Publication: Estratigrafía del Cretácico Superior en las cabezeras de los Valles de Ansó y Roncal (Pirineo Occidental)
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1988
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Sociedad Geológica de España
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Abstract
Se estudian tres columnas estratigráficas del Cretácico superior situadas en el Soum de Léche, Collado de Petrechema y Collado de Petraficha. Abarcan desde el Cenomanense al Maestrichtiense y en ellas se describe su litología y contenido en fósiles a partir del estudio de láminas delgadas.
Discordante sobre el Paleozóico se sitúa un tramo de areniscas azóicas que, por su composición, se asemejan
a la Fm. Arenas de Utrillas (Albense-Cenomanense inf.). El Cenomanense está representado por calizas y calizas arenosas y arcillosas; presenta un reducido espesor (1 a 3 m) y contiene principalmente Prealveolina y Daxia: El Turonense-Coniaciense, con menos de 5 m, está constituido por biomicritas y se ha caracterizado por la presencia de "Rotalina", Tritaxia y Pithonella.
El Santoniense constituye la formación más importante, -conocida como «caliza de los cañones»- con un espesor que llega a superar los 300 m. Se trata de calcarenitas con intracIastos y pellets, con abundante contenido en microfósiles, entre los que destacan Dictyopsella, Dicyclina, Cuneolina, Martiguesia y en la parte superior
Lacazina.
El Campaniense, con unos 55 m, comienza por calizas con nódulos de sílex o areniscas finas muy calcáreas con Monolepidorbis, a las que siguen calcilutitas arcillosas con Globotruncana y Pithonella. El Maestrichtiense comienza por calizas arcillosas con esquistosidad que contienen Globotruncana, Navarella y Rugoglobigerina, seguidas por una serie turbidítica margoso-areniscosa. Se pone de manifiesto la posible existencia de un hiato entre el Turonense y Coniacense. Los niveles del Albense-Cenomanense inf. presentan una disminución del tamaño de grano hacia el sur. La transgresión cenomanense instaura un régimen de ambiente litoral, que en el Turonense y/o Coniacense se mantiene,
evolucionando a un medio de plataforma, posiblemente semiabierta, en el sector meridional. El Santoniense se ha depositado en un medio de plataforma interna carbonática con episodios de carácter arrecifal. Durante el Campaniense y parte del Maestrichtiense el medio es de plataforma externa dando paso a un medio turbidítico. Los nódulos de sílex que presenta el Campaniense en el sector septentrional pasan a silicificaciones dispersas en el sector meridional, aumentando en el mismo sentido la esquistosidad. La recristalización es más manifiesta en los tramos altos del Santoniense del sector meridional. Se muestra también la distribución estratigráfica de los principales micro fósiles determinados en la zona.
Three upper Cretaceous stratigraphic sections from Soum de Léche, Petrechema col and Petraficha col have been studied. These sections begin at the Cenomanian and extend to the Maestrichtian. Their lithology and fossil content have been observed from thin sections. A level of azoic sandstones, similar in composition to those of «Arenas de Utrillas» formation (Albian lower Cenomanian) is situated in discordance over the Paleozoico The Cenomanian is represented by limestones of a moderate thickness (1-3 m), and containing mainly Prealveolina and Daxia. The Turonian-Coniacian, less than 5 m, is represented by biomicrites and is characterized by "Rotalina", Tritaxia and Pithonella. The most important formation is the Santonian -locally called «canyon limestones»- which presents a thickness of more than 300 m. It consists of calcarenites with intracIasts and pellets, containing many microfossils, principally Dictyopsella, Dicyclina, Cuneolina, Martiguesia and (in the upper part) Lacazina. The Campanian (55 m) begins with limestones containing chert nodules or calcareous fine grained sandstones with Monolepidorbis. Above there are cIay calcilutites with Globotruncana and Pithonella. The Maestrichtian begins with schistosed c1ay limestones, containing Globotruncana, Navarella and Rugoglobigerina, which are followed by a turbiditic marly-sandy succesion. A possible gap between the Turonian and the Coniacian has been detected. The Albian lower Cenomanian levels show a reduction of the grain size proceeding in southward direction. The Cenomanian transgression determines a litoral enviroment, which in the Turonian and/or Coniacian has evolved in the southern area to a relatively well communicated shelf. The Santonian deposition developed in an enviroment of internal carbonatic shelf, with episodes of reefal character. During the Campanian and part of the Maestrichtian, the enviroment is an outer shelf which later beco mes a turbiditic one. The chert nodules of the Campanian in the northern area become disperse silicifications in the intermediate sector and dissapear in the southern area. The schistosity increases in the southern direction. Recrystallization is more evident in the upper levels of the meridional sector in the Santonian. The stratigraphic distribution of the most important microfossils of the area is shown.
Three upper Cretaceous stratigraphic sections from Soum de Léche, Petrechema col and Petraficha col have been studied. These sections begin at the Cenomanian and extend to the Maestrichtian. Their lithology and fossil content have been observed from thin sections. A level of azoic sandstones, similar in composition to those of «Arenas de Utrillas» formation (Albian lower Cenomanian) is situated in discordance over the Paleozoico The Cenomanian is represented by limestones of a moderate thickness (1-3 m), and containing mainly Prealveolina and Daxia. The Turonian-Coniacian, less than 5 m, is represented by biomicrites and is characterized by "Rotalina", Tritaxia and Pithonella. The most important formation is the Santonian -locally called «canyon limestones»- which presents a thickness of more than 300 m. It consists of calcarenites with intracIasts and pellets, containing many microfossils, principally Dictyopsella, Dicyclina, Cuneolina, Martiguesia and (in the upper part) Lacazina. The Campanian (55 m) begins with limestones containing chert nodules or calcareous fine grained sandstones with Monolepidorbis. Above there are cIay calcilutites with Globotruncana and Pithonella. The Maestrichtian begins with schistosed c1ay limestones, containing Globotruncana, Navarella and Rugoglobigerina, which are followed by a turbiditic marly-sandy succesion. A possible gap between the Turonian and the Coniacian has been detected. The Albian lower Cenomanian levels show a reduction of the grain size proceeding in southward direction. The Cenomanian transgression determines a litoral enviroment, which in the Turonian and/or Coniacian has evolved in the southern area to a relatively well communicated shelf. The Santonian deposition developed in an enviroment of internal carbonatic shelf, with episodes of reefal character. During the Campanian and part of the Maestrichtian, the enviroment is an outer shelf which later beco mes a turbiditic one. The chert nodules of the Campanian in the northern area become disperse silicifications in the intermediate sector and dissapear in the southern area. The schistosity increases in the southern direction. Recrystallization is more evident in the upper levels of the meridional sector in the Santonian. The stratigraphic distribution of the most important microfossils of the area is shown.