Zusammenfassung
In der Kohlenkalk-Schelffazies des Untertournais bei Velbert (Bundesrepublik Deutschland) lassen sich drei Ooidtypen unterscheiden. Gemeinsam ist diesen Ooiden ein vorwiegend konzentrischer Lagenbau. Dagegen ist das Vorkommen radial-konzentrischer und radial-fibröser Ooide minimal. Die diagenetische Entwicklung der Kohlenkalkooide ist hinsichtlich ihrer chemischen Zusammensetzung deutlich von gleichalten Crinoiden- oder Brachiopodenmaterial zu unterscheiden. Strukturelle Beobachtungen lassen darauf schließen, daß radial-konzentrische und radial-fibröse Ooide ursprünglich aus Aragonit/Hoch-Mg-Calcit bzw. Hoch-Mg-Calcit bestanden. Die Zusammensetzung der Spurenelemente und die Struktur der konzentrisch aufgebauten Ooide hingegen deutet auf eine Fällung von Aragonit, der im Laufe der Diagenese zu Niedrig-Mg-Calcit umgewandelt wurde.
In letzter Zeit wurde in einigen Veröffentlichungen die These vertreten, daß die Mineralogie von nicht-skelett Karbonaten zwischen Calcit und Aragonit in unbekannten Intervallen wechselt. Wechsel des atmosphärischen CO2 Drucks und des Mg/Ca Verhältnisses in den Ozeanen während des Phanerozoikums könnten Vorgänge sein, die die vorherrschende Mineralogie dieser Karbonate beeinflussen. In dieser Arbeit wird der Chemismus der Kohlenkalkooide in Verbindung mit anderen Ooiden und Daten über Mikrite des Mittelpaläozoikums bis heute in bezug auf den Wechsel zwischen »Calcit«- und »Aragonit-Ozeanen« diskutiert. Der Strontiumgehalt der Ooide (¯x = 1010 ppm, Spannbreite: 145–3010 ppm) und der Mikrite (¯x = 841 ppm, Spannbreite: 3–8800 ppm) spricht für eine primäre Aragonitfällung. Zwischen der Ooid/Mikrit Chemie und ihrer Mineralogie gab es keine mögliche statistische Korrelation. Daraus schließen wir, daß während des ganzen Phanerozoikums aragonitische Ooide und mikrite große Bedeutung in flachmarinen Schelfregionen mit Karbonatsedimentation hatten und haben. Das Vorkommen aragonitischer und calcitischer Ooide ist wahrscheinlich an hydraulische und physiko-chemische Vorgänge gebunden, die auf globale Tektonik, eustatische-, klimatische- und atmosphärische Veränderungen zurückzuführen sind. Diese führten dazu, daß die ursprüngliche Geochemie und Struktur diagenetisch verändert wurde.
Abstract
Three ooid types are recognized from the Lower Tournaisian »Kohlenkalk« shelf facies at Velbert, Germany. Ooids from this unit have a predominantly concentric laminae fabric. Radial-concentric and small radial fibrous ooids are minor components to this oolite. The diagenetic response of Kohlenkalk ooid chemistry is significantly different from that observed in contemporaneous crinoid and brachiopod material. Fabric evidence suggests that radial-concentric and radial-fibrous ooids were probably originally aragonite/high-Mg calcite and high-Mg calcite respectively. Fabric and trace elemental chemistries of the concentric fabric ooids suggests that they were originally precipitated as aragonite and subsequently altered to low-Mg calcite.
Recent papers have proposed temporal shifts in the dominant mineralogy of shallow marine non-skeletal carbonates between calcite and aragonite. Changing Phanerozoic atmospheric pCO2 levels and oceanic Mg/Ca ratios may have been factors controlling the dominant mineralogy. The chemistries of the Kohlenkalk ooids in conjunction with other ooid and micrite data spanning the Mid-Paleozoic to Recent are evaluated in context with these temporal shifts between »calcite« and »aragonite seas«. The strontium chemistries of the ooids (¯x = 1010 ppm, range 145–3010 ppm) and micrites (¯x = 841 ppm, range 3–8800 ppm) suggests they had an aragonite precursor mineralogy. No statistical correlation was observed between ooid/micrite chemistries, their mineralogies and the proposed secular trend. Therefore, we suggest that aragonitic ooids and micrites were dominant components of shallow-marine carbonate environments throughout the Phanerozoic. The distribution and abundance of aragonitic and calcitic ooids in the geologic past was probably dependant on local hydraulic, physicochemical, and environmental conditions, areally constrained by global tectonics, eustatic, climatic and atmospheric effects, with significant diagenetic overprinting of the original geochemical and fabric information.
Résumé
Parmi les facies de plate-forme du Tournaisien inférieur, dans le calcaire carbonifère de Velbert (RFA), on distingue trois types d'ooïdes: les plus abondantes présentent une structure lamellaire concentrique; d'autres, en quantité subordonnée, sont radiaires-concentriques et fibro-radiées. En réponse à leur chimisme, les ooïdes du calcaire carbonifère ont connu une évolution diagénétique nettement différente de celle des sédiments à crinoïdes et à brachiopodes de même âge. L'organisation structurale des ooides radiaires-concentriques et fibro-radiées indique que'elles étaient constituées respectivement d'aragonite + calcite magnésienne et de calcite magnésienne. Par contre, la structure et le chimisme des éléments en traces des ooïdes concentriques suggèrent qu'elles ont été d'abord précipitées en aragonite et transformées ensuite en calcite pauvre en Mg.
Selon certains travaux récents, la composition minéralogique des sédiments carbonatés non organo-détritiques aurait fluctué au cours du temps entre l'aragonite et la calcite. Des factures déterminants de ce processus auraient pû être les changements, au cours du Phanérozoïque, de pCO2 dans l'atmosphère et du rapport Mg/Ca dans les océans. Le chimisme des ooïdes du calcaire carbonifère, comparé à celui de micrites et d'autres ooïdes d'âges phanérozoïque moyen à récent est examiné en relation avec ces passages de «mers à calcite» à «mer à aragonite» au cours du temps. Le chimisme du Sr des ooïdes (x = 1010 ppm; intervalle 145–3010 ppm) et des micrites (x = 841 ppm; intervalle 3–8800 ppm) suggère une précipitation primaire d'aragonite. Il n'apparaît aucune corrélation statistique entre le chimisme des ooïdes et micrites, leur minéralogie, et les fluctuations temporelles suggérées. Pour ces raisons, nous pensons que les ooïdes et micrites aragonitiques ont été les composants dominants des milieux carbonatés peu profonds pendant tout le Phanérozoïque. L'abondance et la répartition des ooïdes aragonitiques et calcitiques dans la nature ancienne ont vraisemblablement résulté de conditions locales hydrauliques, physico-chimiques et d'environnement, déterminées par la tectonique globale, et par les changements eustatiques, climatiques ou atmosphériques, qui ont surimposé leur empreinte diagénetique aux caractères structuraux et géochimiques primaires.
Краткое содержание
В шельфовой фации изв естняка нижнего карб она нижне-турнейского яр уса в регионе Вельбер та (Федеративная Республика Германия) установлены три типа ооидов, имеющих гл. обр. концентрическое стр оение; радиально-концентри ческое и радиально-фи брозное строение встречаетс я здесь редко. Диагене тическое развитие известняка нижнего карбона знач ительно отличает его от матер иала криноидов и брах иопод сегодняшнего дня по и х химическому состав у. Изучение структуры разрещает заключить, что исходн ым материалом радиальн о-концентрических и радиальнофиброзных ооидов является араг онит-кальцит с высоки м содержанием магния, и ли кальцит с высоким содержанием магния. Н а основании изучения микроэлементов и строения концентри ческих ооидов пришли к заключению, что здес ь выпадал арагонит, ко торый превращался под влия нием процессов диаге неза в кальцит с низким соде ржанием магния.
В последнее время в не которых опубликован иях высказывается предп оложение, что у карбон атов нескелетного происхождения в неки е интервалы времени происходит смена кал ьцита на арагонит и на оборот. Изменение давления а тмосферного СО2 и соо тношения магния/кальция во вре мя фанерозоя могут бы ть теми процессами, кото рые влияют на состав э тих карбонатных пород. В д анном опубликовании обсуждается химизм ооидов извест няка нижнего карбона и проводится сравнени е с ооидами иного сост ава, а также с данными о микритах о т среднего палеозоя д о сегодняшнего дня с точки зрения сме ны кальцита и арагони та в океанах. Содержание стронция в ооидах / ¯х 1010 ррm; диапазон: 145–3010 ррm / и ми критах / ¯х - 841 ррт; диапазон: 3–8800 ррm / гов орит и первичном выпа дении арагонита. Между хими змом ооидов/микритов и их минералогией ста тистической корреля ции установить не удалос ь. Авторы заключают, чт о во время фанерозоя араг онитовые ооиды и микр иты играли большую роль в мелководных региона х шельфа, где шло образование к арбонатных пород. Поя вление арагонитовых и калыг ятовых ооидов, вероят но связано с гидравличе скими и физико-химиче скими процессами, вызванны ми тектоническими со бытиями в глобальном масштаб е и приводящими к эвст атическим, климатическим и атмо сферным изменениям в то время. Вызванные эт ими изменениями диагенетические процессы оказали бол ьшое влияние на геохи мический состав этих ооидов.
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