The uppermost sediments of the Sabana Formation were deposited during theLast Glacial Maximum, a time when the first humans populated the Bogotá savanna. These hunter-gatherers used the bones of the still extant Pleistocene megafauna asNotiomastodon platensis,Cuvieronius hyodon andEquus 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.
The formation was first defined and named after theBogotá savanna (Sabana de Bogotá) by Hubach in 1957, further described by Van der Hammen in 1973,[1] Guerrero (1992, 1993, 1996) and by Helmens andVan der Hammen in 1995.[2][3][4][5]
During the Mesozoic, the central part of Colombia was a rift basin to the west of theGuyana Shield, where series of marine platform deposits were deposited. The proto-Caribbean, the result of the break-up ofPangea, formed a long seaway into theSouth 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 theHonda 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 offold 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 theMarichuela Formation, underlying the Pliocene and Pleistocene sediments of which the Sabana Formation represents the final chapter.
The Sabana Formation consists mainly of horizontally bedded little consolidated grey and greenishshales withlignite anddiatomites,[3] and fine to coarsesandstones at the edges of the Bogotá savanna.[6] Numerousvolcanic ash deposits are noted in the Sabana Formation.[2] Organic material is preserved in black soils and silts form the terraces of the central part of the savanna.[5] The volcanic ash had as provenance area theCentral Ranges of the Colombian Andes, with probably minor influences from the volcanic areas ofBoyacá (Paipa–Iza volcanic complex). The diatomites are associated with the ash layers, a common feature in the geological record.[7]
The Sabana Formation was deposited in a fluvio-glacial environment, extensive in the central part of the Bogotá savanna and narrow in the northern region
Thedepositional environment has been interpreted aslacustrine (Lake Humboldt) and fluvio-deltaic,[2] with a near-continuous deposition since theLate Pliocene. The Sabana Formation represents the uppermost unit of the lacustrine deposition of Lake Humboldt.[5] 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 theBogotá Fault, influenced the depositional cycles. The middle unit of the formation shows a drying out of the lake and subaerial erosional surfaces.[10] 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 yearsBP. The glacial origin was predominantly theSumapaz Páramo to the south of the Bogotá savanna, with minor snow-capped peaks in theEastern Hills of Bogotá.[11]
The Sabana Formation was deposited during the Pleistocene glaciations and interglacials ("ice ages"). The fluctuations in climate in the Eastern Colombian Andes have been studied around Lake Fúquene at an altitude of 2,540 metres (8,330 ft), to the north of the Bogotá savanna. During theLast Glacial Maximum of the Pleistocene, the paleoecology of the region varied drastically, marking movements of the uppertree line and the types of vegetation. Pollen analysis shows thatpáramo vegetation was abundant from 30 ka to 17,500 years ago, with an increase in Andean forest frequency dated at 15.6 ka. Between 13,000 and 11,000 years BP, a decrease in Andean forest percentage is observed, indicative of a colder climate than before. This period has been named theFúquene stadial. The stadial is followed by an interstadial (Guantivá), with an increase in lake levels of Lake Fúquene.[12] The wetter periods of the interstadial covered earlier paleotopography with humic sediments.[13]
The latest sedimentation phase of the Sabana Formation, evidenced by thesitesEl Abra,Tibitó andTequendama, was accompanied by the first confirmed human settlement in Colombia. Around 12,500 years BP, groups ofhunter-gathererspopulated the rock shelters surrounding the retreating Lake Humboldt. The people of the area hunted the still extant Pleistocene species, and used their remains for the construction of primitive settlements, as bone tools and the skins as clothing. At this stage, thetimber line was 1,000 metres (3,300 ft) lower than today.[26]
During the Holocene, the inhabitants of the Bogotá savanna gradually moved away from the rock shelters as permanent settlements in favour of more open area locations, asChecua andAguazuque. Around 5000 years BP,agriculture became a more dominant phenomenon and the fertile clays mixed with volcanic ash of the Sabana Formation, combined with the bimodal pattern of seasonal precipitation made the Bogotá savanna an ideal area for growing crops.Pottery was used in theHerrera Period, from around 2800 years BP onwards, and the sediments of the Sabana Formation were used for various styles of ceramics, grouped byresearchers based on the colour of the original clays. The northern settlement ofSuesca was an important ceramic producing centre for the people. An advanced civilisation developed in the first and second millennia CE, leading to theMuisca Confederation, a loose collection ofcaciques. The southern Muisca area was centered around the Bogotá savanna with as main settlementBacatá in the middle of the savanna, the namesake of the current capital of Colombia,Bogotá.[27]
With the expansion in the late colonial and early republican era of the Colombian capital to the west and north of the city, the unconsolidated finer sediments of the Sabana Formation became more and more the foundation for construction, leading to problems due to the differential compaction of the sandy and more clay-rich strata.[28]
Type locality of the Sabana Formation on the Bogotá savanna
The Sabana Formation is found at itstype locality in the Funza II well, and covering most of the Bogotá savanna.[2] The newer parts of Bogotá, especially the neighbourhoods north of theAvenida Chile (Calle 72) inChapinero and west of theAutopista Norte (Avenida 30), rest upon the Sabana Formation, where the unconsolidated shales cause frequent fissures in the roads constructed in the Colombian capital. The southeastern part of Bogotá, including the historic centre, rests upon the more competentTunjuelo Formation.[3]
Guerrero Uscátegui, Alberto Lobo (1996),Estratigrafía del material no-consolidado en el subsuelo del nororiente de Santafé de Bogotá (Colombia) con algunas notas sobre historia geológica, VII Congreso Colombiano de Geología - I Seminario sobre el Cuaternario, pp. 1–23
Guerrero Uscátegui, Alberto Lobo (1993),Informe sobre la Cuenca Petrolífera de la Sabana de Bogotá, Colombia, Sociedad Colombiana de Ingenieros, pp. 1–29
Guerrero Uscátegui, Alberto Lobo (1992),Geología e Hidrogeología de Santafé de Bogotá y su Sabana, Sociedad Colombiana de Ingenieros, pp. 1–20
Montoya Arenas, Diana María; Reyes Torres, Germán Alfonso (2005),Geología de la Sabana de Bogotá,INGEOMINAS, pp. 1–104
Gómez Londoño, Ana María (2005),Lo muisca: el diseño de una cartografía de centro. Chigys Mie: el mundo de los muiscas recreado por la condesa alemana Gertrud von Podewils Dürniz in "Muiscas: representaciones, cartografías y etnopolíticas de la memoria",Universidad La Javeriana, pp. 248–291
Van der Hammen, Thomas (1986), "Cambios medioambientales y la extinción del mastodonte en el norte de los Andes",Revista de Antropología, Universidad de los Andes,II:27–34
Hoyos, Natalia; Monsalve, O.; Berger, G.W.; Antinao, J.L.; Giraldo, H.; Silva, C.; Ojeda, G.; Bayona, G.; Escobar, J.; Montes, C. (2015), "A climatic trigger for catastrophic Pleistocene–Holocene debris flows in the Eastern Andean Cordillera of Colombia",Journal of Quaternary Science,30 (3), John Wiley & Sons, Ltd.:258–270,Bibcode:2015JQS....30..258H,doi:10.1002/jqs.2779,hdl:10533/238798
Rutter, N.; Coronato, A.; Helmens, K.; Rabassa, J.; Zárate, M. (2012),Glaciations in North and South America from the Miocene to the Last Glacial Maximum,Springer, pp. 1–67
Scott, David A.; Meyers, Pieter (1994),Archaeometry of Pre-Columbian sites and artifacts, TheGetty Conservation Institute, pp. 1–437
Urrego, Dunia H.; Hooghiemstra, Henry; Rama Corredor, Oscar; Martrat, Belén; Grimalt, Joan O.; Thompson, Lonnie; Bush, Mark B.; González Carranza, Zaire; Hanselman, Bryan Valencia and César Velásquez Ruiz, Jennifer (2016), "Millennial-scale vegetation changes in the tropical Andes using ecological grouping and ordination methods",Climate of the Past,12 (3):697–711,Bibcode:2016CliPa..12..697U,doi:10.5194/cp-12-697-2016,hdl:11245.1/a6c73051-96f3-4abc-ab2d-fa66bce407a0
Villarroel, Carlos; Concha, Ana Elena; Macía, Carlos (2001), "El Lago Pleistoceno de Soatá (Boyacá, Colombia): Consideraciones estratigráficas, paleontológicas y paleoecológicas",Geología Colombiana,26,Universidad Nacional de Colombia:79–93
Zonneveld, Jan Isaak Samuel (1968),Quaternary climatic changes in the Caribbean and N. South America, pp. 203–208
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