How are folds broadly classified ?
Folds tend to be either tectonic or non-tectonic in origin [1]. We tend to think of most folds forming as a result of horizontal compression, some vertical movements or as a result of salt intrusion (tectonic), but many folds form as a result of gravity or even soft sediment deformation (non-tectonic). These can also be of economic significance i.e., traps for petroleum.
Fig. 1. Location of the Hopeman sandstone Outcrop on the southern shores of the Moray Firth, UK.
Where is the outcrop ?
Examples here are given from the Permian Hopeman Sandstone outcrop (Fig. 1) which were first documented in this area by Glennie and Buller [2] although reported earlier in other areas [3]. These are aeolian dunes (known as barchans) that were deposited during the Permian period [4]. In places, the dunes pass up abruptly into what are thought to be air escape structures (Fig. 2) 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 [2]. Air was trapped in the pores in the dune’s core and compressed by the fluvial water outside.
Fig. 2. Deformed dune bedding at the Covesea cliff section at eastern extremity of the outcrop. Dune bedding (about 1 metre high) passes upwards into a unit of homogenised and folded sands.
Saucer-like structures ?
Closer to Hopeman village (GR 15297015) a small number of concave, saucer-like structures occur (Fig. 3) [2]. If you studied these in isolation you may interpret them as tectonic folds. However, they occur as part of a series of soft-sediment deformation structures that form consistently above undeformed dunes sandstones. Unlike the upper unit in Fig. 2, these structures have well preserved, concordant bedding, which has subsided into underlying quicksand. What were the conditions at the time ? Flash flooding and/or a marine transgression ? Deformation has certainly taken place in a sub-aquatic environment as evidenced by the upward gradation of the deformation sequence with little angle discordance into a thin bed capped with shallow water ripple marks [2].
Vertical Escape Structures ?
Another example of soft-sediment folding at the same level is given on Fig 4. At first glance, the folds on the left appear to be tectonic folds [5] but there is some uplift of bedding at B suggestive of air/water escape. There is also a truncation surface (A) just above the level of folding.
Fig 3. Subsidence Saucer. Formed either through sagging of its axis or uplift at flanks. Interestingly, the uppermost beds which contain ripple marks are anticlinal. Photo from Ken Glennie shared with myself in a field guide that we wrote together.
Fig. 4. Small folds in the Hopeman Sandstone. Note the upturn of beds at B suggestive of the air/water escape. ‘A’ is a truncation surface.
Overturned Folds with well defined shear planes
A fold axis shown on Fig. 5 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 on Fig 5 lower) down the windward slope of the dune towards the present sea (Fig. 6).
Air escape along the small geo (1 in Fig. 6), 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 in Fig. 6) was extruded from below.
Fig. 5. Overturned fold formed which must have been damp at the time of deformation. Red rectangle in upper photo shows hammer scale. In lower photo - red dashed line give outline of the fold, green dashed lines are vertical joints. Shear plane in black with direction of shear by red arrow.
Fig. 6. Origin of overturned bedding in the Hopeman Sandstone [4].
References
[1] Online material for examples of tectonic and non-tectonic folds https://slideplayer.com/slide/15901602/#:~:text=CAUSES%20OF%20FOLDING%20b)%20Non%2DTectonic%20Processes%201.,shale%2C%20dipping%20into%20the%20hillside.
[2] 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.
[3] Doe, T.W and Dott, R.H. 1980. Genetic significance of Deformed Cross Bedding - with examples from the Najavo and Weber Sandstones of Utah. Journal of Sedimentary Petrology, v. 50, No. 3, p 793 - 812.
[4] 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.
[5] Peacock, J.D. 1966. Contorted beds in the Permo-Triassic aeolian sandstones of Morayshire. Bull. Geol. Surv. G.B. 24, 157 - 162.