The Sabana Formation (Spanish: Formación Sabana, Q1sa, QTs) is a geological formation of the Bogotá savanna, Altiplano Cundiboyacense, Eastern Ranges of the Colombian Andes. The formation consists mainly of shales with at the edges of the Bogotá savanna lignites and sandstones. The Sabana Formation dates to the Quaternary period; Middle to Late Pleistocene epoch, and has a maximum thickness of 320 metres (1,050 ft), varying greatly across the savanna. It is the uppermost formation of the lacustrine and fluvio-glacial sediments of paleolake Humboldt, that existed at the edge of the Eastern Hills until the latest Pleistocene. The uppermost sediments of the Sabana Formation were deposited during the Last Glacial Maximum, a time when the first humans populated the Bogotá savanna. These hunter-gatherers used the bones of the still extant Pleistocene megafauna as Notiomastodon platensis, Cuvieronius hyodon and Equus neogeus, of which fossils have been found in the Sabana Formation. Knowledge about the formation has been provided by geologists Alberto Guerrero, Thomas van der Hammen and others.
Etymology The formation was first defined and named after the Bogotá savanna (Sabana de Bogotá) by Hubach in 1957, further described by Van der Hammen in 1973, Guerrero (1992, 1993, 1996) and by Helmens and Van der Hammen in 1995.
Regional setting
The Bogotá savanna is a slightly undulated montane savanna in the southwestern part of the Altiplano Cundiboyacense, a high plateau in the Eastern Ranges of the Colombian Andes. The Altiplano was formed during the latest stage of Andean uplift in the Plio-Pleistocene, exposing rocks of mainly Cretaceous to Paleogene ages at surface. A small massif of Paleozoic age is present in the northern part of the Altiplano; the Floresta Massif around Floresta comprising the fossiliferous formations Floresta and Cuche. During the Mesozoic, the central part of Colombia was a rift basin to the west of the Guyana Shield, where series of marine platform deposits were deposited. The proto-Caribbean, the result of the break-up of Pangea, formed a long seaway into the South American Plate, up to Bolivia. During the Late Cretaceous, the Western and Central Ranges of the Colombian Andes began rising, while the Eastern Ranges was still absent. The main phase of tectonic uplift of the Eastern Ranges commenced in the Middle Miocene, marked by a change in paleocurrents of the fluvial deposits of the Honda Group, the most fossiliferous stratigraphic unit of Colombia. Subduction of the Nazca Plate underneath western South America and the resulting compression in the continent created reversal of former extensional faults of the Mesozoic rift basin in the Eastern Ranges. A series of fold and thrust belts, oriented in a north–south to northeast–southwest sense, were formed in the Eastern Andes, uplifting the former marine strata and creating a high plateau between the western and eastern fronts; the Altiplano Cundiboyacense. The tectonic movements of this Andean orogenic phase are reflected in Upper Miocene units as the Marichuela Formation, underlying the Pliocene and Pleistocene sediments of which the Sabana Formation represents the final chapter.
Description
Lithologies The Sabana Formation consists mainly of horizontally bedded little consolidated grey and greenish shales with lignite and diatomites, and fine to coarse sandstones at the edges of the Bogotá savanna. Numerous volcanic ash deposits are noted in the Sabana Formation. Organic material is preserved in black soils and silts form the terraces of the central part of the savanna. The volcanic ash had as provenance area the Central Ranges of the Colombian Andes, with probably minor influences from the volcanic areas of Boyacá (Paipa–Iza volcanic complex). The diatomites are associated with the ash layers, a common feature in the geological record.
Stratigraphy The Sabana Formation in some areas conformably overlies the Subachoque Formation, in other parts unconformably the Tilatá Formation, and is overlain by the alluvium of the Holocene, with the southeasternmost area of the savanna covered and intercalated by the fluvio-glacial deposits of the Tunjuelo Formation. The formation is subdivided into six units of alternating shales and fine sandstones. The age has been estimated to be Middle to Late Pleistocene, based on fission track analysis and radiocarbon dating. The Sabana Formation is time equivalent with the Soatá (upper Sabana), and Sogamoso Formations of the northern Altiplano, and the upper part of the Guayabo Formation of the Llanos Basin.
Depositional environment
The depositional environment has been interpreted as lacustrine (Lake Humboldt) and fluvio-deltaic, with a near-continuous deposition since the Late Pliocene. The Sabana Formation represents the uppermost unit of the lacustrine deposition of Lake Humboldt. At the edges of the lake, numerous deltas of fluvio-glacial origin were present, reflected in the coarser sediments. During periods of stormy climate around the lake, coarser sediments were transported to the interior of the lake. The depositional cycles were geologically speaking fast and the water level of the lake fluctuated greatly during its history. Furthermore, the local tectonic activity of the Bogotá savanna, related to movements of the Bogotá Fault, influenced the depositional cycles. The middle unit of the formation shows a drying out of the lake and subaerial erosional surfaces. The upper part of the Sabana sequence is characterised by fluvial deposits around a retreating Lake Humboldt, estimated at an age of around 30,000 years BP. The glacial origin was predominantly the Sumapaz Páramo to the south of the Bogotá savanna, with minor snow-capped peaks in the Eastern Hills of Bogotá. Present-day, the lakes of Fúquene, Herrera and Suesca are remnants of Lake Humboldt, as well as the many wetlands of Bogotá.
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