Tectonic and sedimentary history of a late Cenozoic intramontane basin (The Pitalito Basin, Colombia)

Abstract The Pitaiito Basin is an intramontane basin (15 × 20 km²) situated at the junction of the Central and Eastern Cordillera in the southern part of the Colombian Andes. Tectonic structures, evolution of the basin and distribution of the sediments suggest that the basin was formed adjacent to an active dextral strike-slip fault. Based on sedimentation rates it is estimated that subsidence started around 4.5 Ma. The basin can be divided into a relatively shallow western part (c. 300 m deep) and a deep eastern part (c. 1200 m deep). The transition between both areas is sharp and is delineated by a NW/SE-oriented fault. The position of this fault is reflected by the areal distribution of the deep non-exposed sediments as well as sediments at the surface: west of this fault the basin infill consists of coarse to medium elastics (conglomerates and sand) whereas in the eastern part fine elastics (clay and peat) are present. The lateral transition between both types of sediment is abrupt and its position is stable in time. The surface and near surface sediments in the Pitalito Basin reflect the last stage of sedimentary infill which came to a halt between 17,000 and 7500 years bp . These sediments were deposited by an eastward prograding fluvial system. The western upstream part of this system differs significantly from that of the eastern part which forms the downstream continuation. The western part exhibits unstable, shallow fluvial channels that wandered freely over the surface which predominantly consists of clayey overbank sediments. The alluvial architecture in the eastern half is characterized by stable channels and thick accumulations of organic-rich flood basin sediments and resembles an anastomosing river. The transition between both alluvial systems also coincides with the N/S-oriented normal fault. Palaeoclimatic conditions over the last c. 61,500 years were determined by means of a pollen record. From c. 61,500 to 20,000 years BP the mean annual temperature fluctuated considerably and decreased by 2–3^oC during the relatively warm periods (interstadials) and by 6–8^oC during the cold periods (stadials) in comparison with modern temperatures. These changes led to a displacement of the zonal vegetation belts from c. 200 m during the stadials to c. 1500 m in interstadial times without significant effects on the fluvial system present in the Pitaiito Basin until c. 20,000 years BP. Around this period the organic-rich eastern flood basins were choked with sediments and peat growth came to an end. Palynological and sedimentological data suggest that around that period the climate was cold (Δ 6–8^oC) and very dry and that a sparse vegetation cover was present around the basin. In these semi-arid climatic conditions the river system changed from an anastomosing pattern to one with ephemeral stream characteristics. This may have lasted until at least 17,000 years BP. Somewhere between 17,000 and 7500 years BP the eastward-flowing infilling river system changed into a NW-flowing erosive river system due to climatic or tectonic control and the present state was reached.