Morphogenesis of the Closed Depressions in Middle Atlas: Case of the Causse of El Menzel, Morocco

Closed depressions are the most typical exokarstic forms of the Moroccan Middle Atlas karst. These features present a great diversity of forms and sizes related to the multiple and often obscure conditions of morphogenesis in several karstic plateaus of the tabular Middle Atlas such as that of El Menzel. This little-known carbonate panel is characterized by an important geodiversity of closed depressions with variable shapes and reliefs: poljes, uvalas, dolines, kamenitzas, and rain pits. The results confirm the tectogenetic nature of several closed depressions of which the most important are: the polje of El Menzel and the polje of Quaçbat Beni Yazgha which are aligned along the major accidents of the Causse oriented NE-SW and NW-SE. The pre-established tectonic heritage Accident Median Moyen Atlasique and Accident Nord Moyen Atlasique, and the compressive tectonics and particularly extensive phases from the upper Miocene to Quaternary during the surrection of the Middle Atlas associated with the pluvial climatic phases have permitted the development of the good potential of karstification as well as the large closed depressions of the Causse

Coupling Field Investigations with the Contribution of Remote Sensing and GIS for the Moroccan Middle Atlas Karst: A Case Study of the El Menzel Causse

Little work has been done on the processes that control karstic phenomena in the Causse of El Menzel. This carbonate section of the Moroccan tabular Middle Atlas has undergone significant karstification due to favorable conditions that contribute to good karstification potential. Our field and remote sensing results confirm the tectogenetic nature of a set of expressions found in the Causse. The maps produced show good agreement between the structural, fluvial, and karstic network, given that the large NE-SW oriented faults in the sector played an inhibiting role in the triggering of karstic processes during the different phases of the Alpine cycle and neotectonics: on the one hand by the fact of weakening the liasic carbonate series sensitivity to dissolution at faulted zones and on the other hand by guiding the action of the external phenomena causing incision and fluvial erosion, forming a system of valleys which simultaneously coincides with the genesis of the karstic forms found in the Causse of El Menzel

New insights into Cenomanian paleoceanography and climate evolution from the Tarfaya Basin, southern Morocco

Highlights • Complete upper Albian to early Turonian climate archive in drilled core from Tarfaya Basin. • Eccentricity pacing of mid Cretaceous OAE isotope excursions. • MCE and OAE2 associated with climate cooling and sea level fall. Abstract A 325 m long continuous succession of uppermost Albian to lower Turonian pelagic (outer shelf) deposits was recovered from a new drill site in the central part of the Tarfaya Basin (southern Morocco). Natural gamma ray wireline logging, carbonate and organic carbon content, bulk carbonate and organic carbon stable isotopes and X-ray fluorescence (XRF)-scanner derived elemental distribution data in combination with planktonic foraminiferal biostratigraphy indicate complete recovery of the Cenomanian Stage. This exceptional sediment archive allows to identify orbitally driven cyclic sedimentation patterns and to evaluate the pacing of climatic events and regional environmental change across the Albian-Cenomanian boundary (ACB), the mid-Cenomanian Event (MCE) and Oceanic Anoxic Event 2 (OAE2) in the latest Cenomanian. The deposition of organic-rich sediments in the Tarfaya Basin, likely driven by upwelling of nutrient-rich water masses, started during the latest Albian and intensified in two major steps following the MCE and the onset of OAE2. The duration and structure of the MCE and OAE2 carbon isotope excursions exhibit striking similarities, suggesting common driving mechanisms and climate-carbon cycle feedbacks. Both events were also associated with eustatic sea level falls, expressed as prominent sequence boundaries in the Tarfaya Basin. Based on the 405 kyr signal imprinted on the Natural Gamma Ray (NGR) and XRF-scanner derived Log(Zr/Rb) records, we estimate the duration of the Cenomanian Stage to be 4.8 ± 0.2 Myr

Unraveling the onset of Cretaceous Oceanic Anoxic Event 2 in an extended sediment archive from the Tarfaya-Laayoune Basin, Morocco

We investigated the onset and development of Cretaceous Oceanic Anoxic Event 2 (OAE2) in a newly drilled core (SN degrees 4) from the Tarfaya Basin (southern Morocco), where this interval is unusually expanded. High-resolution (centimeter-scale equivalent to centennial) analysis of bulk organic and carbonate stable isotopes and of carbonate and organic carbon content in combination with XRF scanner derived elemental distribution reveal that the ocean-climate system behaved in a highly dynamic manner prior to and during the onset of OAE2. Correlation with the latest orbital solution indicates that the main carbon isotope shift occurred during an extended minimum in orbital eccentricity (similar to 400 kyr cycle). Shorter-term fluctuations in carbonate and organic carbon accumulation and in sea level related terrigenous discharge were predominantly driven by variations in orbital obliquity. Negative excursions in organic and carbonate delta C-13 preceded the global positive delta C-13 shift marking the onset of OAE2, suggesting injection of isotopically depleted carbon into the atmosphere. The main delta C-13 increase during the early phase of OAE2 in the late Cenomanian was punctuated by a transient plateau. Maximum organic carbon accumulation occurred during the later part of the main delta C-13 increase and was associated with climate cooling events, expressed as three consecutive maxima in bulk carbonate delta O-18. The extinctions of the thermocline dwelling keeled planktonic foraminifers Rotalipora greenhornensis and Rotalipora cushmani occurred during the first and last of these cooling events and were likely associated with obliquity paced, ocean-wide expansions, and intensifications of the oxygen minimum zone, affecting their habitat space on a global scale

Cretaceous oceanic anoxic events prolonged by phosphorus cycle feedbacks

Oceanic Anoxic Events (OAEs) document major perturbations of the global carbon cycle with repercussions on the Earth’s climate and ocean circulation that are relevant to understand future climate trends. Here, we compare sedimentation patterns, nutrient cycling, organic carbon accumulation and carbon isotope variability across Cretaceous Oceanic Anoxic Events OAE1a and OAE2 in two drill cores with unusually high sedimentation rates from the Vocontian Basin (southern France) and Tarfaya Basin (southern Morocco). OAE1a and OAE2 exhibit remarkable similarities in the evolution of their δ13C excursion with long-lasting negative carbon isotope excursions preceding the onset of both anoxic events, supporting the view that OAEs were triggered by massive emissions of volcanic CO2 into the atmosphere. Based on analysis of cyclic sediment variations, we estimated the duration of the individual phases within the carbon isotope excursions. For both events, we identify: (1) a precursor phase lasting ~ 430 kyr and ~ 130 kyr, (2) an onset phase of ~ 390 and ~ 70 kyr, (3) a peak phase of ~ 600 and ~ 90 kyr, (4) a plateau phase of ~ 1400 and ~ 200 kyr and (5) a recovery phase of ~ 630 and ~ 440 kyr, respectively. The total duration of the positive carbon isotope excursion is estimated as 3400 kyr and 790 kyr and that of the main carbon accumulation phase as 980 kyr and 180 kyr, for OAE1a and OAE 2 respectively. The extended duration of the peak, plateau and recovery phases requires fundamental changes in global nutrient cycles either (1) through excess nutrient inputs to the oceans by increasing continental weathering and river discharge or (2) through nutrient-recycling from the marine sediment reservoir. We investigated the role of phosphorus on the development of carbon accumulation by analysing phosphorus speciation across OAE2 and the mid-Cenomanian Event (MCE) in the Tarfaya Basin. The ratios of organic carbon and total nitrogen to reactive phosphorus (Corg/Preact and Ntotal/Preact) prior to OAE2 and the MCE hover close to or below the Redfield ratio characteristic of marine organic matter. Decreases in reactive phosphorus resulting in Corg/Preact and Ntotal/Preact above the Redfield ratio during the later phase of OAE2 and the MCE indicate leakage from the sedimentary column into the water column under the influence of intensified and expanded oxygen minimum zones. These results suggest that a positive feedback loop, rooted in the benthic phosphorus cycle, contributed to increased marine productivity and carbon burial over an extended period of time during OAEs

Signature of hydrodynamic activity caused by rapid sea level changes in pelagic organic-rich sediments, Tarfaya basin (southern Morocco)

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Post-phosphogenesis processes and the natural beneficiation of phosphates: Geochemical evidence from the Moroccan High Atlas phosphate-rich sediments

International audienceThe Upper Cretaceous-Paleogene sedimentary series of the High Atlas hosts underexplored phosphate-rich sediments and presents an excellent example of how complex interactions between various geological processes control the accumulation and distribution of phosphates. Genetically-linked phosphatic lithofacies from this series were investigated for their mineralogical and geochemical compositions with the purpose of assessing the impact of post-phosphogenesis sedimentary processes on the geochemical behavior of genetically-linked phosphatic lithofacies and understanding the distribution of chemical elements. Sampled phosphatic horizons from five representative sections along the Marrakesh High Atlas borders were investigated using optical and scanning electron microscopies (OM and SEM), X-ray diffraction (XRD), inductively coupled plasma optical emission spectroscopy (ICP-OES), and inductively coupled plasma mass spectrometry (ICP-MS) techniques. Results indicated that the studied lithofacies are composed of a mixture of phosphatic and non-phosphatic particles in variable proportions depending on the facies type. However, the phosphatic fraction is dominated by carbonate fluorapatite (CFA), showing an average CO32− content of 8.40 ± 0.37 wt%. Geochemical results revealed variations in the chemical composition of the different phosphate facies. Low-grade pristine phosphatic sediment (P2O5 < 12%wt.) can be turned by the effects of storms and bottom currents into high-grade granular phosphate beds containing up to 24 wt% P2O5. This natural enrichment is mirrored in the bulk rock by increasing P2O5 concentrations against decreasing detrital phase-associated chemical elements (K, Al, Si, and Ti). Among the investigated lithofacies, karst-filling phosphate show relatively lower uranium and cadmium contents as it experienced a polyphase evolution, including winnowing, submarine reworking, transport, and subaerial weathering, traced using some redox-sensitive proxies such as cerium and uranium. Despite this polyphase sedimentary differentiation, the apatite resisted chemical changes and preserved its original geochemical signature, specifically the rare earth elements signal that reflects oxic seawater conditions. However, the differentiation processes modified the bulk rock composition of phosphates through the preferential leaching of gangue phases (e.g., carbonates). Furthermore, these processes triggered the oxidation of organic matter and sulfides, removing some associated elements of environmental concern, such as uranium and cadmium. Conclusively, the combination of mechanical winnowing/reworking and subaerial chemical weathering is the most effective way of the natural beneficiation of phosphate, both economically and environmentally

Morocco Basin's sedimentary record may provide correlations for Cretaceous paleoceanographic events worldwide.

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