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Burial and Contact Metamorphism in the Mancos Shale

Published online by Cambridge University Press:  01 July 2024

Paul H. Nadeau  and
Robert C. Reynolds Jr.
Paul H. Nadeau*
Affiliation:
Department of Earth Sciences, Dartmouth College, Hanover, New Hampshire 03755
Robert C. Reynolds Jr.
Affiliation:
Department of Earth Sciences, Dartmouth College, Hanover, New Hampshire 03755
*
1Present address: The Anschutz Corporation, 555 17th St., Denver, Colorado 80202.
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Abstract

Clay samples from shales and bentonites in the Mancos Shale (Cretaceous) and its stratigraphic equivalents in the southern Rocky Mountain and Colorado Plateau have been analyzed by X-ray powder diffraction methods. The major clay in the shales is mixed layered illite/smectite, with 20–60% illite layers. The regional distribution of ordered vs. random interstratification in the illite/smectite is consistent with the concept of burial metamorphism in which smectite interlayers are converted to illite, resulting finally in ordered interstratification. The interstratification data correlate with other geologic information, including rank of coal and Laramide tectonic activity. In addition, contact metamorphism of the shale by Tertiary igneous intrusions produced a similar clay suite. Chemical variation within these shales (particularly the presence or absence of carbonate) affected the clay conversion reactions in the interbedded bentonites and the shale itself during the early stages of transformation. In extreme cases, shales and bentonites from a single outcrop may contain clays that range from pure smectite (calcareous shales) to ordered illite/smectite containing ⩾50% illite layers (noncalcareous shales). The use of mixed-layered illite/smectite compositions to infer thermal regimes, therefore, may be misleading unless allowance is made for local chemical controls.

Резюме

Резюме

Образцы глин из сланцев и бентониты из Манкос Сланца (мелового) и их стратиграфические эквиваленты в южных Сколистых Горах и на Плато Колорадо анализировались методом порощковой рентгеновской дифракции. Гдавной глиной в сланцах являлся смешанно-слойный иллит/смектит с 20–60% иллитовых слоев. Местное распределение упорядоченной-случайной внутреннoй стратификации в иллите/смектите соответствует гипотезе метаторфизма захоронений, при котором слои смектита преобразовавались в иллит, результатом чего была упорядоченная внутренняя стратификация. Данные по этой стратификации соответствовали другим геологическим информациям, включая степень науглероживания угля и ларамидо-тектоническую активность. Дополнительно, контактный метаморфизм сланца приводил к образованию подобных систем глин путем третичных изверженных интрузии. Химические изменения внутри этих сланцев (особенно присутствие или отсутствие карбонатов) влияли на реакции преобразования глин во внутринапластованных бентонитах и самых сланцах в течение ранних стадий трансформации. В исключительных случаях сланцы и бентониты из одного обнажения пород могут содержать глины от чистого смектита (известковые сланцы) до упорядоченных иллитов/смектитов, содержащих ⩾50% иллитовых слоев (неизвестковые сланцы). Использование составов смещаннослойных иллитов/смектитов для определения термальных режимов, таким образом, может быть ошибочным, если не принять во внимание местный химический контроль. [Е.С.]

Resümee

Resümee

Es wurden Tonproben aus Schiefertonen und Bentoniten im Mancos Schieferton (Kreidezeit) und aus den stratigraphisch äquivalenten Schichten der südlichen Rocky Mountain und des Colorado Plateau mittels Röntgenpulverdiffraktometer-Methoden untersucht. Das überwiegende Tonmineral in den Schiefertonen ist eine Illit/Smektit-Wechsellagerung mit 20–60% Illitlagen. Die regionale Verteilung von regelmäßigen vs. unregelmäßigen Wechsellagemngen im Illit/Smektit stimmt mit der Vorstellung einer Versenkungsmetamorphose überein, durch die die Smektit-Zwischenlagen in Illit umgewandelt werden, wodurch letztlich eine regelmäßige Wechsellagerung eutsteht. Die Ergebnisse über die Wechsellagerung stimmen mit anderen geologischen Informationen einschließlich Koldearten und laramische Tektonik überein. Darfüberhinaus lieferte die Kontaktmetamorphose der Schierfertone durch tertiäre Intrusionen eine ähnliche Tonabfolge. Chemische Schwankungen innerhalb dieser Schiefertone (vor allem alas Vorhandensein oder Nichtvorhandensein von Karbonat) beeinilussen während der ersten Umwandlungsstadien die Umwandlungsreaktionen des Tons in den zwischengelagerten Bentoniten und im Schieferton selbst. In extremen Fällen können Schiefertone und Bentonite aus eiuem einzigen Aufschluß Tonminerale enthalten, die vom reinen Smektit (kalkhaltiger Schieferton) bis zur regelmäßigen Illit/Smektit-Wechsellagerung mit ⩾50% Illitlagen (kalkfreier Schieferton) reichen. Eine Schlußfoigerung von den Zusammensetzungen der Illit/Smektit-Wechsellagerungen auf thermische Einflüsse kann daher irreführend sein, wenn nicht lokale chemische Untersuchungen in Betracht gezogen werden. [U.W.]

Résumé

Résumé

On a analysé par des méthodes de diffraction poudrée aux rayons-X des échantillons d'argile de shales et de bentonites dans le shale Mancos (crétacé) et duns ses équivalents stratigraphiques dans les Montagnes Rocheuses du sud et sur le plateau du Colorado. L'argile majeure dans les shales est l'illite/smectite à couches mélangées, avec 20–60% de couches d'illite. La distribution régionale d'interstratification ordonnée par rapport à l'interstratificafion sans ordre dans l'illite/smectite est compatible avec le concept de métamorphisme à l'enterrement, dans lequel les intercouches de smectite sont converties à l'illite, résultant finalement en une interstratflication ordonnée. Les données d'interstratification s'accordent avec d'autres renseignements géologiques, y compris le rang de charbon, et l'activité tectonique Laramide. De plus, le métamorphisme par contact du shale par des intrusions ignées a produit la même suite argileuse. La variation chimique au sein de ces shales (particulièrement la présence ou l'absence de carbonate) affecte les réactions de conversion d'argile dans les bentonites interfeuillets et dans le shale lui-même pendant les premiers stages de la transformation. Dans les cas extrèmes, les shales et les bentonites d'un seul affleurement peuvent contenir des argiles qui s’étagent de smectite pure (shales calcareux) à une illite/smectite ordonnée contenant ⩾50% de couches d'illite (shales non calcareux). C'est pourquoi l'emploi de compositions d'illite/smectite à couches mélangées pour impliquer des régimes thermaux peut être trompeur, à moins qu'on ne tienne compte de contrôles chimiques locaux. [D.J.]

Keywords

BentoniteBurial metamorphismIlliteInterstratificationShaleSmectiteX-ray powderdiffraction
Type
Research Article
Information
Clays and Clay Minerals , Volume 29 , Issue 4 , August 1981 , pp. 249 - 259
Copyright
Copyright © 1981, The Clay Minerals Society

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Footnotes

2

Burial metamorphism is defined as low-grade metamorphism without the effects of penetrative deformation (Coombs, 1961; Zen, 1974). Other workers described these clay mineral transformations as diagenetic (Perry and Hower, 1970; Frey, 1970).

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