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.

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