The Effect of Sandstone Composition on Distribution of Tafoni Landforms in the Aghajari Sandstone, Northwest of Masjed Soleyman, Iran

The Aghajari sandstone layers are located in the west of Zagros Mountains from several centimeters thicknesses to maximum 6 meters with carbonating content. Laboratory and fieldwork show high amount of carbonate content through sandstone layers and tafoni and honeycombs (THs) in early layers. In the study area three parameters have the most effective impactful factors in tafoni and honeycombs (THs) including matrix, carbonate content, and porosity. In this study result shows overlays of high ranges of CaCO 3 , porosity, and low matrix in the early layers (especially in A, B, C, D, and H layers) with tafoni and honeycombs (THs). Overall, we conclude that matrix and CaCO 3 (carbonate clast including carbonate lithics, fragment fossils, and Pellet) and porosity have direct relationships and matrix reverse relationships with tafoni and honeycombs (THs) in the Aghajari sandstones layers.


Introduction
In sandstone landscapes, tafoni and honeycombs (THs) are the most common small-scale weathering forms on near-vertical bare rock surfaces [1]. Tafoni are the most important geomorphic landforms which were studied in the world [1][2][3][4][5][6][7]. It was observed that in the Mediterranean climate tafoni and honeycombs (THs) are more common and better developed in arid and semiarid climate conditions [8][9][10]. A wide range of conditions may control the formation and development of tafoni and honeycombs (THs) such as mineralogy [9]. Lithological controls have the greatest effect on the mechanical behavior of stones and are themselves controlled by factors such as mineral composition, type and amount of cement, matrix, and porosity of sandstones [3,[11][12][13][14]. These controls cause large differences on the earth surface [3]. Dissolution of chemically unstable grains such as feldspars and carbonates results in development of porosity in the rocks [15]. Also chemical processes cause changes in the structure of chemical rocks. For example, calcium carbonate is one of the parameters that are highly affected by chemical reactions [16]. So, if these minerals are present in the rocks, they can cause development of specific landforms (such as tafoni and honeycombs (THs)) which are highly affected by chemical processes [2,3,9]. On the other hand, development of porosity in sandstones by dissolution of carbonate cement and calcium carbonate grains causes instability in the rocks and their fragmentation [11,15,17], thus reducing strength of the rocks and their breakdown can result in development of tafoni and honeycombs (THs). So, the type of bedrock and its composition control the rock erosion rate fundamentally [16,18,19]. Porosity and carbonate framework grains, matrix, and cement are four components of sandstones which greatly affect the morphology of these sandstones [16]. Therefore, the present study is an attempt to illustrate the relationship between the CaCO 3 content and role of porosity and matrix in tafoni and honeycombs (THs) development.  of 3.2 km 2 and is located between 32 ∘ 10 17 N and 32 ∘ 11 59 N and 49 ∘ 5 5 E and 49 ∘ 7 5 E. The highest elevation is about 301 m above sea level in the middle and northwestern part while the minimum elevation is about 218 m above sea level in the east and northeast ( Figure 1). The climate of the study area is Mediterranean [20,21], so the semiarid conditions with cool winters and dry summers prevail. Mean annual precipitation and mean annual temperature in the study area are 523 mm and 25.5 ∘ C, respectively [22].

Geological Setting
The Aghajari sandstone layers were produced by erosion of Zagros Mountains and deposited on the rivers and estuary environments [23]. They are a part of the Fars group including Gachsaran (lower Fars), Mishan (middle Fars), and the Aghajari (upper Fars) Formations [24]. The Aghajari sandstone layers are composed of 2 to 5 km thick gray and greenish sandstones [25]. The study area is located in Dezful embayment. The age of the Aghajari sandstone layers is determined as middle Miocene-upper Pliocene [23,26] (Figure 2).

Geomorphology.
Diversity of geomorphic landforms in the Aghajari sandstone layers is one of the most interesting properties of the formation. The most important landforms include crests, rivers, cliffs and bulkheads, stone hillsides, fault line (Figure 3), and various tafoni and honeycombs (THs) including basal tafoni, side tafoni, horn tafoni, and pseudotafoni ( Figure 4). Distribution of landforms indicates that stone hillsides and cliffs and bulkheads have NW-SE orientation throughout the study area. This is the general trend of Zagros Mountains that resulted from continental collision between Arabian and Iranian plates in the late Mesozoic [23,[27][28][29][30][31][32]. The area represented by tafoni and honeycombs (THs) is located in the southern half of the study area.

Materials
In this study, we took samples along eight layers which are named A to H. Samples A1 to A6 were taken from the oldest layer and samples H1 to H6 belonged to the youngest one ( Figure 5). Because the thickness of layers varied from place to place, the sampling interval changes from 50 to 150 meters. Landforms geomorphic map of the study area was prepared    In this reaction the amount of CO 2 of each sample was precisely (up to ±1 cc) recorded. Then 0.1 g of pure calcium carbonate sample (Merck) was used in the same reaction system under the same conditions and then results of CO 2 content were recorded as well.
The content of calcium carbonate in each sample was calculated according to the following formula:

Results
Composition of lithology in the study area includes quartz (34.45%), peloids (29.91%), lithics (mostly carbonate) (16.31%), feldspar (11.98%), and iron oxide (7.23%), and the dominant cements are calcite and dolomite (Figures 6(a), 6(b), and 6(c)). Mean calcium carbonate percentage, matrix, porosity, and carbonate clast in the study area are, respectively, as follows: 48.58%, 62.43%, 18.57%, and 28%. Calcium carbonate zoning map showed five zones with different ranges between a minimum of 26% and a maximum of 68% ( Figure 8). The section with a range of 46-56% occupies more than 60% of total study area. The section with a range of 26-36% comprises the smallest area which is shown by some dots in the map (Figure 8). The effect of CaCO 3 content on tafoni and honeycombs (THs) can be discussed as follows: usually most of tafoni and honeycombs (THs) features are observed in the layers with 36-56% of carbonate content in the region. In this zone, the diversity and frequency of tafoni and honeycombs (THs) depend on amount of CaCO 3 content, porosity, and matrix. Also there is a direct relationship between the number and diversity of tafoni and honeycombs (THs) with carbonate clast and porosity, strong inversely relationship to matrix (Figures 8, 9, and 10).

Discussion
Calcareous sandstones contain a significant quantity (10-50%) of carbonate grains, skeletal fragments, peloids, and ooids, with more than 50% carbonate grains [33]. Carbonate cements in sandstones consist of calcite, dolomite, and occasionally siderite [16]. Peloids (29.91%), calcite cement (19%), dolomite cement, fossils and skeletal fragments, and carbonate lithics in the study area (Figures 6(a), 6(b), and 6(c)) are highly affected by dissolution processes [34] (Figure 6(b)). And they almost represent the total CaCO 3 [33]. Dissolution of chemically unstable grains such as carbonates creates porosity in these rocks [15] (Figures 4(e) and 6(b)). Dissolution often is controlled by soluble minerals, for example, calcium carbonate [35]. The mean content of CaCO 3 (48.58%) indicates a relatively high amount of CaCO 3 in the Aghajari sandstone layers. Moreover, mean variation of CaCO 3 , matrix, and porosity are 12.17%, 37.33%, and 38.4%, respectively (Tables 1, 2, and 3). This comparison showed that the mean variation of CaCO 3 is less than matrix and porosity. By descending porosity and carbonate content and ascending matrix, the number and diversity of tafoni and honeycombs are reduced. Eventually matrix and carbonate content have controlling roles in amount of porosity that affected the tafoni and honeycombs (THs) through sandstone layers.

The Effect of CaCO 3 Content, Porosity, and Matrix on
Tafoni. Generally speaking, tafoni and honeycombs (THs) are observed in five layers (A, B, C, D, and H) (Figures 8,  9, and 10). Fieldwork and survey in the study area represent the fact that diversity and number of tafoni and honeycombs (THs) are high (Figures 4(a), 4(b), 4(c), 4(d), and 4(e) and 8, 9, and 10), and they covered about 65% of study area (a little more than 2 km 2 ) ( Figure 3). As seen in Figures 7(a) and 7(b), using trends, we found a negative association    between either carbonate content and porosity or matrix and porosity. In this regard, we can see that amount of matrix and carbonate content has a controlling role over the amount of porosity. This decreasing trend of porosity leads to decreasing the numbers and diversity of tafoni and honeycombs (THs) in youngest layer. Reversely, in the older layers tafoni and honeycombs (THs) are more observed (Figures 8, 9, and 10). Moreover, porosity zoning, CaCO 3 , and matrix maps show the amounts of 6.6-60%, 26-56%, and 6-80%, respectively (Figures 8 and 10). It is worthy of note that domination of tafoni and honeycombs (THs) is overlays with low ranges of matrix and porosity maps in the region (Figures 9 and 10).

Conclusion
In the Aghajari sandstone layers, mean calcium carbonate percentage, matrix, porosity, and carbonate clast in the study area are, respectively, as follows: 48.58%, 62.43%, 18  honeycombs (THs) and their diversity are concentrated in the layers in which porosity zoning, CaCO 3 , and matrix maps represent amounts of about 6.6-60%, 26-56%, and 6-80%, respectively. Result shows tafoni and honeycombs (THs) are overlays of high ranges of carbonate content, porosity, and low matrix in the early layers (especially in A, B, C, D, and H layers). Almost, matrix, porosity and CaCo 3 (carbonate clast) controlling diversity of tafoni and honeycombs (THs). Overall, there are direct relationships between CaCO 3 (carbonate clast including carbonate lithics, fragment fossils, and Pellet) and porosity and reverse relationship matrix with tafoni and honeycombs (THs) in the Aghajari sandstones.