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A mineralogical and geochemical study of turbidite sandstones and interbedded shales, Mam Tor, Derbyshire, UK

Published online by Cambridge University Press:  09 July 2018

D.A. Spears
Affiliation:
Department of Geology, University of Sheffield, Mappin Street, Sheffield S1 3JD, UK
M.A. Amin
Affiliation:
Department of Geology, University of Sheffield, Mappin Street, Sheffield S1 3JD, UK

Abstract

Eleven shales and fourteen turbidite sandstones from the Mam Tor Beds were analysed chemically and by XRD. The ratio of kaolinite to illite plus mixed-layer clay was higher in the sandstones than in the shales, size fractions demonstrating that this ratio decreased as the grain size decreased. Shales more basinal in character than those of the Mam Tor Beds contain more illite and mixed-layer clay and less kaolinite and it is suggested that there was a lateral variation in clay mineralogy with distance from the shore line due to particle size sorting and that the character of the clay mineral fraction was retained as the turbidity current transported sediment from a nearshore environment deeper into the basin. Support for this model was obtained from the geochemistry which showed that the sandstone matrix differed compositionally from the shales. Systematic variations occurred in the turbidite sandstones but not in the shales which are therefore considered to be non-turbiditic. Only minor mineralogical changes appear to have occurred during diagenesis.

Résumé

Résumé

Onze argiles schisteuses et quatorze turbidites gréseux de Mam Tor ont été soumis aux analyses chimique et par diffraction des rayons X. Le rapport kaolinite sur illite et argiles interstratifiés est plus grand dans les grès que dans les schistes; une analyse granulométrique démontre que ce rapport décroît avec la taille des grains. Des schistes ayant un caractère de dépôt de bassin plus prononcé que ceux des couches de Mam Tor contiennent plus d'illite et argiles interstratifiées et moins de kaolinite. Ceci suggère une variation de la composition minéralogique des argiles avec la distance du rivage due à un classement des particules alors que les caractéristiques de la fraction argileuse sont conservées si le sédiment est transporté par un courant de turbidité du voisinage due rivage au sein du bassin. Ce modèle est confirmé par la géochimie qui montre que la composition de la matrice des grès est différente des schistes. On rencontre des variations systématiques dans les turbidites gréseux et non dans les schistes qui sont alors considérées comme non turbiditiques. Lors de la diagenèse, les changements minéralogiques semblent avoir été mineurs.

Kurzreferat

Kurzreferat

Mittels chemischer und röntgenographischer Analyse wurden elf Schiefertone und vierzehn Turbidit-Sandsteine aus den Mam Tor Beds untersucht. Das Kaolinit-illit Verhältnis und der Anteil wechselgelagerter Tonminerale war in den Sandsteinen höher als in den Schiefertonen. Ein Fraktionierung zeigte, daß dieses Verhältnis im selben Maße abnahm wie sich die Teilchengröße verringerte. Küstenferner abgelagerte Schiefertone als die der Mam Tor Beds enthalten mehr illit und Wechsellagerungsminerale und weniger Kaolinit. Es wird vermutet, daß hier auf Grund einer Teilchensortierung eine lateral Veränderung in der Tonmineralogie mit dem Abstand zur Küstenlinie vorlag, und daß der Charakter der Tonmineralfraktion bewahrt blieb, als die Schwebstoffhaltigen Ströme Sediment vom küstennahen Bereich tiefer in das Becken transportierten. Eine Unterstützung für dieses Modell lieferte die Geochemie, welche zeigte, daß sich die Sandsteinmatrix in ihrer Zusammensetzung von den Schiefertonen unterscheidet. in den Sandsteinen traten systematische Variationen auf, nicht dagegen in den Tonschiefern, weshalb angenommen wird, daß sie nicht umgelagert wurden. Während der Diagenese scheinen nur geringe mineralogische Veränderungen erfolgt zu sein.

Resumen

Resumen

Once pizarras y catorce areniscas turbiditas procedentes de Mam Tor Beds fueron analizadas quimicamente y por DRX. La relación caolinita/ilita + interestratificados resultó ser más alta en las areniscas queen las pizarras, demostrando la separación por tamaño de partícula, que esta relación disminuía a medida que lo hacía el tamaño de grano. Pizarras con más carácter conquifero que las de Mam Tor Beds contienen más ilita e interestratificados y menos caolinita, sugiriendo que hubo en la mineralogia de las arcillas una variación lateral con la distancia a la linea de costa debido a la separación por tamaño de partícula y que el carácter de la fracción arcilla se mantuvo mientras la corriente de turbidez transportaba sedimentos más profundamente en la cuenca desde los alrededores de la costa cercana. La geoquimica apoya este modelo mostrando que la matriz arenisca difiere en composición de la de las pizarras. Se observaron variaciones sistemáticas en las turbiditas pero no en las pizarras que son, por lo tanto, consideradas no turbidíticas. Parece ser que durante la diagenesis sólo tuvieron lugar pequeños cambios mineralógicos.

Type
Research Article
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1981

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