The upper Cenomanian-lower Turonian of the Preafrican Trough (Morocco): Platform configuration and palaeoenvironmental conditions

Abstract : A synthetic study was carried out based on sedimentological, palaeontological, geochemical and mineralogical data of the upper Cenomanian lower Turonian carbonate platform of the Preafrican Trough (eastern Morocco) in order to (1) propose a 3D representation of the platform and constrain the temporal framework of the dysoxic/anoxic episodes recorded during the OAE2, (2) define and discuss the prevailing climate on the platform during this period, and (3) make comparisons with other Cenomanian Turonian platforms. During the late Cenomanian, both before and during the CCIE (Cenomanian Carbon Isotope Excursion), the platform displayed an east west polarity. Three third-order sequences of transgression regression can be defined. Dysoxic conditions were developed in the sediments and the bottom waters of the deepest environment (mid- to outer-ramp setting), in the western part of the platform. Well-oxygenated waters were present in the eastern part of the platform (peritidal zone to mid-ramp environment). The climate was arid before the CCIE, becoming warm with contrasted seasons during the CCIE. This climate is associated with a low palaeoproductivity over the entire platform, along with the presence of photozoan followed by heterozoan carbonate-producers, as found also in other parts of the Saharan platform. However, such conditions are not in accordance with many studies which suggest a wet climate during the CCIE, leading to intense chemical weathering of the continent favouring the appearance of high palaeoproductivity at a global scale and the establishment of dysoxic/anoxic conditions. In the Preafrican Trough, poorly-oxygenated waters spread outwards from the deep basins and covered the platform in response to sea-level rise. Many disturbances are recorded in the platform succession during the early Turonian, after the CCIE. Indeed, just after the C/T boundary, the development of an outer-ramp environment over the entire Preafrican Trough reflects flooding of the platform, linked to the end of the major Cenomanian transgression and the presence of eutrophic conditions which disrupted carbonate-producing organisms. This high palaeoproductivity, due to considerable nutrient input, led to the establishment of highly dysoxic/conditions in the bottom and intermediate waters, causing the disappearance of the majority of the Cenomanian palaeontological groups, except for the opportunist benthic and planktonic foraminifera which proliferated. Part of the nutrient input could be due to the presence of a hot and wet climate that may have led a slight increase in the degree of chemical weathering of the continent. Nevertheless, many lines of evidence, such as the decrease of the detrital influx during the early Turonian and the reduction of the weatherable continental areas after the Cenomanian transgression, suggest the existence of another source of nutrient inputs. Combined with the presence of dysoxic/anoxic conditions during the lower Turonian in several regions of the tropical Central Atlantic, the occurrence of these nutrient inputs may be linked to the volcanism/hydrothermalism of the Caribbean LIP and the mid-ocean ridges.
Type de document :
Article dans une revue
Journal of African Earth Sciences, Elsevier, 2015, 106, pp.1-16. 〈10.1016/j.jafrearsci.2015.03.001〉
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V. Lebedel, C. Lezin, B. Andreu, El M. Ettachfini, Danièle Grosheny. The upper Cenomanian-lower Turonian of the Preafrican Trough (Morocco): Platform configuration and palaeoenvironmental conditions. Journal of African Earth Sciences, Elsevier, 2015, 106, pp.1-16. 〈10.1016/j.jafrearsci.2015.03.001〉. 〈hal-01281807〉



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