Abstract:The coastal sabkha in Ras Gemsa, Red Sea coast with its colonizing microbial mats and biofilms was investigated. The sabkha sediments consist mainly of terrigenous siliciclastic material accompanied by the development of evaporites. Halite serves as a good conduit for light and reduces the effect of intensive harmful solar radiation, which allows microbial mats to survive and flourish. The microbial mats in the evaporite–siliciclastic environments of such sabkha display distinctive sedimentary structures (micr… Show more
“…The degradation of the organic matter together with the interstitial crystallization of gypsum reduces or avoids the preservation of organo‐sedimentary structures in hypersaline environments (Rouchy & Monty, ; Taher, ). On the other hand, the existence of an EPS layer increases the fossilization potential of cyanobacteria through crystallization and cementation (Taher, ), as observed in the studied samples.…”
Section: Discussionmentioning
confidence: 99%
“…In shallow surface brines, when subject to currents, microbial mats unattached to the substrate tend to tear and scour, resulting in tears and folds which are also recognizable in the geological record (Bernier et al ., ). Microbially induced sedimentary structures (MISS) found in Ras Gemsa, an active evaporite–siliciclastic sabkha on the Red Sea Coast of Egypt, appear as salt‐encrusted crinkle mats in the periphery of perennial saline pools, with their origin related to the effect of burial of the microbial mat beneath freshly deposited sand and wind action (Taher, ). Although these features appear similar to the ones in this study, there are no sand deposits in the succession, and therefore this interpretation is not likely.…”
Section: Discussionmentioning
confidence: 99%
“…Inorganic materials in surficial conditions do not have the cohesiveness and flexibility required to generate the folding observed in the studied units. On the other hand, microbial mats, commonly found associated with gypsum beds in saline environments, form cohesive and dense organic‐rich layers that can act as a single unit (Thomas et al ., ) that behaves as a viscoelastic film (Rupp et al ., ) or as a cohesive and flexible mat (Taher, ). In order to determine the relationship between the formation of enterolithic folds and a microbial mat, biosignatures have to be identified, although most of the traces of microbial presence in evaporitic settings are absent or very poorly preserved (Rouchy & Monty, ).…”
Enterolithic structures are stratigraphically localized folds in gypsum beds found in certain saline evaporitic sedimentary units in a wide variety of basins. Different models of formation have been proposed, all related to inorganic processes. These models include: diagenetic transformation of gypsum beds producing either displacive growth of crystals or volume changes; mechanical folding caused by compressional stress; and folding produced by slumping. The analysis of three Cenozoic evaporite sequences in Spain reveals that none of the previous models explains their origin and existence. In these outcrops, gypsum enterolithic structures occur in horizontal beds with parallel troughs and crests of the folds. They appear in shoreline facies of lacustrine environments and did not undergo major diagenetic transformations after the primary lithification of the original sediment. Based on these observations, together with the study of a modern analogue in Minorca, Spain, a new model is proposed for the genesis of enterolithic structures. This new model is based on the existence of a microbial mat exposed to brine concentration–dilution cycles and strong wind events. The high wind flow events enhanced folding of the microbial mat that became subaerially exposed and lithified due to subsequent evaporation. Therefore, the presence of enterolithic structures could be used as an indicator of shallow water environmental conditions subject to variations in brine concentration in areas with strong wind flow events. Previous studies of some evaporitic successions should be revisited, taking into account the proposed model, which would imply new depositional environment interpretations. At the same time, the proposed model could explain the existence of Kinneyia‐type structures, also known as wrinkle structures, formed beneath microbial mats in peritidal zones. Moreover, considering enterolithic structures as microbially induced sedimentary structures could be useful as evidence of microbial life in the ancient geological record and on other planets such as Mars.
“…The degradation of the organic matter together with the interstitial crystallization of gypsum reduces or avoids the preservation of organo‐sedimentary structures in hypersaline environments (Rouchy & Monty, ; Taher, ). On the other hand, the existence of an EPS layer increases the fossilization potential of cyanobacteria through crystallization and cementation (Taher, ), as observed in the studied samples.…”
Section: Discussionmentioning
confidence: 99%
“…In shallow surface brines, when subject to currents, microbial mats unattached to the substrate tend to tear and scour, resulting in tears and folds which are also recognizable in the geological record (Bernier et al ., ). Microbially induced sedimentary structures (MISS) found in Ras Gemsa, an active evaporite–siliciclastic sabkha on the Red Sea Coast of Egypt, appear as salt‐encrusted crinkle mats in the periphery of perennial saline pools, with their origin related to the effect of burial of the microbial mat beneath freshly deposited sand and wind action (Taher, ). Although these features appear similar to the ones in this study, there are no sand deposits in the succession, and therefore this interpretation is not likely.…”
Section: Discussionmentioning
confidence: 99%
“…Inorganic materials in surficial conditions do not have the cohesiveness and flexibility required to generate the folding observed in the studied units. On the other hand, microbial mats, commonly found associated with gypsum beds in saline environments, form cohesive and dense organic‐rich layers that can act as a single unit (Thomas et al ., ) that behaves as a viscoelastic film (Rupp et al ., ) or as a cohesive and flexible mat (Taher, ). In order to determine the relationship between the formation of enterolithic folds and a microbial mat, biosignatures have to be identified, although most of the traces of microbial presence in evaporitic settings are absent or very poorly preserved (Rouchy & Monty, ).…”
Enterolithic structures are stratigraphically localized folds in gypsum beds found in certain saline evaporitic sedimentary units in a wide variety of basins. Different models of formation have been proposed, all related to inorganic processes. These models include: diagenetic transformation of gypsum beds producing either displacive growth of crystals or volume changes; mechanical folding caused by compressional stress; and folding produced by slumping. The analysis of three Cenozoic evaporite sequences in Spain reveals that none of the previous models explains their origin and existence. In these outcrops, gypsum enterolithic structures occur in horizontal beds with parallel troughs and crests of the folds. They appear in shoreline facies of lacustrine environments and did not undergo major diagenetic transformations after the primary lithification of the original sediment. Based on these observations, together with the study of a modern analogue in Minorca, Spain, a new model is proposed for the genesis of enterolithic structures. This new model is based on the existence of a microbial mat exposed to brine concentration–dilution cycles and strong wind events. The high wind flow events enhanced folding of the microbial mat that became subaerially exposed and lithified due to subsequent evaporation. Therefore, the presence of enterolithic structures could be used as an indicator of shallow water environmental conditions subject to variations in brine concentration in areas with strong wind flow events. Previous studies of some evaporitic successions should be revisited, taking into account the proposed model, which would imply new depositional environment interpretations. At the same time, the proposed model could explain the existence of Kinneyia‐type structures, also known as wrinkle structures, formed beneath microbial mats in peritidal zones. Moreover, considering enterolithic structures as microbially induced sedimentary structures could be useful as evidence of microbial life in the ancient geological record and on other planets such as Mars.
“… Figure 7. SEM–EDX images of the microbial mats in the microbial laminated levelling structures. (a–c) The mats are composed of filamentous structures, which resemble filamentous cyanobacteria with EPS from other modern MISS (Taher, 2014). (d) EDX spectrum shows dominant Ca, P and O for phosphate and the existence of carbonaceous materials.…”
Microbially induced sedimentary structures (MISS) are microbial traces in sandy deposits. They have been formed by various modes of microbial behaviour in response to the prevailing physical dynamics in shallow-marine environments since early Archaean time. An association of fossils, phosphatic structures and grains, stromatolites and microbial laminated levelling structures is documented from the Zhongyicun Member of Lower Cambrian strata in the Baideng section dated close to 535 Ma. SEM examination demonstrates that microbial laminated levelling structures are the result of the development of microbial mats composed of filamentous structures. We propose that there was a short hiatus after the dolomite layer deposited when the dolomite layer was weathered to form centimetre-scale valleys firstly, and then microbes accumulated in these valleys and formed the microbial laminated levelling structures.
ResumoO termo 'microbiofácies' possui, mais comumente, uma conotação petrográfica sobre estudos faciológicos de rochas carbonáticas com grãos fósseis em microescala, para o que talvez fosse mais adequado o termo 'biomicrofácies'; todavia esse termo é aqui empregado com uma conotação de biofácies de natureza microbianas. A ocorrência de microbialitos em diversas das lagoas (lagunas) fluminenses vem se revelando extremamente importante para o estudo de processos e fácies carbonáticas de origem microbiana, as quais ganharam evidência depois das descobertas de petróleo na "camada Pré-sal", dessa possível natureza. Dentre tais lagunas, a Lagoa Pitanguinha apresenta não só esteiras microbianas contendo partículas carbonáticas, aqui estudadas, mas também estromatólitos, trombólitos e oncoides. O estudo caracteriza esteiras microbianas (microbialitos) em termos biossedimentológicos, tanto morfologicamente, em campo, quanto em termos de suas texturas e microestruturas presentes, através de microscopia (microscópio petrográfico), com a finalidade de se estabelecer um esquema de classificação de microbiofácies. A Lagoa Pitanguinha formou-se durante o Holoceno, como resultado de uma regressão marinha, que a isolou do mar por um conjunto de cordões praiais estreito, tendo ela desenvolvido condições de hipersalinidade em suas águas devido a condições de aridez local e influxo da cunha salina marinha. Nesse contexto, identificaram-se quatro microbiofácies (MBF-C, Coloforme; MBF-Po, Poligonal; MBF-Pu, Pustular; MBF-O e Oncoidal) em observações de campo em janeiro de 2014, sob condições de nível alto da laguna (ano chuvoso), porém hipersalino, e em janeiro de 2015, sob condições de grande aridez e maior hipersalinidade. As microbiofácies mostram-se controladas peculiarmente pelo aumento da hipersalinidade, à medida em que o nível d'água rebaixa, expondo progressivamente as suas margens, durante o período de forte estiagem. Aponta-se a desidratação da esteira em ambiente subaquoso que se hipersaliniza como mecanismo precursor de fendilhamentos (gretas de sinérese) observados, que se ampliam ou remodelam quando da eventual exposição e desidratação subaérea. Palavras-chave: Microbialito; Esteira Microbiana; Estromatólito; Lagoa Pitanguinha
AbstractThe term 'microbiofacies' has most commonly a petrographic connotation for faciologic studies of carbonate rocks with plenty of fossil grains in microscale, for what it might be more appropriate the term 'biomicrofácies'; however this term is used here with a connotation of a biofacies of microbial nature. The occurrence of microbialites in several lagoons in the southeastern coast of Rio de Janeiro State (namely Região dos Lagos) has been revealing extremely important for the study of carbonate processes and facies of microbial origin, which gained evidence after the discovery of the "Presalt" petroleum province in Brazil, of this putative nature. Among these lagoons, the Lagoa Pitanguinha (here under study) present not only microbial mats containing carbonate particles, but also stromat...
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