Pierre Freytet

Lacustrine and palustrine carbonate petrography: an overview

Lacustrine limestones were formerly identified by their faunistic (limnea, planorbis) and floristic (Charophytes) content. For 30 years, indications of pedogenesis have been found in many lacustrine deposits, and consequently the concept of palustrine limestone was defined.Lacustrine fabrics are not that numerous: varved, laminated, homogeneous, peloidal, brecciated, gravelly, bioturbated (burrows), bioclastic, algal, and stromatolitic. Detrital beds are sometimes present and are interpreted as bottomset deposits. Palustrine fabrics result from exposure and pedogenesis of lacustrine mud. The main processes involved in this evolution are: cracking, with planar, curved, craze and skew planes, colonization by plants resulting in root traces, marmorization (redistribution of iron due to water table fluctuation), and redistribution of carbonates (needles, subspherical or cylindrical vertical nodules, carbonate coatings, early and late diagenetic crystals, Microcodium). Carbonate palustrine features can be associated with other minerals: palygorskite, gypsum, or silica. Alternation of lacustrine sedimentation and exposure/pedogenesis leads to the pseudo-microkarst facies resulting from enlargement of the complex network of root traces and horizontal cracks. The voids in the pseudo-microkarst facies are infilled with a polyphased internal sediment composed of carbonate and vadose silt and phreatic and vadose cements. Traces of exposure and pedogenesis are less in evidence in lacustrine bioclastic sands and algal-stromatolitic limestones. Finally, under certain conditions, the surficial laminar horizon and its associated perlitic crust (ooids) develops on palustrine muds and form a desert stromatolite.

Petrography and paleo-environment of continental carbonate deposits with particular reference to the upper cretaceous and lower eocene of languedoc (Southern France)

Abstract The carbonates studied include several types 1. (1) Lacustrine dolomites which have resulted from the concentration of products leached from neighbouring Paleozoic and/or Mesozoic rocks. Certain well-crystallised limestones contain phantoms of rhombohedra, and are the result of dedolomitisation. 2. (2) Limestones deposited in fluviatile channels, or at the outflow of rivers into lakes, which are always sandy (with corroded quartz) and often contain calcitic intraclasts, bioclasts and oncolites. (Cyanophyceae, Rivulariaceae.) 3. (3) True lacustrine limestones, with an entirely subaqueous diagenetic evolution, which are homogenous, or show evidence of reworking: breccias, gravels, coated gravels. Burrowing animals have left traces of their activity (in particular striotubules∗) in sediments deposited on the bottom lakes. 4. (4) “Paludine” limestones which have resulted from emersion of lacustrine muds, have been affected by soil processes and show various signs of reworking: penetration of roots, burrows, nodulisation∗, marmorisation, etc. All these phenomena occur together giving facies and microfacies which may be very complicated. 5. (5) Silicification, which may occur in all continental limestones, including lacustrine or paludine types. It exhibits various morphological aspects; on a microscopic scale, it is an accumulation of opal and/or thin veins of chalcedony. These different facies are not distributed haphazardly, but constitute sequences which are repeated, with some variations, in stratigraphical series. Lataral variation in the style of vertical sequence in a basin enables the reconstitution of the paleo-environment at a given period.

Lacustrine sedimentation in a semiarid alpine setting: An example from Ladakh, northwestern Himalaya

Abstract The Lamayuru lacustrine strata in Ladakh typify many of the carbonate-rich Pleistocene alpine lakes found in the semiarid environment of the northern Himalaya. Created by a 200-m-thick landslide, the lake was in existence by at least 35,000 yr ago, and may have persisted until 500–1000 yr ago. Represented in the center by thin turbidites and laminated muds, the lacustrine sedimentation along the lake margins and low-relief deltas characteristically displays a marked contrast between (1) clastic lenses representing rapid, sporadic, matrix-poor debris flows and periglacial inputs from the alpine slopes and (2) abundant, diverse, shallow-water, biologically dominated carbonate strata, among which organism-rich, chalky beds and oncolithic and encrusted stem-rich strata predominate. Resemblances of the Lamayuru lacustrine strata and their setting to those of former lakes throughout areas north of the Greater Himalayan crest suggest that the alpine, semi-arid environment would favor diversified, spacially restricted carbonate sedimentation punctuated by occasional clastic influxes. Such a depositional regime contrasts strongly with that found immediately south of the Himalayan crest where more humid conditions promote a more continuous clastic influx into intramontane lakes.

Quaternary changes in the Egyptian shoreline of the Northwestern Red Sea and Gulf of Suez

Abstract On the NW, Egyptian, shoreline of the Red Sea, remarkably preserved Pleistocene-Holocene marine (reefs, beaches, mangrove swamps, gypsum-salinas) and non marine (alluvial fans and wadi terraces) deposits are located within a complex Neogene rift frame. The Early and Middle Pleistocene reefs are uplifted to a maximum of 40 m, subsequent movement being limited to a few metres (except on the S Gulf of Suez shores) which suggests a decreasing tectonic activity of the northern part of the rift. A new series of radiometric dates and precise levelling have demonstrated a short-lived, low stand of relative sea level during the Last Interglacial high stand (5e). At that time, flooding of the Isthmus of Suez has enabled exchanges between Mediterranean and Indian Ocean faunas and the biogeographic limit was temporarily located in the Southern Gulf of Suez, thus explaining the Late Quaternary introduction of Potamides conicus into the Indian Ocean subprovince.

Permian freshwater stromatolites associated with the conifer shoots Cassinisia orobica Kerp et al. — a very peculiar type of fossilization

Foliated shoots of Cassinisia orobica Kerp et al., 1996 are fossilized by a combination of two processes, i.e. by tufaceous/stromatolitic encrustation, preserving their external morphology, and the formation of sparite crystal which include leaf tissues and vascular bundles. Encrustation took place in two phases, interrupted by a period of emersion. The arrangement of the stromatolitic laminations and the aspect of the crystals and their inclusions allow a comparison with some specific recent forms of Schizothrix. On the lake bottom, cavities in the axes and leaves were finally filled with internal sediment, gypsum and diagenetic sparite.

The unusual hydrodynamical behaviour of freshwater oncolites

Algal concretions are frequent in calcareous environments, occuring as individual particles (oncolites) that are easily transported during floods. Several parameters have been measured and calculated for a population of 127 particles from French streams. These oncolites have a high porosity (mean 42.7%) and a low density (mean 1.65), in respect to quartz and calcite. Oncolites have a settling velocity 1.5 times lower than that of quartz grains with the same nominal diameter. The computation of multiple correlations results in a classification in terms of decreasing efficiency: shape coefficients (Corey, ψ, and SI of Sneed and Folk), nominal diameter, density, edge sharpness and roughness. A Cd (drag coefficient) versus Re (Reynolds number) plot of these oncolites displays two different clusters with 1000 < Re < 10,000. One cluster, with 0.6 < Cd < 2.4 is named ‘oncolites’. The other, with a Cd around 0.1, represents ‘encrusted snails’. In the oncolite cluster, no preferential arrangement of points appears and the regression line is horizontal (parallel to the x-axis). In this part of the diagram, the equation has the form Cd = 0Re + b. This b coefficient is an important parameter, because it varies with the values of nominal diameter and density. The various shape coefficients tested (Corey shape factor, sphericity and the Sneed and Folk triangle), the roundness (expressed by the Powers index) and the roughness also exert a slight influence. Multiple correlation calculations allow them to be ranked in function of their influence on the drag coefficient (in decreasing order): density, edge sharpness, nominal diameter, shape coefficients, and roughness. This study is a preliminary approach and many additional measurements on living material are needed to obtain a more pertinent set of data.

Pedological nodules with cone in cone structure in the Permian of Sierra Morena (Spain) and Central Morocco

The occurrence of (1) calcite aureoled quartz, and (2) vertical cylindrical features (“rhizocretions” in literature), and subspherical or unevenly shaped nodules resulting from the coalescence of smaller elements (“nodular calcrete”) is reported in various Permian continental series from Western Europe (Sierra Morena, Spain) and North Africa (Central Meseta of Morocco). These features are developed in flood plain pelites (mudstones) interbedded with gravelly channel material and debris flow deposits. These nodules are interpreted as pedological concretions formed in hydromorphic soils under tropical climatic conditions with contrasting seasons and not under desertic environments.These nodules show the close association of a classical micritic/microsparitic internal sediment with areas where cone in cone structures prevail. The distribution of facies suggests an early pedogenic origin. At this early stage, vegetal roots were alive (probably Cordaitales) and were encrusted by the redistribution of calcitic components in the soil profile resulting from oscillations of the water table. The cone-in-cone structure probably resulted from bacterial activity.

Discovery of Ca oxalate crystals associated with fungi in moss travertines (Bryoherms, freshwater heterogeneous stromatolites)

Buffered decalcification of live moss and liverwort (Hepaticae) travertines resulted in the release of a large number of organisms (bacteria, cyanobacteria, fungi, eukaryotic algae, and small animals), which constitute an “organic mat”; (also called an algal mat or biofilm). This mat is calcified and commonly has laminations, allowing moss travertines to be considered as stromatolitic structures. After decalcification of 300 samples of travertines (using dilute acetic acid), only 9 released Ca oxalate crystals in the form of needle bundles, spherulites, and tetragonal bipyramidal prisms. These crystalline forms are identical to those found in some phanerophytes and soils. Mycelian filaments also exist in travertines mostly composed of algae, and it is possible that Ca oxalate crystals can be formed. However, being metastable, these crystals transform very quickly into calcite by diagenesis, in the same way as aragonite in the stromatolites of saltwater environments.

Palaeoecology of non marine algae and stromatolites: Permian of France and adjacent countries

Abstract Some Permian, continental basins from Algeria, Morocco, France, Italy, Germany and Poland reveal algal remains and stromatolites, mainly during Lower Permian, but also during Middle and Upper Permian. The algae belong to 8 morphogenera and 12 morphospecies. Two species are new. The taxonomic attributions are difficult, even if some species resemble living species. The algae make unorganized masses, or laminated builtups (stromatolites, oncolites, oolites). Algal masses and stromatolites are contained in fluviatile sediments (active or abandoned channels) and lacustrine deposits (playas, ephemeral lakes shorelines, lakes several meters or decameters deep). Considering the sedimentological context, the water salinity could range from karstic springs (hard water) to evaporic ephemeral lakes (playas).

Complex calcitic crystallizations in nostoc parmelioides kütz (Freshwater cyanobacterium): Rhombs around trichomes inside nostoc colonies and epiphytic bacterial microstromatolites

Pluricentimetric colonies of Nostoc parmelioides exhibit two types of calcifications: (1) rhombs, either isolated, chains, or grouped in elongate masses around sheathless trichomes in the interior of the colony; or (2) discs with fibroradiating and concentric fabric, in which radial elements are elongate, stacked, upside‐down, bell‐shaped monocrystals, with concentric laminations in cross section. The discs could be the result of a very peculiar calcification of an epiphytic organism, probably a bacterium. These structures can be referred to as bacterial microstromatolites.