Jonathan Cribb

M. Sc. Thesis

Analogue Modelling of Salt Tectonic Processes Beneath the Shelf and Deepwater Slope of the Eastern Nova Scotian Margin

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The Scotian Basin is situated on the Atlantic continental margin offshore Nova Scotia and is composed of a series of interconnected Mesozoic rift basins. Thick clastic syn rift sediments and late syn rift salt deposits floor these interconnected sub basins. Along the margin, complex salt basin morphologies and variable post rift sedimentation patterns during Jurassic and Cretaceous times have resulted in a laterally variable and diachroneous evolution of salt and sedimentary overburden structures. These characteristic salt features have been used to divide the Scotian margin into five distinctive salt subprovinces (Shimeld, 2004). However, detailed understanding of the link between early post rift salt mobilization, late post rift formation of allochthonous salt bodies, and their relation to depocenter migration within individual sub basins and salt subprovinces of the Scotian margin is yet to be gained. This gap in understanding is underscored by the unsatisfactory results from the recent round of hydrocarbon exploration in the deepwater slope.

For this project, analogue experiments constrained by regional seismic data are used to study evolution of the eastern Scotian margin (Subprovince IV), an area dominated by salt tectonics. Regional GXT NovaSpan seismic line 2000 and GSC seismic line 89 1, spanning the continental shelf to the deep basin (abyssal plain) within Subprovince IV, and the interest of the petroleum industry in this area make this study particularly attractive. The study area is comprised of a characteristic extensive allochthonous salt nappe and detachment system with distinctive rotated strata referred to as the Banquereau Syn kinematic Wedge (BSW). The eastern boundary of Subprovince IV, imaged by Line 2000, displays several wide (~10 km) extensional passive diapirs within the autochthonous basin and a widespread (~65 km wide) climbing salt nappe outboard of the rift basin. Within the western Subprovince IV line 89 1 displays a lack of significant autochthonous salt structures but a similar (~70 km) allochthonous salt detachment system.

New velocity profiles coincident with line 89 1 (Funck et al., 2004) and Seismic interpretation of line 2000 have indicated that the seaward limit of the autochthonous salt basin is uniform within Subprovince IV. The width of the salt basin (~ 90 km) is also comparable to those of surrounding salt subprovinces (III and V). The seismic interpretation of line 2000 has yielded key features of the salt system, implemented in analogue modelling. The salt basin floor (syn rift sediments covering rifted basement blocks) consists of two seaward thinning half graben wedges filled with thick (~ 3 km maximum) late syn rift Argo Salt. Sedimentation rates driving salt mobilization have an overall decreasing trend from the Middle Jurassic to the Late Cretaceous with a maximum of 200m/Ma during the Late Jurassic.

Scaled 4 D physical analogue models have successfully simulated the evolution of characteristic salt tectonic features imaged within line 2000, giving insight into the development of this portion of the Scotian margin. High resolution optical strain monitoring (DIC - Digital Image Correlation) techniques used to quantify model surface deformation indicate that the model can be divided into 4 kinematic salt tectonic domains from the shelf to deep water: (1) salt weld and pillow, (2) extensional diapir, (3) contractional salt massif and canopy, (4) allochthonous nappe and minibasin. Experiments suggest that early post rift sediment input (Middle Jurassic) on a salt basin comprised of two proportional seaward thinning half graben wedges with thick salt deposits led to early widespread mid basin inflation of salt. Experiment strain monitoring data indicate that early (Middle Jurassic) salt ridges developed in response to extension of an overriding sedimentary wedge. Salt ridges widened to form passive diapirs while significantly translating basinward. High sedimentation rates during the Late Jurassic resulted in the seaward extrusion of salt and the basinward translation of an inflated salt massif along a seaward thinning salt basin. This massif was arrested at the salt basin seaward edge during the Early Cretaceous. Overriding sedimentation loading the inflated salt complex during this time is responsible for development and extrusion of the climbing nappe. Early Cretaceous minibasins translated seaward through the extrusion of an open toe salt nappe and the development of passive diapirs.

Analogue experimentation indicated that complex sediment fairway patterns throughout the Cretaceous (model time) were controlled mostly by the topography of allochthonous salt (nappe) and secondary canopy development at the salt basin edge. Here, localized channels (turbidites) transporting sediments seaward form adjacent to salt canopies where subsidence is active. These small potentially sandy deposits beyond the slope of the eastern Scotian margin are the most likely source for untapped reservoir accumulation.

Pages: 278
Supervisor: J. Adam