Palaeocene volcanic activity is represented in west-central Skye, Inner Hebrides, Scotland, by a laterally extensive and thick pile of sub-aerial lavas mainly belonging to the alkali olivine basalt—hawaiite—mugearite—benmoreite—trachyte suite. The lavas are typical of many continental flood basalt suites and were principally fed from fissure eruptions similar to those of present day Iceland. Intercalated with the lavas are rare beds of heterogeneous volcaniclastic material, including breccias, conglomerates, sandstones and mudstones. The sequence forms a major portion of a larger volcanic field preserved within the NNE-SSW-elongated ‘Sea of the Hebrides’ sedimentary basin.Significant hiatuses in the volcanic activity are marked by deep-weathering profiles and thin sedimentary sequences comprising mudstones, ironstones, coals, sandstones and conglomerates. Palaeocurrent indicators and clast lithologies within the clastic sedimentary rocks indicate that erosion of a massif dominated by the Palaeocene Rum Igneous Complex and its roof rocks, c. 20 km to the S, provided abundant detritus to a river system which drained towards the N. Such sedimentary intercalations aid the stratigraphical subdivision of the lava field. Eight lava groups, each most likely with a different focus of fissure eruption, and divisible into mappable formations, together with two sedimentary formations, are recognised.The alkali olivine basalts are typically thin, with a tendency to form compound flows with limited lateral extents, whilst the hawaiites and mugearites are considerably thicker and cover large areas. Only very rarely are flow terminations observed. The original extents of the single benmoreite and rare trachytes cannot be determined from their limited erosional remnants. The more evolved flows tended to occur after brief hiatuses in the volcanic activity, indicated by well-developed lateritic tops to the underlying flows.The youngest preserved lava is a columnar-jointed olivine tholeiite with a MORB-like composition. The flow is at least 120 m thick and apparently ponded in a steep-sided palaeo-valley within the lava field.Three fault trends are recognised: parallel, normal and marginally oblique to the main NW-SEtrending regional dyke swarm, and dissect the lava field into a number of discrete blocks. The more significant of these faults may have been active during the development of the lava field, and in some instances instrumental in controlling the distribution of the flows.Later Tertiary erosion has removed an unknown thickness of material from the upper part of the lava field, the preserved thickness of which is estimated to be about 1·5 km.

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Synopsis The name ‘Staffa Lava Formation’ is proposed for the early sequence of volcanic and associated sedimentary rocks of the Palaeogene Mull Lava Field in the Inner Hebrides, off western Scotland. The Formation is defined as the sequence between the sub-Palaeogene unconformity and the base of the so-called (Mull) Plateau Lava Formation. Observations across the outcrop reveal that the Staffa Lava Formation is a complex, laterally variable sequence of lava facies, hyaloclastites and other breccias, and various interflow sedimentary rocks; also present are peperites and a single occurrence of silicic ignimbrite. The sedimentary rocks and associated palaeo-surfaces represent significant pauses in the volcanism, enabling us to subdivide the Formation into a number of allostratigraphical units termed ‘genetic sequences’. Each sequence is essentially a couplet comprising a ‘basal’ sedimentary unit and an ‘upper’ volcanic unit. In detail, some sequences comprise more than one, laterally restricted, sub-couplet. There are systematic patterns in both the distribution and thickness of the sedimentary and volcanic lithofacies that make up these units. The individual sequences appear to show a strong structural control of thickness, distribution and lithofacies, which we link to palaeo-topographic effects, especially their position within existing Palaeogene fault-controlled valley systems or active, syn-volcanic graben. The presence and significant role played by contemporaneous drainage systems and topography during the evolution of the Staffa Lava Formation is demonstrated by the number and variety of intercalated sedimentary units and the nature and facies of the volcanic products. The clearest example of the latter is the close association of hyaloclastite breccias and thick, impounded lava flows exhibiting classical two-tiered columnar joint sets. Palaeontological data, most especially palynological analysis, allow further detailed ecological characterization of the sedimentary units and palaeo-surfaces, and the development of a model for the early landscape evolution of the lava field. The taxa associated with the sedimentary sequence at the top of the Formation are typical of the early Eocene and contain floral elements characteristic of Paleocene–Eocene Thermal Maximum sites throughout the NE Atlantic region.