Siliceous karst development in the Fontainebleau ...

Siliceous karst development in the Fontainebleau Sandstone (France) Médard Thiry Mines Paris, Géosciences 35 rue St Honoré, 77305 Fontainebleau Cedex, France and Groupe d’Etude, de Recherche et de Sauvegarde de l’Art Rupestre (G.E.R.S.A.R.) e-mail: [email protected] Abstract The flat lying Fontainebleau sandstone tables, embedded in uncemented sand, display numerous siliceous karst morphologies such as vertical and horizontal pipes, karren morphologies, etc., completed with a 10 to 20 m thick weathering profile of bleached sand. These morphologies result from progressive dissolution of the sandstone quartz cement along water-seeping pathways. Dissolution alters the sandstone to loose sand that fills the dissolution features. At the walls of the dissolution structures, a friable centimetric-thick sandstone rim develops within which the overgrowths are only partly dissolved. In some places, the dissolution pipes are lined with secondary silica deposits. Somewhere, the outcrops of this friable sandstone are the sites of pre-historical carvings. Key words Sandstone, dissolution, weathering, morphology, Paris Basin, France. Received: 18.12.2006 Accepted: 11.04.2007 Geological setting The Fontainebleau Sand is of Stampian (Oligocene) age and forms a 50 to 80 m thick unit of fine-grained, well-sorted sand of marine beach and aeolian dune origin (Alimen 1936). South of Paris, the Fontainebleau sand is preserved under the Etampes and Beauce Limestones of the late Oligocene to the basinal Miocene age and forms the scarps of the dissected Beauce Plateau. The upper surface of the Fontainebleau Sand displays parallel ridges, about 10-20 m high and often several tens of kilometres long (Fig. 1), representing aeolian palaeomorphologies. Silicification in the Fontainebleau Sand has produced flat-lying and tightly cemented sandstones, within uncemented loose white sands. The sandstones often form superimposed subhorizontal lenses, ranging from 0.5 to 8 m in thickness and from 10 to about 500 m in length (Fig. 2). The top sandstone level caps the ridges and enhances the elongate morphologies of buttes and plateaus south of Paris. Silicification of these superimposed tables is linked to successively sinking water table positions during the down-cutting of the valleys (Thiry et al. 1988). Sandstone dissolution morphologies Near the outcrop zones, the sandstone tables, still embedded in the encasing sand, display numerous dissolution morphologies, referred to herein as siliceous

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karst. Such dissolution morphologies are often called “pseudokarst” in the geological acceptance of the term; that is karst-like dissolution features developed in a noncalcareous rock (Conrad et al. 1967; Marker 1976; Watson and Pye 1985; Dörr 1993). Distribution of the different morphologies is directly linked to the depth where they develop in the sand. Vertically elongated features of contorted and tortuous shapes occur in the upper pan, beneath the limestone cover (Fig. 3A). In places, such vertical pipes run through the whole thickness of the sandstone pan and show a wide range of sizes, from 40-60 cm up to 10 m in diameter. The most complicate contorded, twisted and tortuous pipe morphologies develop at the top of the uppermost sandstone pans, where the sandstone is nearly totally altered and becomes residual within the loose sand (Fig. 3B, C). More regularly shaped features, especially dissolution furrows and horizontal pipes are at their best in the middle sandstone pans, about 10 m beneath the limestone cover (Fig. 3D, E). Horizontal pipes average 0.2 to 2 m in width and may extend for 20 m and more. The lower pans generally display only incipient or no dissolution morphologies. In places, the top of the sandstone pans show solutional furrows or channels, like karren morphologies, with a 10 cm depth, which may stretch out over 2 to 3 m (Fig. 4A). Such furrows often radiate towards the vertical pipes. They clearly point to groundwater flowing above the impermeable sandstone pan towards the outlets formed by the vertical pipes, which pass through the sandstone pans.

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Fig. 1. Geological map of the Fontainebleau Sand Formation in the Paris Basin. The Fontainebleau Sand forms the scarps of the dissected Beauce Plateau. The upper surface displays parallel ridges representing aeolian palaeomorphologies which are preferentially cemented and thus enhance the elongate morphologies into buttes (after Alimen 1936).

Fig. 2.Sketch showing the layout and the morphologies of the Fontainebleau Sandstone ridges. Notice the predominantly vertically shaped dissolution features in the upper part of the sandstone pans, whereas mostly horizontally shaped features develop at depth.

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Siliceous karst development in the Fontainebleau Sandstone (France)

Fig. 3. Siliceous karst morphological features of the Fontainebleau Sandstone. A – General view of a thick sandstone pan in a quarry. The upper part of the pan is carved out by dissolution of the sandstone, whilst deeper down, the dissolution features become sparser and more regular. Darvault (Seine-et-Marne). B - Residual sandstone features of the top of the pan of picture A (view from above). C – Tortuous and interconnected dissolution pipes. Vayres-sur-Essonnes (Essonnes). D & E – Horizontally elongated dissolution pipes developed in deeper sandstone pans. Notice the carvings in the back of hollow E. (Chantambre, Buno-Bonnevaux Essonnes). Photo M. Berger, M.N. Liron.

Sedimentary structures encased in the sandstone pans may favour dissolution morphologies. This is especially well shown by shrimp (callinas) burrow traces that occur in several layers of the Fontainebleau Sand. The burrows consist of walls built from small clayey pellets, which are less tightly cemented owing to their clay content. These burrows often are the ways for initial dissolution, which then progressively widen and interconnect, finally building up a network of centimetric tortuous pipes (Fig. 4B, C). Preferential dissolutions along contorted sedimentary


beddings (seismite layers described by Cojan and Thiry 1992) may give way to complex shaped dissolution features (Fig. 4D). Dissolution features always come with a 0.5 to 2 cm thick friable sandstone rim (Fig. 4D). This friable sandstone develops by progressive dissolution of the sandstone silica cement. In a few places, the dissolution pipes are lined with secondary silica deposits (Fig. 5A, B). These secondary silica deposits are similar to those developed in fractures of the outcropping sandstone pans and are

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Fig. 4. Development of the dissolution features. A - Karren like dissolution furrows developed in the upper surface of the sandstone pans, where seeping water flows above the impermeable sandstone layer. B - Interconnected dissolution pipes developed along fossil burrows in the sand. C - Detail of the above dissolution pipes; traces of the original burrows are visible. Notice the friable sandstone rim around the dissolution pipes. D - Dissolution features highlifgting contorted sedimentary beddings (seismite layers, Cojan and Thiry, 1992). Gondonnières, Larchant (Seine-et-Marne). Photo M. Berger.

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Fig. 5. Secondary silica deposits in dissolution pipes. Vayres-sur-Essonnes (Essonnes). Photo M. Berger.

Fig. 6. Schematic section through the Fontainebleau Sand showing the silica dissolution features. Dissolution morphologies develop with mainly vertical features in the upper part and more horizontal elongated features deeper in the sand, according to the water seeping pathways. Flint pebbles show also a weathering sequence, according to progressive sinking of the water table during the landscape (ground) incision.

related to the final weathering processes occurring at the outcrop (Thiry 2005). Sandstone weathering The siliceous karst features come with the development of a 10 to 20 m thick weathering profile of bleached


sand (Fig. 6). Flint pebbles contained in the sand are also weathered along this profile: flints at the top are entirely friable, those at the base of the profile are only rimmed with a porous cortex, whereas those included in the sandstone pans remain unweathered. Preservation of the flint pebbles encased in the sandstone pans, provides evidence that the silica dissolution profile developed after silicification. The sandstone lenses are made up of quartz grains with well-developed overgrowths and low residual porosity. The sandstone is very pure, without clay minerals and micas. The detrital grains are clean and in general the impurities underlining the overgrowths are not shown. The overgrowths are sub-euhedral or sutured, and have polygonal contacts (Thiry and Maréchal 2001). The siliceous karst features come with preferential dissolution of the overgrowths (Thiry et al. 1984). Dissolution is often enhanced along the boundary between the overgrowth and the detrital quartz grain. Sandstone pans are almost impermeable and often support a small local perched watertable. Siliceous karst development is triggered by water seeping through the sandstone pans along less impermeable pathways. Water seepage initiates silica dissolution, which in turn enhances the porosity and thus keeps the dissolution going. Dissolution alters the sandstone to loose sand that fills the dissolution structures. At the walls of the dissolution structures, a friable centimetric-thick sandstone rim develops within which the overgrowths are only partly dissolved. In quarry exposures the uncemented sand resulting from sandstone alteration remains in the dissolution morphologies, whereas at the outcrop, the dissolution pipes are cleared of their sand content.

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Fig. 7. Prehistoric carvings in the friable sandstone rim resulting from partial dissolution of the quartz overgrowth’s cement. A - Vayres-sur-Essonnes (Essonnes). B - Chantambre, Buno-Bonnevaux (Essonnes). Photo M.N. Liron.

Similar dissolution features are associated in some places with the development of silica crusts, have been described throughout the world. In South-Africa, dissolution features develop on quartzites (Marker 1976; Martini 1979) and also on granites (Watson and Pye 1985). In central Sahara such dissolution features have been detected in several sandstone and quartzite formations (Conrad 1967), whilst in North America deep weathering and bleaching of the Sioux Quartzite Formation has produced similar features (Austin 1970). The most spectacular siliceous karst features in quartzite are those of the Roraima Formation in Venezuela, where vertical pipes reach several hundreds of meters in diameter and up to 1000 m depth (Zawidzki et al. 1976; Pouyllau 1985; Galan and Lagarde 1988; Dörr 1993;Yanes and Briceno 1993), with very wide galleries, stalactites and secondary crusts formed of opal. All the quoted examples develop in

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very pure sandstones and quartzites, almost totally devoid of clay minerals. They are associated with the formation of very pure white loose sand. Break-up of the ridges The dismantling of the sandstone pans at the outcrop gives way to masses of rockfall on the sandy slopes. These are prismatic blocks that alter and become more and more rounded with typical hard casings (Thiry 2005). Nevertheless, in overhangs and in hollows, the friable walls inherited from the siliceous karst dissolutions are preserved and easily recognizable. These friable walls are the sites of decimetric-sized carvings in several hundreds of large caves as well as in smaller metric sized hollows (Fig. 7). The carvings show human outlines, animals and symbolic geometric figures (Tassé 1982; G.E.R.S.A.R.,



1988; Benard 2005). There is no precise dating available for these carvings, the oldest are ascribed to the Upper Paleolithic, although for the majority of them Neolithic, Bronze-Age and Middle-Age dates are hypothesised. Conclusions The flat laying Fontainebleau sandstone tables, embedded in uncemented sand, display numerous siliceous karst morphologies. These morphologies result from progressive dissolution of the siliceous, quartz cement, which operates in a thick weathering profile of bleached sand. This weathering profile developed in the unsaturated vadose zone, above the water table. Siliceous karst development is triggered by water seeping through the sandstone pans along less impermeable pathways. Dissolution alters the sandstone to loose sand with a transitional zone of a friable centimetric-thick sandstone rim. As the sandstone tables developed in the course of down-cutting of the valleys during the Quaternary, these siliceous karst morphologies are even younger and may developed in a time span of about 10,000 to 100,000 years. Thus, even under temperate climates, considerable masses of silica may be dissolved and removed from the sandstone landscapes by underground waters and rivers. Acknowledgements The author acknowledges Jiři Adamovič from the Academy of Sciences of the Czech Republic, for his constructive comments on the first manuscript and Monique Berger and Marie Nieves Liron for providing the photographs. REFERENCES ALIMEN H. 1936. Etude sur le Stampien du Bassin de Paris. Société géologique de France, Mémoire 31. AUSTIN G.S. 1970. Weathering of the Sioux quartzite near New Ulm, Minnesota, as related to Cretaceous climates. Journal of sedimentary Petrology 40: 184-193. BENARD A. 2005. Aperçu de l’art rupestre des chaos gréseux stampiens du Massif de Fontainebleau (France). Ferrantia 44: 65-68. COJAN I., THIRY M. 1992. Seismically induced deformation structures in Oligocene shallow-marine and eolian coastal sands (Paris Basin). Tectonophysics 206: 79-89. CONRAD G., GÉZE B., PALOC H. 1967. Observations sur des phénomènes karstiques et pseudo-karstiques du Sahara. Rev. de Géogr. Phy. et de Géol. Dyn. 9, 5: 357-370. DÖRR S.1993. Höhlen und andere Karstformen in Quarziten des Guyanaschildes (Venezuela / Tepuis). Pseudokarst oder


Produkt echter verkarstungsprozesses. Materialhehte zur Karst- und Höhlenkunde 13: 1-44. GALAN C., LAGARDE J. 1988. Morphologie et évolution des cavernes et formes superficielles dans les quartzites du Roraima, Vénézuéla. Karstologia 11-12: 49-60. G.E.R.S.A.R.D. 1988. Initiation à l’art rupestre du massif de Fontainebleau, Milly-la-Forêt. MARKER M.E. 1976. Note on some South African pseudokarst. Bol. Soc. Venezolana Espel. 7, 13: 5-12. MARTINI J.E.J. 1979. Karst in Black reef Quartzite near Kaapsehoop, eastern Transvaal. Annals Geol. Survey, Dept. of Mineral and Energy Affairs, Republic of South Afrika 13: 115-125. POUYLLAU M. 1985. Pseudo-karst dans des roches grésoquartzitiques de la formation Roraima. Karstologia 9: 45-52. TASSÉ G. 1982. Pétroglyphes du Bassin Parissien. Supplément à Gallia Préhistoire, CNRS, Paris. THIRY M. 2005. Weathering morphologies of the Fontainebleau Sandstone and related silica mobility. In: Ch. Ries, Y. Krippel (Eds). Sandstone landscapes in Europe. Past, Present and Future. Proceedings of the 2nd Intern. Conference on Sandstone Landscapes, Vianden (Luxembourg), 25-28 May 2005, Ferrantia, Luxembourg 44: 47-52. THIRY M., BERTRANT AYRAULT M., GRISONI J. C. 1988. Groundwater silicification and leaching in sands: Example of the Fontainebleau Sand (Oligocene) in the Paris Basin. Geol. Soc. Am., Bull. 100: 1283-1290. THIRY M., MARÉCHAL B. 2001. Development of tightly cemented sandstone lenses within uncemented sand: Example of the Fontainebleau Sand (Oligocene) in the Paris Basin. Jour. Sedim. Research 71, 3: 473-483. THIRY M., PANZIERA J.P., SCHMITT J.M. 1984. Silicification et désilicification des grès et des sables de Fontainebleau. Evolutions morphologiques des grès dans les sables et à l’affleurement. Bull. Inf. Géol. Bassin Paris 21, 2: 23-32. WATSON A., PYE K. 1985. Pseudokarstic micro-relief and other weathering features on the Mswati Granite (Swaziland). Z. Geomorph. N. F. 29, 3: 285-300. WRAY R.A.L. 1997. A global review of solutional weathering forms on quartz sandstones. Earth-Science Reviews 42, 3: 137-160. YANES C.E., BRICENO H.O. 1993. Chemical weathering and the formation of pseudo-karst topography in Roraima Group, Gran Sabana, Venezuela. Chemical Geology 107: 341-343. ZAWIDZKI P., URBANI F., KOISAR B. 1976. Preliminary notes on the geology of Sarisarinama Plateau, Venezuela, and the origin of its caves. Bol. Soc. Venezolana Espeleologua 7: 29-37. Web link: http://www.cig.ensmp.fr/~thiry

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