This section is basically my geological mantra (some would say rant) surmised from reasonably detailed observations over the years and reliable geochemical information, but certainly not the final word on this subject. Just an educated guess from a decidedly opinionated guy.
“You can’t depend on your eyes when your imagination is out of focus.”
Samuel Clemens-Mark Twain
The Black Hills uplift contains a truly world class karstic terrain, which boasts the second and sixth largest caves on the planet as well as numerous other large and complex caves.
The karst features of this area are decidedly unique, however, in that they typically represent one end member of the hypogene (phreatic) to supergene (vadose) origin cycle for caves.
Much if not most major cave development and enlargement in the Black Hills region is thought to have taken place in the phreatic zone below the water table, with secondary enlargement and modification by vadose processes taking a much smaller role than in other typical karst areas.
Geologically speaking, much of the soluble bedrock in this uplift has until quite recently been covered by one or more hydrologically confining rock strata, and has most likely been under at least moderate thicknesses of cover during much of the speleogenetic process.
As a result, karst features may be evidenced only in the subsurface and the more typical expressions like sinkholes lapies and grikes may be totally absent in the surrounding terrain.
This is not to imply that the region does not have sinkholes and other observable surficial karst features, but only that they are far more limited in distribution than in most other major cave terrains, especially considering the number and size of the cave systems present.
Large phreatic passage Holloch Cave System, Switzerland
Keyhole Shaped Phreatic Tube with vadose "canyon" notch in base Elephant Cave, Wales
- Typical karstic features like sinkholes tend to be restricted to the Pahasapa limestone’s erosional margins nearest to its contact with the overlying Minnelusa formation. These features are associated with relatively recent supergene modification and enlargement of caves that owe their origins to dominantly phreatic processes.
- One of the most conspicuous absences from the Black Hills Karstic picture are the presence of well-developed sinkhole plains. This departure from the norm, however, is a direct consequence of the somewhat unique geological setting in which the most recently speleogenesis has been occurring.
- The cave systems exposed in the Black Hills uplift have not typically developed in a setting with a broad flat water table where fluctuations are primarily parallel to bedding planes as in Kentucky or Indiana. The current period of Karst development in the Black Hills has been within a moderately to locally steeply dipping confined aquifer on the flanks of an uplift with a substantial hydraulic gradient.
There has most certainly been more than one period of speleogenesis within the Madison formation, as there is observable stratigraphic evidence that the overlying Minnelusa formation has in some places infilled sinks and other karst features during its initial stages of sedimentation, and well before the deposition of the superjacent Permian and Jurassic age formations. This older Paleozoic age period of dissolution and cave development has often been referred to as “paleo-karst”.
At least some of the infilling of Paleozoic age karst, however, occurred well after the cycle of sedimentation was completed.
This period of infilling is associated with the concurrent dissolution of the gypsum and anhydrite in the Minnelusa during a much younger period of void formation that was simultaneously occurring within the underlying Madison (Pahasapa) Limestone. This period of karst development, which is responsible for most of the caves and karst found in the region today, is sometimes referred to as “neo-karst”.
- In the Black Hills region, karst features can most commonly be seen in the sides of drainages and on the surrounding bluffs. In some drainages, the sheer volume of observable solution enlarged fractures, voids, pockets, and dog-tooth spar incrustations is staggering. In other drainages these features are more subtle, but wherever a cross section of the upper part of the Pahasapa limestone is exposed they are omnipresent. Most all of these void features have phreatic characteristics and morphologies, and widespread dissolution within the upper 1/3 to 1/2 of the formation is indicated at one or more times since the formations deposition.
- The abundant pore space in the upper portion of the Madison in this area has also been found well out into the surrounding plains in deep drill holes; it is likely the result of older periods of speleogenesis as well as the ongoing process of de-dolomitization recognized throughout the aquifer’s 5 state area.
Exposure of the upper portion of the Pahasapa Limestone in Little Elk Creek canyon showing its' mega-porous character
- The widespread evidence of solutional activity within the Madison Group is what has made it the highly productive aquifer that it is. The extensive volume of interconnected pore space in this aquifer affords for rapid infiltration and recharge within the uplifted source areas of the Black Hills and Bighorn Mountains.
- Most (but arguably not all ) of the Paleozoic age cave systems (aka Paleokarst), appear to have been long ago filled with sediment and precipitated minerals. The bulk of the cave developments currently found in the region are probably post Laramide (< 65 My) in age, and were formed at some time after the Eocene (< 55 My) epoch.
Fold and Fault Related Systems
The fold and fault systems on the margins of the Black Hills have substantially enhanced the porosity and permeability along these discrete and highly localized structural trends. The structural deformation adjacent to faults and folds also provides for the abundant low angle bedding-plane fissures and high angle stress fractures needed to develop large scale cross and between channel flow paths. Within these zones of enhanced porosity and permeability, any actively forming and/or preexisting phreatic channels can undergo significant enlargement and develop interconnections into complex three dimensional networks with a substantially greater volume.
The complex geometric style of these caves suggests that passage development involved numerous interconnected phreatic loops in a confined aquifer setting. This environment implies one in which there was significant structural preparation, a substantial hydraulic gradient, high head pressure, variable but slow flow rates and a base level outlet to the system at significant distance from the source area of the solutions.
A substantial percentage of the passages in Black Hills caves tend to have low, flattened and anastomosing phreatic characteristics, and it is likely that the overlying confining layers were still largely in place during their formation. These passages are generally parallel or subparallel to bedding planes and do not typically “daylight” against the upper contact of the limestone in epigenetic sinks and/or other surface related features.
Many of the larger caves in the Black Hills uplift are labyrinthine style systems, with multiple levels and a highly complex series of interconnecting passages and loops. The passage geometries in these complex caves often mimic the trends of faults, folds and cross structures within the immediate area, suggesting a strong structural control on their development. A partial list of the complex multilevel caves currently explored in the Black Hills region includes such examples as:
Jewel Cave
Bethlehem Cave
Wind Cave
Brook’s Cave
Stagebarn Cave
Wildcate Cave
Reed’s Cave
Nameless Cave
The largest explored Black Hills caves tend to be spatially associated with major structural trends nearest to the flanks of the uplift. Many large (and small) cave geometries closely follow the trends of these major features which strongly suggests that a substantial period of their growth was post-deformational.
The diagram below illustrates an idealized monoclinal fold along the margins of the Black Hills Uplift. This situation represents a structural regime similar to that present during the speleogenesis of Stagebarn, Bethlehem and Wonderland caves. Although the example illustrated here is that of a monocline, similar types of structural regimes are also expected to be associated with any major fold, fault, dome or subsidence basin located marginal or internal to the Black Hills uplift.
- If you look closely at this diagram, you will notice that both surface derived (epigene/supergene) and subsurface (hypogene) thermal waters are shown as potential sources for the speleogenetic solutions. These two end member represent the current thoughts on the nature of the driving force for the cave forming process.
- The temperature, geochemistry and source of the solutions involved in the most recent period of speleogenesis continue to be arguable (imagine that!), as are the depth below the water table/piezometric surface and/or the degree of confinement for the host aquifer. The structural control, on the other hand, is hard to refute in most every moderate to large sized cave in the region, regardless of the source and nature of the solutions involved.
The following two diagrams show caves that are spatially associated with major folds and illustrate that the expected tension and shear directions associated with these major structures parallels the geometry of a significant number of the phreatic passages in these caves. This correlation suggests that these caves are post deformational and that their speleogenesis can be related, at least in part, to the nearby structures.
Stagebarn Caverns showing phreatic passage alignment with predicted tension and shears directions associated with the adjacent Stagebarn Canyon Monocline.
Bethlehem Cave showing phreatic passage alignment with the predicted tension and shears directions associated with the nearby Piedmont Monocline.
The anticlinal flexture of the monocline is directly south approximately 1500′. The synclinal flexture is unmapped, but likely trends through the cave area.
The Jewel Cave Structural System
This overlay of the surface structural map of the jewel cave area with the mapped passages in the cave shows a very close correspondence between the fault and passage trends.
- North of the main fault system and its’ splays on the northern margins of the cave network, all the known caves are only a few hundred feet in length at best.
- South of the fault system there is a complex of over 194 miles (and growing) of interconnected passages.
- The majority of these passages have orientations that correspond closely to fault and fold orientations and their associated stress related fissures and fractures.
The Wind Cave Structural System
The Wind Cave system is spatially associated with a series of folds and faults associated with the Pringle Fault to the north. The cave is dominantly developed on the NE limb of an anticline. Surface structures are difficult to map in this area due to large percentage of unconsolidated sediments that cover the bedrock in this area. All of the major structures mapped on the surface or underground, however, are oriented in a manner consistent with their having been a strong control on the development of the cave’s phreatic network. (My opinion…there are others)
The following diagram shows a closeup of the Wind Cave passage network. All of the major NW trending passages/galleries parallel the Anticline and Syncline that trend through the cave system. Most all of the cross passages are similarly oriented to the southwest, which is parallel to the Pringle fault and a series of associated faults close to the southern end of the mapped network.
Source of the Solutions
Ascension (Hypogene) vs. Descension (Supergene)
With respect to groundwater, the genesis of Black Hills caves has been attributed to a number of different theories and combinations thereof. These include:
- Dissolution by typical surface derived (epigenetic or supergene) carbonic acid enriched precipitation, as is the origin of most other karst areas.
- Dissolution due to the recharge and fluctuations thereof near the uppermost edge of the confined artesian Madison aquifer.
- Dissolution due to mixing of surface related and formation waters within the Madison formation.
- Attack from “below” by hypogene (subsurface derived) waters ascending and/or migrating laterally into the confined Madison aquifer.
The available evidence (in my opinion) suggests that varying combinations of “hypogene” waters and supergene solutions are responsible for the most recent period of speleogenesis.
- Hypogene solutions (thermal of otherwise) refers to waters that are derived from other than the immediately overlying “recharge area” and that have ascended or moved laterally along structural and stratigraphic pathways through the aquifer and resulted in cave development. This usage of hypogene adheres to a strictly hydrogeological definition of the term, irrespective of any geochemical or thermal implications. As such, these waters may or may not be of a thermal nature, and can have either concentrated or dilute chemical compositions. The defining criteria is that the dissolution process does not involve waters derived from the immediately overlying recharge area.
- Surface derived supergene/epigene carbonic acid enriched solutions from soil zone and atmospheric sources have doubtless been responsible for Black Hills cave genesis in some areas, as well. The term epigene (or more properly, supergene) refers to those processes that originate at the surface and work their way downward. This mode of speleogenesis is the most generally accepted means of cave development for karst areas worldwide, but it is certainly not the only way that caves can form, nor is it the most prevalent mode in this region.
As to the thermal. non-thermal aspects of any of the ascending or laterally migrating solutions involved, the Black Hills currently has a relatively high geothermal gradient and probably has had since at least the Eocene epoch, so it is not unreasonable to expect the involvement of thermal waters at some level in episodes of speleogenesis since that time.
As for the geochemical nature of the lixiviants involved or the involvement of any “hydrothermal” solutions or concentrated formation waters, that question is open to speculation, but is certainly not unreasonable. There are many areas within the present day Madison aquifer with formation waters that qualify as brines and there is widespread evidence of hydrothermal solutions and their affects in the northern hills and Edgemont areas, so the possibility cannot be discounted.
I have a bit more observation and geochemical testing to do before I rant on further on that subject, but I rather lean towards the involvement of a thermal hypogene component due to its simplicity and a personal bias towards the validity and liberal application of Occam’s razor until proven otherwise.
Definitely more about this later… particularly if I can arrange for some fluid inclusion studies of dog-tooth spar from more than just one Black Hills cave…. like I said, this page is a work in progress.
Final (for now) Thoughts
The Madison (Pahasapa) limestone contains over 400 known caves within the Black Hills area. At least two of these caves contain more than 100 miles of mapped passageways each and are currently ranked as the 2nd and 6th longest cave systems known on earth. Many others in the hills region are a mile or more in length, and the list is growing every year.
- The full extent of most of the hill’s larger caves systems are as yet unknown, and the potential for further exploration and discovery is without any doubt, second to none, both within these caves and elsewhere within the Hills.
In my opinion, we most likely have the largest cave system in the world under us… maybe even two or three of the top five. We just need to keep pushing those leads, walking those ridges, looking in the right places and using our heads and the best available science to guide our efforts.