Hypogene Speleogenetic Evidences in theDevelopment of Cova des Pas de Vallgornera(Mallorca Island, Western Mediterranean)

J.J. Fornós, A. Ginés, J. Ginés, F. Gràcia, A. Merino, J. Cifre, and F. Hierro

Abstract In the southern part of the island of Mallorca, and developed in UpperMiocene reefal limestones, the Cova des Pas de Vallgornera is an exceptional coastalcave. Littoral mixing dissolution processes represent the most important speleoge-netic mechanism to be considered in the eogenetic karst platform where it devel-ops. Nevertheless, part of the cave consists of an extensive network of galleriesthat show morpho-sedimentary features pointing to a possible participation of hy-pogene speleogenesis. Its morphological assemblage illustrates the typical coastalkarstification, a noticeable meteoric water recharge and a possible deep recharge ofhypogenic origin. Solutional features related to rising flow are abundant. The pres-ence of vents and some related speleothems (crusts and rims) must be highlighted,together with Mn and Fe rich deposits hosting several minerals not observed untilpresent in other caves of the region. Given the monotonous surface geology aroundthe cave, it is suspected that ascending hypogene solutions could have reacted withthe host rock to form a unique mineral assemblage. These deep-seated phenom-ena could be associated with the feeble geothermal anomalies existing in the area,related to SW-NE faults.

1 Introduction

Cova des Pas de Vallgornera, located on the coast of Mallorca Island (W-Mediter-ranean), is an extensive maze cave partially drowned by brackish phreatic waters.It lies in the natural area of Migjorn, a littoral karst region built up by an Upper

J.J. Fornós, A. Ginés, J. Ginés, J. Cifre, F. HierroUniversitat de les Illes Balears, Palma (Mallorca), Spain, e-mail: joan.fornos@uib.es

F. Gràcia, A. MerinoFederació Balear d’Espeleologia, Palma (Mallorca), Spain

349B. Andreo et al. (Eds.), Advances in Research in Karst MediaDOI 10.1007/978-3-642-12486-0, © Springer 2010

350 J.J. Fornós et al.

Miocene reefal limestone sequence. Until recently, the speleogenetic mechanismsassociated with the littoral mixing zone were considered responsible for the excava-tion of most caves in Migjorn area (Ginés and Ginés 1992; Gràcia et al. 2007) beingremarkable the extensive participation of breakdown processes in its present-dayvolumetric configuration (Ginés and Ginés 2007).

Recent discoveries of new passages and breakdown chambers are evidences ofa complex speleogenesis. Apart from the dissolution in the freshwater-seawater mix-ing zone, large quantities of allochthonous infillings have been found related to animportant surface recharge. Moreover, solutional features observed in the new sec-tions of the cave include rising channels, ceiling channels, feeders and cave rims(Merino 2007); these morphologies together with the mineralogy of some deposits,may be tangible evidence of hypogenic processes (Ginés et al. 2009).

2 Geological Setting and Cave Description

Upper Miocene tabular deposits, constituted by a complex reef sequence (Fig. 1),outcrop all along the southern and eastern coast of Mallorca, shaping the flat karstregion called Migjorn where the cave is located (Fornós and Gelabert 2004). This

Fig. 1 The plan pattern of Cova des Pas de Vallgornera shows clear relationships with the faciesdisposition within the Upper Miocene Reef Complex. The Messinian Terminal Complex (CT)outcrops only on the sea cliffs of the area

Hypogene Speleogenetic Evidences in the Development of Cova des Pas de Vallgornera 351

post-orogenic littoral carbonate platform that fringes and onlaps the folded mesozoicbasement, has been affected by Neogene normal faults, disposed in a SW-NE trend,having produced a relative uplift of the area.

The cave has a current development longer than 63,000 m (Fig. 1). The only ex-isting entrance to the cave connects with a series of breakdown chambers quicklyattaining the phreatic level that shows an assemblage of aquatic galleries decoratedwith speleothems. A narrow passage at one of the furthest ends allows the connec-tion with important extensions, which begin with several great chambers disposedin a NW-SE trend; the dimensions attained by the biggest of them are 200 per 80 m.The inner part of the cave shows an irregular maze pattern where several main gal-leries can be differentiated, running parallel more than 1 km in a SW-NE direction(Merino et al. 2008).

The general morphology of the endokarstic system can be divided into twowell-individualised parts: a seaward or outer part of the cave where breakdownand spongework passages are dominant and, on the other hand, an inland or innerpart characterised by joint-guided galleries (Fig. 1) shaped with dissolution forms.Breakdown processes are ubiquitous all along the cave, especially in the seawardpart of the cave.

As a function of the lithological and hydrogeological characteristics of theseyoung carbonate rocks, a clear dichotomy in the morphogenetic assemblages arisesbetween those parts of the cave excavated in the reef front facies and the inner gal-leries developed in the back reef facies (Ginés et al. 2009). Cave zones showing gen-eralised breakdown processes occur in the highly porous reef front facies, where thecorals are omnipresent being affected by intense differential dissolution phenomena.

3 Morphological and Mineralogical Evidencesfor Hypogene Speleogenesis

The presence of rising channels is frequent in the galleries, being related to solu-tional ascending processes (Klimchouk 2007). These forms range from millimetresto some decimetres in width (Figs. 2a and b) and develop vertically along the over-hanging walls of the galleries. Some of these morphologies are similar to the bubbletrails (Palmer 2007; Audra et al. 2009), consisting in rising channels originated bythe release of dissolved gases (presumably CO2, in this case) that contribute to theaggressiveness of shallow phreatic waters. As far as morphologies of larger dimen-sions, small chambers and galleries in the cul-de-sac occur in the inner passages ofthe cave, showing in their floors impenetrable vertical conduits that could have actedas feeders (Klimchouk 2007).

Particular Mn and Fe rich blackish deposits occurring as detritic fine grainedsediments occur on the floors and on the subhorizontal walls of the passages, al-ternating sometimes with reddish clay and silty materials. These deposits have alsobeen also observed as very thin black coatings, more or less crystallized and/or in-durated, covering the walls of galleries or filling the rock fissures or, even formingbubbles like hard crusts that develop over its rocky hollowed floors.

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Cave rims and vents have been identified (Merino 2007) in the inner galleriesof the cave (Fig. 2c). Cave rim morphology resembles a shell; both sides of theseprojections are strikingly different. While the outside is rough and coralline, theinside is smooth, like the inside of the tube below the rim. Small and whitish crys-tallizations on the rim’s edge have been spotted. Rims occur in the upper borders offeeder-like solutional vertical hollows or vents which are narrow inaccessible pas-

Fig. 2 Morphological features related to hypogene speleogenesis consist of sharp solutional as-cending grooves whose widths are millimetric (a), as well as wider rising channels (b). Caverims (c) grow on the upper part of vertical vents occurring on the floors of some galleries

Fig. 3 BSE images of some minerals present at the cave rims and vent walls: a nordstrandite,b strontianite, c ankerite, calcite and barite and d celestine

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sages that link two different levels of galleries. Its walls are smooth and covered byweathered limestone and mineral deposits.

The nearby areas of the vents normally show mineral crusts (Fig. 2c) and scat-tered mineralizations, different from the common speleothem minerals found in themallorcan cave environment (Onac et al. 2005). They form minute crystalline pre-cipitates as well as degradative weathering products on the rock walls with non-frequent chemical elements, where a series of rare mineral species have been iden-tified (Fig. 3). Huntite, monohydrocalcite, strontianite, siderite, todorokite, celes-tine, and various clay minerals, apart from gypsum are very abundant (Merino et al.2009). There are also completely unexpected minerals such as barite, nordstran-dite, maghmite or paralstonite. Besides, on the outermost part of the vent, tiny,well-formed transparent celestine crystals have been observed mainly on the wallbedrock where gypsum has also been found.

4 Discussion and Conclusions

Cova des Pas de Vallgornera remains clearly individualised with respect to the nor-mal trends of the endokarst in Migjorn region. The plan development is really com-plex, in general terms, and fits in the maze caves category as defined by Palmer(1991, 2007).

Some morphological features as well as specific deposits point to the involvementof other speleogenetic mechanisms working together with the coastal mixing zoneprocesses (Ginés et al. 2009). Hypogene speleogenesis seems to have played animportant role in the excavation of the cave.

Whereas the spatial disposition and morphology in the seaward section of thesystem is like those in other coastal mixing caves of Migjorn, in the inland passagesthe pattern corresponds to a shallow phreatic 2D maze, in which the presence ofsolutional features presumably corresponding to a basal recharge is remarkable. Thepattern of the inner galleries of Cova des Pas de Vallgornera seems to correspondto a hydrogeological situation leading to the generation of a shallow phreatic maze,related to a local boost of the water aggressiveness. This situation is compatible witha hypogenic recharge, associated with local geothermal anomalies, and the mixingwith the coastal aquifer installed in this eogenetic karst platform.

Geothermal anomalies in the unconfined aquifer of the southern part of Migjornregion are linked to the neogene extensional faults that would have come to sustainthe participation of hypogene speleogenesis in shaping the Cova des Pas de Vallgo-rnera. Thermal waters with temperatures up to 51.6 °C have been documented in theunconfined sea level controlled aquifer, in the vicinity of Llucmajor village (Lópezand Mateos 2006). The basal recharge documented in the area must be related toregional groundwater circulation coming from the mesozoic folded basement out-cropping in the central mountains of the island.

One of the most conspicuous facts is the presence of deposits rich in Mn and Fe,elements that are moderately soluble in anoxic deep waters and easily precipitate

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as oxides and hydroxides in oxigenated shallow aquifers (Palmer 2007). Also, thechemical composition of minerals forming the rims and crusts with the presence inquite high amounts of non-frequent chemical elements such as S3C, Si4C, KC, Sr2C,or even Ti4C, Ni2C, Zr4C, Cu2C, Cr3C, Ba2C, and La in different proportions pointstowards an hypogenic influence into the crusts and cave rims. Their genesis must berelated to the participation of episodic deep- seated processes in the geomorpholog-ical evolution of the cave.

Acknowledgements This work has been supported by MICINN-FEDER research projectsCGL2006-11242-C03-01/BTE and CGL2009-07392.

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