Regional stratigraphy and subsidence of Orphan Basin near the time of breakup and implications for rifting processes

Abstract

The stratigraphic, subsidence and structural history of Orphan Basin, offshore the island of Newfoundland, Canada, is described from well data and tied to a regional seismic grid. This large (400 by 400 km) rifted basin is part of the non-volcanic rifted margin in the northwest Atlantic Ocean, which had a long and complex rift history spanning Middle Jurassic to Aptian time. The basin is underlain by variably thinned continental crust, locally <10-km thick. Our work highlights the complex structure, with major upper crustal faults terminating in the mid-crust, while lower crustal reflectivity suggests ductile flow, perhaps accommodating depth-dependent extension. We describe three major stratigraphic horizons connected to breakup and the early post-rift. An Aptian-Albian unconformity appears to mark the end of crustal rifting in the basin, and a second, more subdued Santonian unconformity was also noted atop basement highs and along the proximal margins of the basin. Only minor thermal subsidence occurred between development of these two horizons. The main phase of postrift subsidence was delayed until post-Santonian time, with rapid subsidence culminating in the development of a major flooding surface in base Tertiary time. Conventional models of rifting events predict significant basin thermal subsidence immediately following continental lithospheric breakup. In the Orphan Basin, however, this subsidence was delayed for about 25-30 Myr and requires more thinning of the mantle lithosphere than the crust. Models of the subsidence history suggest that extreme thinning of the lithospheric mantle continued well into the post-rift period. This is consistent with edge-driven, small-scale convective flow in the mantle, which may thin the lithosphere from below. A hot spot may also have been present below the region in Aptian-Albian time.

Kinematic Evolution of the Southern North Atlantic: Implications for the Formation of Hyperextended Rift Systems

“…This high extends northward to the Orphan Knoll. The age of crustal extension and necking in these subbasins is ill constrained, but stratigraphic studies and well calibration provide some insight (Dafoe et al, ; Gouiza et al, ). After Late Triassic‐Early Jurassic rifting in the East Orphan and the Jeanne d'Arc Basin (Enachescu, ) (Figure ), two main extension phases have been suggested.…”

“…After Late Triassic‐Early Jurassic rifting in the East Orphan and the Jeanne d'Arc Basin (Enachescu, ) (Figure ), two main extension phases have been suggested. The first one, during the Late Jurassic, is supposed to mainly affect the eastern subbasin, whereas the second phase, during Early Cretaceous, was likely more intense in the western subbasin (Dafoe et al, ; Enachescu, ; Gouiza et al, ). Seismic velocities from seismic refraction studies suggest a 5–8 km thick continental crust in the thinnest part of the subbasins and do not present evidence for large areas of exhumed mantle (Chian et al ; Watremez et al, ).…”

Enigmatic Deep-Water Mounds on the Orphan Knoll, Labrador Sea

“…Small-scale depressions and possible erosional or dissolutional scour features are seen in layers 1 and 2 on the top of the NE Orphan Knoll mound (Figures 8, 9, 12). The depressions might have an origin in subaerial dissolution in the Cenomanian to Maastrichtian unconformity (Dafoe et al, 2017), similar to the dissolution of Cretaceous chalks of the Wyandot Formation on the Scotian Shelf (Wielens et al, 2002). Alternatively, depressions might result from submarine fluid escape (Land et al, 1995;Wielens et al, 2002).…”

“…The Knoll rises 2 km above the adjacent Labrador Sea ocean basin. Orphan Basin was affected by several phases of Mesozoic rifting culminating in the Late Jurassic-Early Cretaceous hyperextension (Enachescu et al, 2005;Dafoe et al, 2017). Continental breakup east of Orphan Knoll followed during the Mid-to Late Cretaceous, with the oldest definite magnetic anomaly on the seaward side of Orphan Knoll being C34n (84 Ma, Santonian, Srivastava et al, 1988).…”

“…With available data, it is difficult to further characterize the faulting style. They may represent normal, possibly listric faults, structurally similar to the extensional faulting within the Orphan Basin (Dafoe et al, 2017). Alternatively, the faults may be parallel to the NNW-trending fault-bound northeastern face of Orphan Knoll, although this is not supported by orientations of groups of mounds in the bathymetry (Figure 5D).…”