3D facies architecture of flood basalt provinces and their internal heterogeneity: examples from the Palaeogene Skye Lava Field, The

The cliff section case study is divided into zones of geological intrafacies. Each geological intrafacies is represented by a range of rock property values (Fig. 7). The geological interpretation is schematically represented in terms of a primary velocity structure to highlight the geophysical heterogeneity (Fig. 1 Ic). Quantification of the image into pixel locations and numbers allows calculation of the areas of each intrafacies by thresholding the quantified pixels. The statistical variability in the geophysical rock properties can therefore be quantified for any cliff section using a combination of the image analysis and rock property data such as that presented in Figure 7. The most localized and most profound lateral geological intrafacics changes are those of the dyke and lava feeder. Although the rock properties of the dyke are very similar to those of other massive zones (e.g. sills), dykes are considered as geophysically distinct in such sections as they are vertical structures. High-velocity vertical structures affect the propagation of seismic waves differently, as only part of the wavefrpnts accelerates through the massive zone. The seismic waves on either side of the dyke structure are relatively attenuated in the surrounding medium, resulting in broken wavefronts.

The cliff section is dominated by compound-braided type olivinc-phyric basalts containing highly vesicular basal and flow top zones and massive core zones up to 1.5 m thick. Scismically, vesiculated zones exhibit low compressional-wave velocities of 3.0-5.4 km s^sup -1^ (Planke & Cambray 1998) and low bulk densities of 2.56-2.72 g cm^sup -3^. In contrast, the cores have

The development of thin boles on lava flow tops suggests that there were many minor extrusion hiatuses during the eruption of the flows in this sequence. Although thin (

Discussion

Lava volumes and eruption styles in the Skye Lava Field

The metre- to kilometre-scale structure of the Skye Lava Field in west-central Skye provides information about the lateral continuity, geometry and stacking patterns present in the architecture of the volcanic rocks. In this study, lava field scale observations suggest that we can divide the lavas into three main facies types, each with characteristic architectural sequences. The variations in the facies sequences have been established from vertical section logging through the lava field. The styles of volcanism have directly affected the architectures present throughout the sequence. The vertical change in architecture can be related to the evolution of the lava field. More primitive, olivine-rich flows assume complex architectures of the compound-braided system, effused passively, but constantly, with only minor hiatuses. Increasingly evolved lava flows are more vertically and laterally homogeneous, yet these flows are more laterally extensive and thicker. This observation is important as it suggests both an effusion rate and compositional link to the style of volcanic facies present.