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Production Geoscience Research Unit, Department of Geology & Petroleum Geology, King’s College, University of Aberdeen, Aberdeen AB9 2UE, UK
Late Jurassic sandstones form important potential reservoirs within the West Shetland Basin. These strata are thin (4–84 m) and broadly equivalent in age to the Kimmeridge Clay Formation. They were deposited predominantly in deep marine environments although subaerial and shallow marine facies are also present. The Rona Sandstone forms the lowermost of three sandstone units within the Upper Jurassic succession. Data from 23 exploration wells show, however, that it is a heterogeneous unit ranging from coarse-grained pebbly sandstones to mudstones and, because of this wide lithological variation, the Rona Member is considered a more appropriate name. The facies identified (cohesionless debris flow, high and low density turbidites, upper and lower shoreface) and their vertical transitions are considered typical of fan delta deposition, initially in a subaerial environment and later under progressively more marine conditions. Coarse-grained sediments (debris flow and high density turbidites) of the Rona Member are restricted to isolated topographic lows within Lewisianoid basement whereas finergrained sediments (low density turbidites) have a wide distribution across the basin and cover much of the Rona Ridge. Previous models have suggested that the coarse-grained sediments were derived from active fault scarps. However, to explain the facies distribution it is suggested that footwall uplift on the Rona and Judd Faults (Rona Ridge and associated basement highs) was responsible for the supply of coarse-grained material to isolated fan deltas. Transgression eventually resulted in: flooding of the basement highs, cut off of sediment supply and deposition of finer-grained sediments. Deposition of the Rona Member is interpreted to have been controlled by the gradual transgression of residual islands of Lewisianoid basement.
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