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

Breakouts. Breakouts are an essential part of the pahoehoe inflation system, and are considered to be geophysically similar to inflated pahoehocs or pillow lavas. Thin alternating massive and vesicular zones typify each of these intrafacies types, as noted by their intrafacies components. Breakouts in Talisker Bay pahoehoe flows are observed to verge to the SW (Fig. 8g).

Lava feeder tubes and pipes. Rhythmically banded inflated basalts are cross-cut by large, ovate, concentrically banded lava structures in several parts of the bay area (Fig. 9). Several such structures also sit passively within the inflated flows (Fig. 8h) and are laterally impersistent, forming a low percentage of the bulk rock volume. These are interpreted to be lava tube conduits feeding the toe of the lava field. To the south of Loch Eynorl, structures similar to those described below are also observed in cliff sections. In the north-facing cliff section [NG 13125 82998] a large feeder tube reveals strong concentric banding and truncates the surrounding inflated sheet flows forming an erosive channel-like body (Fig. 9a). On the headland, another feeder consists of concentric layers of holocrystalline, olivine-rich basalt with a core region of c. 1.5m × 2.0m. The sheet flows are passively intruded by this pipe-like feature: banding is folded both above and below (Fig. 9b). This suggests thai the sheet flows into which the structure was intruding were still hot and able to deform plastically. The evidence for comparable bodies in the inflated stratigraphy suggests these bodies to be types of high heat flux lava feeder tubes, feeding the toes of the advancing lava field and inflating the basalts from within. The orientation of the feeders is of a shallow plunge towards the SW, consistent with the breakouts and pahoehoe textures. The most prominent lava feeder tube lies in the sea stack section [NG 13119 82997], where it truncates the surrounding lavas and invades them with basaltic veins. Strong striations on the tube walls plunge at the same attitude as the tube, indicating these to be a product of the magma movement (Fig. 10). The presence of a large number of lava tubes and pipes suggests that there was a well-established, high heat flux plumbing system active in the lower sequence lavas. The dimensions of the feeders imply that the Talisker Bay section may be proximal to a magma source that was erupting relatively small volumes from the direction of Loch Harport. The volcanic system may be considered to be analogous to those of modern-day Hawaii or Iceland, where the slow eruption rates (

Intrafacies and geophysical properties; a case study

In this section, we demonstrate how a cliff outcrop of lavas may be divided into zones of varying geological intrafacies, and how this geological heterogeneity relates to the geophysical properties of the sequence.

A case study outcrop section at the south side of Talisker Bay [NG 3122 2995] was selected for the high quality of exposure revealed in the lower architectural sequence of the 3D model described above (Fig. 11). Architectural complexity in the geology is at it highest level in this sequence, making it an ideal place to observe the juxtaposition of a variety of architectural intrafacics, which have major implications for geophysical studies. Studying the intrafacics components present in parts of the geological architecture allows prediction of the possible geophysical variability present in the outcrop section (Fig. II). To quantify the geophysical variability in the geological architecture, we must link the intrafacies present with geophysical rock properties (Fig. 7). Below, we present this case study using intrafacies as a system for predicting geophysical heterogeneity from the geology.