Recent developments on morphometric analysis of karst caves

Nowadays, the use of computers and the digital techniques for survey processing easily allows to carry out morphometric analysis of caves and karst systems. Morphometric indices have been proposed by many authors in order to describe the genetic features of some particular types of caves, suchas coastal ones, maze caves or isolated voids. Large cave systems can be analyzed throughthe reconstruction of a 3D model. The digital model can be used to evaluate the role of the structural setting or the evolution stages throughthe recognizing of particular levels where epi-phreatic passages are more developed. These levels are particularly significant when a lithological or structural control can be excluded. Some morphometric indices, obtained by the ratios of simple dimension parameters of caves, allow the statistical analysis of large databases, collected in the regional speleological inventories, in the attempt to recognize different geomorphic populations of caves. Further studies and a greater number of analysis could allow to use these morphometric indices to distinguishgenetic categories of caves and to get a useful instrument for the study of the evolution of karst areas

Recent developments on morphometric analysis of karst caves

Nowadays, the use of computers and the digital techniques for survey processing easily allows to carry out morphometric analysis of caves and karst systems. Morphometric indices have been proposed by many authors in order to describe the genetic features of some particular types of caves, suchas coastal ones, maze caves or isolated voids. Large cave systems can be analyzed throughthe reconstruction of a 3D model. The digital model can be used to evaluate the role of the structural setting or the evolution stages throughthe recognizing of particular levels where epi-phreatic passages are more developed. These levels are particularly significant when a lithological or structural control can be excluded. Some morphometric indices, obtained by the ratios of simple dimension parameters of caves, allow the statistical analysis of large databases, collected in the regional speleological inventories, in the attempt to recognize different geomorphic populations of caves. Further studies and a greater number of analysis could allow to use these morphometric indices to distinguishgenetic categories of caves and to get a useful instrument for the study of the evolution of karst areas

Surface and subsurface drainage evolution of the Corfino and Soraggio karst areas (Tuscany, Italy)

The Pania di Corfino and Ripa di Soraggio are two minor karst areas in Tuscany, having a surface of only 11 km2, but contain more than 100 known caves. Some caves are old epi-phreatic passages testifying to a discontinuous lowering of base level in the two major valleys that cross the carbonate outcrops: the Serchio di Soraggio and the Fiume rivers, respectively located along the NWand SE borders of the massif. The spatial-altimetric distribution of major caves, whichare found on a vertical range of a few hundreds of meters, and their relationships withthe position of surface alluvial deposits have allowed to infer a first evolutionary framework of karst during the late Quaternary. If we refer to a simple model, where fluvial deposition occurs mainly during cold stages and incision during warm stages, the discrete distribution of cave passages suggests that the different epi-phreatic phases are the responses to the alternation of cold and warm periods. In any case, the re-organization of the river network induced by the tectonic uplift had a relevant effect on cave systems. First, the underground diversion of surface drainage enhanced the downcutting of NWand SE peripheral streams, whichreceived a larger quantity of water throughkarst springs due to the favored morpho-structural setting. Successively, the backward piracy of the allogenic catchments of the karst systems by surface tributaries led to the dewatering of caves and to the present situation

Hydrogeological and geochemical overview of the karst aquifers in the Apuan Alps (Northwestern Tuscany, Italy)

Apuan Alps, in north-western Tuscany (Italy), have a very complex geological structure. For this reason karst springs show very different behaviours according to the geological setting of aquifer systems. More than 80 springs are fed by carbonate aquifers; flow rates (Q) range from 10 to 1600 L/s, in average, temperatures range from 8 to 15 °C. Deep and very slow groundwater flow feed some low-thermal springs (20-30 °C). Major karst springs (Q > 100 L/s) are concentrated in two altimetry ranges, one from 200 to 300 m a.s.l. in the seaward side (SW-NW) and a second one from 500 to 600 m a.s.l. on the inner (NE-SE) side of the mountain range. Most of the springs are the final destination of large karst systems developed in meta-dolomite and marbles characterized by a very rapid flow. Some springs have a regular regime and are fed by bathyphreatic systems in metamorphic rocks or by carbonate aquifers with a major contribution of fissured drainage in non-metamorphic rocks. Large physical-chemical variations, both in space and time, are observed as a consequence of lithological heterogeneity, mixing processes and hydrodynamic conditions. Most waters are of the Ca-HCO3 type, but Ca-SO4 and Na-Cl facies are also present. A wide range of electrical conductivity is recorded, with values between 0.1 μS/cm and 10 μS/cm. Significant differences in the average isotopic signature (e.g. δ18O from -5.5 to -8.5‰) of the "base-flow" are registered due to the variability of hydrogeological basins dimension and their distribution in terms of altitude range and side. In some cases, springs with similar chemical features and located close to each other, point out very different isotopes signature, thus highlighting complicated flow path of groundwater. Furthermore, different seasonal evolutions of isotopic signatures are registered

Descrizione morfologica e mineralogica della grotta ipogenica sulfurea di Montecchio (Grosseto, Toscana)

La Grotta di Montecchio (254 T/GR, presso Saturnia, provincia di Grosseto) si apre a quota 320 m, in corrispondenza di un limitato affioramento di Calcare Massiccio, prospiciente la valle del Fiume Albegna. Benché nota da tempo, finora non sono stati fatti studi dettagliati di tipo speleogenetico su questa particolarissima cavità. Questa grotta, che si sviluppa per oltre 1700 m, è infatti caratterizzata dalla presenza di una falda di acqua con T poco sopra i 30°C e con un elevato contenuto in solfato, alla profondità di circa 100 metri dall’ingresso: si tratta quindi di una grotta ipogenica attiva. Di particolare interesse è l’abbondanza di gesso, a partire da circa 40 metri di profondità, prodotto dalla reazione tra acido solforico e roccia carbonatica. Tale processo, che è avvenuto in condizioni aeree (ossigenate), ha creato anche una serie di morfologie particolari come le piccole tasche di sostituzione e le bubble trails. La parte inferiore della cavità presenta depositi di calcite flottante di spessore anche superiore a 1 metro che testimoniano lo stazionamento di acque, probabilmente termali, in ambienti con notevole evaporazione legata a correnti d’aria. Undici campioni di prodotti di alterazione della roccia sono stati sottoposti ad analisi diffrattometrica ed esaminati al microscopio elettronico dotato di microsonda per analisi chimiche semiquantitative. Tali analisi hanno evidenziato la presenza di materiali residuali (quarzo, feldspato), alcuni ossidati (ematite, goethite e boehmite), ma soprattutto materiali di alterazione come clorite ed illite. Di particolare interesse sono i prodotti di alterazione acida sia della roccia carbonatica (gesso) sia delle argille (alunite, jarosite e caolinite). Questi minerali sono strettamente legati alle fasi speleogenetiche della grotta, essendo formati dall’interazione della roccia madre con i vapori sulfurei provenienti dalla falda termale.The Montecchio cave (254 T/GR, close to Saturnia, Grosseto province) opens at 320 m asl, in a small outcrop of Calcare Massiccio, close to the Albegna river. Although known since a long time, there are no speleogenetical studies on this very special cave. This cave, with a development of over 1700 m, is characterised by the presence of the thermal aquifer at a depth of 100 m below the entrance. The water has a temperature of over 30°C and high sulphate content. The cave can thus be considered an active hypogenic system. The cave hosts large gypsum deposits, starting from 40 m below the entrance. This gypsum derives from the reaction between sulphuric acid and the carbonate hostrock. This process, that occurs in the vadose (oxygen rich) environment, has created a set of typical morphologies such as replacement pockets and bubble trails. The lower part of the cave hosts calcite cave raft deposits over 1 m thick, evidence of standing, probably thermal, water pools in strongly evaporative areas related to strong air currents. Eleven samples of weathering products have been sampled and analysed at the X-ray diffractometer and investigated under SEM/EDAX. These mineralogical analyses have identified residual minerals (quartz, feldspar), some oxides and hydroxides (hematite, goethite and boehmite), but especially weathering products such as chlorite and illite. The products of acid weathering are particularly interesting, like those derived from calcite (gypsum) and from clays (alunite, jarosite and caolinite). These minerals are strictly related to the speleogenesis of the cave, having formed by the interaction between the host rock and the sulphuric vapors rising from the thermal aquifer

The influence of light attenuation on the biogeomorphology of a marine karst cave: a case study of Puerto Princesa Underground River, Palawan, the Philippines

Karst caves are unique biogeomorphological systems. Cave walls offer habitat for microorganisms which in-turn have a geomorphological role via their involvement in rock weathering, erosion and mineralisation. The attenuation of light with distance into caves is known to affect ecology, but the implications of this for biogeomorphological processes and forms have seldom been examined. Here we describe a semi-quantitative microscopy study comparing the extent, structure, and thickness of biocover and depth of endolithic penetration for samples of rock from the Puerto Princesa Underground River system in Palawan, the Philippines, which is a natural UNESCO World Heritage Site. Organic growth at the entrance of the cave was abundant (100% occurrence) and complex, dominated by phototrophic organisms (green microalgae, diatoms, cyanobacteria, mosses and lichens). Thickness of this layer was 0.28 ± 0.18 mm with active endolith penetration into the limestone (mean depth = 0.13 ± 0.03 mm). In contrast, phototrophs were rare 50 m into the cave and biofilm cover was significantly thinner (0.01 ± 0.01 mm, p < 0.000) and spatially patchy (33% occurrence). Endolithic penetration here was also shallower (< 0.01 mm, p < 0.000) and non-uniform. Biofilm was found 250 m into the cave, but with a complete absence of phototrophs and no evidence of endolithic bioerosion. We attribute these findings to light-induced stress gradients, showing that the influence of light on phototroph abundance has knock-on consequences for the development of limestone morphological features. In marine caves this includes notches, which were most well-developed at the sheltered cave entrance of our study site, and for which variability in formation rates between locations is currently poorly understood