Eric Jean Davaud

Tyrrhenian coastal deposits from Sardinia (Italy): a petrographic record of high sea levels and shifting climate belts during the last interglacial (isotopic substage 5e)

Abstract Detailed petrographic and sedimentological analysis of Tyrrhenian coastal deposits from four sites in Sardinia provides new data about climate and sea-level changes during the early part of the last interglacial (isotopic substage 5e) and shows that facies analysis is not only useful for the identification of ancient depositional environments, but also represents a powerful tool for evaluating past climatic conditions and sea-level forcing mechanisms. Two shallowing-upward sequences (sensu Ward, W.C., Brady, M.J., 1979. Strandline sedimentation of carbonate grainstones, Upper Pleistocene, Yucatan Peninsula, Mexico. Am. Assoc. Pet. Geol. Bull. 63, 362–369), commonly separated by an erosion surface and displaying complex geometric relationships, occur above modern sea level at several locations along the shorelines of Sardinia. These sequences include upper shoreface to backshore deposits and were formed during separate sea-level highstands attributed to isotopic substage 5e. Our recent revision of these deposits indicates that each sequence presents distinctive petrographic characteristics. The older succession essentially contains mixed calcarenites, rich in quartz grains and lithic fragments, whereas the younger sediments are dominated by carbonate bioclasts such as red algae, coral and mollusk fragments. Both units apparently accumulated in a similar depositional setting, but during different climatic regimes. The petrography of the lower unit reflects a temperate-humid period characterized by increased weathering and continental erosion, whereas the composition of the upper one suggests a dryer, and possibly warmer, climate favoring active carbonate production. The disparity in petrographic composition between sequences probably records the migration of atmospheric circulation cells, i.e. a shift of climate belts, before and after the ∼128-ka insolation maximum. Our climatic data are coherent with the calculated insolation curves for isotopic substage 5e and thus support the Milankovitch hypothesis for the origin of Pleistocene climates. In contrast, our sedimentological data clearly show the occurrence of two distinctive highstands of sea level during this time period, which does not agree with the hypothesis. It follows that the relation between orbitally induced insolation variations, climate and sea level is not necessarily straightforward.

Carbonate cements in Holocene beachrock: example from Bahiret et Biban, southeastern Tunisia

Abstract Holocene beachrock has been studied on the northern and western shores of Bahiret el Biban, a shallow restricted lagoon in southeastern Tunisia. The beachrock consists of horizontal, cemented layers which are 2–15 cm thick. Early cementation took place at the water table below beach ridges, where geochemical, hydrodynamic and possibly also microbiological conditions favored rapid precipitation of aragonite and/or high-Mg calcite. Locally, superimposed low-Mg calcite cements point to subsequent freshwater influence. At one site, six vertically stacked beachrock beds are found. Total-rock 14C dating shows that the highest beachrock bed is older than the one (of the same petrographic composition) situated at the present groundwater level. This implies a downward progression of cementation which probably monitored the sea level fall following a local highstand about 3000 years ago.

Sequential evolution and diagenesis of Pleistocene coral reefs (South Sinai, Egypt)

Abstract The uplifted Pleistocene coral reefs in South Sinai represent two major depositional sequences. They generally form two morphological terraces, which locally are tilted and offset by still active faulting. The major reef sequences are composed of several small-scale sequences. Corals grew rapidly when accommodation space was created by eustatic sea-level rise or fault-block subsidence. As the accommodation decreased, coral rubble and siliciclastic sands prograded over reefal and lagoonal sediments, thus creating a shallowing-upward sequence. Terrigenous input was further stimulated by a more humid climate during interglacial times. 230 Th/ 234 U dating places the older reef cycle between 350,000 and 270,000 years B.P., which corresponds to the interglacial period of isotope stage 9. The younger reef cycle has been dated between 140.000 and 60,000 years B.P. (isotope stage 5). An intermediate, relic sequence found in only one outcrop can be attributed to isotope stage 7. Correlation of the small-scale sequences is difficult: age-dating is not precise enough, and local tectonic activity in many cases overruled the smaller eustatic fluctuations. The younger reef sequence commonly exhibits marine cements, whereas the older sequence was exposed to freshwater dissolution and cementation. An important feature is the locally pervasive dolomitization especially of the older reef. Oxygen and carbon isotope values suggest that the dolomite formed in a seawater-dominated mixing zone. Multiple phases of dissolution, cementation, and dolomitization point to a very complex diagenetic history.

Formation of Holocene Limestone Sequences by Progradation, Cementation, and Erosion: Two Examples from the Bahamas

ABSTRACT Holocene limestones on North Bimini and Joulter Cays (Bahamas) reveal sedimentary structures that are directly comparable to structures in the modern beach environment. A complete sequence comprises shallow subtidal, intertidal, and supratidal deposits. Holocene and equivalent modern sedimentary features are found on more or less the same topographic levels. Freshwater cementation (phreatic and vadose) takes place after accretion and progradation of the shore. Subsequent erosion exposes the consolidated sediments. Secondary cementation in the intertidal zone leads to beachrock formation. Continued erosion may break off blocks of beachrock and cliffs, which will be incorporated into the basal part of the sequence during a new phase of progradation. A freshwater-cemented sequence going from shallow subtidal to supratidal and eolian deposits does not, therefore, necessarily imply sea-level changes, but can be created by a cycle of progradation, cementation, and erosion.

Recent Freshwater Ooids and Oncoids from Western Lake Geneva (Switzerland): Indications of a Common Organically Mediated Origin

ABSTRACT The shallow-water sediments of western Lake Geneva are composed mainly of sand-size coated grains ranging from oncoids to ooids. In surficial sediments the coatings consist of anhedral microcrystals of low-Mg calcite and often show a well developed intercrystalline porous network containing bacteria and residues of degraded mucus. Nucleation occurs in close association with organic films formed by mucus produced by filamentous cyanobacteria and diatoms. Similarities between oncoid and ooid cortex (crystal size, shape and arrangement) indicate that the processes controlling the building of these types of particle are identical. In subsurface sediments the microstructure of the coatings changes considerably: the intercrystalline porosity is reduced by aggrading neomorphism and the micritic porous and organic-rich coatings are transformed into a dense xenotopic mosaic with scattered molds of filamentous cyanobacteria. This occurrence of ooidal-oncoidal sands in Lake Geneva may help in interpreting ancient lacustrine formations and brings additional information on the processes of genesis and diagenesis of coated grains.

Post-mortem onshore transportation of epiphytic foraminifera; recent example from the Tunisian coastline

ABSTRACT The barrier-island system that connects Jerba Island to the Tunisian coastline is composed mainly of carbonate bioclastic sands. Well-preserved tests of dead epiphytic foraminifera are abundant on the foreshore and backshore, but they are scattered and poorly preserved on the shoreface, where very fine sand-size eolian quartz, peloids, and broken bioclasts are the main components. The reason for this paradoxical pattern is that most epiphytic foraminifera are very easily removed as suspended load by storm-generated currents and concentrated on beaches, washover fans, and small eolian dunes, kilometers away from their original life environment. Similar bioaccumulations are common in the fossil record and have often been interpreted incorrectly as prolific biocoenoses.

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 influenced 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 difference 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 affected 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.

The carbonate-evaporite lagoon of Al Dakhirah (Qatar): an example of a modern depositional model controlled by longshore transport

Abstract The southern coast of the Arabian Gulf is considered a good example of an arid carbonate depositional system. The objective of this field investigation is to better elucidate the dynamic controls on the character and pattern of the depositional belts in a carbonate-evaporite lagoonal setting. The interpretation of this lagoon is based on combined field examination and laboratory analyses of surface samples, and this has resulted in an updated depositional model for carbonate-evaporite lagoonal settings. This study highlights the effects of sediment transport by longshore currents, with the formation of sand spits that gradually closed the lagoon during their southwards migration. The associated narrow and elongated back-barrier settings consist of tidal-dominated muddy sediments affected by a network of tidal channels. This wave-dominated high-energy system has generated several carbonate barrier and back-barrier units that migrated southwards and seawards during the last few thousands of years. This updated depositional model significantly differs from shoreline-parallel facies tracts of the classical Trucial Coast model, where the dominant wind is orientated perpendicular to the coastline. Conversely to the sediment belts of the Trucial Coast, the Al Dakhirah lagoon displays a strongly asymmetrical pattern of the sediment belts, marked by a southwards and seawards migration.

Geoman: A FORTRAN program for the management of geological thin section data

Abstract A program for treating large numbers of qualitative and/or quantitative geological thin section data has been developed. Data are coded, stored in a direct access data bank, and extracted selectively for treatment. The system is easy to handle and flexible, that is the user can choose descriptive attributes (variables) and select parameters for different computational operations. The codification and program can be applied to all rock types. The treatment includes: Loading of data bank, Updating of data bank, Selection of samples and variables, Edition of samples, Variable distribution in geological sections, Histograms, Scatter diagrams, Association analysis and Correspondence analysis.