Submarine fans of Late Palaeocene and Early Eocene age form important hydrocarbon reservoirs in the Bruce-Beryl Embayment, northern North Sea. The Early Eocene fans are the main reservoirs in the Forth-Gryphon oilfields and in the giant Frigg gasfield. Significant oil discoveries have also been made in Late Palaeocene fans. Forth and Gryphon lie on the flanks of the Crawford Anticline, a drape structure that developed during the Palaeocene above the crest of a Mesozoic tilted fault block. The Early Eocene fans pinchout against the flanks of the anticline implying continued growth of the structure throughout the Eocene. Growth was accompanied by the development of major gravity slides that detached in a sequence of altered, basaltic tephras at the base of the Eocene sequence. Seismic-scale, post-depositional deformation (sandstone diapirism and the intrusion of clastic sills and dykes) connected with this sliding dramatically modified the original depositional geometries of the fans. A detailed account of the deformation features, illustrated with core, wireline log and three-dimensional seismic data is presented together with a discussion of their exploration/appraisal significance.

SUMMARY The Woo Dale Limestones Formation is the oldest of the exposed shelf limestone units on the Derbyshire Block. It occurs in four inliers: Peak Forest, Wye Valley, Hartington and Alstonfield. Three members are recognised; the lowest, the Vincent House Member, is present throughout the area and consists of packstones and grainstones with a diverse bioclast assemblage. It represents deposition in a shallow open shelf environment. The Topley Pike Member comprises principally mudstones and wackestones with a restricted fauna. Fenstrate fabrics, desiccation cracks and pseudoanticlines indicate deposition and early diagenesis in a tidal flat environment. The uppermost member, the Dam Dale Member consists of dark-coloured packstones and grainstones with a bioclast assemblage intermediate in diversity between the Topley Pike and Vincent House Members. It represents deposition in a partially restricted lagoon. Tidal flat deposits occur higher in the sequence to the south (Wye Valley, Hartington) than in the north (Peak Forest). This is thought to indicate that tidal flats migrated roughly southwards with time as a result of progradation. The contact of the Woo Dale Limestones with the overlying Bee Low Limestones appears conformable at Peak Forest but unconformable at Wye Valley and Hartington. The relationship at Alstonfield is uncertain.

Dolomite occurs in open‐shelf packstones and grainstones of the Vincent House Member of the Woo Dale Limestones Formation (Lower Carboniferous) in the Wye Valley area, east of Buxton, Derbyshire, Central England. Two stages of dolomitization can be recognized. Stage 1 consists of limpid, non‐luminescent dolomite lacking detectable iron and manganese. Stage 2 comprises cloudy, inclusion‐rich, orange‐red luminescing dolomite with significant iron and manganese. The contact between the two stages is irregular and suggests a hiatus between dolomite growth stages. Dolostones lacking any unreplaced limestone may contain up to 35% calcite cement rilling intercrystal porosity. This cement shows a characteristic zonal sequence and is normally in optical continuity with the surrounding dolomite crystals. In some cases, precementation compaction of the dolomite mosaic caused fracturing of the crystals. Sherds of dolomite on the floors of pore‐spaces then provided the nucleus for growth of calcite cement. The petrographic character and stratigraphic distribution of the dolomite suggest that dolomitization occurred at depth, during burial of the Woo Dale Limestones in the Late Carboniferous. Dolomitizing solutions, expelled from basinal shales, moved up‐dip along permeable limestones or along the sub‐Carboniferous unconformity. Fluids responsible for the generation of stage 2 dolomite may have been contaminated by ions such as iron and manganese released from the alteration of volcanic rocks which occur beneath the Carboniferous Limestone in the Wye Valley area.

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