Understanding the diagenesis of speleothems is important on account of the fact that such deposits are often used for determining palaeoclimate parameters and for estimating the ages of speleothem growth. Impressive speleothem deposition of Vallesian age occurred in an immense palaeokarst network in the Western Desert, Egypt, the age of formation being determined on the basis of mammalian biochronology (fossils found in spelean clastic deposits intercalated between speleothems). Many of the Egyptian speleothems have been pervasively recrystallised internally, but their outer surfaces are usually well preserved except in the formations which were buried in clastic deposits, in which case the entire speleothem can be recrystallised. The recrystallisation results in large crystals (up to 20 cm diameter) growing radially outwards from the centre of stalagmites and stalactites, or at right angles to the outer surface of flowstone deposits. It is clear that crystal growth occurred without change of volume. Although the recrystallisation of speleothems in the Western Desert of Egypt resulted in the development of unusually large calcite crystals, it does indicate that diagenesis may be an important process that needs to be taken into account before speleothems in other karst systems can be used as raw material for unravelling palaeoclimatic and geochronological parameters. The gross morphology of the Egyptian speleothems is described in order to put on record the effects of diagenesis on them. The geochemistry of the speleothems remains to be studied.

The south Adriatic shelf offshore of the predominently carbonate Apulian coast is characterized by a peculiar rough topography interpreted as relic karst formed at a time of lower sea level. The study area covers a surface of about 220 km2, with depths ranging from 50 to 105 m. The most relevant and diagnostic features are circular depressions a few tens to 150 m in diameter and 0.50 to 20 m deep thought to be dolines at various stages of evolution. The major doline, Oyster Pit, has its top at about 50 m water depth and is 20 m deep. It is partly filled with sediments redeposited by episodic mass failure from the doline’s flank. Bedrock samples from the study area document that Plio-Pleistocene calcarenites, tentatively correlated with the Calcarenite di Gravina Fm, are a prime candidate for the carbonate rocks involved in the karstification, although the presence of other units, such as the Peschici or Maiolica Fms, is not excluded. The area containing this subaerial karst landscape was submerged about 12,500 years ago as a result of the postglacial transgression over the continental shelf.

We have analyzed the geoelectric response produced by three cavities cut into different geological substrata of granite, phyllite, and sandstone that had previously been characterized by direct methods. We also examined a mining void excavated in granite. In each case, we applied three different geoelectric arrays (Wenner-Schlumberger, Wenner and dipole-dipole) and several inter-electrode spacings. The survey results suggest that electrical resistivity tomography is a viable geophysical tool for the detection and monitoring of mining voids and other subsurface cavities. The results vary depending on a wide range of factors, such as the depth and diameter of the cavity, the multi-electrode array used, the inter-electrode spacing, the geological model, and the density of the data. The resolution capacity of the Wenner- Schlumberger array for the detection of these cavities was greater than that of the Wenner array and slightly better than the dipole-dipole. There is a direct relationship between inter-electrode spacing and diameter of the cavity. In general, we observed a loss of resolution as the distance between the electrodes increased. The most efficient detection was achieved when the inter-electrodes distance was less than or equal to the diameter of the cavity itself. In addition, cavity detection became increasingly less precise with its depth beneath the surface. Cavities with a radius of about 1.5 m were located by both the Wenner- Schlumberger method and the dipole-dipole at depths of more than 4.6 m, which means that prospecting can be carried out at depths 3 times the radius of the cavity.

Fluorescent dye tracer breakthrough curves (TBCs) obtained from quantitative traces in karst flow systems record multiple processes, including advection, dispersion, diffusion, mixing, adsorption, and chemical reaction. In this study, TBCs were recorded from small, bench-scale physical models in an attempt to isolate, understand, and quantify some of these processes under full-pipe flow conditions. Dye traces were conducted through a suite of geometries constructed out of Pyrex glass. These geometries consisted of (1) linear conduits, of varying length and diameter, (2) single and dual mixing chambers, and (3) a single chamber with an immobile region. Each glass system was connected to a constant flow apparatus. Dye was then injected with a syringe, allowed to flow through the system, and be naturally or artificially mixed in the process. Solute breakthrough was recorded in a scanning spectrofluorophotometer and the resulting TBC was analyzed. Independent variables examined in each of the three settings were discharge (Q) and dye concentration (Co). Artificial mixing rates (RM), induced by magnetic stirrers in settings (2) and (3), were also considered. Initial runs varied Q from 0.75 to 1.25 mL/s, with constant RM ranging from 0 to 360 revolutions per minute (rpm). Preliminary data yield realistic-looking breakthrough curves with steeply rising leading edges, a peak, and an asymmetric, exponential tail. Analysis of laboratory variables with respect to hydraulic parameters extracted from each TBC suggests that discharge and mixing rate alone can differentiate conduit complexity at the laboratory scale.

The classic hypothesis of G. Horn’s (1935) subglacial speleogenesis as an explanation of the relatively small diameter cave conduits in the Scandinavian marble stripe karst is reviewed. Recent work, including accurate cave mapping and morphological analysis, radiometric dating of cave deposits, chemical kinetics experiments and computer simulations have challenged the old theory. Scandinavia has relatively small caves that often have surprisingly high ages, going beyond the limit of Th/U dating. The high ages are apparently compensated by correspondingly slow wall retreat rates in the icecontact regime, and longer periods when the caves were inactive. Ice-contact speleogenesis varied in time and space, in pace with waxing and waning of wet-based ice. Maze or labyrinth morphology appears as a characteristic feature of caves ascribed to these processes.

Sudden cover-collapse sinkhole (doline) development is uncommon in the karstic Cretaceous-age Edwards limestone of central Texas. This paper presents a case-study of a sinkhole that formed within a stormwater retention pond (SWRP) in southwest Austin. Results presented include hydrogeologic characterizations, fate of stormwater, and mitigation of the sinkhole. On January 24, 2012, a 11 cm (4.5 in) rainfall filled the SWRP with about 3 m (10 ft) of stormwater. Subsequently, a sinkhole formed within the floor of a SWRP measuring about 9 m (30 ft) in diameter and 4 m (12 ft) deep. About 26.5 million liters (7 million gallons) of stormwater drained into the aquifer through this opening. To determine the path, velocity, and destination of stormwater entering the sinkhole a dye trace was conducted. Phloxine B was injected into the sinkhole on February 3, 2012. The dye was detected at one well and arrived at Barton Springs in less than 4 days for a minimum velocity of 2 km/day (1.3 mi/day).Review of pre-development 2-foot topographic contour and geologic maps reveals that the SWRP was built within a broad (5,200 m2; 6 acre), shallow depression bounded by two inferred NE-trending fault zones. Photographs taken during SWRP construction showed steep west-dipping bedrock in the northern SWRP wall. Following collapse of the sinkhole, additional hydrogeologic characterization included excavation to a depth of 6.4 m (21 ft), surface geophysics (resistivity), and rock coring. Geologic materials consisted mostly 89of friable, highly altered, clayey limestone consistent with epikarst in-filled with terra rosa providing a cover of the feature. Dipping beds, and fractured bedrock support proximity to the mapped fault zone. Geophysics and surface observations suggested a lateral pathway for stormwater flow at the junction between the wet pond’s impermeable geomembrane and compacted clay liner for the retention pond. The collapse appears to have been caused by stormwater down-washing poorly consolidated sediments from beneath the SWRP and into a pre-existing karst conduit system.

Mitigation of the sinkhole included backfill ranging from boulders to gravel, a geomembrane cover, and reinforced concrete cap. Additional improvements to the SWRP included a new compacted clay liner overlain by a geomembrane liner on the side slopes of the retention pond.

Permian bedded salt is widespread in the Anadarko Basin of western Oklahoma and the Texas Panhandle, where partial or total dissolution of the shallowest salt in some areas has resulted in subsidence and/or collapse of overlying strata. Groundwater has locally dissolved these salts at depths of 10–250 m. The distribution (presence or absence) of salt-bearing units, typically 80–150 m thick, is confirmed by interpretation of geophysical logs of many petroleum tests and a few scattered cores. Salt dissolution by ground water is referred to as “salt karst.”Chaotic structures, collapse features, breccia pipes, and other evidence of disturbed bedding are present in Permian, Cretaceous, and Tertiary strata that overly areas of salt karst. The dip of Permian and post-Permian strata in the region normally is less than one degree, mainly towards the axis of the Anadarko Basin. Where strata locally dip in various directions at angles of 5–25 degrees or more, and underlying salt units show clear evidence of dissolution, these chaotic dips must result (mostly, if not totally) from subsidence and collapse into underlying salt-dissolution cavities.Gypsum karst and resultant collapse of overlying strata have been proposed in many parts of the Anadarko Basin. However, the gypsum beds typically are only 1–6 m thick and more than 100 m deep, and cannot contribute to disruption of outcropping strata—except where they are within 10–20 m of the surface.Typical areas of disturbed bedding comprise several hectares, or more, with outcrops of moderately dipping strata—as though large blocks of rock have foundered and subsided into large underground cavities. Other examples of disturbed bedding are small-diameter breccia pipes, or chimneys, that extend vertically up from salt-karst cavities, through several hundred meters of overlying strata. The best evidence of these chimneys are collapsed blocks of Cretaceous strata, chaotically dropped some 50 m, or more, that are now juxtaposed against various Permian formations on the north flank of the Anadarko Basin. Any study of surface or shallow-subsurface geology in the Anadarko Basin must consider the influence of subsurface salt karst on the structure and distribution of overlying rocks

Volumetric depletion of a subsurface body commonly results in the collapse of overburden and the formation of enclosed topographic depressions. Such depressions are termed sinkholes in karst terrains and pit craters or collapse calderas in volcanic terrains. This paper reports the first use of computed X-ray microtomography (?CT) to image analog models of small-scale (~< 2 km diameter), high-cohesion, overburden collapse induced by depletion of a near-cylindrical (“stock-like”) body. Time-lapse radiography enabled quantitative monitoring of the evolution of collapse structure, velocity, and volume. Moreover, ?CT scanning enabled non-destructive visualization of the final collapse volumes and fault geometries in three dimensions. The results illustrate two end-member scenarios: (1) near-continuous collapse into the depleting body; and (2) near-instantaneous collapse into a subsurface cavity formed above the depleting body. Even within near-continuously collapsing columns, subsidence rates vary spatially and temporally, with incremental accelerations. The highest subsidence rates occur before and immediately after a surface depression is formed. In both scenarios, the collapsing overburden column undergoes a marked volumetric expansion, such that the volume of subsurface depletion substantially exceeds that of the resulting topographic depression. In the karst context, this effect is termed “bulking”, and our results indicate that it may occur not only at the onset of collapse but also during progressive subsidence. In the volcanic context, bulking of magma reservoir overburden rock may at least partially explain why the volume of magma erupted commonly exceeds that of the surface depression.

Water temperature is a non-conservative tracer in the environment. Variations in recharge temperature are damped and retarded as water moves through an aquifer due to heat exchange between water and rock. However,within karst aquifers, seasonal and short-term fluctuations in recharge temperature are often transmitted over long distances before they are fully damped. Using analytical solutions and numerical simulations, we develop relationshipsthat describe the effect of flow path properties, flow-through time, recharge characteristics, and water and rock physical properties on the damping and retardation of thermal peaks/troughs in karst conduits. Using these relationships, one can estimate the thermal retardation and damping that would occur under given conditions with a given conduit geometry. Ultimately, these relationships can be used with thermal damping and retardation field data to estimate parameters such as conduit diameter. We also examine sets of numerical simulations where we relax some of the assumptions used to develop these relationships, testing the effects of variable diameter, variable velocity, open channels, and recharge shape on thermal damping and retardation to provide some constraints on uncertainty. Finally, we discuss a multitracer experiment that provides some field confirmation of our relationships. High temporal resolution water temperature data are required to obtain sufficient constraints on the magnitude and timing of thermal peaks and troughs in order to take full advantage of water temperature as a tracer.

Calcite spar (crystals >1 cm in diameter) are common in limestone and dolostone terrains. In the Guadalupe Mountains, New Mexico and west Texas, calcite spar is abundant and lines small geode-like caves. Determining the depth and timing of formation of these large scalenohedral calcite crystals is critical in linking the growth of spar with landscape evolution. In this study, we show that large euhedral calcite crystals precipitate deep in the phreatic zone (400–800 m) in these small geode-like caves (spar caves), and we propose both are the result of properties of supercritical CO2 at that depth. U-Pb dating of spar crystals shows that they formed primarily between 36 and 28 Ma. The 87Sr/86Sr values of the euhedral calcite spar show that the spar has a signifi cantly higher 87Sr/86Sr (0.710–0.716) than the host Permian limestone (0.706–0.709). This indicates the spar formed from waters that are mixed with, or formed entirely from, a source other than the surrounding bedrock aquifer, and this is consistent with hypogene speleogenesis at signifi cant depth. In addition, we conducted highly precise measurements of the variation in nonradiogenic isotopes of strontium, 88Sr/86Sr, expressed as 88Sr, the variation of which has previously been shown to depend on temperature of precipitation. Our preliminary 88Sr results from the spar calcite are consistent with formation at 50–70 °C. Our fi rst U-Pb results show that the spar was precipitated during the beginning of Basin and Range tectonism in a late Eocene to early Oligocene episode, which was coeval with two major magmatic periods at 36–33 Ma and 32–28 Ma. A novel speleogenetic process that includes both the dissolution of the spar caves and precipitation of the spar by the same speleogenetic event is proposed and supports the formation of the spar at 400–800 m depth, where the transition from supercritical to subcritical CO2 drives both dissolution of limestone during the main speleogenetic event and precipitation of calcite at the terminal phase of speleogenesis. We suggest that CO2 is derived from contemporaneous igneous activity. This proposed model suggests that calcite spar can be used for reconstruction of landscape evolution