The final phase of tropical lowland conditions in the axial zone of the Eastern Cordillera of Colombia: Evidence from three palynological records

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Abstract

Deformation of the Eastern Cordillera, as a double-verging thrust belt that separates the Magdalena Valley from the Llanos Basin, is a defining moment in the history of the northern Andes in South America. Here we examine the age and depositional setting of the youngest stratigraphic unit in three sectors of the Eastern Cordillera: (i) the Santa Teresa Formation (western flank), (ii) the Usme Formation (southern central axis), and (iii) the Concentración Formation (northeastern central axis). These units were deposited prior to the main Neogene deformation events. They represent the last preserved record of lowland conditions in the Eastern Cordillera, and they are coeval with a thick syn-orogenic deposition reported in the Llanos Basin and Magdalena Valley. Based on palynological data, we conclude that the upper Usme Formation was deposited during the Bartonian-earliest Rupelian? (Late Eocene-earliest Oligocene?); the Concentración Formation was deposited during the Late Lutetian-Early Rupelian (Middle Eocene to Early Oligocene), and the upper Santa Teresa Formation was accumulated during the Burdigalian (Early Miocene). These ages, together with considerations on maximum post-depositional burial, provide important time differences for the age of initial uplift and exhumation along the axial zone and western foothills of the Eastern Cordillera. The switch from sediment accumulation to erosion in the southern axial zone of the Eastern Cordillera occurred during the Rupelian-Early Chattian (Oligocene, ca 30 to ca 26 Ma), and in the northeastern axial zone occurred prior to the latest Chattian-Aquitanian (latest Oligocene-Early Miocene ca 23 Ma). In contrast, in the western flank, the switch occurred during the Tortonian (Late Miocene, ca 10 Ma). In addition, we detected a marine transgression affecting the Usme and Concentración formations during the Late Eocene; coeval marine transgression has been also documented in the Central Llanos Foothills and Llanos Basin, as evidenced by the similarity in floras, but not in the western foothills. Our dataset supports previous sedimentological, geochemical and thermochronological works, which indicated that (i) deformation in the Eastern Cordillera was a diachronous process, (ii) the sedimentation along the axial zone stopped first in the south and then in the north during the Oligocene, (iii) depositional systems of the axial zone and central Llanos Foothills kept partly connected at least until the Late Eocene, and (iv) Miocene strata were only recorded in adjacent foothills as well as the Magdalena and Llanos basins.

Highlights

► Sedimentation cease was not a simultaneous event along the Eastern Cordillera (EC). ► The Usme Formation is dated as Late Eocene-earliest? Oligocene. ► The Concentración Formation is dated as Middle Eocene-Early Oligocene. ► Upper sediments from the Santa Teresa Formation are dated as Early Miocene.

Introduction

The northern Andes Mountains are the result of a complex interaction between the continental South American plate and the oceanic Caribbean and Nazca plates (Fig. 1). One consequence of this multistage orogeny was the asynchronous building of three different mountain belts -the Western, Central, and Eastern Cordilleras- throughout the Mesozoic and Cenozoic (Barrero, 1979; Etayo et al., 1983; Villamil, 1999). Deformation of the Eastern Cordillera (EC), as a double-verging orogen with intermontane basins along the axial zone (Cooper et al., 1995; Cortés et al., 2006), was of particular significance in the geological history of northern South America as it modified river drainages (Guerrero, 1997, Hoorn et al., 1995, Hoorn et al., 2010), promoted vertical surface uplift in the last 3–5 m.y. (Gregory-Wodzicki, 2000; Mora et al., 2008b), and created new habitats, such as páramos and cloud forests in the axial zone of the EC (Hooghiemstra, 1984, Hooghiemstra, 1988, Van der Hammen et al., 1973). Tectono-sedimentological, geodynamic and thermochronological studies have shown that the Eastern Cordillera in the Colombian Andes was already tectonically active during the Early Paleocene (Bayona et al., 2008; Parra et al., 2012) and the Late Eocene-Early Oligocene (Bande et al., 2012, Bayona et al., 2008, Gómez et al., 2005, Gómez et al., 2003, Horton et al., 2010b, Mora et al., 2010a, Nie et al., 2010, Parra et al., 2009a, Parra et al., 2009b, Saylor et al., 2011, Toro et al., 2004). During the Late Neogene, the tectonic activity in the Eastern Andes rapidly increased (Duque-Caro, 1990; Helmens and Van der Hammen, 1994; Hooghiemstra and Van der Hammen, 1998; Hoorn et al., 1987; Mora et al., 2010b; Shephard et al., 2010; Taboada et al., 2000), generating a major exhumation pulse of the EC in the last ∼7 m.y. (Bayona et al., 2008; Cortés et al., 2006; Mora et al., 2010b). These pulses led to changes in regional climate (Ehlers and Poulsen, 2009; Insel et al., 2009; Sepulchre et al., 2009) and the sedimentary regimes in the adjacent lowlands which resulted in the establishment and evolution of the Amazon River (Figueiredo et al., 2009, Figueiredo et al., 2010, Hoorn, 1994, Hoorn et al., 1995, Hoorn et al., 2010, Hoorn and Wesselingh, 2010, Shephard et al., 2010) and increased biodiversity in western Amazonia (Hoorn et al., 2010).

Both the onset of double-verging deformation of the EC and consequent isolation of the Magdalena River Valley from the Llanos region are critical to the development of the northern Andes. In order to understand the final phase of the sedimentation within the EC, we have dated the youngest sediments preserved in three different synclines located (1) along the western flank of the EC (Santa Teresa Formation in the Guaduas Syncline), (2) in the southern axial zone of the EC (Usme Formation in the Usme Syncline, Bogotá High Plain), and (3) in the northeastern axial zone of the EC (Concentración Formation in the Floresta Syncline, footwall of the Soapaga Fault) (Fig. 1). In addition, we have provided temporal estimates of the switch from burial to exhumation through the assessment of the magnitude of post-depositional burial based on recently published paleothermal and thermochronometric data. Finally, we have reviewed several depositional and kinematic models to evaluate the degree of connectivity between the basins during the Late Eocene to Early Miocene.

Section snippets

Lithostratigraphic setting

The Late Eocene to Miocene evolution of the EC is recorded in three different large synclines located across the EC (Table 1). The youngest sediments, which are preserved at the core of each syncline, also represent the youngest sediments preserved in the EC, other than the Pliocene lake and alluvial sediments from the Bogota High plain (Helmens, 1990; Helmens and Van der Hammen, 1994; Hooghiemstra and Van der Hammen, 1998; Hoorn et al., 1987). All stratigraphic sections were measured near the

Methods

A total of 41 samples from the three formations (Table 1) were processed for palynological analysis following standard procedures (Traverse, 2007). All samples were digested by using 10% HCl and 40% HF. Each sample was sieved using 10 μm and 100 μm meshes; finally, permanent montages were prepared. Light microscopy was used to examine the palynological content and at least 100 grains were counted per slide. Morphological features were compared with descriptions and illustrations from various

Santa Teresa Formation

Palynological matter was recovered only from the upper 266 m of the Santa Teresa Formation. The palynoflora includes a large abundance of ferns spores (e.g. Psilatriletes and Laevigatosporites) and palms (Mauritiidites). Fungal remains are common throughout the entire section, also a 78.5 m thick interval with abundant Pediastrum and Botryococcus algae was observed (from 549.74 m to 628.24 m) (Fig. 3). Neither mangrove elements nor marine palynomorphs were observed (Fig. 3). The most abundant

Age

We found that the upper sediments of the Santa Teresa Formation (corresponding to the uppermost 266 m) have a high probability of belonging to the upper part of palynological zone T-12 H. lunarensis and zone T-13 E. maristellae (Burdigalian, Early Miocene, 19–16 Ma) (Fig. 4, Fig. 5). Earlier stratigraphic, malacological and palynological studies considered the formation as having accumulated during the Oligocene (De Porta and De Porta, 1962), Late Oligocene (Acosta and Ulloa, 2001), Oligocene

Overburden

Vitrinite reflectance (Ro) data in strata from the Santa Teresa Formation in the Guaduas Syncline show Ro values of 0.54–0.60% (Gómez, 2001; Gómez et al., 2003), which represent maximum temperatures of ∼95–105 °C for heating rates of 2–10 °C, according to the kinetic model of Burnham and Sweeney (1989). In the absence of evidence of other heating mechanisms, we calculate that 2.5–4.5 km of overburden, with a thermal gradient of 20–30 °C/km and a surface temperature of 25 °C -similar to

Stepwise cessation in sediment preservation along the Eastern Cordillera

The transition from burial to exhumation occurred first in the southern central axial zone of the Eastern Cordillera (Usme Formation) during the Rupelian-Early Chattian (ca 30–ca 26 Ma). Subsequently, sediment accumulation stopped along the northeastern axial zone of the Eastern Cordillera (Concentración Formation) not before the Late Chattian (ca 23 Ma); and finally, it was not until the Late Miocene (ca 10–6 Ma) that exhumation was initiated along the western flank of the Eastern Cordillera

Regional implications

Three different tectonic scenarios have been proposed to explain the Cenozoic tectonic development of the axial zone of the EC and adjacent Magdalena and Llanos basins to the west and east, respectively. According to the first model, the EC corresponds to a large foreland basin system, including what are now the Llanos and the Magdalena Valley basins (Cooper et al., 1995; Villamil, 1999). Initial breakup of the foreland basin occurred during the Late Oligocene, with a complete segmentation by

Conclusions

Biostratigraphic data from the youngest sediments preserved in all three synclines indicate different ages associated with the uppermost formations in each stratigraphic sequence. We estimated the age of the sediments from the Usme Formation (Usme Syncline) as Bartonian-earliest Rupelian? (38–33 Ma), from the Concentración Formation (Floresta Syncline) as Late Lutetian-Early Rupelian (39–31.5 Ma), and uppermost Santa Teresa rocks (Guaduas Syncline) as Burdigalian (19–16 Ma). In addition, the

Acknowledgments

We thank the Colombian Petroleum Institute, the Smithsonian Tropical Research Institute, the Corporación Geológica Ares, and Colciencias for their financial, logistic, and technical support. Thanks to Andrés Pardo (Universidad de Caldas) for the palynological analysis of most of the samples of the Concentración Formation. We specially thank J. Saylor, B. Horton, J. Kellogg and an anonymous reviewer for their helpful comments. Natasha Atkins improved readability of the manuscript.

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