Salty Matters

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Salt Dissolution (3 of 5): Natural Geohazards

John Warren - Tuesday, October 31, 2017


Introduction

Surface constructions and other anthropogenic activities atop or within evaporite karst terranes is more problematic than in subcopping carbonate terranes due to inherently higher rates of dissolution and stoping (Yilmaz et al., 2011; Cooper and Gutiérrez, 2013; Gutiérrez et al., 2014). Overburden collapse into nearsurface gypsum caves can create stoping chimneys, which break out at the surface as steep-sided dolines, often surrounded by broader subsidence hollows. Such swallow-holes, up to 20 m deep and 40 m wide, continue to appear suddenly and naturally in gypsum areas throughout the world.

Unlike the relatively slow formation of limestone karst, gypsum/halite karst develops on a human/engineering time-scale and can be enhanced by human activities (Warren, 2016, 2017). For example, in 2006, the Nanjing Gypsum mine in China broke into a phreatic cavity in a region of gypsum karst, driving complete flooding of the mine in some three days. Associated groundwater drainage caused a sharp drop in the local piezometric level of up to 90 m in a well in nearby Huashu village. Resultant ground subsidence severely damaged nearby roads and buildings (Wang et al., 2008). In Ukraine, dewatering of gypsum karst to facilitate sulphur mining substantially increased the rate of gypsum dissolution and favoured the expansion of sinkholes within an area affected by the associated cones of water-table depression (Sprynskyy et al., 2009). Natural evaporite karst enhanced by intrastructure focusing of drainage creates the various scales of problem across the Gypsum Plain of West Texas and New Mexico (Stafford et al., 2017).

Although halite is even more susceptible to rapid dissolution than gypsum, it typically is not a major urban engineering problem; large numbers of people simply do not like to live in a climate that allows halite to make it to the surface. However, in the Dead Sea region, the ongoing lowering of the water level encouraged karstic collapse in newly exposed mudflats and has damaged roads and other man-made structures (Frumkin et al. 2011; Shviro et al., 2017). Catastrophic doline collapse atop poorly managed halite/potash mines and solution brine fields is an additional anthropogenically-induced or enhanced geohazard in developed regions is discussed in detail in Warren, 2016 (Figure 1).


Gypsum karst is a documented natural hazard in many parts of Europe (Figure 2), and similar areas of shallow subcropping gypsum are common in much of the rest of the world (Table 1). For example, areas surrounding the city of Zaragoza in northern Spain are affected, as is the town of Calatayud (Gutiérrez and Cooper, 2002; Gutiérrez, 2014). Gypsum dissolution is responsible for subsidence and collapse in many urban areas around northern Paris, France (Toulemont, 1984), in urban areas in and around Stuttgart and other towns peripheral to the Harz Mountains in Germany (Garleff et al., 1997), in Pasvalys and Birzai in Lithuania (Paukstys et al., 1999), in the Muttenz-Pratteln area in northwestern Switzerland (Zechner et al., 2011), in the Perm area of Russia (Trzhtsinsky, 2002), in the Sivaz region of Turkey (Karacan and Yilmez, 1997), in the region centred on the city of Mosul in northern Iraq (Jassim et al., 1997) and in a number of areas of rapid urban development in eastern Saudi Arabia (Amin and Bankher, 1997a, b). Large subsidence depressions caused by gypsum dissolution in China have opened up in the Taiyuan and Yangquan regions of Shanxi Coalfield and the adjacent Hebei Coalfield.


Variation in the watertable level, induced by groundwater pumping or uncontrolled brine extraction, can be an anthropogenic trigger for dolines surfacing. As the watertable declines it causes a loss of buoyant support to the ground, it also increases the flow gradient and water velocity, which facilitates higher rates of crossflow and deeper aquifer recharge in subsequent floods and so reduces the geomechanical strength of the cover and washes away roof span support (Figures 1, 3). Dolines can also be associated with groundwater quality issues. Collapse dolines or sinkholes are frequently used as areas or sumps for uncontrolled dumping industrial and domestic waste. Because of the direct connection between them and the regional aquifer, uncontrolled dumping can cause rapid dispersion of chemical and bacterial pollutants in the groundwater. In the case of Riyadh region Saudi Arabia, a lake of near-raw sewage has appeared in Hit Dahl (cave) and is likely related to the increased utilisation of desalinated water for sanitation and agriculture (Warren, 2016). In the Birzai region of Lithuania numerous sinkholes developed in Devonian gypsum subcrop are in direct connection across the regional hydrology. Accordingly, the amount of agricultural fertilizer use is limited to help protect groundwater quality.

One of the problems associated with rapid surfacing of evaporite collapse features is that any assignment of sinkhole cause will typically lead to an assignment of blame, particulary when anthropogenic infrastructure has been damaged or destroyed by the collapse, or lives may have been lost. Areas of natural evaporite karst are typically areas of relatively shallow evaporites. Shallow evaporites make such regions suitable for extraction via conventional or solution mining. When a collapse does occur in a mined area, one group (generally the miners) has a vested interest in arguing for natural collapse, the others, generally the lawyers and their litigants, will argue for an anthropogenic cause. The reality is usually a combination of natural process enhanced to varying degrees by human endeavours. In the examples in this section, much of the driving process for the collapse is natural, while the cause of any unexpected karst-related disaster is typically geological ignorance combined with political/community intransigence. See Chapter 13 for a further discussion of karst and stope examples that include collapses and explosions where the anthropogenic drivers can dominate.

Problems in the Ripon area, Yorkshire, UK

The town of Ripon, North Yorkshire, and town’s surrounds experiences the worst ongoing gypsum-karst related subsidence in England (Figures 3, 4; Cooper and Waltham, 1999). Some 43 events of subsidence or collapse in the caprock over the Ripon gypsum have occurred over the last 160 years, within an area of 7 km2 (Figures 4). This gives a mean rate of one new sinkhole every 26 years in each square kilometre. Worldwide, the highest documented event rate occurs in Ukraine, in an area of thin and weak clay caprocks above interstratal gypsum karst, where new sinkholes appear at a rate of 0.01 to 3.0 per year per km2 (Waltham et al., 2005). In the Ripon area, numerous sags and small collapses also typify surrounding farmlands. Subsidence features are typically 10-30m in diameter, reach up to 20m in depth and can appear at the surface in a matter of hours to days (Figure 3). To the east of the town, one collapse sinkhole in the Sherwood Sandstone is 80 m in diameter and 30 m deep, perhaps reflecting the stronger roof beam capacity of the Sherwood Sandstone.

When a chimney breaks through, the associated surface collapse is very rapid (Figure 3 b-e). For example, one such subsidence crater, which opened up in front of a house on Ure Bank terrace on 23rd and 24th April, 1997, is documented by Cooper (1998.) as follows (Figure 3b).

“...The hole grew in size and migrated towards the house, to measure 10m in diameter and 5.5m deep by the end of Thursday. Four garages have been destroyed by the subsidence. This collapse was the largest of one of a series that have affected this site for more than 30 years; an earlier collapse had demolished two garages on the same site, and a 1856 Ordnance Survey map shows a pond on the same site. The hole is cylindrical but will ultimately fail to become a larger, but conical, depression. As it does so, it may cause collapse of the house, which is already damaged, and the adjacent road. The house and several nearby properties have been evacuated and the nearby road has been closed. The gas and other services, which run close to the hole, have also been disconnected in case of further collapse.”

Cooper (1998) found the sites of most severe subsidence in the Ripon area (including the house at Ure Terrace and in the vicinity of Magdelen's Road) are located at the sides of the buried Ure Valley, an area where the significant volumes of water seeps from the gypsum karst levels into the river gravels (Figure 4). In 1999 the Ure Terrace sinkhole was filled using a long conveyor belt that was cantilevered over the hole so that no trucked needed to back up close to the sinkhole opening. The hole was surcharged to a height of 0.5m. The hole remains unstable, but the collapse of the fill is monitored to document fill performance and the fill is periodically topped up. After the sinkhole was filled, the road adjacent to the sinkhole was re-opened and the site of the sinkhole fenced. The severely damaged Field View house remains standing next to the sinkhole. The nearby Victorian Ure lodge was not directly damaged by the 1997 sinkhole, but its western corner fell within the council-designated damage zone, and was left unoccupied. It fell into disrepair and was subsequently demolished (Figure 3b). A similar fate befell houses damaged by the surfacing of collapse sinkholes in and around Magdelen's Road, which is located a few hundred metres from Ure Terrace (Figure 3c-e). Shallow subcropping Zechstein gypsum (rehydrated anhydrite) occurs in two subcropping bedded units in this area, one is in the Permian Edlington and the other is in the Roxby Formation (Figure 4b). Together they form a subcrop belt about a kilometre wide, bound to the west by the base of the lowest gypsum unit (at the bottom of the Edlington Formation) and to the east by a downdip transition from gypsum to anhydrite in the upper gypsum-bearing unit of the Roxby Formation. The spatial distribution of subsidence features within this belt relates to joint azimuths in the Permian bedrock, with gypsum maze caves and subsidence patterns following the joint trends (Cooper, 1986). Most of the subcropping gypsum is alabastrine in the area around Ripon, while farther to the east, where the unit is thicker and deeper, the calcium sulphate phase is still anhydrite.

Fluctuations in the watertable level tied to heavy rain or long drought are thought to be the most common triggering mechanism for subsidence transitioning to sinkhole collapse. Many of the more catastrophic collapses occur after river flooding and periods of prolonged rain, which tend to wash away cavern roof span support. Subsidence is also aggravated by groundwater pumping; first, it lowers the watertable and second, it induces considerable crossflow of water in enlarged joints in the gypsum. When recharged by a later flood, the replacement water is undersaturated with respect to gypsum.


Thomson et al. (1996) recognised four hydrogeological flow units driving karst collapse in the Ripon area (Figure 4):

1) Quaternary gravels in the buried valley of the proto-River Ure

2) Sherwood Sandstone Group

3) Magnesian Limestone of the Brotherton Fm. and the overlying/adjacent gypsum of the Roxby Fm.

4) Magnesian limestone of the Cadeby Fm. plus the overlying/adjacent gypsum of the Edlington Fm.

Local hydrological base level within this stratigraphy is controlled by the River Ure, especially where the buried Pleistocene valley (proto-Ure) is filled by permeable sands and gravels, as these unconsolidated sediments, when located atop a breached roof beam, are susceptible to catastrophic stoping to base level (Figure 4). In the area around Ripon the palaeovalley cuts down more than 30 m, reaching levels well into the Cadeby Formation, so providing the seepage connections or pathways between waters in all four units wherever they intersect the palaeovalley. There is considerable groundwater outflow along this route with artesian sulphate-rich springs issuing from Permian strata in contact with Quaternary gravels of the buried valley (Cooper, 1986, 1995, 1998).

The potentiometric head comes from precipitation falling on the high ground of the Cadeby formation to the west and the Sherwood Sandstone to the east. Groundwater becomes largely confined beneath glacial till as it seeps toward the Ure Valley depression, but ultimately finds an exit into the modern river via the deeply incised sand and gravel-filled palaeovalley of the proto-Ure. Waters recharging the Ure depression pass through and enlarge joints and caverns in the gypsum units of the Edlington and Roxby Formations, so the highest density of subsidence features are found atop the sides of the palaeovalley. This region has the greatest volume of artesian discharge from aquifers immediately beneath the dissolving gypsum bed. Although created as an active karst valley, the apparent density of subsidence hollows is lower on the present Ure River floodplain than the surrounding lands as floodplain depressions are constantly filled by overbank sediments (Figure 4b).

Cooper (1998) defined 16 sinkhole variations in the gypsum subsidence belt at Ripon, all are types of entrenched, subjacent and mantled karst. Changes in karst style are caused by; the type of gypsum, the nature and thickness of the overlying deposits, presence or absence of consolidated layers overlying the gypsum and the size of voids/caverns within the gypsum.

To the west of Ripon, the gypsum of the Edlington Formation lies directly beneath glacial drift. These unconsolidated drift deposits and the loose residual marl atop the dissolving gypsum gradually subside into a pinnacle or suffusion (mantled) karst. But between Ripon town and the River Ure, the limestone of the Brotherton Formation overlies the Edlington Formation. There the karst develops as large open caverns beneath strong roof spans (entrenched karst). Ultimate collapse of the roof span creates rapid upward-stoping caverns in loosely consolidated sediment. Stopes break though to the surface as steep-sided collapse dolines or chimneys with sometimes catastrophic results. A similar entrenched situation is found east of the Ure River but there karstified gypsum units of both the Edlington and the Roxby formations are involved.


There are also thick beds of gypsum in the Permian Zechstein sequence that forms the bedrock in the Darlington area. In this area, subsidence features attributed to gypsum dissolution are typically broad shallow depressions up to 100 m in diameter, and the ponds, known as Hell Kettles, are the only recognized examples of steep-sided subsidence hollows around Darlington (Figure 5). Historical records suggest that one of the ponds formed in dramatic fashion in AD 1179 (Cooper 1995). The southern pond appears to be the most likely one to have formed at that time because it is many metres deep and is fed from below by calcareous spring water that is rich in both carbonate and sulphate. The 2D profiles have revealed evidence of foundering in the limestone of the Seaham Formation at depths of c. 50 m (Figure 5; Sargent and Goulty, 2009). The foundering is interpreted to have resulted from dissolution of gypsum in the Hartlepool Anhydrite Formation at ≈ 70 m depth. The reflection images of the gypsum itself are discontinuous, suggesting that its top surface has karstic topography. The 3D survey also acquired and interpreted by Sargent and Goulty (2009) reveals subcircular hollows in the Seaham Formation up to 20 m across, which are again attributed to foundering caused by gypsum dissolution.


Problems with Miocene gypsum, Spain

Karstification has led to problems in areas of subcropping Miocene gypsum in the Ebro and Calatayud basins, northern Spain (Figure 6). Cliff sections and road cuts indicate the widespread nature of karstification in the gypsum outcrops and subcrops in Spain (Figure 7b) Areas affected are defined by subsidence or collapse in Quaternary alluvial overburden and include; urban areas, communication routes, roads, railways, irrigation channels and agricultural fields (Figure 7a; Soriano and Simon, 1995; Elorza and Santolalla, 1998; Guerrero et al., 2013; Gutiérrez et al., 2014). In the region there can be a reciprocal interaction between anthropic activities and sinkhole generation, whereby the ground disturbance engendered by human activity accelerates, enlarges and triggers the creation of new sinkholes. Subsidence is particularly harmful to linear constructions and buildings and numerous roads, motorways and railways have been damaged (Figure 7a, b). Catastrophic collapse and rapid karst chimneying into roads and buildings can have potentially fatal consequences. For example, several buildings have been damaged around the towns of Casetas and Utebo. In the Portazgo industrial estate some factories had to be pulled down due to collapse-induced instability (Castañeda et al., 2009). A nearby gas explosion was attributed to the breakage of a gas pipe caused by subsidence. The local water supply is also disrupted by subsidence and pipe breakage so that 20,000 inhabitants periodically lose their water supply. The most striking example of subsidence affecting development comes from the village of Puilatos, in the Gallego Valley. In the 1970's this town was severely damaged by subsidence and abandoned before it could be occupied (Cooper 1996).


Collapse affects irrigation channels in the countryside with substantial economic losses (Elorza and Santolalla, 1998). In 1996 a doline collapse surfaced and cut the important Canal Imperial at Gallur village. New dolines often form near unlined irrigation canals. The ongoing supply of fresh irrigation waters to field crops can also encourage sinkhole generation in the fields. Though not directly visible, natural sinkholes also form in the submerged beds of river channels cutting regions of subcropping gypsum.

On December 19th, 1971, a bus fell from a bridge into the Ebro River at Zaragoza, near where the ‘San Lazaro well’ (a submerged gypsum sinkhole) is located (Figure 8a). Ten people lost their lives in this accident , while the remainder of the passengers were rescued, after being stranded on the bus roof in the flowing river for some hours (Figure 8b). After survivors were rescued, river waters washed the bus from the foot of the bridge supports into the nearby 'San Lazaro well (collapse sinkhole) in the water-covered floor of the river. Nine of the ten bodies in the bus were never found, although the bus was later recovered from the sinkhole. Locals suggested that bodies were carried deeper into the various interconnect phreatic sinkhole caverns fed by this losing stream.


Karstification in the Zaragoza region is characterised by the preferential intrastatal dissolution of glauberite bed, which are more soluble than the gypsum interbeds, this leads to collapse and rotation of gypsum blocks and river capture (Guerrero et al., 2013).

Sometimes even well-intentioned attempts to remediate culturally significant buildings under threat of evaporite karst collapse can exacerbate collapse problems. Gutiérrez and Cooper (2002) cite examples from the city of Calatayud, Spain. Subsidence-induced differential loading across doline edges drives the tilting of the 25-metre high tower (mudéjar) of the San Pedro de Los Francos church, which leans towards and overhangs the street by about 1.5 metres. (Figure 9) In places, the brickwork of the church indents the pre-existing tower fabric, which probably dates from the 11th Century or the beginning of the 12th Century. This indentation and the non-alignment of the church and the tower walls indicates that most of the tower tilting occurred prior to the construction of the church. In 1840, the upper 5m of the tower was removed and the lower part buttressed for the safety of the Royal family, who visited the town and stayed in the palace opposite. On 3rd June 1931, San Pedro de Los Francos church was declared a “Monument of Historical and Artistic value.” Due to its ruinous condition, the church was closed to worship in 1979. Micropiling to improve the foundation was started in 1994, but this corrective measure was interrupted when only half of the building was underpinned. Very rapid differential settlement of the building took place in the following year, causing extensive damage and aggravating the subsidence problem.


Colegiata de Santa María la Mayor was constructed between the 13th and 18th centuries, it has an outstanding Mudéjar (a 72 m high tower) and numerous Renaissance features; it is considered the foremost monument in the city of Cataluyud. As with the San Pedro de los Francos Church, recent micropiling work, applied to only one part of the cloister, has been followed by alarming differential movements that have drastically accelerated the deterioration of the building. Large blocks have fallen from the vault of the “Capitular Hall” and cracks up to 150 mm wide have opened in the brickwork of the back (NW) elevation, which has now been shored up for safety. The dated plaster tell-tales placed in these cracks to monitor the displacement demonstrate the high speed of the deformation produced by subsidence in recent years. On the afternoon of 10 September 1996, the fracture of a water supply pipe flooded the cloisters and the church with 100 mm of muddy water. Ten years earlier a similar breakage and flood had occurred. These breaks in the water pipes are most likely related to karst-induced subsidence. Once they occur, the massive input of water to the subsurface may trigger further destruction via enhanced dissolution, piping and hydrocollapse (Gutiérrez and Cooper, 2002).


Gypsum karst in Mosul, Iraq

A similar quandary of multiple areas of structural damage from gypsum-induced subsidence affects large parts or the historic section of the city of Mosul in northern Iraq (Jassim et al., 1997). The main part of its old quarter is over a century old and some buildings are a few hundred years old. Mosul lies on the northeastern flank of the Abu Saif anticline and near to its northern plunge (Figure 10a). It was built on the western bank of the Tigris River on a dip slope of Middle Miocene Fatha limestone that is directly underlain by bedded gypsum and green marl (equivalent to Lower Fars Formation). Houses in the old city were built on what seemed to be at the time a very sound rock foundation.

Water distribution in the city was done on mule back in the early part of last century and the estimated water consumption did not exceed 10 litres per person per day (Jassim et al., 1997). Discharge from households was partly to surface drainage and partly to shallow and small septic tanks. The modern piped system of water distribution did not start until the 1940s, resulting in a sudden increase in water consumption (presently around 200 litres per person per day) and it was not associated with a complementary sewer system. Increased water consumption meant larger and deeper septic tanks were dug at the perimeter of buildings (which never seemed to fill) resulting in a dramatic increase in water percolating downwards, water that was also more corrosive than previously due to the increased use of detergents and chlorination. This water passes through the permeable and fractured limestone to the underlying gypsum. On its way through the limestone it enlarges and creates new dissolution cavities, but eventually finds its way into the older gypsum karst maze, which is then further widened as water drains back into the Tigris (Figure 10b). Caverns in the gypsum enlarge until the roof span collapses. Since the 1970s more and more buildings in the old city have fractured and many are subject to sudden collapse. The problem is further intensified due to the expansion of the city in the up-dip direction (west and southwest) including the construction of industrial, water-dependent centres with integrated drainage. Water seeping/draining from these newly developed up-dip areas eventually passes under the old city before discharging in the Tigris river. The process was slightly arrested in the 1980s by the completion of a drainage system for the city, but the degradation of the old city continues.

Coping: man-made structures atop salts

The towns of Ripon in the UK and Pasvales and Birzai in Lithuania house some 45,000 people, who currently live under the ongoing threat of catastrophic subsidence, caused by natural gypsum dissolution (Paukstys et al., 1999). Special measures for construction of houses, roads, bridges and railways are needed in these areas and should include: incorporating several layers of high tensile heavy duty reinforced plastic mesh geotextile into road embankments and car parks; using sacrificial supports on bridges so that the loss of support of any one upright will not cause the deck to collapse; extending the foundations of bridge piers laterally to an amount that could span the normal size of collapses; and using ground monitoring systems to predict areas of imminent collapse (Cooper 1995, 1998).


Dams to store urban water supplies are costly structures and failure can lead to disaster, large scale mortality and financial liability (for example, Cooper and Gutiérrez, 2013). For example, at two and a half minutes before midnight on March 12, 1928, the St. Francis Dam (California) failed catastrophically and the resulting flood killed more than 400 people (Figure 11). The collapse of the St. Francis Dam is considered to be one of the worst American civil engineering disasters of the 20th century and remains the second-greatest loss of life in California’s history, after the 1906 San Francisco earthquake and fire. The collapse was partly attributed to dissolution of gypsum veins beneath the dam foundations. The Quail Creek Dam, Utah, constructed in 1984 failed in 1989, the underlying cause being an unappreciated existence of, and consequent enlargement of, cavities in the gypsum strata beneath its foundations.

Unexpected water leakage from reservoirs, via ponors, sinkholes and karst conduits, leads to costly inefficiency, or even project abandonment. Unnaturally high hydraulic gradients, induced by newly impounded water, may flush out of the sediment that previously blocked karst conduits. It can also produce rapid dissolutional enlargement of discontinuities, which can quickly reach break-through dimensions with turbulent flow. These processes may significantly increase the hydraulic permeability in the region of the dam foundation, on an engineering time scale.

Accordingly, numerous dams in regions of the USA underlain by shallow evaporites either have gypsum karst problems, or have encountered gypsum-related difficulties during construction (Johnson, 2008). Examples include; the San Fernando, Dry Canyon, Buena Vista, Olive Hills and Castaic dams in California; the Hondo, Macmillan and Avalon dams in New Mexico; Sandford Dam in Texas; Red Rock Dam in Iowa; Fontanelle Dam in Oklahoma; Horsetooth Dam and Carter Dam in Colorado and the Moses Saunders Tower Dam in New York State. Up to 13,000 tonnes of mainly gypsum and anhydrite were dissolved from beneath a dam in Iraq in only six months causing concerns about the dam stability (Figure 13). In China, leaking dams and reservoirs on gypsum include the Huoshipo Dam and others in the same area. The Bratsk Dam in eastern Siberia is leaking, and in Tajikistan the dam for the Nizhne-Kafirnigansk hydroelectric scheme was designed to cope with active gypsum dissolution occurring below the grout curtain. Gypsum karst in the foundation trenches of the Casa de Piedra Dam, Argentina and El Isiro Dam in Venezuela, caused difficult construction conditions and required design modifications.


Another illustration of the problems associated with water retaining structures and the ineptitude, or lack of oversight, by some city planners comes from the town of Spearfish, South Dakota (Davis and Rahn, 1997 ). As discussed earlier in this chapter, the Triassic Spearfish Formation contains numerous gypsum beds in which evaporite-focused karst landforms are widely documented across its extent in the Black Hills of South Dakota (Figure 12). The evaporite karst in the Spearfish Fm. has caused severe engineering problems for foundations and water retention facilities, including wastewater stabilization sites. One dramatic example of problems in water retention atop gypsum karst comes from the construction in the 1970s of now-abandoned sewage lagoons for the City of Spearfish.

Despite warnings from local ranchers, the Spearfish sewage lagoons were built in 1972 by city authorities on alluvium atop thick gypsum layers of Spearfish Formation. Ironically, at one point during lagoon construction, a scraper became stuck in a sinkhole and required four bulldozers to pull it out. Once filled with sewage, within a year the lagoons started leaking badly; the southern lagoon was abandoned after four years because of ongoing uncontrollable leaks, and the northern lagoon did not completely drain, but could not provide adequate retention time for effective sewage treatment. Attempts at repairs, including a bentonite liner, were ineffective, and poorly treated sewage discharged beneath the lagoon’s berm into a nearby surface drainage. The lagoons were abandoned completely in 1980. This was after a US $27-million lawsuit was filled in 1979 by ranchers whose land and homes were affected by leaking wastewater. A mechanical wastewater treatment plant was constructed nearby on an outcrop of the non-evaporitic Sundance Formation. The engineering firm that designed the facility without completing a knowledgeable geological site survey was reorganised following the lawsuit.

Likewise, the development of Chamshir Dam atop Gascharan Formation outcrop and subcrop in Iran is likely to create ongoing infrastructure cost and water storage problems (Torabi-Kaveh et al., 2012). The site is located in southwest of Iran, on Zuhreh River, 20 km southeast of Gachsaran city. The area is partially covered by evaporite formations of the Fars Group, especially the Gachsaran Formation. The dam axis is located on limestone beds of Mishan Formation, but nearly two-thirds of the dam reservoir is in direct contact with the evaporitic Gachsaran Formation. Strata in the vicinity of the reservoir and dam site have been brecciated and intersected by several faults, such as the Dezh Soleyman thrust and the Chamshir fault zone, which all act in concert to create karst entryways, including local zones of suffusion karst. A wide variety of karstic features typify the region surrounding the dam site and include; karrens, dissolution dolines, karstic springs and cavities. These karst features will compromise the ability of Chamshir Dam to store water, and possibly even cause breaching of the dam, via solution channels and cavities which could allow significant water flow downstream of the dam reservoir. As possible and likely partial short term solutions, Torabi-Kaveh et al. (2012) recommend the construction of a cutoff wall and/or a clay blanket floor to the reservoir

Difficulties in building hydraulic structures on soluble rocks are many, and dealing with them greatly increases project and maintenance costs. Gypsum dissolution at the Hessigheim Dam on the River Neckar in Germany has caused settlement problems in sinkholes nearby. Site investigation showed cavities up to several meters high and remedial grouting from 1986 to 1994 used 10,600 tonnes of cement. The expected life of the dam is only 30-40 years, with continuing grouting required to keep it serviceable.

Grouting costs in zones of evaporite karst can be very high and may approach 15 or 20% of the dam cost, currently reaching US$ 100 million in some cases. In karstified limestones grouting is difficult, yet in gypsum it is even more difficult due to the rapid dissolution rate of the gypsum. Karst expansion in limestone occurs on the scale of hundreds of years, in gypsum it can be on the order of a decade or less. Grouting may also alter the underground flow routes, so translating and focusing the problems to other nearby areas. In the Perm area of Russia, gypsum karst beneath the Karm hydroelectric power station dam has perhaps been successfully grouted, a least in the short term, using an oxaloaluminosilicate gel that hardens the grout, but also coats the gypsum, so slowing its dissolution. The Mont Cenis Dam, in the French Alps, is not itself affected by the dissolution of gypsum. However, the reservoir storage zone is leaking and photogrammetric study of the reservoir slopes showed ongoing doline activity over gypsum and subsidence in the adjacent land.


Probably the worst example tied to and evaporite karst hazard is the significant dam disaster waiting to happen that is the Mosul Dam in Iraq (Figure 13; Kelley et al., 2007; Sissakian and Knutsson, 2014; Milillo et al., 2016). It is ranked as the fourth largest dam in the Middle East, as measured by reserve capacity, capturing snowmelt from Turkey, some 70 miles (110 km) north. Built under the despotic regime of Saddam Hussein, completed in 1984 the Mosul Dam (formerly known as Saddam Dam) is located on the Tigris river, some 50 km NW of Mosul.

The design of the dam was done by a consortium of European consultants (Sissakian and Knutsson, 2014), namely, Swiss Consultants group, comprising: Motor Columbus; Electrowatt; Suiselectra; Societe Generale pour l’Industrie. The construction was carried out by a German-Italian consortium of international contractors, GIMOD joint venture, comprising: Hochtief; Impregilo; Zublin; Tropp; Italstrade; Cogefar. The consultants for project design and construction supervision comprised a joint venture of the above listed Swiss Consultants Group and Energo-Projekt of Yugoslavia, known as MODACON.

As originally constructed the dam is 113 m in height, 3.4 km in length, 10 m wide in its crest and has a storage capacity of 11.1 billion cubic meters (Figure 13b). It is an earth fill dam, constructed on evaporitic bedrock atop a karstified high created by an evaporite cored anticline in the Fat’ha Formation, which consists of gypsum beds alternating with marl and limestone (Figure 13a, 14). To the south, this is same formation with the same evaporite cored anticlinal association that created all the stability problems in the city of Mosul (Figures 10). The inappropriate nature of the Fat’ha Formation as a foundation for any significant engineering structure had been known for more than a half a century. Then again, absolute rulers do not need to heed scientific advice or knowledge. Or perhaps he didn’t get it from a well-paid group of Swiss-based engineering consultants. As Kelley et al. (2007) put it so succinctly....“The site was chosen for reasons other than geologic or engineering merit.”

The likely catastrophic failure of Mosul Dam will drive the following scenario (Sissakian and Knutsson, 2014); “... (dam) failure would produce a flood wave crest about 20 m deep in the City of Mosul. It is estimated that the leading edge of the failure flood wave would arrive in Mosul about 3 hours after failure of the dam, and the crest of the flood wave would arrive in Mosul about 9 hours after failure of the dam. The total population of the City of Mosul is about 3 million, and it is estimated that about 2 million people are in locations within the city that would be inundated by a 20 m deep flood wave. The City of Baghdad is located about 350 km downstream of Mosul Dam, and the dam failure flood wave will arrive after 72 hours in Baghdad and (by then) would be about 4 m deep.”



The heavily karsted Fat’ha Formation is up to 352 m thick at the dam and has an upper and lower member. The lower member is dominated by carbonate in its lower part (locally called “chalky series”) and is in turn underlain by an anhydrite bed known as the GBo. Gypsum beds typify its upper part,and the evaporite interval is capped by a limestone marker bed. The upper member, crops out as green and red claystone with gypsum relicts, around the Butmah Anticline. Thickness of individual gypsum beds below the dam foundations can attain 18 m; these upper member units are intensely karstified, even in foundation rocks, with cavities meters across documented during construction of the dam (Figure 14). Gypsum breccia layers are widespread within the Fatha Formation and have proven to be the most problematic rocks in the dam’s foundation zone. The main breccia body contains fragments or clasts of limestone, dolomite, or larger pieces of insoluble rocks of collapsed material. The upper portion of the accumulation grades upward from rubble to crackle mosaic breccia and then a virtually unaffected competent overburden. Breccia also may form without the intermediate step of an open cavity, by partial dissolution and direct formation of rubble. As groundwater moves through the rubble, soluble minerals are carried away, leaving insoluble residues of chert fragments, quartz grains, silt, and clay in a mineral matrix. These processes result in geologic layers with lateral and vertical heterogeneity on scales of micro-meters to meters.

High permeability zones in actively karsting gypsum regions can form rapidly, days to weeks, and quickly become transtratal. So predicting or controlling breakout zones via grouting and infill can be problematic (Kelley et al., 2007; Sissakian and Knutsson, 2014). For example, four sinkholes formed between 1992 and 1998 approximately 800 m downstream in the maintenance area of the dam (Figure 13a). The sinkholes appeared in a linear arrangement, approximately parallel to the dam axis. Another large sinkhole developed in February 2003, east of the emergency spillway when the pool elevation was at 325 m. The Mosul Dam staff filled the sinkhole the next day, with 1200 m3 of soil. Another sinkhole developed in July 2005 to the east of the saddle dam. Six borings were completed around the sinkhole and indicated that the sinkhole developed beneath overburden deposits and within layers of the Upper Marl Series. Another cause for concern at Mosul Dam in recent years is a potential slide area reported upstream of the dam on the west bank. The slide is most likely related to the movement of beds of the Chalky Series over the underlying GBo (anhydritic) layer.

To “cope” with ongoing active karst growth beneath and around the Mosul Dam, a continuous grouting programme was planned, even during dam construction, and continues today, on a six days per week basis. It pumps tens of thousands of tons of concrete into expanding karst features each year (Sissakian and Knutsson, 2014; Milillo et al., 2016). The dam was completed in June 1984, with a postulated operational life of 80 years. Due to insufficient grouting and sealing in and below the dam foundation, numerous karst features, as noted above, continue to enlarge in size and quantity, so causing serious problems for the ongoing stability of the dam. The increase in hydraulic gradient created by a wall of water behind the dam has accelerated the rate of karstification in the past 40 years.

Since the late 1980s, the status of the dam and its projected collapse sometime within the next few decades has created ongoing nervousness for the people of Mosul city and near surroundings. All reports on the dam since the mid 1980s have underlined the need for ongoing grouting and monitoring and effective planning of the broadcasting of a situation where collapse is imminent. For “Saddam’s dam” the question is not if, but when, the dam will collapse. To alleviate the effects of the dam collapse, Iraqi authorities have started to build another “Badush Dam” south of Mosul Dam so that it can stop or reduce the effects of the first flood wave. However this new dam has a projected cost in excess of US$ ten billion and so lies beyond the financial reach of the current Iraqi government. Problems related to the dam increased with the takeover of the region by the forces of ISIL.

Today, the Mosul dam is subsiding at a linear rate of ~15 mm/year compared to 12.5 mm/year subsidence rate in 2004–2010 (Milillo et al., 2016). Increased subsidence restarted at the end of 2013 after re-grouting operations slowed and at times stopped. The causes of the observed linear subsidence process of the dam wall can be found in the human activities that have promoted the evaporite–subsidence development, primarily in gypsum deposits and may enable, in case of continuous regrouting stop, unsaturated water to flow through or against evaporites deposits, allowing the development of small to large dissolution cavities.

Large vertical movements that typified the dam wall have resulted from the dissolution of extensive gypsum strata previously mapped beneath the Mosul dam. Increased subsidence rate over the past five years has been due to periods when there was little or no regrouting underlying the dam basement. Dam subsidence currently seems to follow a linear behavior but on can not exclude a future acceleration due to increased gypsum dissolution speed and associated catastrophic collapse of the dam (Milillo et al., 2016).

Given the existing geologic knowledge base in the 1980s, in my opinion, one must question the seeming lack of understanding in a group of well-paid consultant engineering firms as to the outcome of building such a major structure, atop what was known to be an active karstifying gypsum succession, sited in a location where failure will threaten multimillion populations in the downstream cities. The same formation that constituted the base to the Mosul dam was known at the time to be associated with ground stability problems atop similar gypsum-cored anticlines in the city of Mosul to the south. Even more concerning to the project rationale should have been the large karst cavities in highly soluble gypsum that were encountered a number of times during feasibility and construction of the dam foundations (Figure 14). Or, perhaps, as Lao Tzu observed many centuries ago, “ ...So the unwanting soul sees what’s hidden, and the ever-wanting soul sees only what it wants.”

Canals, like dams, that leak in gypsum karst areas can trigger subsidence, which can be severe enough to cause retainment failure. In Spain, the Imperial Canal in the Ebro valley, and several canals in the Cinca and Noguera Ribagorzana valleys, which irrigate parts of the Ebro basin, have on numerous occasions failed in this way. Similarly, canals in Syria have suffered from gypsum dissolution and collapse of soils into karstic cavities. Canals excavated in such ground may also alter the local groundwater flow (equivalent to losing streams) and so accelerate internal erosion, or the dissolution processes and associated collapse of cover materials. In the Lesina Lagoon, Italy, a canal was excavated to improve the water exchange between the sea and the lagoon. It was cut through loose sandy deposits and highly cavernous gypsum bedrock, but this created a new base level, so distorting the local groundwater flow. The canal has caused the rapid downward migration of the cover material into pre-existing groundwater conduits, producing a large number of sinkholes that now threaten an adjacent residential area.

Pipelines constructed across karst areas are potential pollution sources and some may pose possible explosion hazards. The utilization of geomorphological maps depicting the karst and subsidence features allied with GIS and karst databases help with the grouting and management of these structures. In some circumstances below-ground leakage {Zechner, 2011 #26} from water supply pipelines can trigger severe karstic collapse events. Where such hazards are identified, such as where a major oil and gas pipeline crosses the Sivas gypsum karst in Turkey, the maximum size of an anticipated collapse can be determined and the pipeline strength increased to cope with the possible problems.


Solving the problem?

Throughout the world, be it in the US, Canada, the UK, Spain, eastern Europe, or the Middle East, it is a fact that weathering of shallow gypsum forms rapidly expanding and stoping caverns, especially in areas of high water crossflow, unsupported roof beams, and unconsolidated overburden and in areas of artificially confined fresh water. Rapid karst formative processes and mechanism will always be commonplace and widespread (Table 2). Resultant karst-associated problems can be both natural and anthropogenically induced or enhanced. It is fact that natural solution in regions of subcropping evaporites is always rapid, and even more so in areas where it is encouraged by human activities, especially increased cycling of water via damming, groundwater pumping, burst pipes, septic systems, agricultural enhancement and uncontrolled storm and waste water runoffs to aquifers.

Typically, the best way to deal with a region of an evaporite karst hazard is to map the regional extent of the shallow evaporite solution front and avoid it (Table 3). In established areas with a karst problem the engineering solutions will need to be designed around hazards that will typically be characterised by short-term onsets, often tied to rapid ground stoping/subsidence events and quickly followed by ground collapse. If man-made buildings of historical significance are to be restored and stabilized in such settings, perhaps it is better to wait until funds are sufficient to complete the job rather than attempt partial stabilization of the worst-affected portions of the feature. Significant infrastructure (including roads, canals and dams) should be designed to avoid such areas when possible or engineered to cope with and/or survive episodes of ground collapse.

A piecemeal approach to dealing with evaporite karst can intensify and focus water crossflows rather than alleviate them. In the words of Nobel prizewinner, Shimon Peres; “If a problem has no solution, it may not be a problem, but a fact - not to be solved, but to be coped with over time.”


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Dissolving salt (1 of 5): Landforms

John Warren - Thursday, August 31, 2017

 

Introduction

This series of five articles discuss effects of nearsurface evaporite dissolution at various scales and times, in terms of; 1) landform expression, 2) evaporite-hosted caves and speleothems 3) natural evaporite karst geohazards 4)Anthropogenically enhanced karst features, 5) economic associations tied to evaporite paleokarst (hydrocarbon and metal).

As salt dissolves in the subsurface, it creates void spaces of various sizes and shapes into which overlying strata can drape or brecciate, so creating characteristic landforms at both local (m to km) and regional (km to tens of km) scales (Figure 1). Water sources producing these features can be shallow unconfined meteoric or marine, confined shallow or deeply circulating meteoric, or hypogene, basinal and hydrothermal. Solution-related evaporitic landforms can be either active karst or palaeokarst features. Paleokarst refers to ancient karstic features that are no longer active and tied to ancient basin flushing hydrologies and now buried land surfaces. Active karst is responding to the modern ambient hydrology and typifies landscape elements discussed in the following section.


Evaporite solution karst landforms overprint nearby carbonate or evaporite strata, with many descriptive features and terms are common to textures and geometries across both host lithologies. Evaporite-related karst includes many varieties of karren, sinkholes (dolines and caves), blind valleys, poljes and subsidence basins (Ford and Williams, 2007; Warren, 2016). Bedrock exposures of gypsum or salt at a finer scale are sculpted into irregular curved and grooved surfaces called rillenkarren or other varieties of gravity-oriented solution flutes (Macaluso and Sauro, 1996; Stenson and Ford, 1993).

Local-scale karst geometries that define evaporite-related subsidence range in lateral extent from the metre-scale cones of suffosion karst to collapse cones to kilometre-scale doline depressions (Figure 1). In regions of soil-mantled outcrops of bedded evaporites, these features tend to occur at higher densities, within or near broad subsidence valleys. In the context of the lateral extents of evaporite-related landforms, it is also useful to separate landforms associated with the dissolution of bedded evaporite versus dissolution landforms and namakier residues that occur above variably active salt diapir crests. Features related to dissolving diapirs and namakiers are discussed in an earlier article (Salty Matters, March 10, 2015).

The geometry of the karst set-up hydrology is distinct in sediments above diapiric versus bedded evaporite substrate, and this article focuses on landforms related to a dissolving substrate made up of shallow dipping salt beds. The next section outlines characteristics feature of evaporite karst at the metre to kilometre scale, as defined in Figure 1. Then we will compartmentalise these features using the classification of Gutiérrez et al., 2008, and finally look at three of the more distinctive regions responding to the dissolution of subcropping evaporite beds.

Evaporite solution dolines

Dolines are closed circular to elliptical hollows or depressions, often funnel shaped, with diameters ranging from a few metres to a few kilometres and depths from a metre or so, to hundreds of metres (Figure 1; Ford and Williams, 2007). They indicate subsidence and collapse in underlying salt or carbonate units. Valley sides in larger subsidence dolines can be steep and expose karstified gypsum, or can be gentler slopes covered by soil.

Larger dolines may enclose one or more smaller sinkholes and can be further subdivided into, suffusion, subsidence sag, and collapse dolines (Figure 1). A collapse sinkhole is initiated by collapse of a roof span into an underlying solution cavity. A subsidence sag doline indicates a more diffuse broader and gentler lowering of the ceiling above the dissolving bed. Suffosion dolines are small-width high-density karst features (1-2m diameter) covered and filled with soil and debris that has washed or fallen into closely-spaced fissures cutting into the evaporite bed. They typically indicate that a dissolving salt mass is very close to the landsurface.

Dissolution, collapse and suffosion processes are more active, more rapid, more frequent and more noticeable in regions of shallow evaporite units, compared to carbonate terranes at the same depths. For shallow weathering evaporites versus carbonates, at roughly the same depths in the Perm and Bashkir regions of Russia, Gorbunova (1979) reported doline densities of 32 and 10/km2 respectively.


Suffosion dolines

Suffosion in a karst terrane describes the downward migration of unconsolidated cover deposits through voids. High levels of dispersed impurities (mostly clays and muds, often dolomitic and other dissolution residues) in a rapidly dissolving evaporite mass, means a soil or carapace of insoluble residues quickly covers the subcropping top and edges of a dissolving salt mass (Figures 1a, 2). Downwash transport into growing fissures and voids in the shallow underlying evaporite is via downwashing of fine particles carried by percolating waters, cohesionless granular flows, viscous sediment gravity flows (non-Newtonian), freefall of particles, and sediment-laden water. The carapace is continually undermined by ongoing rapid solution, as residual debris is continually and rapidly washed into the doline crevices into the growing cavities in the evaporite. This creates a unique soil-covered dimpled landscape, which is typified by a high-density doline terrain where the dissolving evaporite is only a metre or so below the landsurface.


Densities of up to 1000 suffosion dolines/km2 occur in many evaporite-cored fold axes or at subcrop contacts of evaporites with other outcropping lithologies. Densities of 1100-1500/km2 have been documented in evaporite karst in the Italian Alps and in regions to the west of Sivas, Turkey (Figure 3; Belloni et al., 1972; Kacaroglu et al., 1997). The inherently high solubility of evaporite salts explains similar densely-packed schlotten depressions and large karren shafts seen in gypsum karst of Antigonish County, Nova Scotia, where extrapolated doline densities in the latter case range up to 10,000/km2 (Martinez and Boeher, 1997). Extrapolated, because such dense networks do not extend over any more than a square kilometre or two and are typically found near retreating escarpment edges underlain by shallow subcropping salts. Such high densities tend to occur in more humid rather than arid areas, with high hydraulic gradients so that the resulting suffosion dolines have diameters around 5 m or less.


Subsidence Dolines

Solution (subsidence) dolines, as described in much of the literature tend to be larger down-warped doline craters or bowl-like subsidence depressions (Figure 1c). In contrast to the numerous small steep-sided suffosion and collapse dolines formed atop shallow subcropping shallow evaporites, these larger, bowl-like dolines can have diameters of 100-500 m and depression depths of 10-20 m or more. Most subsidence sag dolines have a well-developed soil cover and a thick sediment fill, with dissolving salt units found depths measured in tens to hundreds of metres below the land surface (Figure 4). Compared to collapse and suffusion dolines, subsidence or sag dolines have lower angle to near flat slopes into the deeper parts of the doline hollow, the doline walls at the outcrop level are usually hosted in non-evaporites, with the dissolving salt bed lying some distance below the landsurface. Compared to suffusion karst, subsidence dolines occur in regions of more deeply buried evaporites (tens of metres) including; Italy (Belloni et at. 1972; Burri 1986; Ferrarase et al., 2002), Spain (Gutiérrez, 1996) and the Pecos Valley of West Texas and New Mexico (Gustavson et al., 1982; Davies, 1984a, b; Quinlan et al., 1986). At the larger end of the scale of subsidence doline development, a subsidence doline merges into a subsidence basin (Figure 1d). The latter is a large solution hollow that had created enough accommodation space to be considered a small sedimentary basin, generally filling with varying combinations of fluvial lacustrine and other continental sediments.


Collapse dolines

Collapse dolines are steep-sided sinkholes, often defined by cave entrances that contain large blocks of roof material (Figure 1c). They form when solution of an underlying evaporite bed creates a roof span that can no longer be supported by the overlying lithology. Collapse doline walls are frequently asymmetrical; one wall is steep, and the other one is gentle (Figure 5). Soil covered doline floors, when not water covered, tend to display either concave-up or flat geometries. Apart from blocks of the collapsed roof span, active doline floors can be veneered by thin collapse breccias in a matrix of insoluble residues. The high density of dolines in areas of evaporite subcrop and the ubiquity of the associated breccias indicates the inherently higher solubility of the evaporite salts compared with interbedded and overlying carbonates. Active collapse sinkholes atop shallow bedded evaporites typify terminations of dry arroyos in many deserts and may also line up along active or former river courses, as in Bottomless Lake State Park, New Mexico (Figure 6).


Sinkhole Classification

Following the definitions of Waltham et al. (2005), Gutiérrez et al. (2008) constructed a nongenetic classification of subsidence dolines or sinkholes (Figure 7). It is based on two observations that refer to the material affected by downward gravitational movements (cover, bedrock or caprock) and the primary type of process involved (collapse, suffosion or sagging). The classification also applies to both evaporite and carbonate karst. The term “cover” refers to allogenic unconsolidated deposits or residual soil material, bedrock to karst rocks and caprock to overlying non-karst rocks. Collapse indicates the brittle deformation of soil or rock material either by brecciation or the downward migration deposits through conduits and its progressive settling, while sagging is the ductile flexure (bending) of sediments caused by the lack of basal support. In practice, more than one material type and several processes can be involved in the generation of sinkholes. These more complex sinkholes can be described using combinations of the proposed terms with the dominant material and/or process followed by the secondary one (e.g. cover and bedrock collapse sinkhole, bedrock sagging and collapse sinkhole).


The cover material may be affected by any of three subsidence mechanisms. The progressive corrosional lowering of the rockhead may cause the gradual settling of the overlying deposits by sagging, producing cover sagging sinkholes. An important applied aspect is that the generation of these sinkholes does not require the formation of cavities These depressions arc commonly shallow, have poorly defined edges and may reach several hundred metres across.

Cover deposits may migrate downward into fissures and conduits developed in the rockhead by action of a wide range of processes collectively designated as suffusion: downwashing of particles by percolating waters, cohesion-less granular flows, viscous gravity flows (non-Newtonian), fall of particles, and sediment-laden water flows. The downward transport of the cover material through pipes and fissures may produce two main types of sinkholes depending on the rheological behaviour of the mantling deposits. Where the cover behaves as a ductile or loose granular material, it may settle gradually as undermining by suffosion progresses. This creates sags and slumps in the overburden materials. Where cover behaves in a more brittle manner, collapse breccias form.

Sinkholes that result from the combination of several subsidence processes and affect more than one type of material are described by combinations of the different terms with the dominant material or process followed by the secondary one (e.g. bedrock sagging and collapse sinkhole). The mechanism of collapse includes any brittle gravitational deformation of cover and bedrock material, such as upward stoping of cavities by roof failure, development of well-defined failure planes and rock brecciation. They define suffosion as the downward migration of cover deposits through dissolutional conduits accompanied by ductile settling. Sagging is the ductile flexure of sediments caused by differential corrosional lowering of the rockhead or interstratal karstification of the soluble bedrock. Sag plays a major role in the generation of sinkholes across broad areas underlain by shallow dissolving evaporites (not included in previous genetic classifications mostly based on carbonate karst). Likewise, collapse processes are more significant in extent and rate in areas underlain by evaporites than in carbonate karst, primarily due to the order of magnitude greater solubility of the evaporites and the lower mechanical strength and ductile rheology of gypsum and salt rocks (Warren, 2016).


Broader-scale evaporite landforms

There are some unique aspects to regional landform and doline density associations above dissolving evaporites because; 1) halite, and to a lesser extent anhydrite, units are many times more soluble than carbonate or siliciclastic strata, and 2) matrix in most salt units away from its retreating edges tends to remain impervious even as salt masses approach the landsurface (Figure 8; melting block of ice analogy, see Warren, 2016). Such features form at the basin-scale in regions where an ancient salt bed is moving from the mesogenetic to the telogenetic realm, especially in regions where mountains are growing adjacent to the uplifted evaporite basin. The broader-scale landscape features not seen in carbonate karst in similar regional settings are; 1) laterally migrating subsidence basins (Figure 8a) and 2) regions of breccia chimneys (Figure 8b).

Subsidence basins

A subsidence basin is a large (tens of kilometres width) solution hollow that creates enough accommodation space to be considered a small sedimentary basin with widths measured in tens of kilometres (Figure 8a). In a continental setting, the basin is filling with varying combinations of fluvial and lacustrine sediments.

Subsidence troughs are large-scale elongate depositional depressions created by interstratal solution along the dissolving edge of shallow dipping ancient uplifted salt bodies. The largest solution-induced depositional basins tend to occur along the margins of the great interstratal halite deposits, creating a solution form that may be represented by a shallow retreating salt slope at the surface, with a laterally migrating monoclinal drape of beds in the vicinity of a salt scarp, which is defined the dissolving edge of the underlying salt bed or beds. Subsidence basins are filled or partly filled by clastic sediments (Olive, 1957; Quinlan, 1978; Simpson, 1988). If the subsidence zones atop a retreating salt edge lack a significant volume of sediment fill, it called is a subsidence trough, as seen in the vicinity of the outcropping edge of the Jurassic Hith Anhydrite in Saudi Arabia (see part 2).

Salt-edge leaching of shallow dipping salt underscores a set of self-perpetuating processes. Fractures created by the collapse of overburden and intrasalt beds contribute to an expanding accommodation space generated by salt cavities and the lateral retreat of the salt scarp. The ongoing salt dissolution provides new fissures and sinks that act as additional conduits for further percolation of meteoric or upwelling of undersaturated basinal waters into the salt. This, in turn, instigates yet more salt solution in the vicinity of the collapse and typically, so creating an elongate corridor of subsidence at the surface, which can extend for many kilometres parallel to the dissolving salt edge

Hence, large solution-induced depositional basins and monoclines outline the edges of the great interstratal salt deposits of the world. Depression corridors some 5-500 km long, 5-250 km wide, with up to 100 to 500 m of subsidence induced relief and sediment fill define saline karst plains along the edges of bedded and dissolving saline giants in the Devonian evaporite subcrop of Canada (Figure 9b; De Mille et al., 1964; Tsui and Cruden 1984; Christiansen and Sauer, 2002; Tozer et al., 2014; Broughton et al., 2017), the Permian Basin of New Mexico, Oklahoma and Texas (Figure 9a; Anderson, 1981; Davies, 1984a, b; Bachman, 1984), the Perm region of Russia (Gorbunova, 1979) and the Jurassic Hith region of Saudi Arabia (Amin and Bankher, 1997a, b;). Associated regolith and sediment infill typically reduces the regional solution edge landscape to a few tens of metres of relief.

Salt Breccia Chimneys

Undersaturated waters not only influence the dissolving edges of a shallow-dipping salt bed, but can also cut up through the salt as a series of salt chimneys, located well out in the salt basin, with diameters between 20 and 250 m. Most are plumb vertical, with reported heights ranging from tens of metres to kilometers. Higher examples have usually stoped upward through one or more cover formations, which may include siliciclastics, coal measures, extrusive volcanics, etc. (Figure 9b). Breccia pipes in an evaporite mass may exhibit one of four possible states (Figure 8b): a) Active, and propagating upwards towards the surface, with fault and collapse bounded edges, but not yet expressed at the surface - a blind chimney; b) Active or inactive, expressed at the surface as a closed depression or a depression with a surface outflow channel; c) Inactive, and buried by later strata - paleokarst chimney; d) Inactive and standing up as a positive relief feature on the landsurface, because the breccia (generally cemented) is more resistant to weathering than the upper cover strata - so forming a residual pipe with positive relief. Some upstanding features are firmly cemented and resistant structure, such as the castiles above an erosional plain in the Delaware Basin of Texas (Hill, 1990)

Downward flexure in the uppermost strata atop a growing pipe tends to form a bowl of subsidence and a sag doline structure.

In some intrasalt pipes, the halite or gypsum is entirely removed, leaving only an accumulation of insolubles and collapsed breccia blocks. In others, portions of the less soluble salts can remain in the pipe at the level of the mother salt. Mature pipes are typically filled by a jumble of intrasalt and suprasalt breccia blocks. In some, a lower brecciated zone is succeeded by an upper zone in which the overlying strata failed as a coherent block, settling downwards via a cylindrical pattern of steep to vertical faults with downthrows of up to 200 m. Such variably filled and zoned salt breccia chimneys have created one type of barren zone in the potash ores of the Prairie Evaporite in Canada, and we shall discuss this in detail in part 5(economic associations).

Most breccia pipes originate via a point-source dissolution breakthrough of a halite bed (occasionally gypsum or anhydrite) and are located above a fracture junction, an anticlinal crest, or a buried reef that channels groundwater into a local area in the salt (Figure 8b). Hence, salt breccia chimneys and pipes form best where there is an artesian head to create subsalt pressures. In the Western Canada Sedimentary Basin the head comes from the adjacent uplifted Rocky Mountains, while the Delaware and Guadalupe Mountains play a similar role in the Delaware Basin. A pipe creates anomalous chemical interfaces with adjacent and overlying strata and so may be targets for the later precipitation of economic ores, including uranium and base metals (part 5).


Delaware Basin, West Texas

The Delaware Basin of West Texas and southeast New Mexico, in the southwest portion of the Permian Basin, contains bedded evaporites of the Late Permian Castile, Salado and Rustler Formations (Figure 9a). Outcrops and subcrops of these three formations constitute an area of widespread subsidence troughs, collapse sinks, dolines and breccia chimneys, all created by ongoing removal of underlying bedded Permian salts. The Delaware is an eastward-dipping basin, mainly surrounded by the Capitan Reef and its equivalents (Figure 9a). The original extent of the evaporites in the basin was much greater than today due to ongoing salt dissolution.

The area called the “Gypsum Plain” of Texas lies to the west of the Pecos River and comprises about 2,600 square kilometres of subcropping gypsum of the Castile Formation (Olive, 1957; Quinlan, 1978; Kirkland and Evans, 1980; Anderson, 1981; Stafford et al., 2008a,b; Holt and Powers, 2010). South of Carlsbad and Carlsbad Caverns, the plain exhibits many small examples of solution subsidence troughs, typically 0.7–15 km in length, 100–1500 m wide, but no more than 5–10 m deep (Figure 9a: Stafford et al., 2008a). The plain is also where the “Castile” landforms are found and indicate an overprint of bacterial and thermochemical sulphate reduction (Kirkland and Evans, 1976). Additional gypsum outcrops are present to the east in the Rustler Hills and to the north in Reeves County, Texas. Permian strata in region of the "Gypsum Plain" once contained significant volumes of halite that were dissolved well before the evaporite succession reached the landsurface.


In the centre of the Delaware Basin, the thick evaporites of the Castile and Salado Formations retain their halite beds as do the thinner evaporites of the overlying Rustler Formation. All are underlain by relatively permeable carbonates and siliciclastics, including some prolific hydrocarbon reservoirs in Permian backreef of the Central Basin Platform (Figure 10). Reef mounds, fractures and faults in these underlying sediments have provided focused conduits for upward stoping breccia chimneys through the buried evaporites as well as the subsurface formation of now-exhumed Castiles to the west. An eastward-flowing deeply-circulating regional artesian hydrology, in combination with centripetal escape of buoyant hydrocarbon-rich basinal waters, drives the formation of these chimneys, with their surface expressions occurring in areas such as the Wink Sink (chain of breached chimneys) and the Gypsum Plain (Castiles).

The upper sides of the shallow dipping salt beds are also affected by the hydrologies of the zone of active phreatic circulation. The Pecos River has migrated back and forth across the top of subcropping evaporites for much of the Tertiary. Its ancestral positions drove substantial salt dissolution, now evidenced by large sediment-filled subsidence basins and troughs in the centre of the Delaware Basin (Figures 9b, 10). The regional eastward dip of the Delaware basin sediments means first halite, and then gypsum has disappeared along the updip eastern edge of the basin. Relatively undisturbed salt remains along the more deeply buried western side of the basin that abuts and covers the Central Basin Platform. Bachman and Johnson (1973) estimate the horizontal migration rate of the dissolution front across the basin as high as 10-12 km per million years so that more than 50% of the original halite is gone. Multiple smaller examples of sediment-filled subsidence troughs occur at the edge of the gypsum plain of the Delaware Basin south of Carlsbad Caverns, where depositional troughs, 0.7 to 15 km in length, 100 to 1,500 m wide and no more than 5-10 m deep, are well documented, as are subsidence sinks within the subsidence swales, such as Bottomless Lake, which is a region where the watertable intersects a collapse chimney (Figure 10c; Quinlan et al. 1986).

The San Simon swale is a 25 km2 depression defining a residual karst feature atop the Capitan Reef on the northeastern margin of the Basin (Figures 9a, 10a, b). San Simon Sink sits atop a subsidence chimney within the San Simon swale; it is the lowest point in the depression and is some 30 m deep and 1 km2 in area. It, in turn, encloses a secondary collapse sink some a few hundred metres across and 10 metres deep (Figure 10b). During a storm in 1918, the San Simon sinkhole formed as a gaping hole about 25 metres across and 20 metres deep in the lower part of the sink. In one night, nearly 23,000 cubic metres of soil and bedrock disappeared into the collapse cavern. Annular rings that cut the surface around the San Simon sinkhole today suggest ongoing subsidence and readjustment of the sinkhole is still occurring in response to earlier collapses. The position of the San Simon sink over the Permian reef crest led Lambert (1983) and many others to suggest that the sinkhole originated as a groundwater cavity breakthrough, atop a series of stoping reef-focused breccia pipes or chimneys. Sinkhole breakouts, which can emerge in a matter of hours, continue to form across this dissolution basin, in some case aided by poorly-monitored brine extraction operations and improperly-cased water wells. But the majority of the sinkholes in the Delaware Basin are natural, not anthropogenically enhanced (Land, 2013).


Nash Draw is a southwesterly trending depression or swale, some 25 km long and 5-15 km wide, at the northern end of the Delaware Basin with its sump in a salt lake (Poker Lake) at the southern end of the draw (Figure 11). The underlying evaporitic Rustler Formation and parts of the Salado Formation have largely dissolved so that more than 100 caves, sinks, fractures, swallow-holes, and tunnels make up a complex local karst topography in the Draw, which is still active today (Figure 11b, c; Bachman, 1981, Powers et al., 2006; Goodbar and Goodbar, 2014).

An extensive drilling program conducted for the nearby WIPP site (now a low-level radioactive waste repository) showed that natural dissolution of halite in the Rustler and upper Salado formations is responsible for the subsidence and overall formation of Nash Draw (Lambert, 1983; Holt and Powers, 2010). To the immediate west of Nash Draw, the WIPP/DOE drilling program defined the formation of a solution trough in the Dewey Lake Redbed; it was created by preferential leaching of halite beds in the Rustler Formation, with interstratal anhydrite and breccia residuals (Figure 12). This dissolution occurred at a level some 400 metres above the salt-encased storage level of the WIPP waste isolation facility and so is not considered a significant risk factor in terms of longterm site stability (Holt and Powers, 2010). A heated scientific (and at times not so scientific!) debate of just how deep surface karst penetrates into the bedded halite of the Salado Formation in the vicinity of the WIPP site continues today.


Further north, near the subcropping western edge of the Northwest Permian Shelf, is Bottomless Lakes State Park, located some 20 kilometres southeast of Roswell. Encircling the lakes are the gypsum, halite and dolomitic redbeds of the Artesia Group and San Andres Formation. Away from the lakes are numerous other sinkholes and collapse dolines, most of which are circular, steep walled or vertical holes, 50-100 metres across and 30-60 metres deep, with the greatest density of features aligned along the eastern side of the Pecos River floodplain (Figure 6). Water in the various sinks that make up the Bottomless Lakes is crystal clear and brackish to saline (6,000-23,000 ppm), attesting to its passage through subsurface layers of gypsum and salt. Although the lakes are around 30 metres deep, dark-green moss and algae coat the bottoms giving the impression of great depth and hence the name of the park (Lea Lake; Figure 10d.). To the west, many of the playas in depressions near Amarillo in the High Plains of Texas have a similar genesis as solution depressions atop dissolving Permian salt, but most do not intersect the regional watertable and so do not hold permanent surface water (Paine, 1994). Further south, near Carlsbad Caverns, there are other subsidence chimneys that form lakes where they intersect the watertable and so are also locally known as “Bottomless” (Figure 9a).


Western Canadian karst

The distribution and timing of chimneys and subsidence troughs created by the subsurface dissolution of the Prairie evaporite are well known and tied to the distribution of oil sands and the quality of potash ores (part 5). Dissolution drape features are more pronounced nearer the retreating edges of the thick multilayered subsurface Devonian salt succession while breccia chimneys occur above the buried salt successions (Figure 9b). For example, the Rosetown Low and the Regina Hummingbird Trough accumulated more than 100 metres of depression trough and drape sediment during interstratal dissolution of the underlying Prairie salt (Devonian) in southern Saskatchewan, especially in Cretaceous time (Figure 13a,b; DeMille et al., 1964; Simpson, 1988). This evaporite-related Cretaceous subsidence is also the principal control on the distribution of the Athabasca oil sands in subsidence basins along the eastern side of the evaporite extent (Tozer et al., 2014). Uplift of the ancestral Rocky Mountains likely created the potentiometric head that drove much of the subsalt aquifer flow. As in the Delaware Basin, the positions of sub-salt reefs and pinnacles focused many of the upwells of deeply circulated meteoric water that ultimately created solution breccia layers and breccia chimneys (Figure 8b).

In various circum-salt subsidence troughs, now filled or partially filled with late Mesozoic and Tertiary sediment thicks, the concurrence of a supra-unconformity thick, adjacent to the sub-unconformity feather-edge of a bedded salt sequence, is at a scale that is easily recognised in seismic and constitutes one of the classic signatures of a salt collapse-induced hydrocarbon trap (Figure 8a). For now, we will focus on the influence of dissolving evaporites on the modern Canadian landscape in regions of active karst, but we will return to the topic of economic hydrocarbon associations with paleokarst in this basin in part 5.

Evaporite karst domes, and laterally extensive solution breccia units, tied to a waxing and waning cover of glacial ice and permafrost, were first documented in Canada in northern Alberta and the Northwest Territories (e.g. De Mille et al., 1964). Karst domes remain as surface features of positive relief once the surrounding evaporite mass has completely dissolved and are outlined by megabreccia with caverns. Unlike breccia chimneys, the cores of evaporitic karst domes can expose blocks from below, as well as above the original bedded and folded evaporite level. The evaporitic karst domes in western Canada are related to the stratigraphic level of former bedded salts; elsewhere in the world others are the remains of now dissolved salt thrusts, diapirs and allochthons (see diapiric breccias and rauhwacke discussion in Warren, 2016). Hence, domes and residual units are dramatic landscape features in the gypsiferous terrain of northern Alberta, Canada (Wigley et al., 1973; Tsui and Cruden, 1984). Similar features, tied to the dissolution of bedded evaporites, typify the Arkhangelsk gypsum-residue karst region inland of the Barents Sea coast of Russia (Korotkov, 1974).

Canadian karst domes range from 10 to 1000 m or more in length or diameter, and can rise to 25 m above the surrounding land surface. Many domes are highly fractured, with individual overburden blocks displaced by heaving and sliding, with the residual gypsum showing well-developed flow foliation.

At the extreme end of disturbance and dissolution range, the domes breccias are megabreccias; positive relief features made up of collapsed or even upthrust jumbles of large blocks, some the size of houses. The largest reported Canadian megabreccia example is in a steep-limbed anticline that extends along the shore of Slave Lake for a distance of 30 km. It is up to 175m in height with a brecciated crestal zone that is marked by a ‘chaotic structure and trench-like lineaments’ (Aitken and Cook 1969).

These brecciated landforms develop upon a distinctive geological association in the central Mackenzie Valley region, NWT, at a stratigraphic level equivalent to anhydrites and halites in the deeper subsurface (Hamilton and Ford, 2002 ). In widespread outcrops the breccia unit is formally named the Bear Rock Formation, when covered by consolidated strata it is termed the Fort Norman Fm (Meijer Drees, 1993; Law, 1971). It is centred in Late Silurian-Early Devonian strata and defines an outcrop and subcrop belt more than 50,000 km2 in extent. In core, the Fort Norman Formation is 250-350 m or more in thickness. It consists of a thin upper limestone (Landry Member), a central Brecciated Member and, in some cores, undisturbed lower sequences of inter­bedded dolostones and anhydrite remnants. It is conformably overlain by 90-150 m of limestones and calcareous shales (Hume Formation-Eifelian). This evaporite dissolution breccia is possibly derived from the dissolution of not one, but several subcropping Devonian salt layers. That is, although not much discussed in the literature, the mother level may not be tied to a single stratigraphic layer.

Typical till-covered collapse and subsidence karst of the Bear Lake Formation can develop through the Hume and higher formations as a consequence of interstratal dissolution of the salt layers in the Fort Norman and equivalents, where meteoric groundwater circulation and sulphate dissolution have been recorded at core depths as great as 900 m (Figure, 14, 15).


In the NWT the Bear Rock Formation is considered to host to much of this widespread solution breccia, which up to 250 m thick. If present, the Landry Member is a brecciated limestone no more than 20 m thick. The main breccia level forms a visually set of outcrops made up of chaotic, vuggy mixtures of limestone, dolomite, and dedolomite clasts, variably cemented by later calcites, with small residual clasts and secondary encrustations of gypsum. Pack breccias in the Bear Creek and its equivalents displaying rubble fabric (predominant), crackle or mosaic fabric, are common and cliff-forming (Stanton, 1966; James & Choquette, 1988). Float breccias are rarer and tend to be recessive. Meijer Drees (1985) classifies the Bear Rock Formation as a late diagenetic solution breccia created by meteoric waters. Hamilton (1995) shows that calcite, dolomite and sulphate dissolution, plus dedolomitisation with calcite precipitation, are continuing today. They are re-working older breccia fabrics, creating new ones, as well as forming a suite of surficial karstic depressions and subsidence troughs ranging from metres to several km in scale.

The Canadian example constitutes an important set of observations that also relate to many other regions with widespread, basin-scale evaporite dissolution breccias, namely; evaporite solution breccias are multistage and can encompass significant time intervals. Similar-appearance evaporite solution breccias can form at different times in a basin's burial history, from different superimposed undersaturated hydrologies. Individual breccias in any single sample are typically responses to multistage, multi-time diagenetic-fluid overprints. This is also why potash ores in the Prairie evaporite preserve evidence of multiple times of potash mobilisation and mineralogy (part 5; Warren, 2016).

Detailed evaporite karst landform studies have focused on Bear Rock Mountain (type area) and the Mackay Range, which are outlying highlands on the east and west banks respectively of the Mackenzie River, and terrain between Carcajou Canyon and Dodo Canyon in the Mackenzie Mountains (Hamilton, 1995). These sites were covered by the Laurentide Ice Sheet (Wisconsinan) but were close to its western, sluggish margin. They are at the boundary between widespread and continuous permafrost in the ground today and can display some year-round groundwater circulation via taliks. Thaw/freeze and solifluction processes compete with ongoing evaporite dissolution to mould the topography.


Regionally, the principal karst landforms hosted in and above the Bear Rock Formation are varieties of sinkholes, blind valleys, solution-subsidence troughs and fault-bounded depressions (e.g. Figures 14, 15). Sinkholes range from single colla­pse features a few metres in diameter to merged or compound dolines up to 1.5 km2 and 100 m deep. Smaller individual sinkholes and collapse dolines may retain seasonal meltwater ponds, and there are permanent lakes draining to marginal ponors in some of the larger subsidence troughs. Blind valleys have developed where modern surface streams flow for several km into the evaporite karst zone from adjoining rocks. There are many relict, wholly dry valleys that may have been created by glacial meltwaters. Subsidence troughs have developed at the surface along contacts between the Bear Rock breccias and underlying, massive dolostones that are typically gently dipping. In the centre of the Mackay Range and at Bear Rock itself are solution depressions formed where the breccias make-up hanging-wall strata on steeply inclined fault planes on the collapse or pipe edge. The Mackay example is 3.2 km long, 1.0 km wide and-160 m deep (Ford, 1998).

Where patches of the Landry Member survive above the main breccia level(s), they are often broken into large, separate slabs that tilt into adjoining depressions in sharply differing directions, creating a very distinctive topography of dissolution draping and block rotation (see also Dahl Hit in Warren, 2016). Some slabs are rotated through 80-90°. Ridges (inter-sinkhole divides) that are wholly within the main breccias often display stronger cementation, represented by pinnacles as much as 30 m high The many sinking streams pass through the permafrost via taliks and emerge as sub-permafrost springs at stratigraphic contacts or topographic low points.

According to Hamilton (1995), the variety and intensity of karst landform assemblages on the Bear Rock Formation are like no other in Canada, and he notes he had not seen or read of very similar intensive karst topographies elsewhere. He attributes their distinctiveness to repeated evaporite dissolution and brecciation, with dedolomitisation and local case hardening, throughout the Tertiary and Quaternary, with these processes occurring multiple times in mountainous terrains subject to episodes of glaciation, permafrost formation and decay, and to vigorous periglacial action.

The spectacular karst domes that typify dolines and glacially associated surfaces atop an evaporite subcrop and are particularly obvious in regions of anticlinal salt-cored structures within regions of widespread permafrost. The accumulation of ground ice in initial fractures in the evaporite layer probably contributes to the heaving, folding and other displacement of breccia fragments in the dome. Tsui and Cruden (1984) attributed the examples that they studied in the salt plains of Wood Buffalo National Park Canada (Lat. 59-60°N) to hydration processes operating on subcropping bedded gypsum during the postglacial period. Ford and Williams (2007) argued such features indicate local injection of gypsum residuals during times of rapidly changing glacial ice loading. Whether they are created by glacial unloading/reworking or are a type of gravitationally-displaced dissolution breccia in a permafrost region is debatable; that in Canada they are a widespread type of evaporite karst residue is not. They characterise those parts of the permafrost-influenced region defined by dissolution of shallow subcropping folded Devonian salts in Northern Alberta, where the evaporites are typically exposed in anticlinal crests are today still retain fractionated gypsum residuals.

And so, in addition to widespread breccias hoisting karst domes, there are numerous natural collapse dolines atop dissolving shallow salt beds. A classic example is the water filled doline some km NE of Norman Wells (Figure 15).


Holbrook Anticline, Arizona

Subsidence driven by natural salt solution at depth generates regional-scale drape or monoclinal folding in strata atop the retreating salt edge. This is the corollary of the formation of a subsidence trough (Figure 16a). The 70-km long dissolution front in the Permian Supai salt of Arizona is defined by more than 500 sinkholes, fissures, chimneys and subsiding depressions some 40-50m across and 20-30m deep (Neal, 1995; Johnson, 2005). Away from the main dissolution zone the northeasterly-dipping Schnebly Hill Formation (aka Supai Salt) is composed of up to 150 metres of bedded halite, with local areas of sylvite along its northern extent (Figure 16a). Atop the solution front, there are a number of topographic depressions with playa lakes that in total cover some 300 km2. Salt-dissolution induced features to include areas known as; The Sinks, Dry Lake Valley, and the McCauley Sinks (Neal et al. 1998; Martinez et al. 1998). The Sinks region includes more than 20 steep-sided caprock sinkholes, with some that are more than 100 metres across and up to 30 metres deep (Figure 16b). The McCauley Sinks are the likely surface expression of compound breccia pipes and chimneys (Neal and Johnson, 2002).

Regional expanses of Supai Salt removal produce a regional gravity depression, largely coincident with a surface topographic depression, where there is as much as 100 metres of collapse and topographic displacement. The solution front is essentially coincident with the updip end of the Holbrook Anticline, a flexure defined by dip reversal in an otherwise northeasterly dipping succession (Figure 16a).

Rather than orogenically driven, the Holbrook Anticline is a subsidence-induced monoclinal flexure created by the northeasterly migrating dissolution front (Figure 16a). It may be the largest single solution-collapse fold structure in the world (Neal, 1995). The reverse dip of the flexure directly overlies the salt-dissolution front and marks the location of two major collapse depressions known as The Sinks and Dry Lake Valley, both occur where salt is within 300 m of the surface (Peirce, 1981). Although it has periodically held surface water, reports of several hundred acre-feet of flood water in the Dry Lake Valley playa draining overnight supports the notion of active fissure and cavern formation related to salt removal at depth. Major surface drainage events took place in 1963, 1979, 1984 and 1995, with more than 50 new sinkholes forming in the valley during that period. The continuing rapid appearance of new sinkholes testifies to the ongoing nature of dissolution in the underlying evaporites. According to Neal (1995), dissolution front features began forming in the landscape in the Pliocene and continue to form today.

The caprock collapse sinkholes centre in the Coconino Sandstone bed that overlie the salt, which is located 200-300 metres below the at-surface features. These caprock sinkholes define regions of focused breccia pipe development and discreet upward cavity migration (chimney stoping). The underlying salt is bedded and there are no indications of halokinesis anywhere in the basin. Sinkhole regions lie just ahead of a dissolution front that is migrating downdip, driven by the widespread dissolution of halite.

Other karst features attributed to evaporite dissolution in the Holbrook basin are; pull-apart fissures, graben sinks (downward-dropped blocks), breccia pipes and plugs, and numerous small depressions with and without sinkholes (Neal et al., 1998, 2001). Interestingly, many of the “karst” features occur in sandstones, not limestones; such caprock collapse sinkholes can form wherever pervasive dissolution has removed the underlying salt, independent of caprock lithology. The presence of the more than 500 karst features in the Holbrook basin, some of which formed in days, evokes practical karst hazard and infrastructure concerns, even in such a sparsely populated region (Martinez et al., 1998).

Implications

Evaporite-related karst landforms are in many ways similar carbonate karst features. But, the much higher solubilities of halite and other evaporite salts compared to limestone and dolomite means there are additional features unique to regions of salt dissolution.

In the subsurface, a bedded evaporite can be composed of thick impervious halite with intrasalt beds composed of anhydrite dolomite and calcite. The edges of this halite bed can dissolve even in the deep mesogenetic (burial) realm, wherever the edge of the salt is in contact with undersaturated waters. Insoluble residues bands start to form and can take the form of a basal anhydrite or a fractionated caprock. If the rise of undersaturated water is focused, a breccia cavern can form, and once it breaches the salt, the cavity will contain collapse blocks of the less soluble intrasalt beds. The cavity can then stope to the surface, forming a breccia pipe. A transtratal breccia pipe can rise through kilometres of overburden before attaining the surface. There is no equivalent process-response in the carbonate realm.Subsidence basins, troughs and megabreccia plains are also features that owe their origins to the rapid dissolution rates inherent to salt bed-fresher water contacts

In a similar fashion, the rapid dissolution of halite in the shallower parts of a salt basin means the evaporite karst features will transition from mesogenetic or bathyphreatic cavern formation to meteoric phreatic to vadose effects. This is the emphasis of the second article in this series, which will discuss processes forming vadose and phreatic caves in evaporites.

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