Littérature scientifique sur le sujet « Coralline red-algae facies »

In this paleoecological study we focused on coralline red algae-rich shoal grainstone facies in the Lower Miocene (Aquitanian) carbonates of the Qom Formation in the Vartun section in central Iran. The identified coralline red algae are mainly non-geniculate, although very rare geniculate forms of corallines were also recognized in thin section analysis. The identified algae are represented by Melobesioideae (Lithothamnion cf. valens, Lithothamnion cf. rovereoti, Lithothamnion cf. peleense, and Lithothamnion spp.), Mastophoroideae (Neogoniolithon sp., Spongites spp., and Spongites cf. fruticulosus), Sporolithon spp. (Sporolithon spp. and Sporolithon cf. airoldii), and geniculate coralline (Corallina). The algal taxa mainly display fruticose and lumpy growth forms. The dominance of the former is consistent with high water energy in the identified shoal environment. The presence of Lower Miocene algae indicates that deposition of carbonates took place in warm tropical to subtropical waters in a euphotic marine environment during the Aquitanian. Likewise, high water energy and sediment agitation resulted in taphonomic features such as fragmentation, abrasion, and disarticulation.

The carbonate rocks of the Shella Formation (Middle Eocene) belonging to the Jaintia Group in the Jaintia Hills of Meghalaya are represented by two sandstone units in alternation with three limestone units, viz. Lakadong Limestone, Umlatdoh Limestone and Prang Limestone respectively in chronological order. Umlatdoh Limestone, the middle limestone unit of Shella Formation is conformably underlain by the Lakadong Sandstone and overlain by Narpuh Sandstone. Samples from the Umlatdoh Limestone were collected from the outcrop on the Jowai-Badarpur Road, about 1 km southwest of Lumshnong. Calcareous algae have been recovered from four samples and two distinct facies have been recognized. One of the facies is dominated by non-geniculate coralline red algae and benthic foraminifera. The non-geniculate corallines are represented by Lithothamnion and Mesophyllum (Family Hapalidiaceae), Lithoporella (Family Corallinaceae) and Sporolithon (Family Sporolithaceae). The other facies is rich in calcareous green algae along with benthic foraminifera. The green algal genera belong to families Dasycladaceae, Udoteaceae and Halimedaceae. Previously, a green algal assemblage was recorded from the Umlatdoh Limestone Member of Shella Formation exposed in the low lying hills between Sutnga and Litang valleys of Jaintia Hills. However, this is the first report on the occurrence of non-geniculate coralline red algae from the Umlatdoh Limestone Member of the Shella Formation. Based on the algal-foraminiferal assemblages, interpretation has been made on the palaeoenvironment and palaeobathymetry.

Abstract Acorn barnacles are sessile crustaceans common in shallow-water settings, both in modern oceans and in the Miocene geological record. Barnacle-rich facies occur from polar to equatorial latitudes, generally associated with shallow-water, high-energy, hard substrates. The aim of this work is to investigate this type of facies by analysing, from the palaeontological, sedimentological and petrographical points of view, early Miocene examples from Northern Italy, Southern France and South-western Peru. Our results are then compared with the existing information on both modern and fossil barnacle-rich deposits. The studied facies can be divided into two groups. The first one consists of very shallow, nearshore assemblages where barnacles are associated with an abundant hard-substrate biota (e.g., barnamol). The second one includes a barnacle-coralline algae association, here named “barnalgal” (= barnacle / red algal dominated), related to a deeper setting. The same pattern occurs in the distribution of both fossil and recent barnacle facies. The majority of them are related to very shallow, high-energy, hard-substrate, a setting that represents the environmental optimum for the development of barnacle facies, but exceptions do occur. These atypical facies can be identified through a complete analysis of both the skeletal assemblage and the barnacle association, showing that barnacle palaeontology can be a powerful tool for palaeoenvironmental reconstruction.

Autochthonous carbonate build-ups are mainly formed by encrusting coralline algae, which are one of the most important carbonate sediment contributors in the benthic communities of the Mediterranean area. They represent one of the most productive ecosystems in temperate regions and currently develop on Mediterranean hard and soft bottoms with a patch distribution along the coast. Several types of assemblages have been described so far, due to the high heterogeneity of coralline growth-forms, distributed from the intertidal down to 160 m water depth, from rhodolith-beds to coralligenous build-ups. Coralline red-algae build-ups are frameworks with 3-D structure that serve as shelter and provide storm protection by buffering wave action along coastlines. For this reason coralline build-ups have high potential of preservation. However, few fossil examples have been described in the literature and their evolution in the context of an eustatic cycle has seldom been modeled in detail. Wide marine terraces are preserved in the area of the Crotone peninsula, Ionian Calabria, southern Italy. They are related to the interplay between Pleistocene sea-level changes and the progressive uplift of the Calabrian arc from the middle Pleistocene onwards. These terraces overlie a Plio-Pleistocene slope succession (Cutro Marly Clay Fm). The present paleoecological study aims at describing biogenic marine terrace deposits from Capo Colonna, correlated to Marine Isotopic Stage5.1 and 3. Stratigraphic sections have been measured and sampled at each visible facies change. Thin sections have been prepared for red algae identification. The deposits up to 10 m thick consist of a Plio-Pleistocene slope clay unit overlain unconformable by a transgressive erosional surface and a shallow-marine succession, in which well-developed Pleistocene temperate red-algal reefs and their associated deposits are dominant. Stratigraphic sections have been measured and sampled at each visible facies change. Thin sections have been prepared for red algae identification. Bioclastic calcarenite units have been disaggregated in a conservative way to conduct paleontological, sedimentological, chemical and statistical analyses. Species identification of bryozoans has been provided for facies dominated by this taxon. The coralligenous build-ups are the most common facies, formed metrical structures, and developed as coralligenous de plateau, or alternatively, as coralligenous on hard substrate. The coralligenous build-ups of Capo Colonna form algal reefs dominated by Mesophyllum alternans (Foslie) Cabioch & Mendoza and Titanoderma pustulatum (Lamouroux) Nägeli usually alternated with bryozoan crusts. Other red algae species like Lithophyllum stictaeforme (Areschoug) Hauck, Phymatolithon calcareum (Pallas) W. H. Adey & D. L. McKibbin and Neogoniolithon sp. occur. Moreover crustose coralline algae (CCA) are the dominant biogenic-carbonate producers, and form other facies in the deposits, like prâline rhodoliths or maërl bed. The occurrence of other CCA facies led to a high detailed interpretation of the temporal evolution of marine terrace deposits, in the framework of the Mediterranean benthic bionomy. The coralligenous build-ups of Le Castella are dominated by Mesophyllum alternans (Foslie) Cabioch & Mendoza and Titanoderma pustulatum (Lamouroux) Nägeli usually alternated with bryozoans crusts. Other red algae species like Lithophyllum stictaeforme (Areschoug) Hauck, Phymatolithon calcareum (Pallas) W.H.Adey & D.L.McKibbin and Neogoniolithon sp. rarely occur. The dominance of M. alternans in both marine terrace coralligenous build-ups suggests infralittoral paleoenvironmental conditions. The molluskan assemblages in the packstone occupies the base of the deposits represent a Posidonia meadows paleoenvironment, whereas the grainstone associated to algal build-ups vary from typical coralligenous (C) to coastal detritic (DC), to an infralittoral environment in the uppermost unit, with species linked to AP/HP biocoenoses. The paleontological results have been framed in the context of a genetic-stratigraphic interpretation of the marine terraces to reconstruct a temporal evolution of both marine terrace deposits.