Hydrodynamic behaviour of Nummulites: implications for depositional models

Large benthic foraminifers are considered to be good indicators of shallow marine carbonate environments in fossil series. Over the last 50 years, the palaeoenvironment of Tertiary Nummulites accumulations has been a matter of debate, particularly because of difficulties in interpreting these deposits, and in this way, the absence of analogues in present-day seas does not help. The aim of this paper is to insight the different ways Nummulites tests and clasts may accumulate according to their hydrodynamic behaviour. Based on experimental measurements and on SEM observations, it appears that the high primary skeletal porosity of Nummulites made them easily transportable. The calculated threshold shear velocities confirm that large-sized Nummulites can be moved by weak wave-driven currents. This peculiar hydrodynamic behaviour of Nummulites could explain the diversity of depositional models. Depending on local hydrodynamic conditions, autochthonous Nummulites deposits can be preserved as in situ winnowed bioaccumulations or be accumulated offshore, onshore or alongshore, away from the original biotop

Facies characteristics and diversity in carbonate eolianites

Carbonate eolian dunes can form huge sand bodies along the coasts but are seldom described in the pre-Quaternary record. The study of more than 600 thin-sections collected in present-day, Holocene and Pleistocene dunes from Sardinia, Crete, Cyprus, Tunisia, Morocco, Australia and Baja California confirms that these deposits can be easily misinterpreted as shallow marine at core or thin-section scale. The classical eolian criteria (fine-grained and well-sorted sands) are exceptional in carbonate dunes because the diversity of shapes and densities of carbonate particles lowers the critical shear velocity of the sediment thus blurring the sedimentary structures. Wind carbonate deposits are mainly heterogeneous in size and often coarse-grained. The paucity of eolianites in the pre-Quaternary record could be due to misinterpretation of these deposits. The recognition should be based on converging sedimentological and stratigraphic elements at core scale, and diagenetic (vadose diagenesis, pedogenetic imprints) and petrographical (grain verticalization, scarcity of micritic envelopes, broken and/or reworked foraminifera) clues in thin-section. Bioclastic or oolitic grainstones showing evidence of vadose diagenesis or pedogenetic imprints, should always be suspected of having an eolian origi

Geometry and internal discontinuities of an Ypresian carbonate reservoir (SIT field, Tunisia)

Les calcaires ypresiens à nummulites présentent un important potentiel pétrolier, en particulier en Tunisie centrale. Ce travail comprend deux études parallèles: sur le terrain, dans le massif du Djebel Ousselat (région de Kairouan) et sur les données du champ pétrolier de Sidi el Itayem (SIT), près de la ville de Sfax. Les reconstitutions en trois dimensions des corps sédimentaires pour les deux cas d'étude ainsi que pour les niveaux diagénétiques et les "rock types" pour le réservoir principal de SIT ont été effectués. Le Djebel Ousselat montre des affinités avec un système détritique, avec des corps progradants épais et des niveaux condensés riches en dents de requin au sommet des corps. Au contraire, Sidi el Itayem montre plus d'affinités avec un modèle de type production carbonaté "in situ", avec développement des corps pendant les périodes de bas niveau marin relatif et durant les phases transgressives

External geometiy of nummulite accumulation within a sequence stratigraphy framework (Sidi El Itayem hydrocarbon field, Ypresian, Tunisia)

Hasler Claude-Alain, Davaud Eric. External geometiy of nummulite accumulation within a sequence stratigraphy framework (Sidi El Itayem hydrocarbon field, Ypresian, Tunisia). In: Géologie Méditerranéenne. Tome 28, numéro 1-2, 2001. Anatomy of Carbonate Bodies / Anatomie des corps carbonates. International Meeting / Colloque international. Marseille, 9-12 mai 2001, France, sous la direction de Marc Floquet, Jérôme Hennuy et Jean-Pierre Masse. pp. 89-92

Anatomy, internal heterogeneities, and early fracture network of a Pleistocene carbonate coastal dune (Rejiche Formation, southeastern Tunisia)

Although eolian deposits are known to record the dominant winds, secondary conditions such as wind reversals during wintertime can also be observed in the petrographic composition and facies succession. Thus, eolian deposits are used here as a local paleoclimatic proxy. The spatial distribution of the depositional facies, early diagenetic imprints, and early fracture network of a coastal Pleistocene eolian ridge in southeastern Tunisia is described using a small-scale GIS model. Facies analysis indicates that coastal dune systems record seasonal cycles. The fracture density and directions are strongly inXuenced by the depositional facies type. Laminated facies present a higher fracture density compared to more homogeneous facies and show only one major fracture direction, while the more homogeneous facies display a bimodal distribution. Such a diVerence between these two groups is explained by the heterogeneous distribution of the early calcite cement within the laminated facies. No tectonic activity or overlying strata have aVected the Pleistocene dunes under study. Therefore, the mechanism responsible for the fractures could only be related to the own weight of the eolianite and to its internal or underlying lithologic heterogeneity

Catastrophic event recorded among Holocene eolianites (Sidi Salem Formation, SE Tunisia)

A high-energy deposit cuts through the early Holocene eolianites of the Sidi Salem Formation which forms a ridge along the southeastern coast of Tunisia. The sedimentary structures as well as the paleo-altitude and paleo-location of the outcrop state for a subaqueous deposition by an unusually large catastrophic event. Regarding its age and the related uncertainties, it could be either an exceptional storm, or a landslide or impact triggered tsunami. The mega-tsunami of the 8000 BP collapse of the Valle del Bove valley (Etna Volcano) could be this event, for its matching age and calculated run-up height

Hydrodynamic behaviour of Nummulites: implications for depositional models

Large benthic foraminifers are considered to be good indicators of shallow marine carbonate environments in fossil series. Over the last 50 years, the palaeoenvironment of Tertiary Nummulites accumulations has been a matter of debate, particularly because of difficulties in interpreting these deposits, and in this way, the absence of analogues in present-day seas does not help. The aim of this paper is to insight the different ways Nummulites tests and clasts may accumulate according to their hydrodynamic behaviour. Based on experimental measurements and on SEM observations, it appears that the high primary skeletal porosity of Nummulites made them easily transportable. The calculated threshold shear velocities confirm that large-sized Nummulites can be moved by weak wave-driven currents. This peculiar hydrodynamic behaviour of Nummulites could explain the diversity of depositional models. Depending on local hydrodynamic conditions, autochthonous Nummulites deposits can be preserved as in situ winnowed bioaccumulations or be accumulated offshore, onshore or alongshore, away from the original biotope