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

Each of the intrafacies components displayed in Figure 6 may be observed without the use of specialist equipment at the outcrop or field specimen scale. Our classification aims to maintain consistency in scale, such that these intrafacies components may be integrated to form larger, lava flow scale intrafacies associations as discussed below.

The intrafacies offload basalt architecture

Intrafacies may be interpreted, to a large extent, on the basis of the natural association of the architectural intrafacies components outlined above (Fig. 7). These are similar in scale to the features described as architectural elements by Miall (1985), and are of the same scale as the subdivision of lava flows by Self et cil. (1997). Although several of the intrafacies in Figure 7 contain similar intrafacies components, they are usually distinguished by a combination of the components within the geometries observed. In the following section, field examples of flood basalt intrafacies are cited from Talisker Bay.

Intrafacies examples in the sequence of Talisker Bay

In the flood basalt sequence of Talisker Bay, we can observe most of the intrafacies listed in Figure 7. Heterogeneities in the distribution of rock properties through such igneous sequences may strongly affect the performance of geophysical remote sensing techniques as a result of the variability in their rock properties over centimetre to metre scales (Planke & Cambray 1998). The key geological intrafacies that affect the ability of geophysical methods to image through igneous successions are highlighted in Figure 7, but particular facies to note are:

(1) boles: high attenuation (Q^sub s^ and Q^sub p^) in seismic surveying;

(2) flow tops and bases: low-velocity, low-density zones;

(3) flow cores and massive sheets: high-velocity, high-density zones;

(4) sills: thick high-velocity zones with high bulk density;

(5) dykes: vertical high-velocity, high-density sheets.

It is important to recognize how the varied geological intrafacies of the volcanic sequence may affect remote sensing techniques, as they are volcanic sequence geophysical heterogeneities. These invariably cause seismic waveform scattering and degeneration as a result of factors such as surface geometry (rugosity) and acoustic impedance contrasts through the lavas. These heterogeneities reduce the quality and resolution of seismic data and also affect gravity modelling. The major intrafacies features of the Talisker Bay rocks are now discussed, together with their geophysical implications.

Dykes. Although Talisker Bay is 89 km SW of the axis of the most intense dyking seen in the Skye Swarm, several dykes are exposed in the cliffs and the surrounding hillsides (Fig. 8a). The dykes assume the common trend of NW-SE and cut through the entire volcanic succession. A compound dyke cuts the north-facing cliff section containing evidence of multiple injection episodes. The walls are columnar jointed and aphanitic, and internally the massive core is texturally zoned. The geophysical rock properties of the dyke are interpreted to be similar to those of a massive degassed sheet flow or sill; however, the steeply dipping orientation of dykes severely affects acquisition of seismic data.