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Le passage Cénomanien - Turonien en Tunisie : Biostratigraphie des foraminifères planctoniques et des radiolaires, chimiostratigraphie, cyclostratigraphie et stratigraphie séquentielle

Abstract : High-resolution foraminiferal biostratigraphic, chemostratigraphic and cyclostratigraphic (using time-series) analyses were undertaken on the Bahloul Formation, in eight spaced sections Bargou (COK and AZ), Jerisa (CES), Guern Halfaya (GH), Hammem Mellegue (HM), Kherij (KJ); Mahjouba (KS) and Oued Hissi (OH) which preserve the worldwide expression of the Cenomanian-Turonian Oceanic Anoxic Event (OAE-2). We have collected 430 samples from the eight studied sections This study on the Bahloul black shales reveals that the OAE-2 was not a sudden event, but rather a long-term global one. In all the studied sections, the high resolution biostratigraphical analysis allows identifying five subzones of the standard biostratigraphic scheme. (1) Rotalipora cushmani zone, (2) Whiteinella archaeocretacea zone, (3) Helvetoglobotruncana Helvetica zone. The ability to apply same subzones in both Tethyan and transitional/Boreal provinces during the C-T boundary has important palaeobiogeographic and palaeoecologic implications. The palaeoecological framework shows that OAE-2 development is composed of four successive phases. In addition, a faunal turnover during the W. archaeocretacea zone interval is recorded. However, this does not result in an obvious loss in diversity that can be related to a major sea level rise. The high-resolution chemostratigraphic analyses for the Late Cenomanian-Early Turonian Bahloul Formation provide new insight on the palaeoceanographic evolution of the southern Tethyan margin. Relative low abundance of related terrigenous Ti/Al and K/Al ratios and enrichment of some productivity proxies such as Ba, Cu, and Ni (organic matter related trace elements) suggests that the Bahloul, deposited during a relative short period (0.5 Ma), was of relatively elevated primary productivity and minimal detrital input. While higher D* values concurrent with lower Ti/Al ratios are interpreted as caused by enhanced fluvial material contribution, due to more humid climate during the OAE-2. Enhanced humidity triggered probably fluvial influxes, resulted in a sluggish water circulation and consequent anoxic/euxinic conditions favouring the preservation of organic matter at the bottom. Enrichments in redox-sensitive trace metals U, V, and Mo in the Bahloul Formation deposits and redox indices, such as V/(V+Ni), U/Th, V/Cr, and Ni/Co, indicate that oxygen-restricted conditions prevailed during the Late Cenomanian - earliest Turonian times and correlate well with relative abundances of some foraminiferal and radiolarian paleo-environmental relevant indicators. High Baxs values and Uauth may indicate anoxic conditions at least at the water- sediment interface during the Bahloul Formation deposition and provide information about low to moderate sulphate-reduction reactions. Time series analysis has been performed to shed light on its Milankovitch-like cyclicity. This analysis was tested and applied on two foraminiferal species: the biserial Heterohelix, an oxygen minimum zone (OMZ) dweller, and the triserial Guembelitria, a eutrophic surface dweller. Average sedimentary rates and the duration of the OAE2 in each studied section were estimated. Alternatively, the fluctuations of these two opportunistic species can be related mainly to both precessional (23-19 kyr) and eccentricity (414-106 kyr) cyclicity suggesting that changes in surface water fertility were linked to the Milankovitch parameters.
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https://tel.archives-ouvertes.fr/tel-00626628
Contributor : Mohamed Soua <>
Submitted on : Monday, September 26, 2011 - 3:58:56 PM
Last modification on : Monday, September 26, 2011 - 4:04:19 PM
Long-term archiving on: : Tuesday, December 27, 2011 - 2:30:37 AM

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  • HAL Id : tel-00626628, version 1

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Soua Mohamed. Le passage Cénomanien - Turonien en Tunisie : Biostratigraphie des foraminifères planctoniques et des radiolaires, chimiostratigraphie, cyclostratigraphie et stratigraphie séquentielle. Sciences de la Terre. Faculté des Sciences Mathématiques, Physiques et Naturelles de Tunis, 2011. Français. ⟨tel-00626628⟩

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