Integrated photogrammetry, lava geochemistry and palynological re-evaluation of the early evolution of the topographically constrained Mull Lava Field, Scotland

Abstract

Photogrammetry was used to elucidate complex strata relationships between isolated outcrops of the Palaeocene lava fields of SW Mull, part of the North Atlantic Igneous Province. Subsequent sampling for lava geochemistry and palynology was undertaken based on interpretation of these photogrammetry models. Coherent units of Plateau- and Staffa-type lavas were identified using lava geochemistry, in particular using rare earth elements (REEs), divisions supported by multivariate statistics. Lavas with three different REE compositional clusters were identified within the Staffa magma type and four within the Plateau type of SW Mull. Understanding the distribution of these lava types was achieved using the interpreted photogrammetry models and analysis of prominent interbedded sedimentary rock units and their correlative unconformities. Probably the most renowned rocks of SW Mull are the thick columnar jointed lavas, including those of the Isle of Staffa. REE geochemistry reveals that lavas of both Staffa- and Plateau-type geochemistry occur as columnar jointed facies associated with what has previously been attributed to the Staffa Lava Formation. Instead, the SW Mull Lava Field was initiated by eruption of Plateau-type lava into a fault-controlled valley. Subsequent eruptions of Staffa-type lavas partially infilled this structure, which was finally overfilled by a thick succession of younger Plateau-type lavas. The geochemical characteristics of this Plateau–Staffa–Plateau lava succession indicate that magma reservoirs deep in the crust were succeeded by shallow melts during a period of crustal extension. This phase of shallow melting induced topographical instability and formation of sedimentary interbeds and correlative unconformities that characterise the Staffa-type lava succession. Subsequent compressive tectonics forced a return to deep crustal melts. Interbed palynofloral compositional data indicate that eruption of the later Plateau-type lavas likely took place within a period of ~2.5 million years during the Selandian to early Thanetian period.