Study Area
Jebel Shams is the higest peak in Oman in the Al Hajar mountains (Fig 1). Driving through this area you are surrounded by vast Karst topography in limestone (Mid Cretaceous Natih Fm). This forms as a result of the dissolution of calcium carbonate by rainwater.
Fig. 1 (a) Study area in Oman in the Al Hajar Mountains (b) Jebel Shams is the highest peak in Oman and on the flanks of an eroded anticline.
From a distance it looks like a vast landscape of monotonous blocks of limestone (Fig 2a). Close up we see a distinctive pattern of blocks surrounded by joints (Fig 2b) which contain numerous geological structures. The solution enhanced fractures are often regional joints. In the UK, I’ve heard these being called “grykes” and the blocks themselves “clints”. The shape of the clints is dependant upon the frequency and pattern of grykes. https://www.clarelibrary.ie/eolas/coclare/places/the_burren/limestone_pavements.htm
The surface of the “clints” have a very distinctive weathering pattern which looks like elephant skin (Fig 3). Its more official name is Rillenkarren [1] which I learnt from a collegue. When I saw this pervasive pattern first at this outcrop, I wondered if it was a result of tectonism ? Plumose markings perhaps ? But nothing to do with tectonism as is a weathering (post tectonic) pattern. It forms through dissolution of limestone, although other factors (environmental) such as slope and soil cover may play a role in their evolution [1]. They are part of a spectrum of karsitic structures such as dolines, sinkholes, caves & pitts.
Fig 2. (a) Karst topography in Jebel Shams area (b) Close up of a regional joint network. Note the sheet-like matrix blocks, numerous calcite veins, shells and weathering patterns.
Fig. 3. MAP VIEW: Rillenkarren on the surface of a limestone block, Natih Fm (Oman). The limestone in (b) is a shelly limestone. Note the calcite veins running across the more recent rillenkarren structures.
It seems that these weathering patterns are ubiquitous, often occuring at much larger scales than the above examples and form upstanding ridges in Madagascan carbonates towering above the treeline. Included are equally impressive examples from Italy in Fig 4.
Fig. 4. Larger examples of rillenkarren from Tiscali in Italy.
https://www.flickr.com/photos/banco_imagenes_geologicas/5020973753
The tension gashes in Fig. 5. are much older and are tectonic structures. We looked at those in a previous blog…http://www.ogilviegeoscience.co.uk/blog/2021/10/6/veins-as-shear-indicators
The veins in Fig. 5 are arranged in a zone. Some have a characteristic sigmoidal shape showing that they rotate from they moment they form (simple shear mechanism) - this geometry can be used to determine the sense of shear - in this case it is sinistral (to the left).
There are some younger structures which cross-cut the veins - these have a relatively small displacement as indicated by the blue arrows on Fig. 5. Interestingly, the movement at this later time is right lateral or dextral. Holland et al [1] state that there are several generations of fault and fracture sets and that calcite cement heals the fractures before the next sets were formed.
Fig 5.MAP VIEW: Calcite veins in limestone. Some have characteristic sigmoidal shape indicating a sinistral shear direction (red arrows). Later dextral faults with a subtle displacement are shown alongside the blue arrows.
Another example from the same locality is given in Fig. 6. The sigmoidal shape is not as well developed as in the previous example - neverthess a sinistral sense of shear can still be established.
Fig 6. MAP VIEW: Another example, although the shapes are not sigmoidal.
In conclusion..
A variety of recent and old structures can be found on the karst limestone blocks of the Natih Formation, Oman. These include solution enhanced joints, rillenkarren and tension gashes.
References
[1] Shah, R.A, Jeelani, G, Goldscheider, N. 2018. Karst Geomorphology, Cave Development, and Hydrogeology in the Kashmir Valley, Western Himlaya, India. Acta Carsologica, 47/1, 5 - 21, Postojna 2018.
[2] Holland, M, Urai, J.L, Muchez, P, Willemse, E.J.M. 2009. Evolution of fractures in a highly dynamic thermal, hydraulic, and mechanical system - (I) Field observations in Mesozoic Carbonates Jabal Shams, Oman Mountains. GeoArabia, vol 14, no. 1, 2009, 57-110.