Petrografía, geocronología y geoquímica de las ignimbritas de la formación Popayán, en el contexto del vulcanismo del suroccidente de Colombia
Fecha
2010
Autores
Torres Hernández, María Patricia
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Editor
Universidad EAFIT
Resumen
Desde 1.932 se está discutiendo acerca de la estratigrafía y la composición de los depósitos y rocas que constituyen la Formación Popayán, surgiendo varias propuestas estratigráficas que se han venido actualizando de acuerdo a la evolución de los conceptos que, con el transcurrir del tiempo, ha tenido la vulcanología. A partir de la caracterización de los depósitos de la Formación Popayán se plantea que está constituida primordialmente por depósitos de flujos de ignimbritas y por espesos depósitos de flujos de ceniza y bloques de hasta 200 m de espesor que rellenaron un paleorelieve, construido sobre rocas del Paleozoico al Paleoceno, conformando la Meseta de Popayán. Se propone que la sección estratigráfica tipo de la Formación Popayán sea la levantada en la quebrada Sombrerillo, localizada al norte de la Meseta de Popayán en el Municipio de Morales, de muy fácil acceso, y donde está perfectamente representada ésta formación. Las columnas estratigráficas levantadas en la Chorrera del Club Campestre del municipio de Popayán y en la quebrada La Chorrera- río Hondo, al sur de la meseta, en el municipio de El Tambo, se consideran secciones estratigráficas de referencia. En la sección estratigráfica de la quebrada Sombrerillo se observa la base de la Formación Popayán conformada por depósitos de flujos de ignimbritas con un espesor máximo observado de 100 m; estas ignimbritas están suprayacidas por una serie de depósitos de flujos piroclásticos y depósitos vulcano sedimentarios, con un espesor máximo observado de 100 m, sobre los que se desarrolló un paleosuelo de color rojo que se propone constituye el techo de la Formación de Popayán. Los análisis geoquímicos ubican las ignimbritas de la Formación Popayán en las series magmáticas calcoalcalinas, siendo su composición riolítica alta en potasio. Estas características las comparten con las ignimbritas de la Formación Guacacallo ubicadas en el valle geográfico del río Magdalena en el departamento del Huila; las dos formaciones tienen, igualmente, características cartográficas, geomorfológicas y estratigráficas comparables y esto permite su correlación. De acuerdo con los estudios petrográficos realizados, los depósitos de flujos de ceniza y bloques estratigráficamente localizados hacia el techo de las ignimbritas son de composición andesitica. Los volúmenes estimados para las ignimbritas de las formaciones de Popayán y Guacacallo son de aproximadamente 250 Km3, sin considerar los volúmenes de las cenizas acompañantes o co-ignimbritícas, cuyos cálculos pueden ser iguales a los de las ignimbritas propiamente dichas. Estos volúmenes son indicadores de la gran significancia que tiene el vulcanismo riolítico en el segmento volcánico central de la Cordillera Central de Colombia. Así mismo, su volumen y el alto contenido en potasio permite suponer que la génesis de los magmas que generaron los mantos ignimbríticos de las Formaciones de Popayán y Guacacallo está relacionada con la corteza continental. Las ignimbritas denominadas en este trabajo de Pisojé Alto, ubicadas en el piedemonte occidental de la Cordillera Central y que bordean la Meseta de Popayán, de acuerdo con su información geoquímica, son de composición traquiandesitíca-andesitica, altas en potasio y hacen parte de las series calcoalcalinas. Estas ignimbritas, de composición andesitica, no son correlacionables con las de la Formación Popayán y se propone que hacen parte de la Formación Coconuco. De acuerdo a las dataciones de las ignimbritas del suroccidente de Colombia existentes en la literatura, y a las dataciones de 40Ar/Ar39 realizadas en esta investigación, consideradas referentes estratigráficos debido al alto contenido de argón atmosférico, el vulcanismo riolítico alto en potasio, del suroccidente de Colombia, se extiende desde hace 7.1±0.3 M.a hasta 2.1±0.4 M.a y sobre él se está construyendo el vulcanismo reciente.
ABSTRACT: From 1932 there has been discussion of the stratigraphy and composition of deposits and rocks that constitute the formation of Popayán; a number of proposals have emerged in accord with the progressive evolution of concepts in the field of volcanology. The formation of Popayán was constituted of ignimbrites flows, ash deposits and 200 m blocks which filled a paleorelief, constructed between the Paleozoíco and the Paleocene, forming the plateau of Popayán. It is proposed that the typical stratigraphic formation of Popayán is the one which was shaped in the Sombrerillo Creek, located north of the plateau of Popayán in the municipality of Morales, easy access, where this formation is very well represented. The stratigraphic column erected on the Chorrera stream at the Country Club located in the municipality of Popayán and the one on the Hondo River, to the South of the plateau, in the municipality of El Tambo, are considered stratigraphic sections of reference. In the stratigraphic section of the Sombrerillo Gorge, composed of ignimbrites deposits with an observed maximum thickness of 100 m, the base of the Popayán formation can be observer. These ignimbrites are superimposed by a number of pyroclastic deposits and volcanic sediment, with a maximum observed thickness of 100 m, upon which was developed a regional red paleosuelo extension proposed as the roof of the formation of Popayán. Geochemical analyses locate the ignimbrites of the Popayán formation in magmatic calco-alkaline series, with riolitic composition high in potassium. These characteristics are share with the Guacacallo ignimbrites located in the geographic Valley of the Magdalena River in the Department of Huila; these two formations also have comparable cartographic, geomorphologic and stratigraphic features that allow their correlation. According to petrography studies, deposits of ash flow and block stratigraphic located towards the ceiling of the ignimbrites are of andesitic composition. Estimated ignimbrites volumes for the Popayán and Guacacallo formations are approximately 250 Km3, without considering the accompanying ash or co-ignimbrites, which may be calculated as equal to the actual ignimbrites volume. These volumes indicate the significance of riolitic volcanism in the central volcanic segment of the Central Cordillera of Colombia. The total volume and high potassium content allow it to be assumed that the genesis of the magmas that generated the formations of Popayán and Guacacallo ignimbrites mantel is related to the continental crust. The ignimbrites denominated in this work Upper Pisojé, located on the western foothills of the Cordillera Central and bordering the plateau of Popayán, according to their geochemical information are of traquiandesitic-andesitic composition, high in potassium and are part of the calco-alkaline series. These ignimbrites are not correlated with the Popayán formation and it is proposed that they belong to the Coconuco formation. High in potassium, southern Colombian West riolitic volcanism extends from 7. 1± 0. 3 up to 2 M.a. ±0. 4 M.a upon which recent volcanism is being built.
ABSTRACT: From 1932 there has been discussion of the stratigraphy and composition of deposits and rocks that constitute the formation of Popayán; a number of proposals have emerged in accord with the progressive evolution of concepts in the field of volcanology. The formation of Popayán was constituted of ignimbrites flows, ash deposits and 200 m blocks which filled a paleorelief, constructed between the Paleozoíco and the Paleocene, forming the plateau of Popayán. It is proposed that the typical stratigraphic formation of Popayán is the one which was shaped in the Sombrerillo Creek, located north of the plateau of Popayán in the municipality of Morales, easy access, where this formation is very well represented. The stratigraphic column erected on the Chorrera stream at the Country Club located in the municipality of Popayán and the one on the Hondo River, to the South of the plateau, in the municipality of El Tambo, are considered stratigraphic sections of reference. In the stratigraphic section of the Sombrerillo Gorge, composed of ignimbrites deposits with an observed maximum thickness of 100 m, the base of the Popayán formation can be observer. These ignimbrites are superimposed by a number of pyroclastic deposits and volcanic sediment, with a maximum observed thickness of 100 m, upon which was developed a regional red paleosuelo extension proposed as the roof of the formation of Popayán. Geochemical analyses locate the ignimbrites of the Popayán formation in magmatic calco-alkaline series, with riolitic composition high in potassium. These characteristics are share with the Guacacallo ignimbrites located in the geographic Valley of the Magdalena River in the Department of Huila; these two formations also have comparable cartographic, geomorphologic and stratigraphic features that allow their correlation. According to petrography studies, deposits of ash flow and block stratigraphic located towards the ceiling of the ignimbrites are of andesitic composition. Estimated ignimbrites volumes for the Popayán and Guacacallo formations are approximately 250 Km3, without considering the accompanying ash or co-ignimbrites, which may be calculated as equal to the actual ignimbrites volume. These volumes indicate the significance of riolitic volcanism in the central volcanic segment of the Central Cordillera of Colombia. The total volume and high potassium content allow it to be assumed that the genesis of the magmas that generated the formations of Popayán and Guacacallo ignimbrites mantel is related to the continental crust. The ignimbrites denominated in this work Upper Pisojé, located on the western foothills of the Cordillera Central and bordering the plateau of Popayán, according to their geochemical information are of traquiandesitic-andesitic composition, high in potassium and are part of the calco-alkaline series. These ignimbrites are not correlated with the Popayán formation and it is proposed that they belong to the Coconuco formation. High in potassium, southern Colombian West riolitic volcanism extends from 7. 1± 0. 3 up to 2 M.a. ±0. 4 M.a upon which recent volcanism is being built.
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1 CD-ROM : il., mapas