Air Escape Deformation

The (Permian age) Hopeman Sandstone (Inner Moray Firth, UK) was deposited as large-scale aeolian dunes. In places, the dunes pass up abruptly into what are thought to be air escape structures (Figure 1) formed during the fluvial flooding of the dunes (shortly after deposition) when they were unconsolidated. The timing could coincide with that of the Zechstein (Sea) transgression of the Central North Sea Basin (Glennie & Buller, 1983). Air was trapped in the pores in the dune’s core and compressed by the fluvial water outside.

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Figure 1. Deformed dune bedding - Covesea cliff section of the Hopeman Sandstone, Inner Moray Firth, UK. Large scale dune bedding (about 1 metre high) passes upwards into a slightly thicker unit of homogenised sands.

Figure 1. Deformed dune bedding - Covesea cliff section of the Hopeman Sandstone, Inner Moray Firth, UK. Large scale dune bedding (about 1 metre high) passes upwards into a slightly thicker unit of homogenised sands.

The most impressive deformation structure can be found in the cliffs below the Covesea coastguard station (Figure 2).

Figure 2. Vertical air escape structure (c. 20 m high) on Hopeman Sandstone cliffs just below the Covesea coastguard station. (Ken Glennie in the foreground). From Ogilvie et al (2000).

Figure 2. Vertical air escape structure (c. 20 m high) on Hopeman Sandstone cliffs just below the Covesea coastguard station. (Ken Glennie in the foreground). From Ogilvie et al (2000).

A fold axis shown on Figure 3 indicates that the top few metres of the sandstone was inverted while still a soft sand. Such a structure could form only if the sand had sufficient capillary strength (provided, as in sand castles, by damp sands whose pores were still filled with air) to prevent collapse. Sand uplifted by air escaping is interpreted to have slid (shear plane) down the windward slope of the dune towards the present sea (Figure 4).

Figure 3. Fold formed by water escape in aeolian dune sandstones (Permian Hopeman Sst, Inner Moray Firth, UK)

Figure 3. Fold formed by water escape in aeolian dune sandstones (Permian Hopeman Sst, Inner Moray Firth, UK)

Air escape along the small geo (1), south of the axis of overturning, caused uplift of adjacent damp bedding on the northern (windward) flank of the dune. Being unstable, the uplifted bedding then rolled back and slid down to the north along a shear plane, perhaps helped by local subsidence as sand at (2) was extruded from below.

Figure 4. Origin of overturned bedding in the Hopeman Sandstone. From Ogilvie et al (2000).

Figure 4. Origin of overturned bedding in the Hopeman Sandstone. From Ogilvie et al (2000).

References

Glennie, K.W. & Buller, A.T. 1983. The Permian Weissliegend of NW Europe: The partial deformation of Aeolian dune sands caused by the Zechstein transgression, Sedimentary Geology , v.35, pp.43–81.

Ogilvie, S.R. Glennie, K.W. Hopkins, C. 2000. Excursion guide 13: The Permo-Triassic sandstones of Morayshire, Scotland. Geology Today, Sept-Oct, 194-199.