Resumen de Estratigrafía, procedencia, subsidencia y exhumación de las unidades paleógenas en el Sinclinal de Usme, sur de la zona axial de la Cordillera Oriental

Germán Alonso Bayona Chaparro, Omar Camilo Montenegro Castillo, Agustín Cardona Molina, Carlos Alberto Jaramillo, Felipe Lamus Ochoa

• español

  Se efectuaron análisis estratigráficos, procedencia y termocronología en los 2200 metros de la sucesión Paleógena en el sector sur de la zona axial de la Cordillera Oriental (Sinclinal de Usme), permiten establecer la evolución tectónica y paleogeográfica de este sector de la Cordillera Oriental. El análisis sedimentológico permite identificar un dominio de ambientes continentales con leves incursiones salobres y desarrollo de pantanos. La composición de las areniscas cambia, de base a techo, de cuarzoarenitas a sublitoarenitas para las formaciones Guaduas y Cacho, y a ublitoarenitas y litoarenitas feldespáticas para la Formación Bogotá. Siguiendo hacia el techo, la composición tiende a ser más cuarzosa en la unidad más superior de la Formación Bogotá, y en las formaciones Regadera y Usme son cuarzoareniscas. La asociación de minerales pesados sigue la misma tendencia de madurez composicional de las areniscas, reportándose minerales inestables (hornblenda, talco, clinozoisita) en la parte media de la Formación Bogotá; este nivel consiste de depósitos volcaniclásticos con circones volcánicos de edad 56.2 ± 1.6 Ma. La población de edades de circones detríticos, la presencia de glauconita retrabajada, el retrabajamiento de polen de edad Campaniano-Maastrichtiano en la base de la Formación Bogotá y en la Formación Regadera, y el cambio composicional de las areniscas entre las formaciones Cacho-Bogotá y Regadera-Usme nos permite inferir una procedencia del Occidente (Cordillera Central y estructuras occidentales de la Cordillera Oriental) para las formaciones Cacho y Bogotá (Paleoceno inferior-Eoceno inferior). Las areniscas cuarzosas de las formaciones Regadera y Usme (Eoceno medio � Oligoceno inferior) proceden de cobertera cretácica involucradas en estructuras al oriente del Sinclinal de Usme. Las tasas de subsidencia tectónica del Paleoceno-Eoceno inferior son más altas que las registradas durante el Eoceno medio-Oligoceno temprano. Esta disminución en la tasa de subsidencia coincide con el inicio de la exhumación en el Oligoceno temprano y el cese del registro sedimentario en la zona axial de la Cordillera.

• English

  Stratigraphic, provenance and thermochronological analyses in the 2.2 km thick Paleogene succession of the southern axial zone of the Eastern Cordillera (Usme Syncline, Figura 1) allow proposing a tectonic and paleogeographic evolution of this region. Four stratigraphic units were studied along the Mochuelo creek: the upper Guaduas Formation, and the complete sections of the Cacho, Bogotá and Regadera formations (Figura 2). The top of the Paleogene succession (upper segment of the Usme Formation) was surveyed in the Buenavista creek (Figura 1). Sedimentological and palynological analyses indicate the dominance of continental environments for most of the Paleogene succession with short periods of lacustrine deposition and brackish-water incursions in the lower Cacho and upper Regadera formations (Figura 2 and Figura 3). The pollen association in the Bogotá Formation indicates a late Paleocene-early Eocene age for this unit, whereas the contact with the Regadera Formation is in the early-middle Eocene range. Late Eocene pollen association was identified at the top of the Regadera Formation, and the pollen association in the Usme Formation assigned an early Oligocene age (Figura 4). Provenance analysis includes the integration of sandstone petrography, heavy mineral association, and detrital zircon geochronology (Figura 5). Sandstone compositional maturity decreases up section from quartzarenites and sublitharenites in Guaduas and Cacho formations to litharenites and feldspathic litharenites in the Bogotá Formation (Figura 6), with a volcaniclastic interval in the middle of the Bogotá Formation. In contrast, compositional maturity increases upsection from the uppermost Bogotá Formation to the Regadera and Usme formations (Figura 6). Provenance diagrams, lithic fragment distribution and petrofacies analysis of the Bogotá and Regadera formations suggest at least two different source areas (Figura 7 and Figura 8). Heavy mineral association change similarly to the sandstone composition, being the volcaniclastic interval in the middle Bogotá Formation with the highest concentration of unstable minerals such as hornblende, talc, clinozoisite, etc (Figura 8 and Figura 9); volcanic zircons of this interval reports an U/Pb age of 56.2 ± 1.6 Ma (Figura 10). In addition to the Paleocene ages, other population ages (mainly 70-90 Ma; 220-290 Ma) reported in one sample of the Cacho and four samples of the Bogotá formations clearly indicate a source from the Central Cordillera of Colombia (Figura 10) since early Paleocene time. Detrital zircon ages older than 500 Ma increased in the sample of the Regadera Formation; these ages were interpreted as reworking of the cretaceous sedimentary cover. This shift in sandstone composition, heavy mineral association and detrital zircon ages is coincident with a change from high subsidence rates in late Paleocene � early Eocene time to an interval of low subsidence rates in middle to late Eocene time (Figura 11). Detrital zircon population, reworked cretaceous pollen and glauconite and the change in sandstone composition indicate provenance from westerly uplifts during the Paleocene-early Eocene (Central Cordillera and nearby uplifts with cretaceous cover), (Figura 12a). Source areas for middle Eocene-lower Oligocene Regadera and Usme formations were composed of quartzose cretaceous rocks located to the east (Figura 12b); at this interval the rate of tectonic subsidence decreased in comparison with Paleocene-early Eocene rates of subsidence (Figura 11). Onset of exhumation in early Oligocene time (Tabla 7) coincides with culmination of deposition along the axial zone of the Eastern Cordillera.