نوع مقاله : مقاله پژوهشی
1 دانشیار ژئومورفولوژی دانشگاه شهید باهنر کرمان
2 گروه زمین شناسی دانشگاه شهید باهنر کرمان
عنوان مقاله [English]
The "badland" term refers to regions that have soft and poorly consolidated material outcrops, limited vegetation, reduced or no human activity and a wide range of geomorphic processes (Martinez- Morillo and Nadal- Romero, 2018). A specific character of these landscapes is their high rates of evolution. Although, these landscapes may be observed in any climate, but they commonly are seen in arid and semiarid areas. Lithology plays a key role in their formation. The most common lithologies creating badlands are mud rocks and marine marls (Faulkner, 2013), because such factors as fine particles and richness in Na+ cause their particles to be dispersed and become prone to erosion and piping. The main particles in badland- forming lithologies include clays and silts, since they are suitable for weathering, dispersion, piping and slope instability (Faulkner, 2016). Clay particles play a major role in permeability, contraction- expansion capacity, slaking, fissility and weathering profile of the materials creating badlands. Two- layer clays such as montmorillonite are more susceptible to dispersion than other clays.
There are four main type of badland, which include calanchi, biancane, pinnacle and pseudokarstic. The kind of badland formed in a specific area depends on such diverse factors as lithology, tectonic activity, climate, as well as topographic slope.
Pseudokarstic badlands are much less common than the other three types, mainly because they need vertical and horizontal pipes for development. These badlands have several apparent similarities with true karsts and this is why they are called pseudokarstic.
The prerequisite for formation of these badlands is development of pipes. Intensified erosion at the mouth of vertical pipes creates funnel- shaped depressions similar to miniature solution dolines. Enlargement of horizontal pipes may cause their roofs to collapse and collapse depressions take form. At a larger scale, it is possible that the entire roof of a horizontal pipe to collapse and a collapse gully will form.
Materials and Methods
In this study, the following investigations were undertaken.
1. Field investigation for discovering the characteritics of studied badlands.
2. The landforms constituting these landscapes were identified.
3. The hydrological system of the badlands was investigated.
4. The processes creating this landscape were studied.
5. A model for formation of the studied badlands is presented.
6. Morphomeric measuremens of landforms were undertaken.
7. The studied pseudokarstic badlands are compared with true karsts.
The study area lies at at a distance of about 85 km at the east of Kerman city. The playa surface on which the badland landscape has formed gently slopes towards the north and is 9.1 km in length and 1.7 km in average length.The south Golbaf playa is situated at the southern part of NNW-SSE trending Golbaf tectonic valley. This valley in bounded from the west and the east, respectively, by Abbarik and Sekonj mountains.
The average amounts of annual precipitation and temperature in south Golbaf playa are about 110 mm and 17 ºC, respectively. Therefore, it expriences a dry climate .The south Golbaf playa is a small pull-apart basin created by tectonic tension and subsidence resulting from right-step condition in Golbaf fault zone (Walker and Jackson, 2002). Actually, this playa is the product of silt, clay and some sand deposition by incoming ephemeral streams.
Results and Discussion
The material on which the studied badlands are formed is a loam deposit equivalent with mud rock. This material is fit for occurrence of badlands. Its minerals include gypsum, quartz, calcite, dolomite, albite, chlorite and montmorillonite. The exchangeable sodium percentage (ESP) value of these mud rocks is higher than 10, indicating a dispersible material suitable for piping.
There are a set of landforms which constitute the studied badlands. They include: erosion gullies, funnel- shaped depressions, collapse depressions and collapse gullies. The main processes operating on these badlands include weathering, erosion, slope failure, shrink and swell, dissolution, transport and precipitation. The main product of weathering is a thin regolith covering the loams. The operating processes acting on these badlands include rain splash, sheet wash, rill erosion and bank erosion in gullies. Additionally, tunnel erosion operates in the pipes which cause them to be enlarged. Pipe collapse, solifluction and creep are other processes affecting these badlands.
The studied badlands have formed from erosion of muds deposited in a pull apart basin which was formed as a result of right- step condition in two segments of the Gowk active fault. The thicknesses of these muds is 20m at maximum. This flat depositional environment became an erosional environment as a result of the depression being filled of muds and, the consequent overflowing of floods from the northwestern border of the depression. The resulting steep segment played as a knickpoint which moved backwards into the playa's flat environment, and incision of water courses. Incision of the playa surface created erosion gullies. The gullies devided the playa into a number of separated blocks which became dried as a result of drainage by erosion gullies. Drainage, ultimately, led to contraction of muds and creation of dessication fissures. Penetration of surface runoffs into these fissures, and the consequent erosion, created the pipe system. Also, intensified erosion at the mouth of vertical pipes created the funnel- shaped depressions. Morphometrically, the average depth and diameter of depressions are 1.6 and 2.8 meters, respectively. The studied pseudokarstic badlands have several similarities and differences with true karsts.
The studied pseudokarstic badlands have formed in mud rocks deposited in a small and shallow tectonic basin at 85 Km east of Kerman city, SE Iran. This environment shifted from depositional to erosional, as a result of being filled of sediments which caused overflow of the floods from a point at the north of the basin. The steep gradient resulting from overflowing played as a knickpoint which migrated upstreain. This led to erosion of playa surface by the passing flows. As a result, the playa deposits became dry and a network of contraction fissures were formed in them. The arrival of surface runoff into these fissures was led to depevopment of the pipe system.