A long core (11 10 m) drilled at Montcornet (northeastern Paris Basin) provides early Jurassic magnetostratigraphic data coupled with biochronological control. About 600 paleomagnetic samples were obtained from a 148-m-thick series of Hettangian and Sinemurian rocks. A composite demagnetization using thermal (up to 300øC) followed by alternating field technique (up to 100 mT) is used to separate the magnetic components. A low unblocking temperature component (<250øC) with an inclination of about 65 ø is interpreted as a present-day field overprint. The characteristic reinanent component with both normal and reversed antipodal directions is then isolated between 5 and 70 mT. Eighty-one polarity intervals are recognized in this study. The higher reversal frequency of the late Hettangian/early Sinemurian time interval contrasts with a lower reversal rate observed in the rest of the early Liassic. A rough mean estimate of about 5 reversals/m.y. can be proposed for the earliest Jurassic. These results represent a significant contribution to the magnetic polarity reversal timescale for a time interval hitherto poorly known and add to the magnetic reversal frequency curve of the last 350 m.y. 1988; Steiner et al., 1989; Witte and Kent, 1990; Ogg and Steiner, 1991; Gallet et al., 1992, 1993, 1994], and the late Permian [Haag and Heller, 1991;Heller et al., 1995). Despite superimposition of high frequency variations, these results document an increasing trend of the reversal rate from the end-Kiaman reversed superchron to late Jurassic time. This pattern of change in reversal frequency compares to the one that followed the long normal chron of late Cretaceous age (Gallet
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.
The Upper Cretaceous-lower Paleocene terrestrial formations from the Aix-en-Provence basin offer a highresolution record of the effects of climate changes. These units were deposited in a basin with low topographic relief under climate conditions that varied from subhumid to semiarid. Sedimentation during subhumid periods was characterized by accumulation of carbonate mud in the main lake and aggradation of floodplains during overbank floods. Carbonate-rich paleosols, which occur throughout the subhumid succession, contain authigenic minerals with interstratified illite-smectite and smectite.Although subhumid conditions were dominant during deposition, the occurrence of five semiarid episodes can be documented on the basis of facies, paleomagnetic signal, and mineralogic associations of rocks deposited across the paleolandscape. The lake-margin environment was most sensitive to climate change. Dolomite and gypsum crystals, authigenic smectite and palygorskite, and secondary fine-grained hematite grew within rocks deposited along the lake margin under semiarid conditions. The mineralogic transformations resulted in a distinct paleomagnetic record composed of a lt component (200 to 400uC) and an associated chemo-detrital ht component (up to 600uC). During the semiarid episodes, sedimentation in floodplain environments was reduced, allowing development of mature smectite or smectite-palygorskite paleosols.Semiarid episode 1 occurs within the Calcaire de Rognac Formation, semiarid episodes 2 and 3 lie within the Upper Argiles Rutilantes Formation, and semiarid episodes 4 and 5 are just below and within the Calcaire de Vitrolles Formation. Recognition of correlative deposits with a distinct paleomagnetic signal allows correlation between the continental successions of Provence and the geomagnetic polarity time scale. The lithostratigraphic units, as well as the distribution of dinosaur oospecies are largely diachronous, representing a few millions of years.Semiarid episodes 1 to 3 occurred during the Early Maastrichtian. No semiarid episodes are recorded for the cooler interval that defined the Middle-Late Maastrichtian. Semiarid episodes 4 and 5 correspond to the warmer periods that preceded and followed the 500-ky-long interval containing the Cretaceous-Tertiary boundary.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.