Week5 - Unconformities
Unconformities represent periods of non-deposition or erosion in the rock record. A number of types of unconformities can be recognised, each with a different mode of development (Figure 1).
A
B
C
D
Fig. 1. Processes involved in the formation of unconformities (from Boulter 1989)
The simplest (and most cryptic)type of unconformity is where there is no angular discordance between the beds above and below the unconformity, but a time gap is identifiable. This may be because of a change in relative sea-level that caused deposition to stop for a time (Figure 1C), or because of active erosion that removed previously deposited beds (Figure 1D).
Unconformities are also generated during tectonic events that lead to folding (Figure 1B), such as collisional mountain building events – in these cases the unconformity may be very obvious indeed. An intermediate type of unconformity-generating process is where small-scale tectonic activity, such as fault-block rotation, causes tilting and erosion of beds prior to renewed deposition (Figure 1A).
The principal types of unconformity are illustrated in Figure 2 below. An angular unconformity involves tilting of the underlying beds prior to erosion and deposition of new beds above them. A heterolithic unconformity is where igneous and/or metamorphic rocks have been eroded and sedimentary rocks deposited upon them.
Fig. 2. The principal types of unconformity. Non-depositional unconformities are sometimes also referred to as disconformities (from Boulter 1989)
Unconformities in which marine rocks are deposited above the surface of non-deposition typically have planar unconformity surfaces, because the erosion surface is planed off by the advancing marine transgression. In contrast, unconformities developed in terrestrial environments may have considerable erosional relief, and are sometimes referred to as landscape unconformities (Figure 3).
A good example of this type is the Lewisian Complex–Torridon Group unconformity in NW Scotland, which has a total relief up to 600 m, with vertical cliffs in the unconformity up to several hundred metres high in places.
Fig. 3. A non-planar unconformity of buried landscape type, where a deeply eroded surface has been infilled by later sedimentation (from Boulter 1989).
Unconformities create distinctive map patterns, and a key skill in the interpretation of geological maps is to be able to recognise these patterns in two-dimensions, to be able to visualise them in 3D, and to be able to interpret them in four dimensions.
The two principal patterns to recognise are overstep and overlap (also called onlap) (Fig. 4),which refer to the geometry of beds above the unconformity surface relative to those beneath it. If the oldest bed above an unconformity cuts across several beds below the unconformity, because the latter are dipping more steeply, this is referred to as overstep. In contrast, overlap occurs where the depositional area is enlarging because of a sea-level rise and/or an enlargement of the subsiding part of the basin. As a result, in overlap unconformities, successive beds above the unconformity occupy a larger area than the underlying ones (Figure 4).
Fig. 4. Overstep and onlap above unconformity surfaces. The unconformity surface may also be marked by coarse-grained sedimentary rocks such as conglomerates – sometimes referred to as a ‘basal conglomerate’ (from Boulter 1989).
In plan view (map view) unconformities are readily recognised by the truncation of boundaries.The truncation of a geological boundary on a map can occur for one of only three reasons: 1) faulting, 2) unconformities or 3) discordant igneous intrusions. The truncation of beds at a non-faulted junction where the beds are in contact with other sedimentary or extrusive igneous rocks can only indicate an unconformity. Unconformities may themselves be folded by later events (Fig. 5).
Fig. 5. A planar unconformity surface that has been folded by later tectonic activity. A cross-section of this geometry would give unequivocal evidence of two phases of deformation (from Boulter 1989).
Construction of subcrop maps beneath unconformities
It is often useful to be able to predict the position of a given geological boundary beneath an unconformity, where it is covered up by the younger rocks above the unconformity (Fig. 6).
Fig. 6. The subcrop of a truncated fold beneath a planar unconformity (from Lisle 2004).
Fig. 7. A, Map of a coal seam that is overlain with angular unconformity by Permian sandstones. B, The subcrop of the coal beneath the sandstones (from Lisle 2004).
For planar unconformity surfaces this is a relatively simple exercise since the intersection of the boundary with the unconformity is the intersection of two planes and can thus be plotted in the same way as a fold hinge (the intersection of two fold limbs) or fault cutoff lines (the intersection of a boundary with a fault plane). Stratum contours are constructed for the base of the lowest bed above the unconformity (= the unconformity surface) and for the geological boundary that is truncated by the unconformity. Once this is done, the intersections of stratum contours of equal height for the two surfaces are marked. The subcrop will extend from the point of truncation of the boundary at the unconformity on the map through the successive points of intersection of the stratum contours (Fig. 8).
Fig. 8. The construction of the subcrop of a geological boundary beneath a succession that unconformably overlies it (from Lisle 2004).
(Sincere thanks to Dr Carl Stevenson of the University of Birmingham for providing this document)