Late Cretaceous paleoenvironmental evolution and sea-level history of the Tarfaya Basin, SW Morocco : Evidence from XRF scanner-derived elemental records, benthic and planktonic foraminifera and bulk carbonate stable isotopes
Lithological evidence, benthic foraminiferal census counts, and high-resolution X-ray fluorescence (XRF) scanner-derived elemental data were integrated with planktonic foraminiferal biostratigraphy and bulk carbonate stable isotopes to retrace the Turonian to early Campanian paleoenvironmental evolution and sea-level history of the Tarfaya Atlantic coastal basin (SW Morocco). The lower Turonian is characterized by impoverished benthic foraminiferal assemblages, which reflect an impingement of the oxygen minimum zone on the shelf during a sea-level highstand. The appearance of low-oxygen tolerant benthic foraminiferal assemblages in the middle to upper Turonian indicates an improvement in bottom water oxygenation, probably linked to offshore retraction of the oxygen minimum zone during a regressive phase. From the late Turonian to Santonian, the presence of benthic foraminiferal with low diversity suggests relatively impoverished oxygenation in bottom water along the shelf. Three long-term oscillations in the abundance of terrigenous elements (increase of Al, Ti, K, Si and Fe normalized against Ca) are shown during the Coniacian and Santonian. This interval, which roughly corresponds to the Coniacian-Santonian Anoxic Event (OAE-3), is characterized by overall oxygen depleted to anoxic conditions at the sea-floor (indicated by the high organic carbon content, the presence of laminations and by low manganese/sulphur, high vanadium/calcium and bromine/calcium ratios in XRF scanning records). The lower Campanian transgression, only recorded in the southern part of the Tarfaya Basin, coincided with substantial deepening, enhanced accumulation of fine-grained clay-rich hemipelagic sediments and improved oxygenation at the sea-floor (highest diversity and abundance of benthic foraminiferal assemblages and low values of log(Mn/S)). The sea-level changes reconstructed in the Tarfaya Basin are correlated to the global eustatic changes. Two major unconformities (U1/U2 and U3), which punctuate the upper Turonian to lower Campanian succession in Tarfaya SN°1 and 2, are correlative to the base of the Merchantville III and Magothy III sequence boundaries of Miller et al., (2004) and Mizintseva et al. (2009), respectively. Stable isotope data of bulk carbonates (outcrop sections and composite cores) are correlated to the English Chalk, the Niobrara Formation (US Western Interior Seaway) and to the stacked carbon isotope reference curve of Wendler (2013). The Tarfaya carbon isotope curve reveals in particular the Navigation Event in the Coniacian, the Haven Brow, the Horseshoe Bay and the Buckle Events in the Santonian as well as the Santonian/Campanian Boundary Event.
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