The ingress of meteoric fluids into the crests of salt structures typically results in the formation of anhydrite-dominated caprock. The migration of connate fluids up the margins of salt structures has also been documented, although the products related to this are rarely sampled in the deep subsurface. We use 3D seismic reflection and borehole data to document the geometry and lithology of a salt diapir. Seismic data suggest that a weld is developed along the diapir stem, although borehole data indicate that the stem consists of an inner, c. 1500 m thick, halite-dominated zone, and an outer, c. 250 m thick, anhydrite-dominated ‘sheath’. The anhydrite may have formed early in the basin history, as a depositional anhydrite or crestal caprock. An alternative interpretation is that the anhydrite represents ‘lateral caprock’, which formed late in the basin history in response to the migration of NaCl-poor fluid up the margins of the diapir and dissolution of halite. The possibility that lateral caprock may form adjacent to salt structures has implications for understanding the patterns and vigour of groundwater flow in sedimentary basins. Furthermore, our study shows the margins of steep-sided salt structures may be misidentified by several hundred metres if time-migrated seismic reflection data are used.

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