In shallowmarine carbonate successions of the Phanerozoic, evidence for shorter-lived subaerial exposure stages of ancient carbonate seafloors is commonly found in the presence of small-scale epi-karst solution pits in discontinuity surfaces. Under favorable conditions, these solution features are accompanied by soil features including calcretes or root traces, alveolar septal structures, petrographic evidence such as pendant cements, circumgranular cracks or pisoliths, bleaching and staining of carbonate rocks or circumstantial geochemical evidence. Perhaps more often, however, ancient carbonate successions lack undisputable evidence for meteoric karsting. Using a well exposed case example from the Aptian of Oman, we here document that the superficial visual field inspection of solution pits in discontinuity surfaces may lead to erroneous interpretations. Outcrops at
JabalMadar, a diapiric structure, allowfor an in-depth analysis of dissolution features in the regionally extensive top Shu'aiba discontinuity. The solution pits discussed here were investigated for their stratigraphic position, their orientation relative to bedding planes, diagenetic and petrographic features and their potential relation to extensional fractures related to the updoming of Jabal Madar. The mainmessage brought forward, is that under burial conditions, spatially localized, hypogenic carbonate leachingmay formfeatures that are easily mistaken for ancient meteoric epikarst. These features preferentially form in interstratal positions where fractured, massive carbonate rocks are capped by a major discontinuity surface overlain by non-fractured argillaceous sediments. Thus,while dissolution–reprecipitation processes in the burial phreatic realmare omni-directional in permeable carbonates, low-permeability, argillaceous fines are not, or to amuch lesser extent, prone to chemical corrosion.
As a consequence carbonate-aggressive burial fluids leach out pits at the carbonate–shale interface. These appear to protrude perpendicular as bowl-shaped depressions into the underlying limestones and are – in the case examples documented here – preferentially aligned along factures. These findings have significance for the interpretation of ancient epikarst features in shallow marine carbonate successions

A typical small-scale epikarst ecosystemusually consists of an epikarst zone, soil and vegetation. In this study, to determine the hydro-eco-geochemical effects of an epikarst ecosystem in subtropical humid area, the samples of vegetation, soil, soil microbes, rainfall, throughfall, stem flow, soil water and epikarst springs of Nongla Village, Mashan County, Guangxi in China were collected and analyzed. The research results have shown in the epikarst ecosystem, the conductivity, temporary hardness and total carbon increased continuously in hydro-ecochemical cycle; the vegetation–soil system conducted the transformation and transference of carbon in hydro-ecochemical cycle; the vegetation layer was the major source for organic carbon, while the soil layer was of the important chemical field for the conversion of organic/inorganic carbon and HCO3 –, which would affect the epikarst dynamical system; for most ions, the vegetation layer and shallow soil layer presented more leaching effect than absorption, in contrast, the deep soil layer behaved oppositely. The vegetation layer and shallow soil layer leached ions, and deep soil layer absorbed them. With the plant community presenting in a positive succession, the epikarst ecosystem trended to be stabilized gradually, which made the hydro-eco-geochemical effects to be adjusted and controlled more effectively

Whereasmost karstic cavesworldwide are formed by carbonic acid, a small but significant number of sub-surface cavities are the product of sulfuric acid speleogenesis (SAS). In the Eastern Alps, no cave has so far been attributed to this type. In this multidisciplinary studywe demonstrate that Kraushöhle in northern Styriawas indeed formed by SAS. The cave pattern shows individual chambers, 3D-mazes and blind galleries, as well as typical SAS morphologies such as cupolas, gypsum replacement pockets, corrosion notches and convection niches. “Ceiling pendant drip holes” are described here for the first time and these corrosion features are fully consistent with the SAS model. Other features of Kraushöhle include thick gypsum deposits with strongly depleted δ34S values and other minerals – mostly sulfates – indicating highly acidic conditions. We also studied acid–rock interaction processes giving rise to widespread corrosion and concomitant replacement by gypsum. Petrographic and geochemical analyses reveal the presence of a distinctive alteration layer of highly increased porosity at the interface between the host limestone and the secondary gypsum. Dissolution and replacement of the limestone was fast enough to prevent the development of C and O isotopic alteration halos but resulted in selective leaching of elements. This stable isotope signal is thus different from the pronounced isotope gradient commonly observed in CO2-dominated hypogenic caves. Petrographic observations reveal that the limestone–gypsum replacement was a nearly constant volume process.Whereasmost karstic cavesworldwide are formed by carbonic acid, a small but significant number of sub-surface cavities are the product of sulfuric acid speleogenesis (SAS). In the Eastern Alps, no cave has so far been attributed to this type. In thismultidisciplinary studywe demonstrate that Kraushöhle in northern Styriawas indeed formed by SAS. The cave pattern shows individual chambers, 3D-mazes and blind galleries, as well as typical SAS morphologies such as cupolas, gypsum replacement pockets, corrosion notches and convection niches. “Ceiling pendant drip holes” are described here for the first time and these corrosion features are fully consistent with the SAS model. Other features of Kraushöhle include thick gypsum deposits with strongly depleted δ34S values and other minerals – mostly sulfates – indicating highly acidic conditions. We also studied acid–rock interaction processes giving rise to widespread corrosion and concomitant replacement by gypsum. Petrographic and geochemical analyses reveal the presence of a distinctive alteration layer of highly increased porosity at the interface between the host limestone and the secondary gypsum. Dissolution and replacement of the limestone was fast enough to prevent the development of C and O isotopic alteration halos but resulted in selective leaching of elements. This stable isotope signal is thus different from the pronounced isotope gradient commonly observed in CO2-dominated hypogenic caves. Petrographic observations reveal that the limestone–gypsum replacement was a nearly constant volume process.