North Sea basins, is the lateral continuation of a base Late Valanginian unconformity. This megasequence boundary separates lower Valhall conglomerates, sandstones and marlstones from the Scapa Member sandstones and conglomerates at the base of the Scapa syncline. The Late Cimmerian rift megasequences of the study areas are characterized by basal and upper packages of relatively coarse siliciclastic sediments partially enclosing a middle package of basinal shales with condensed sections. The relatively widespread deposition of sandstones and, more locally, conglomerates at the base and top of the rift megasequences is attributed to relatively low topographic relief and subsidence rates at the initiation of extension and again in the latter stages of rifting. Local structural responses to extensional stress and wide ranges of distance from clastic source areas resulted in variations in sedimentary response at the start and finish of rifting. The deposition of basinal shales in condensed sections over wide areas during the mid-rift period is attributed to entrapment of coarse sediment along active faults and basin margin areas in response to peak rates of fault block rotation and subsidence. Post-rift subsidence in both study areas was characterized by a general decrease in grain size of siliciclastic debris, a halt in significant fault block rotation, and initiation of regional “sag” basins in response to decay of the thermal anomaly associated with rifting. The post-rift “sag” basin of the Outer Moray Firth area was dominated by deposition of a condensed marlstone section. Onset of thermal subsidence in the Jeanne d'Arc Basin was marked by a rapid transgression and widespread deposition of a bioclastic-rich marine limestone. Differential compaction modified post-rift subsidence patterns in both areas.