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Análisis Litoestratigráfico De La Formación Cerro Azul (Mioceno Superior) En La Provincia De La Pampa

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Análisis Litoestratigráfico De La Formación Cerro Azul (Mioceno Superior) En La Provincia De La Pampa Revista de la Asociación Geológica Argentina 67 (2): 257 - 265 (2010) 257 ANÁLISIS LITOESTRATIGRÁFICO DE LA FORMACIÓN CERRO AZUL (MIOCENO SUPERIOR) EN LA PROVINCIA DE LA PAMPA Graciela VISCONTI1, Ricardo N. MELCHOR1,2, Claudia I. MONTALVO1, Aldo M. UMAZANO1,2 y Elena E. DE ELORRIAGA1 1 Universidad Nacional de La Pampa, Facultad de Ciencias Exactas y Naturales. E-mail: [email protected] 2 INCITAP (CONICET-UNLPam) RESUMEN La Formación Cerro Azul fue definida en 1980 para incluir a las sedimentitas continentales pliocenas (limolitas arenosas y are- niscas limosas) que afloran de manera discontinua en casi todo el ámbito de la provincia de La Pampa. No obstante, varias in- vestigaciones paleontológicas realizadas a partir de la segunda mitad de la década del 80’ han permitido ubicar geocronológi- camente a la unidad en el intervalo 10 Ma a 5,8-5,7 Ma. El objetivo del trabajo es realizar un análisis de las características li- toestratigráficas de la Formación Cerro Azul de acuerdo al Código Argentino de Estratigrafía. Se propone un lectoestratoti- po para la unidad, consistente en el perfil de Algarrobo del Águila y un perfil auxiliar en cerro El Morro. También se estable- cieron las relaciones estratigráficas con otras formaciones. Se interpreta un paleoambiente depositacional de llanura, donde alter- nan depósitos de loess con numerosos paleosuelos, detectándose escasos depósitos lacustres en la base y pocos cursos fluviales. Palabras clave: Formación Cerro Azul, Mioceno, Huayqueriense, La Pampa. ABSTRACT: Lithostratigrafic analysis of Cerro Azul Formation (Upper Miocene), La Pampa. Cerro Azul Formation was defined in 1980 to include the Pliocene continental sedimentary rocks (sandy silt and silty sand) that appear in discontinuous outcrops in al- most all La Pampa province. Nevertheless, several paleontological investigations carried out starting from the second half of the decade of the 80' have allowed establish the geochronology of this unit between10 Ma to 5,8-5,7 Ma. The objective of the work is to carry out litostratigraphic studies of the Cerro Azul Formation to fit whit the Stratigraphic Argentinean Code. It intended a lectoestratotipe for the unit; include the profile Algarrobo del Águila and auxiliary profile in Cerro El Morro. The stra- tigraphic relationships with other formations are analyzed. A dominant aeolian paleoaenvironmental is interpreted, composed of alternating loess deposits with paleosol horizons, being detected scarce lacustrine deposits at the base and few fluvial courses. Keywords: Cerro Azul Formation, Miocene, Huayquerian, La Pampa. INTRODUCCIÓN 1963) y Formación La Pampa (Giai 1975). mentos arenosos de la Formación Río Negro, Particularmente en la provincia de La pudiéndose aquí distinguir una parte inferior La Formación Cerro Azul (sensu Linares Pampa, Llambías (1975), en un informe que correspondería a la Formación Arroyo Cha- et al. 1980) es una unidad arealmente muy inédito para la Dirección de Minas de esa sicó. Debido a que en el resto de la provincia, por extendida en el ámbito de la provincia de provincia, denominó y describió a esta falta de la Formación Río Negro no se puede co- La Pampa (Fig. 1). Originalmente se le sucesión como Formación Cerro Azul. nocer estratigráficamente si las rocas pertenecen a asignó una edad pliocena sobre la base de Dentro de ella incluyó a las sedimentitas sectores inferiores a la Formación Río Negro o las relaciones de campo, sin mayores pre- compuestas fundamentalmente por “li- superiores, de modo que la denominación de For- cisiones ni designación de un estratotipo. mos arenosos pardo rosados a rojizos, sin estra- mación Cerro Azul pretende cubrir este déficit y Las sedimentitas que la componen aflo- tificación interna o muy poco visible, con frecuen- referirse exclusivamente a su carácter litológico. ran en el sureste de la provincia de Men- tes clastos de arcillas rojizas del tamaño de una El hallazgo de restos fósiles determinará en cada doza, un amplio sector de la provincia de nuez, dispuestos horizontalmente. Presenta for- caso la edad correspondiente” (Llambías 1975: La Pampa y oeste de la provincia de Bue- ma de erosión tipo loess, con barrancas y caña- 32). nos Aires (Folguera y Zárate 2009). Se- dones abruptos. En la parte superior, los sedi- Linares et al. (1980) retomaron la descrip- gún los autores que las han mencionado, mentos están reemplazados por tosca, que llegan ción de la unidad y le asignaron una po- recibieron distintas denominaciones ta- a tener hasta dos metros de espesor” (Llambías sible edad pliocena sobre la base de las les como: Formación Araucana (Döering 1975: 31). También mencionó que “aflo- relaciones de campo con las formaciones 1882), Formación Pampeano (Stappen- ran en todo el ámbito de la provincia de La infrayacentes (ej.: Formaciones Río Ne- beck 1913, Salso 1966), Formación Epe- Pampa y en la parte oriental (laguna Blanca gro y Arroyo Chasicó). Estos autores la cuén (Pascual 1961, Pascual y Bocchino Grande) se intercalan en forma de cuña los sedi- reconocieron en diversas localidades, ta- 258 G. VISCONTI, R. N. MELCHOR, C. I. MONTALVO, A. M. UMAZANO Y E. E. DE ELORRIAGA les como: laguna Blanca Grande, barran- cas situadas al oeste de Algarrobo del Águila y limitantes con el río Salado, en los cerros Azul, Bayo, El Chancho, La Bota y sierra Chata (Fig. 1). Visconti et al. (1996) y Goin et al. (2000) realizaron estudios sedimentológicos en la Formación Cerro Azul en distintas lo- calidades de La Pampa, reconociendo tres asociaciones de facies sedimentarias: una inferior, de ambiente lacustre, y dos en el sector medio y superior, donde al- ternan eventos eólicos loéssicos, desarro- llo de paleosuelos y escaso retrabajo flu- vial. Estos estudios permitieron estable- cer correlaciones estratigráficas prelimi- nares (Visconti et al. 1996, Goin et al. 2000) para afloramientos distantes de esta uni- dad. También reconocieron como perte- necientes a la Formación Cerro Azul a los afloramientos de Salinas de Hidalgo, asignados previamente a la Formación Epecuén (Goin et al. 2000). Este trabajo tiene por finalidad efectuar un análisis litoestratigráfico detallado de la Formación Cerro Azul, siguiendo las normas establecidas por el Código Ar- gentino de Estratigrafía (Comité Argen- tino de Estratigrafía 1992). Se incluye la designación y ubicación de un lectoestra- totipo para la unidad, la descripción lito- lógica, el ambiente sedimentario, la edad y una breve reseña del contenido paleon- tológico. MARCO GEOLÓGICO Figura 1: Esquema de los afloramientos de las Formaciones Cerro Azul, Río Negro, Arroyo Chasicó, Gran Salitral y Barranca Final, ubicación del lectoestratotipo de la Formación Cerro Azul en Alga- rrobo del Águila, del perfil auxiliar en cerro El Morro y localidades mencionadas en el texto: 1) lagu- Los afloramientos de la Formación Ce- na Callaqueo; 2) laguna Blanca Grande; 3) laguna Colorada Grande; 4) Estancia La Chola; 5) Salinas rro Azul se disponen en un área donde la Grandes de Hidalgo (modificado de Linares et al. 1980 y de Folguera y Zárate 2009). En el mapa de estructuración y composición del basa- ubicación de la República Argentina puede verse la distribución de la Formación Cerro Azul de Fo- lguera y Zárate (2009). mento corresponde principalmente a Sierras Pampeanas Occidentales y Orien- lacionadas con la apertura del océano gro (Mioceno-Plioceno). Se interdigita con tales, que incluye rocas ígneas y meta- Atlántico a partir del Jurásico Superior- ésta última en la cuenca del Colorado mórficas de edades pampeanas (Protero- Cretácico (Juan et al. 1996). (Visconti et al. 2002, de Elorriaga y Vis- zoico medio, Paleozoico inferior y me- La Formación Cerro Azul se ha deposita- conti 2005) y yace sobre los depósitos dio; Linares et al. 1980) y famatinianas (Pa- do sobre las rocas del basamento cristali- marinos de la Formación Macachín (Sal- leozoico inferior; Sato et al. 1999, 2000). no y es parte del relleno sedimentario ne- so 1966) en la Cuenca homónima (Salso Al este de la provincia se desarrollan las ógeno de las cuencas nombradas, habién- 1966, Giai y Tullio 1998, de Elorriaga y cuencas de Quehué (Kostadinoff y Llam- dose reconocido su presencia sobre las Tullio 1998). bías 2002) y de Macachín (Salso 1966) y sedimentitas marinas de la Formación Ba- Los afloramientos expuestos son saltua- al sur la Cuenca del Colorado (Kaasschie- rranca Final (Mioceno Medio-Superior) y rios, generalmente se desarrollan en pla- ter 1965, Yrigoyen 1975), estructuras re- fluvio-eólicas de la Formación Río Ne- nicies y cárcavas, que cubren amplias ex- Litoestratigrafía de la Formación Cerro Azul... 259 tensiones de la provincia de La Pampa. anaranjado pálido (5YR 7/2), selección (37º 22’ S; 65º 28’ O) y hacia el este se in- En la parte central y oriental de la provin- moderada, textura matriz sostén y ce- terna en la provincia de Buenos Aires, cia, la unidad aflora en las márgenes de mento carbonático en la mayoría de los donde los afloramientos han sido clásica- “valles” con rumbo ENE, denominados casos. De acuerdo a su composición se mente asignados a la Formación Epe- valles transversales (Cano et al. 1980, Cal- las clasifica como wackes líticas (siguien- cuén (Goin et al. 2000). mels et al. 1996), o sistema de depresiones do la clasificación de Dott 1964, modifi- El espesor máximo de los depósitos aflo- transversales (Calmels y Casadío 2005). El cada por Pettijohn et al. 1972, 1987) o li- rantes es de aproximadamente 50 metros origen de dichos “valles” ha sido discuti- toarenitas feldespáticas (de acuerdo a Folk (medido en Algarrobo del Águila), mien- do por distintos autores; algunos como et al. 1970). Las arcilitas presentan un co- tras que en el subsuelo se la ha encontra- Tapia (1931), Terraza et al. (1981), Kosta- lor rojo moderado (5R 4/6) y general- do en perforaciones hasta los 180 m de dinoff et al. (2001) suponen un origen ero- mente están laminadas. profundidad (Giai y Tullio 1998).
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