Samarium-neodymium isotopic constraints on the provenance of the Brent Group

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Abstract
Samarium-neodymium data for the Brent Group of Gullfaks oilfield provide a detailed record of provenance. The Broom Formation, yielding relatively high Sm-Nd ages in the range 1700–1800 Ma, is interpreted as being derived from a proximal southwesterly source on the Shetland Platform. The Rannoch and lower Etive Formations, having lower Sm-Nd provenance ages (1550–1650 Ma) than the Broom, are interpreted in terms of easterly provenance involving transport westwards to Gullfaks by marine currents. The upper Etive Formation returns to the proximal southwesterly source. This source area grew southwards during initial deposition of the Ness Formation to encompass the Forties volcanic centre resulting in young Sm-Nd provenance ages in some Ness strata. Provenance ages in the upper Etive-Ness section thus have a large range from 1300 Ma to 1800 Ma. It is therefore suggested that the Brent Group was deposited by at least two major river systems: one draining an easterly source in southern Norway and the other draining a proximal southwesterly and southerly source in the Shetland Platform and the central North Sea.