Fjord sedimentation as an analogue for small hydrocarbon-bearing fan deltas

Abstract

Summary Fjord basins have sediment accumulating at rates between 0.1 and more than 1000 mm a −1 with sea-level fluctuations up to 50 mm a −1 , lithologies varying from poorly sorted conglomerates to unimodal sands and muds, carbon contents from less than 1.0% to more than 10% for methane-producing sediments found in some anoxic basins, and variably consolidated and sensitive sediment. Four major mechanisms affect the rate and style of basin infilling: (i) hemipelagic sedimentation of particles carried seaward by river plumes; (ii) delta-front progradation, particularly as affected by bed-load deposition at river mouths; (iii) proximal slope bypassing, primarily by turbidity currents and cohesionless debris flows; (iv) down-slope diffusive processes, mainly creep and slides, that work to smear previously deposited sediment into deep water. Fjords, with their rugged topography and dynamic history of basin infilling, are prone to slope failures. Release mechanisms include sediment loading, ground motion from earthquakes, giant waves, sea-level fluctuations and changes in the mass physical properties of basin sediments. Slope failures occur on a semicontinuous basis and with displaced volumes of 10 3 –10 6 m 3 , or with much less frequency (1000 year intervals) but affecting larger volumes of sediment (10 6 –10 9 m 3 ). Although slope failures are common to the delta front, they may occur anywhere along the prograding prodelta surface, along the basin margins and over basement highs. Sediment accumulations in fjords have many similarities to small hydrocarbon-bearing basins, such as those of the North Sea, that once received submarine fans, often experienced anoxic conditions and contained high rates of sedimentation. Fjords typically contain more sand and conglomerate than other commonly employed modern analogue environments such as the Indus, Mississippi or Amazon fans. Two stratigraphic plays are suggested on the basis of fjord-infilling processes. The first involves the semicontinuous failure of foreset sands where, through progradation, gravity flow sands form an areally extensive reservoir (up to 10 km wide, 100 m thick and 100 km long) sandwiched between basinal muds and prodelta muds. The second play involves the catastrophic failure of a major portion of the prodelta where reservoir-quality sands (from either the basin-head delta or marginal delta-beach complexes) infill a slide-formed megachannel (up to 500 m wide, 50 m thick and tens of kilometres long). These megachannel sands would be underlain by faulted and disturbed mud blocks, and capped by conformable hemipelagic muds.