Continental France and Belgium during the early Cretaceous: paleoweatherings and paleolandforms

Abstract

During the early Cretaceous, successive tectonic phases and several sea level falls resulted in the emersion of the main part of western Europe and the development of thick “lateritic” weathering. This long period of continental evolution ended with the Upper Cretaceous transgressions. During this period, the exposed lands displayed a mosaic of diverse morphologies and weathered landscapes. Bauxites are the most spectacular paleoweathering features, known for long in southern France. Recently, new residual outcrops have been identified, trapped in the karstic depressions of the Grands Causses. Other bauxitic formations, containing gibbsite, have also been recognised, occurring with the Clay-with-Jurassic-cherts in the southeastern border of the Paris Basin. These bauxitic formations overlay Jurassic limestone and are buried beneath Upper Cretaceous marine deposits. The recognition of bauxites up north into the southern Paris Basin significantly widens the extension of the Lower Cretaceous bauxitic paleolandscapes. On the Hercynian basements thick kaolinitic weathering mantles occur. They have been classically ascribed to the Tertiary. The first datings of these in situ paleosoils, by means of paleomagnetism and/or radiogenic isotopes, record especially early Cretaceous ages. This is the case for the “Siderolithic” formations on the edges of the French Massif Central, but also for the kaolinitic profiles in the Belgian Ardennes. In the Flanders, the Brabant basement is deeply kaolinised beneath the Upper Cretaceous cover. These paleosoils show polygenetic evolutions. The relief of these basement paleolandscapes may have been significant. There where probably high scarps (often of tectonic origin) reaching 200 m in elevation or beyond, as well as wide surfaces with inselbergs, as in the present day landscapes of tropical Africa and South America. On the Jurassic limestone platforms occur diverse kaolinitic and ferruginous weathering products. Around the Paris Basin they show various facies, ranging from kaolinitic saprolites to ferricretes. Due to the lack of sedimentary cover, the age of these ferruginous and kaolinitic weathering products has been debated for long, most often allocated to the Siderolithic sensu lato (Eocene-Oligocene). Recent datings by paleomagnetism have enabled to date them (Borne de Fer in eastern Paris Basin) back also to the early Cretaceous (130 ± 10 Ma). These wide limestone plateaus show karstified paleolandforms, such as vast closed and flat depressions broken by conical buttes, but also deep sinkholes in the higher areas of the plateaus and piedmonts. The depth of the karst hollows may be indicative of the range of relative paleoelevations. Dissolution holes display seldom contemporaneous karst fillings, thus implying that the karstland had not a thick weathering cover or that this cover had been stripped off before or by the late Cretaceous transgression. Nevertheless, some areas, especially above chert-bearing Jurassic limestone or marl, show weathering products trapped in the karst features or as a thick weathering mantle. In the Paris Basin, the Wealden gutter looked like a wide floodplain in which fluvio-deltaic sands and clays were deposited and on which paleosoils developed during times of non-deposition. The edges of the gutter were shaped as piedmonts linked up with the upstream basement areas. The rivers flowing down to the plain deposited lobes of coarse fluvial sands and conglomerates. The intensity of the weathering, the thickness of the profiles and their maturation are directly dependent on the duration of the emersion and the topographic location relative to the gutter. Near the axis of the gutter, where emersion was of limited duration, the paleoweathering features are restricted to rubefaction and argillization of the Lower Cretaceous marine formations. On the other hand, on the borders of the basin and on the Hercynian basement, where emersion was of longer duration, the weathering profiles are thicker and more intensively developed. The inventory of the Lower Cretaceous paleoweathering features shows the complexity of the continental history of this period. Moreover, the preserved weathering products are only a part of this long lasting period, all the aspects relative to erosion phases are still more difficult to prove and to quantify. In this domain, apatite fission tracks thermochronology (AFTT) can be helpful to estimate the order of magnitude of denudation. Residual testimonies and subsequent transgressions may enable to estimate relative elevations, but in return, we presently have no reliable tool to estimate absolute paleoelevations. In the work presented here, the inventory enabled to draw a continental paleogeographic map showing the nature of the weathering mantles and the paleolandscape features, just as paleoenvironments and paleobathymetry presently appear on marine paleogeographic maps. For the future, the challenge is to make progress in dating the paleoweathering profiles and especially in the resolution of these datings, in order to correlate precisely the continental records with the different events which trigger them (eustatism, climate, regional and global geodynamics). The final goal will be to build up a stratigraphic scale of the “continental geodynamic and climatic events” in parallel with “sequential stratigraphy” in the marine realm.

Plant-dominated assemblage and invertebrates from the Lower Cenomanian of Jaunay-Clan, western France

“…1 and 2). At the Bergère locality (JCB), the paleokarst surface displays a partlypreserved paleosol profile resembling those observed on the Jurassic rings of the Paris Basin (Thiry et al, 2006), consisting of 20 to 60-cm-thick ochreous clay overlaid by a 20-cm-thick ferruginous crust made of iron pisolites. The profile is overlaid by a 1-m-thick and 10-m-wide pocket of green-gray mudstones that fills a small depression of the paleokarst surface (Fig.…”

The origin and timing of multiphase cementation in carbonates: Impact of regional scale geodynamic events on the Middle Jurassic Limestones diagenesis (Paris Basin, France)

“…The first evidence of widespread sub-aerial exposure, soil development and karstification of the Jurassic limestones appears during the lowermost Cretaceous (Corroy, 1925;Magniez et al, 1980;Guillocheau et al, 2000;Quesnel, 2003;Thiry et al, 2006;Vincent et al, 2007;Theveniaut et al, 2007). This emergence corresponds to the amalgamation of two unconformities (the Jurassic/Cretaceous and Lower/Upper Berriasian boundaries) known as the Late Cimmerian Unconformity (LCU; Guillocheau et al, 2000).…”

Elucidating the formation of terra fuscas using Sr–Nd–Pb isotopes and rare earth elements

“…These old saprolite systems represent a polycyclic evolution due to successive geological and pedogenetic processes that affected the profiles (Thiry et al, 2006). Their evolution may be related to paleoweathering during long periods of emersion, which likely reached back to the Cenozoic and even the Mesozoic -as it is the case for the sites in eastern France, Belgium and Luxembourg (Quesnel, 2003).…”

“…Some of these weathering mantles contain huge amounts of iron ores and associated metals and have also been of economic interest and mined since even before the Middle Age until the 20th century over the whole western Europe (Thiry et al, 2006). The cause of their trace metal enrichments has to our knowledge not been elucidated.…”