Salty Matters

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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 Paleokarst refers to ancient karstic features that are no longer active and tied to ancient basin flushing hydrologies and now buried land surfaces.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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