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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.

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A mid-Cretaceous to Late Cretaceous volcanic province, named here the Charlie-Gibbs Volcanic

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Assessing the temporal significance of invertebrate ichnofossils is essential in interpreting ancient organism behaviors, depositional settings, and bioturbation and sedimentation rates. The trace fossil, Macaronichnus segregatis, is known to represent the work of deposit-feeding polychaetes and commonly occurs as a pervasive structure in shallow-marine sandstone deposits. This study uses the polychaete Euzonus mucronata, which produces M. segregatis-like structures, as a modern analogue to the trace-making counterparts. Field measurements from Pachena Beach, Vancouver Island, Canada, included assessment of population densities and worm behaviors. Volumetric burrowing rates were obtained from a thinwalled aquarium constructed in the laboratory. The burrowing rate calculated for 5 Euzonus (0.089 cm 3 /hr) was extrapolated to populations (approximately 1,400-5,000 worms/m 2 ) estimated from Pachena Beach, which require 70-300 days to completely rework 0.1 m 3 of sediment. Calculated rates are dependent upon the limitations of simulating a natural setting in an aquarium, the population density assessment, and the particular characteristics of the worm population and foreshore at Pachena Bay. However, these initial estimates can still be applied to rock record examples such as the Macaronichnus segregatis found in the Appaloosa Sandstone of Alberta, Canada. In this unit, ancient worms persisted in dense populations and reworked sediment at a rate that exceeded deposition during overall foreshore aggradation.