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

Journal of the Geological Society, Nov 2004 by Single, Richard T, Jerram, Dougal A

Abstract:

Quantifying the facies architecture of flood basalt provinces is important as it can be used to understand the physical volcanology and rock property variations throughout the lava sequence. The 3D facies architecture and internal heterogeneity of the Skye Lava Field, for example, provides important insights into the evolution of the British Tertiary Igneous Province, and valuable information to aid in the exploration of potential offshore reservoirs underlying significant flood lavas along the North Atlantic margin. The volcanic stratigraphy of the Talisker Bay area of Skye comprises: (1) lower compound-braided lavas (flow lobes

Keywords: Skye Lava Field, flood basalts, 3D modelling, rock properties, structure.

Flood volcanism represents the largest outpourings of volcanic material that the Earth has experienced during the Palaeozoic (Mahoney & Coffin 1997). These occurred as short-lived, punctuated events that potentially had a profound effect on the Earth's ecosystem on a regional and global scale (Wignall 2001, and references therein), and also provide information about the dynamic behaviour of the Earth's interior (e.g. Courtillot et al. 1999; Hawkesworth et al. 1999). Flood volcanic sequences (commonly known as flood basalts) are often subdivided into stratigraphie packages by variations in the geochemical signatures across lava fields in both vertical and lateral space (e.g. Saunders et al. 1997; Marsh et al. 2001), and less commonly by sedimentary units (e.g. Anderson & Dunham 1966; Williamson & Bell 1994). In these and other cases, the detailed internal architecture of the flood basalt stratigraphy has often been neglected, and our understanding of the physical geometries present within flood basalts is limited to a few recent studies (e.g. Self & Keszthelyi 1997; Jerram et al. 2000; Planke et al. 2000; Jerram 2002; Peate et al. 2003).

Additional motivation for characterizing the detailed physical features of flood basalts comes from the substantial recent interest in subvolcanic hydrocarbon reservoirs by the petroleum industry (Jerram et al. 1999). This interest has increased the requirement for understanding the 3D organization of volcanic successions so as to enhance geophysical subvolcanic investigation methods. Fundamental to this cause is the development of an understanding of the internal and external shape of flood basalts and of the internal variability of facies and rock properties within the architecture (Planke et al. 2000; Jerram & Robbe 2001; Jerram 2002). Central to the study of facies variations on all scales in flood basalts is the characterization of these potential facies variations on the scale of individual lava flows, as these form the building blocks of the larger-scale flood basalt sequence (Jerram 2002).

In the present study we characterize the 3D facies architecture of a portion of the Palaeogene Skye Lava Field, concentrating on the lava flow stacking patterns and the visible internal facies heterogeneities preserved within individual flows, and using the Minginish district as a case study. Detailed geospatial digital mapping techniques are used to build a 3D model of the lavas. To quantify the internal facies heterogeneities, we have devised a classification scheme for small-scale facies interpretation of flood basalts with emphasis on their geophysical importance and implications. This scheme is referred to as the intrafacies classification scheme. In discussion, the aims are to demonstrate how this scheme may be utilized by geologists to estimate geophysical rock properties and develop our understanding of the facies building blocks that constitute flood basalt sequences.

Lava field scale facies architecture of the Minginish district

Geologists and geophysicists aim to accurately constrain the architecture and facies variations at the lava flow scale. To achieve this, case study areas require the lava sequence to be well exposed in 3D outcrops. In the United Kingdom, the British Tertiary Igneous Province comprises lavas, central complexes and dyke swarms of Palaeogene age along the western coast of the British Isles. The British Tertiary Igneous Province can therefore provide detailed case studies on flood basalt facies architecture and the structure of associated basins through well-exposed 3D sections. The general structure of the Sea of the Hebrides Basin (Butler & Hutton 1994) and the Skye Lava Field (England 1994) have been documented; however, studies of the internal facies architecture and structure of the igneous sequences are scarce excepting the more detailed stratigraphie work of Williamson & Bell (1994). Within the British Tertiary Igneous Province, Skye provides an ideal starting point for flood basalt facies studies for the following reasons.

(1) Skye contains substantial well-exposed sections through a flood lava sequence, including dykes, sills and a central volcanic complex.

(2) Geochronology is well constrained by field relationships and various dating techniques (Pearson et cd. 1996; Jolley 1997; Hamilton et ill. 1998).