Abstract

Upper Aptian deep-water sandstones in the Witch Ground Graben of the North Sea have been extensively drilled and cored in the Britannia Field. The upper part of the Britannia Sandstone Member comprises a series of thick (~ 3.8 to 16.5 m) sandstone beds that thin and pinch out against a paleoslope formed by the Fladen Ground Spur to the north. Deposition was from unusually large-volume flows that carpeted the deep basin floor and base of the lateral slope with sand. The flows were able to heal significant seafloor topography produced during periods of local slope collapse. The recognition of widespread early remobilization is new and results in a revised event-bed correlation scheme and casts doubt on relationships between spatial patterns of grain-size distribution, facies distribution, and paleoflow used to underpin previous models of deposit emplacement. The beds are here interpreted as the deposits of large-volume, axially dispersing turbidity currents, probably some 70 km from source, and within 25 to 35 km of their run out limit to the south-southeast of the Britannia Field. Single event beds contain a wide range of sand-rich facies (deposited progressively from turbulent suspension), and clay-rich facies, previously termed "slurry" deposits (deposited in cycles of en masse deposition from dilute laminar flow alternating with phases of suspension fallout from turbulent flow). Along the northern fringe of the system clay-rich facies dominate, but pass southward over six or seven km into progressively cleaner sand-rich facies. This trend in facies distribution is at a high angle to the inferred SE-dispersing paleoflow and is thought to reflect lateral suppression of flow turbulence from flow axes to flow margins controlled by relative changes in flow depth. In addition, the vertical arrangement of facies within beds always shows sand-rich facies at bed bases, passing upward often via distinctive "banded" facies into clay-rich facies, indicating a temporal flow transformation from dominantly turbulent to dominantly laminar, with an intermediate phase of alternating laminar and turbulent flow. This temporal transition in inferred flow conditions is similar to the inferred on-axis to off-axis spatial transition in conditions. Thus single clay-rich sediment gravity flows can evolve in both time and space to develop different rheological zones, of turbulent, transitional, and laminar character. In addition, certain debritic, clay-rich facies are now interpreted to result from post-depositional remobilization from off the confining slope to the north, and must be differentiated from the primary suite of depositional facies.

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