Cretaceous climate change evidenced in the Senegalese rock record, NW Africa

Abstract

Climate change directly impacts the source, mode and volume of sediment generation which can be observed in the rock record. To accurately model source to sink systems, in addition to hinterland geology, tectonics and transport distance, a thorough comprehension of the climate is essential. In this study we evaluate the role of climate on Cretaceous sediment delivery into the Senegal Basin, NW Africa, using data recorded from extensive sampling of basinal sediments. This is achieved through the mineralogical characterisation by X-ray diffraction and ¹⁴⁶Nd/¹⁴⁴Nd and ⁸⁶Sr/⁸⁸Sr isotopic analyses, which are correlated against existing, climate, tectonic and oceanographic models. Examples of climatic indicators include the change from predominantly smectitic deep marine basinal-clays recorded from the Cretaceous in DSDP wells 367 and 368 to clays with increased illite and kaolinite content, observed during the Albian and Cenomanian-Turonian, interpreted to be representative of higher humidity following the kaolinisation of hinterland source-rocks. Another climate indicator is the observation of palygorskite in deep-marine sediments, noted to be indicative of ocean anoxia related to the authigenesis of marine-smectite, a product of warm saline bottom waters and increased abundancy of silicon. The increase in salinity is interpreted to be a biproduct of elevated temperatures throughout the Cenomanian and increased denudation of the North Atlantic circumjacent continental evaporite-belts. Increase in silicon (biogenic) is related to a result of ocean-wide mass extinction of foraminifera during OAE2 triggered by the eruption of the Caribbean large igneous province. The results suggest that Cretaceous climate evolution of Senegal can be divided into four stages: 1. Berriasian-Barremian; an arid-period with monsoonal weather producing modest fluvial systems restricted to coastal regions. 2. Aptian-Albian; the establishment of a paleo-Intertropical Convergence Zone began to increase global temperature and humidity as recognised by the increase in kaolinite content. 3. Cenomanian-Turonian; the Cretaceous Thermal Maximum hothouse period incurring exceptional temperatures and humidity. This is represented as an antithetical shift in clay mineralogy from chlorite-illite to smectite-kaolinite throughout most of the onshore and nearshore basinal sediments. 4. Coniacian-Maastrichtian; transitional from tropical-to-tropical swamp-like conditions evidenced by increased onshore basin sediment capture and a shift in vegetation to aquatic-fern species. The impact of climate change throughout the Cretaceous produced dynamic shifts in both river size and source-catchment, witnessing exception rates of denudation during the hotter and more humid periods, which climaxed during the Cenomanian and Turonian as a result of the Cretaceous Thermal Maximum. This eroded sediment was deposited in both the onshore and offshore basins during the mid-late Cretaceous but became increasingly restricted to the onshore segment of the basin during the Late Cretaceous.