Tectonic and climatic controls on late quaternary sedimentary processes in a neotectonic intramontane basin, the Pitalito Basin, South Colombia

The present study deals with the influence of tectonics and climatic changes on sedimentation patterns in the Quaternary Pitalito Basin (lat. 1°52'N, long. 76°02'W). This intramontane sedimentary basin is 15 km in width and 20 kin in length and is located in the Eastern Cordillera of the southern Colombian Andes at an altitude of c. 1300 m.
Chapter 1 discusses the scope of this study.
In Chapter 2 general information is given about the regional tectonic setting of the basin, its morphological features, the present climate around the basin and the present-day zonal vegetation of the Colombian Andean region.
Chapter 3 starts with the description of a gravity survey carried out to examine basin geometry. The basin consists of a shallow western part in which the basement is situated at 300-400 m depth, and a deep eastern part with the basement at 1000-1200 m depth. This geometry is controlled by tectonic structures: (1) an active strike-slip fault with right lateral displacement along the northern boundary of the basin, (2) a relatively passive southern fault system and (3) a NW/SE-oriented fault which separates the shallow western part from the deep eastern part and which is situated at the present course of the Guarapas river. By extrapolation from the known sedimentation rates for the last 60,000 years, subsidence could have started c. 4.5 Ma ago. The second part of Chapter 3 discusses the geoelectric characteristics of the upper 200 m of the non-exposed sedimentary infill. Coarse to medium clastics (cobbles, gravel and sand) are restricted to the shallow western part, whereas clay and peat predominate in the deep eastern part. The lateral transition between the two lithofacies is situated along the present course of the Guarapas river.
In Chapter 4 the near-surface and surface basin sediments are described. These sediments have been studied in exposures and borings. They represent the last stage of sedimentary infill by a northeastward-heading fluvial system. The top of these sediments has an approximate age of 17,000 years B.P. The sediments in the shallow western region of the basin represent the proximal part of the fluvial system. They consist of widely scattered lenticular channel deposits enveloped by extensive and thick inorganic overbank deposits. The sediments in the east form the distal part of the fluvial system. They are made up by laterally fixed sandbodies whereas the adjacent flood basin deposits are characterized by the occurrence of organic strata which are several meters thick. These different types of alluvial architecture are ascribed to different fluvial dynamics. The western, proximal component is intermediate between braided and anastomosing river types and is characterized by channels migrating laterally due to avulsion. The distal, eastern part shows an anastomosing pattern and is characterized by laterally fixed channels. The transition between the two river types is equally situated along the line of the Guarapas river.
In Chapter 5 two pollen records are described which were obtained from the organicrich deposits in the eastern part of the basin. The records revealed the paleoecological and especially climatic conditions during the last 60,000 years around the Pitalito Basin. From ≈60,000 years B.P. to ≈20,000 years B.P. mean annual temperature fluctuated considerably and decreased, in comparison with modem temperatures, with about 3°C during the relatively warm periods (interstadials) to about 6°-8°C during the coldest periods (stadials). There is no evidence for a significant variation in climatic humidity during the registered period. The changing temperatures led to a downward displacement of the vegetation belts amounting c. 500 m during the interstadials and c. 1500 m during stadial times, in comparison with the present position. In spite of these environmental changes, the fluvial system present in the Pitalito Basin probably did not change significantly until c. 20,000 years B.P. At that time the eastern flood basins became choked with clastic sediments and peat growth ended. Sedimentological and palynological data suggest a change from relatively humid climatic conditions to a semi-. and climate which is characterized by torrential rains and a sparse vegetation cover. These semi-arid conditions lasted from ≈20,000 to at least ≈17,000 years B.P. Somewhere between 17,000 and 7000 years B.P. basin infill came to an end and the rivers incised in their own sediments. The river changed its course 90° to an outlet in the NW. There is no palynological record of this interval. Around 7000 years B.P. peat began to develop again in the northeastern part of the basin due to tilting of the basin floor. The palynological record registers the prevalence of present-day climatic conditions from 7000 years B.P. onward (mean annual temperature c. 20°C; annual rainfall c. 1200 mm). A somewhat warmer period is recorded around 5000 years B.P.; mean annual temperature was about 2°-3°C higher than today.
In Chapter 6 a synthesis of the data of the former chapters is given.

The following conclusions may be drawn:
- The intraniontane Pitalito Basin developed as a result of extension along a fault wedge which forms part of the Garzón-Suaza fault. It is the first extensively described pullapart basin in the Colombian Andean region, Tectonics played a decisive role in the differentation of the fluvial system
- The effects of climatic changes during the depositional history are superimposed upon those caused by tectonic activity. During a considerable part of the Last Glacial (60,000-20,000 years B.P.) the fluvial system in the Pitalito Basin was not affectedby the vertical shifts of the zonal vegetation belts. The start of especially dry climatic conditions around 20,000 years B.P., possibly combined with tectonic activity, caused a dramatic change in the sedimentation patterns. The maximal estimated temperature decline during the Pleniglacial at the altitude of the Pitalito Basin is possibly in the order of 6°-8°C compared to modem temperatures.
- The thick pile of sedimentary infill and the presence of well-preserved pollen at this elevation make the Pitalito Basin a very suitable site for deep borings to record Late Cenozoic vegetational changes at low elevation.