Pierre-Jean Combes

Typologie, cadre géodynamique et genèse des bauxites françaises

ResumeSur les cratons europeen et iberique, les bauxites de haut niveau se forment durant de longues lacunes stratigraphiques, au Cretace inferieur, caracterisant une emersion precoce et un recouvrement tardif. En se rapprochant des marges, dans des domaines de bas niveau, en bordure du bassin axial pyreneen, les lacunes stratigraphiques diminuent : les bauxites sont incorporees dans les plates-formes carbo-natees urgoniennes. Dans ce cadre stratigraphique et paleogeographique, la typologie et le mode de formation des gisements sont lies au degre de stabilite controle par les mouvements tectoniques et les oscillations eustaliques. Sur les cratons (types Sardaigne et Languedoc-Provence), la bauxite apparait par ferrallitisation a partir, le plus souvent, des niveaux alumineux de la couverture sedimentaire post-hercynienne. Les modalites de genese dependent du taux de soulevement : faible, autochtonie avec alteration sur place des marnes; modere, parallochtonie avec surimposition verticale et laterale, dans...

Mise en évidence d'un sillon marin à brèches paléocènes dans les Pyrénées centrales (Zone interne métamorphique et Zone nord-pyrénéenne)

Abstract The discovery, in several localities, of significant planktonic Foraminifera, Danian–Selandian in age (P1c–P3 interval), both in the matrix of polygenic, post-metamorphic and post-tectonic debris-flow breccias (Comus Breccia Fm.) and in argillaceous hemipelagites, interbedded within these breccias, evidences the occurrence, during Palaeocene, of a longitudinal east–west marine trough, particularly open towards the Atlantic Ocean, within the present Internal Metamorphic Zone (and adjacent North-Pyrenean areas) of Central Pyrenees, from Aude to Haute-Garonne. This new dating shows the major importance of the Upper(most) Cretaceous compressions in the structuration of the tectorogenic axis of the range.

Pedogenic and Karstic Features at the Boundaries of Bathonian Depositional Sequences in the Grands Causses Area (Southern France): Stratigraphic Implications

ABSTRACT Several exposure surfaces (D1 to D6) underlain by paleosols, paleokarstic surfaces, and subsurface paleokarsts were identified in the Middle to Upper Bathonian Calcaires a Stipites and Dolomies II Formations in southern France. Two kinds of paleosols with different degrees of maturity were recognized: simple ferruginous crusts, capping very irregular bed surfaces, and a rooted horizon. The paleokarstic surfaces are marked by nodular horizons and paleocaves. On the Cevennes shoal, the paleokarsts discontinuities are associated with synsedimentary tectonic processes, which did not extend into the overlying Dolomies II Formation. Subsurface paleokarsts were observed in the Cirque du Bout du Monde (on the Cevennes shoal) within the Calcaires a Stipites Formation, just beneath surface D5. They are characterized by stronger brecciation of the beds. Most of the paleokarstic discontinuities described in this study correspond to the boundaries of four third-order depositional sequences, Bt 1 to Bt 4 (D1 at the base of Bt 1; D2, D3, D4, and D5 capping Bt 1, Bt 2, Bt 3, and Bt 4, respectively; D6 at the top of the Dolomies II Formation). D1, D2, and D3 paleokarsts are geographically limited to the Grands Causses Graben, whereas D4, D5, and D6 are present only on the Cevennes shoal. Geographic trends of paleokarsts confirm the transgressive trend of sequences Bt 3 and Bt 4, and of the overlying Dolomies II Formation towards the shoal. D6 paleokarstic features were also observed within the uppermost part of the Dolomies II Formation in the Horst de Saint-Bresson. The latter transgressive process provides evidence for subaerial exposure of this paleostructure during the latest Bathonian-Callovian interval, induced by tectonic uplifts.

Latest-Cretaceous/Paleocene karsts with marine infillings from Languedoc (South of France); paleogeographic, hydrogeologic and geodynamic implications

The Latest Cretaceous/Paleocene paleokarsts from Bas-Languedoc (South of France) are characterized by : 1) exokarstic paleosurfaces (sink holes, pinnacles, canyons) which are mostly superimposed onto the Late Jurassic limestones and partly filled up with breccias and sandy clays containing Paleocene planktonic foraminifera; 2) endokarstic cavities filled up by sandy pelites and laminated mudstones with similar micropaleontological assemblage. All these cavities constitute three cut-and-filled paleokarstic systems. A general model of formation for this polyphase system is proposed. These paleosurfaces are generated by a Latest Cretaceous tectonic phase and Paleocene movements associated with eustatic rises inducing rapid floodings. During Upper Danian-Lower Selandian times three successive base levels drops, whose cumulated duration could be as long as 3 Ma, have induced several hundred metres of incisions within the exposed Jurassic carbonate series. A paleogeographic reconstruction shows three main marine gulfs which extend towards NE the EW Pyrenean thrust belt (“Pyrenean Paleocene Trough”, PPT) located on the axis of the future Pyrenees. The similarity of the facies and micropaleontological content in the two domains suggests connexions between the marine Paleocene deposits of Bas-Languedoc and the PPT. It could thus explain the succession of the same tectono-eustatic events. We propose, as an hypothesis, that the rapid sea-level changes, as recorded by the imbricated karstic paleomorphologies, could be induced by the closing and the opening of a strait in the Eastern part of the PPT acting as a sill. This would be controlled by tectonic movements along the active orogenic axis of the Pyrenean Range and eustatic variations of the Paleocene World Ocean. The karstic systems developed during these low-stand episodes may have been later reactivated after the Early Selandian and more specifically during the Messinian desiccation event. This late evolution may have generated deep flooded karsts, now situated below the present sea level. Such karsts, partly inherited from Paleocene, could be important aquifers of economic interest.

Altérites et bauxites, témoins des marges européenne et ibérique des Pyrénées occidentales au Jurassique supérieur — Crétacé inférieur, à l'ouest de la vallée d'Ossau (Pyrénées-Atlantiques, France)

Abstract Alterites and bauxites from Atlantic Pyrenees cropping out immediately west of the Ossau Valley (‘North-Pyrenean Zone’ in the literature) give information, by means of their ages within the Upper Jurassic-Aptian interval and of their depositional conditions (allochthonous or autochthonous) on the successive steps of the Iberian and European margins, about the extensional motions linked to the Lower Cretaceous rifting (opening of the Axial Basin). Pyrenean data are also interpreted in a larger Tethyan context where principal factors are the changes of climate, atmospheric CO2-O2 ratio and sea level.

Présence de Paléocène marin dans les Grands Causses (France)

Identification of Palaeocene marine sediments in the Grands Causses (France). In the Grands Causses, incised valleys, lapies, fissures and sinkholes inherited from successive polyphase karstifications were filled by Palaeocene marine sediments overall assigned to the P1c–P3 interval (Upper Danian–Lower Selandian). These sediments are distributed into three detritic facies, generated by extensional tectonics controlling karstic and erosional processes. Upper Cretaceous marine fossils known within these facies are interpreted as reworked from hypothetically pellicular deposits. The probable palaeogeographic connection with the Pyrenean Palaeocene ‘Breccia trough’ supposes the presence of a SE–NW ‘ria’ running across the continental areas of Lower Languedoc and draining towards the northwest the marine waters of the Palaeocene transgression as

Paléosurfaces d'érosion et paléokarsts dans la série de passage Crétacé/Tertiaire des Corbières occidentales (Aude, France)

Abstract The continental series of Uppermost Maastrichtian/‘Lower Paleocene’ includes 6 palaeosurfaces induced by the erosion of alluvial marls and the karstification of lacustrine/ palustrine limestones within a subtropical climate characterized by a moderate pluviometry. An estimation of the whole gap gives a relatively high extension which must be taken into consideration in the stratigraphic studies and the search for the iridium anomaly at the K/T boundary. The evidenced discontinuities are helpful for correlations between continental and marine domains as they may refer to low stand episodes.

Reply to comment on Latest-Cretaceous/Paleocene karsts with marine infillings from Languedoc (South of France) ; paleogeographic, hydrogeologic and geodynamic implications

Our paper 1) describes in detail three successive paleokarstic incisions formed in a continental environment, each one later filled with sediments containing marine fauna. The altitude distribution of the analysed outcrops implies successive base-level falls and rises with an amplitude of hundreds of metres. 2) Incidentally, it occurs that the youngest marine fauna found in the successive paleokarst fillings is early Paleocene in age ; we therefore assume that in spite of the very rare occurrence of older reworked benthic fauna, the successive phases of karst formation and marine filling occurred during early Paleocene. Surprisingly, Bilotte et al.’s comment neither question the former point, nor the geological processes responsible for such an original setting. We would have welcomed any constructive suggestion in order to solve this outstanding problem, which, we believe, is now the most exciting scientific question to be addressed. Instead, the virulent comment focuses on the age of the sedimentary filling and challenges the Paleocene age we have determined. Even more surprisingly, the comment never refers to, nor it brings additional or contradictory observations on the specific localities analysed in the paper, but rather criticises some of our previous publications dealing with different geographical locations such as the French and Spanish Pyrenees. This results in a confusing case, in which scientists unfamiliar with our previous papers can hardly decipher the argument, unless they recover the dozen incriminated papers and as many counter-publications put forward by Bilotte and coauthors. We therefore reply to Bilotte et al.’s comment without expanding on the localities and topics relevant to previous works published elsewhere.

Geodynamics of Bauxites in the Tethyan Realm

Bauxites, particularly the karst-type deposits, are closely related to the history of the Tethys. Four main modes of origin can be distinguished: autochthonous, parautochthonous, parallochthonous, and allochthonous. A simple classification of the deposits is proposed according to their location in relatively high level, intracontinental domains, on pericontinental lowlands or on insular platforms. The secondarily redeposited bauxite accumulations are considered a separate group. Bauxite formation in the Tethys area was a discontinuous process, which began during the Late Permian in the area of the present Middle East and progressed westward following the gradual separation of Laurasia and Gondwana. It reached a peak during the Mid-Cretaceous and two minor peaks in the Malm and the Paleocene. A further major peak occurred during the Late Miocene in the Caribbean area. Study of these periods shows that several favorable factors promoted the formation of bauxite: 1. Tropical and subtropical, humid, monsoon climate, a low equator-to-pole temperature gradient, and a high atmospheric CO2 and O2 content. 2. High eustatic sea level, leading to the submergence of the coastal lowlands and contributing to the development of a humid climate. 3. Tectonic instability. Most bauxite deposits in the Tethys area are allochthonous. The instability favored the erosion of the weathering profiles and the accumulation of clastic bauxitic material. 4. Paleogeography. The distribution of the bauxite deposits followed the opening of the Tethys sea in a westerly direction. The wedge-shaped coastline guided the warm ocean currents and the clouds westward, where the carbonate platforms and the bauxite deposits could form under the most favorable climatic conditions.