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

Shorthand descriptive classification schemes in sedimentary sequences are in common use and have been developed from studies of fluvial systems (e.g. Alien 1983; Miall 1985). These schemes provide a method by which a field or well-site geologist (who may be studying core samples or drill cuttings) can interpret sedimentary facies by studying how descriptive architectural elements are associated in a series of rock units. In the classification for fluvial systems for example, centimetre-scale lithofacies are described and allocated a 'lithofacies code'. Subsequently, the associations of these lithofacies in the geological record are used to interpret the architectural elements and facies evolution of the succession.

In igneous sequences large textural variations, and subsequently rock property variations, exist within the individual facies units themselves, caused by, for example, the organization of distributions of vesicles within lava flow units. We can use the term 'intrafacies' to describe these variations within individual igneous structures, both intrusive and extrusive. Classification schemes for igneous architectural facies are uncommon, although Jerram (2002) and Planke et al. (1999, 2000) have provided systems of architectural subdivisions on the scales of kilometres to tens of kilometres, respectively. Self et al. (1997) have discussed several internal features of lava flows such as megavesicles, vesicle sheets and pipe vesicles. The associations of many of the observations of Self et al. (1997) have been used to invoke lava sequence development models, but are limited in their ability to predict heterogeneities in the geophysical properties of rocks.

The intrafacies scheme is useful as it provides a systematic shorthand notation for describing features in lava sequences. The work incorporates some of the concepts of Self et al. (1997), but aims to build a system that can provide a rapid interpretation tool for the field and well-site geologist as an aid to interpretation of igneous facies, which can be based solely on field or borehole observations. The system aspires to provide a distinct classification for rock property analysis so that pctrophysicists or geophysicists may improve the characterization of igneous sequences in both seismic and potential field data (gravity and magnetic studies). Small-scale components of the classification characterize the most basic level of lava sequence heterogeneity, typically at the centimetre to metre scale. Subsequently, the associations of these components are discussed as the intrafacies of igneous successions, along with their geophysical implications for subvolcanic investigation.

Intrafacies components offload basalt architecture

A series of intrafacies components have been recognized from observations made in the case study area of Talisker Bay. The components are the most fundamental level of field observation and form the constituent elements of intrafacies and the largerscale architecture of flood basalts (Fig. 6). The emphasis lies in description of architectural styles and textures at this smallest scale of investigation, noting features such as vesiculation, fracturing, presence of palaeosol beds (boles) and an account of the shape of the features and how they are juxtaposed. Interpretation of the intrafacies or geophysical rock properties is not essential at this stage in the characterization. An architectural intrafacies component is denoted by a letter within square brackets. For example, [v] denotes vesiculation and [J] indicates the descriptive component of regular jointing. At any particular part of an outcrop, several components will be present. The combination and association of the components are subsequently used to help constrain the metre-scale intrafacies interpretation.