Let me introduce...the Aburrá Valley

The Andean Mountains in Colombia are divided into three ranges, Western, Central, and Eastern cordilleras. The Aburrá Valley is located in the northern portion of the Central Cordillera. It extends approximately between latitudes 6°00’N and 6°30’N and longitudes 75°15’W and 75°45’W. As shown in Figure 2.1, highly-populated areas extend along the central portion of the valley. The valley constitutes a geographical, economical and urban unit called the Area Metropolitana del Valle de Aburrá (Aburrá Valley Metropolitan Area). This unit includes eight municipalities: Medellín, the second largest and one of the most important cities of Colombia; Itagüi and Bello, important industrial and population centers; and Barbosa, Caldas, Copacabana, Girardota, La Estrella, Sabaneta, and Envigado.

The area is located in the tropical zone. Rainfall over the year is distinctly bimodal, with maximum precipitation in April - May and October - November. Annual rainfall varies from 1000 mm at the borders of the district to 3000 mm in the central and northern areas (Fig. 2.2).

Topographical outline
The entire catchment area[1] of the Aburrá Valley is 1152 km2. This valley is constituted by the Medellín River, which flows north to northeast in the floor of the valley. The Aburrá Valley is surrounded by erosional landsurfaces (Santa Elena around 2750 m above sea level (a.s.l.), San Pedro around 2800 m a.s.l. and remnants of an older erosion surfaces (Padre Amaya Hill and Boquerón Hill 3100 m a.s.l.).

Figures show elevation and topographic outline maps of the Aburrá Valley. The valley constitutes a large depression zone N - S from the Alto de Minas to La Estrella. From the latter, the valley changes, trending NE - SW up to Envigado, then turns N - S up to Bello, and finally trends NE - SW from Bello to Barbosa.

The Aburrá Valley has a length of 65 km, and ranges in altitude from 1000 m and 3000 m in elevation. Morphologically the valley is a narrow valley at its northern and southern extremities. The central portion is wider, with a flat bottom composed of alluvial sediments, with a width of up to 8 km. Elevation of the valley floor ranges from 1,800 m in Caldas town to 1,000 m a.s.l. in Baborsa town with a longitude up to 8 km, which correspond to alluvial plains.

The river profile is characterized by the presence of remarkable knickpoints at Ancon Sur, Ancon Norte, El Hatillo, and La Montera. The Medellín River has around 87 tributaries on its eastern flank and 107 tributaries on its western flank. The valley includes some small basins such as Santa Elena, La Ayurá, Doña Maria, and La Iguaná. Other significant basins are La Miel, La Doctora, Piedras Blancas, Ovejas, La García, La Salada, La Valeria, La Guayabala, La Picacha, La Madera, Guasimal, and La Mata.

Geomorphological classification
The Aburrá Valley comprises a deep valley in mountainous areas of the Central Cordillera. The inner morphology is characterized by alluvial plains along the Medellín River, bounded by gentle to moderate step-like slopes. The geomorphological characteristics of the valley changes slightly along the Medellín River. It is a narrow and asymmetrical valley with relatively steep slopes in the southern portion and becomes a smooth and wider valley with gentle to moderate slopes in its central portion. In the north it is a narrow and symmetrical valley, bounded by very steep slopes. Local tributary valleys in the central valley are wide and strongly dissected.

Distribution of very steep slopes is restricted to cliffs and scarp of slope movement along the upper valley areas. The gentler slopes in the lower valley may correspond to depositional surfaces formed by debris flows or as fans. According to GSM (2002), the geomorphological surfaces of the Aburrá Valley can be roughly divided into four major groups on the basis of slope configurations, slope steepness, degree of incision of the streams, and general appearance: (1) scarps, (2) ridges and hills, (3) gentle slope surfaces, and (4) fluvial plain; Figure 2.6 shows simplified morphological classification map by GSM (2002).

Scarps
Extremely steep surfaces formed on rocks are defined as scarps. They are located in the upper slope, bordering erosional surfaces. Small slope deposits consisting of angular cobbles without matrix are found associated with the principal scarps.

Ridges and hills
Most ridges are sharp with convex crests. Slopes along ridges are straight and very steep (>30°), and are partially dissected. Based on relative heights above the valley floor, ridges and hills are conveniently divided into high, middle, low, sharp ridges, and isolated hills.

The ridges are elongated mostly in the direction perpendicular to the Medellín River. The hill units present rounded profiles and convex or flat crests. Some isolated hills or inselbergs located on the valley floor or in the middle parts of the slopes belong to this group.

Gentle slope surfaces
Gentle surfaces extend to both sides of the central plain along the Medellín River. These surfaces consist of slope deposits and alluvial fans, forming gently to moderately sloping terrains. The slope morphology is straight, concave, and little- to moderately-dissected.

Gentle surfaces are distributed on the middle and lower parts of the valley flanks. A typical cross section is represented by staircase shaped slopes. Remarkable differences are recognized among step levels in degree of dissection, inclination, and relative height from the Medellín River.

Alluvial plains
Alluvial plains comprises flat surfaces on the valley floor. They include alluvial terraces, which are some 10 to 15 m above the modern base level. Alluvial plains are the result of building up of the valley floor by alluvium sediments.

These surfaces are distributed along the Medellín River in the central valley, ranging from 1 to 4 km in width. Over these areas are located the larger part of Medellín city, Envigado and Itagui. In the north of the valley the alluvial plains are long and narrow, bounded by steeper slopes.

Geological outline
Figure 2.7 shows a simplified geological map of the Aburrá Valley, here compiled on the basis of previous work by Restrepo and Toussaint (1984), Ortiz (2002), and Rendón (2003). The northern Central Cordillera is mainly composed of a Paleozoic metamorphic rocks, which was named the Central Cordillera Polymetamorphic Complex (CCPC). This form a metamorphic belt composed of gneisses, schists, and amphibolites. Ultrabasic rocks such as dunites and gabbros are thrust over the metamorphic rocks during Cretaceous times (Toussaint and Restrepo, 1984). They comprise an ophiolite complex of oceanic origin distributed with N – S trend, crossing the Aburrá Valley.

Triassic and Cretaceous plutonic bodies of acid to intermediate composition are widely distributed intruding the metamorphic rocks, such as the Antioqueño Batholith and the Altavista and Ovejas Stocks (Restrepo and Toussaint, 1984). These basement

Rocks are mainly distributed in mountainous zones. In the lower and middle slopes they are partly covered by Tertiary and Quaternary sediments. These sediments are slope deposits, and are widely distributed, forming gentle slopes in the Aburrá Valley. They are defined as a complex of old debris flow deposits, and other fluvial deposits. Alluvial deposits are also distributed along the Medellín River, forming alluvial plains, especially from Itagui to Copacabana, as shown in Figure 2.7.

The rocks and sediments distributed in the Aburrá Valley can be roughly divided into seven geological units: metamorphic, granitoid, ophiolitic, volcano – sedimentary, Tertiary sediments, slope deposits, and alluvium deposits, as shown in Figure 2.7.

Metamorphic rocks
This sequence includes gneisses, schists, and amphibolites. Gneissic rocks crop out in the southern portion of the valley (Alto de Minas, La Miel Stream), the west (La Iguaná Stream), the east (Las Palmas, La Ayurá Stream), and in the north (Las Peñas, Ancon Norte).

Gneisses rocks present a defined banding formed by quartz and feldspar. The residual profile contains high concentration of quartz and is around 15 m thick along gentle slopes. Although the surfaces formed on this weathered material are generally stable due to homogenization of the physical properties, numerous landslides associated with the human intervention have occurred nevertheless (PNUD, 1998).

Gneisses and amphibolites are intercalated in the northern and eastern Aburrá Valley. These sequences are banded with grayish color, and locally show migmatite textures. The mineralogy consists of quartz, feldspar and micas. Residual profiles along the northern valley reach 9 m in thickness; and are characterized by corestones embedded within a sandy matrix (GSM, 2002).
Schistose rocks are mainly distributed in the southern Aburrá Valley (Alto de Minas, Caldas Ancon Sur), and in the eastern part (Padre Amaya Hill, Boquerón Hill). These schists are fine-grained, and contain muscovite, chlorite, and graphite. The weathering profiles of these rocks are characterized by irregular horizons, and may be sandy to fine-grained, according to the percentage of quartz in the fresh parent rock.

Amphibolites are mainly distributed toward the eastern Aburrá Valley (Santa Elena high - plain), to the north (Bello, Girardota ), to the south (Caldas), and in the eastern Medellín city. These rocks vary from medium to fine in grain size, and foliated to massive in texture. The composition is homogeneous, contain hornblende and leucocratic minerals. The weathering profiles are silty, reaching thickness around 30 m in the northern valley. Slope surfaces formed by these rocks do not show evident signs of instability, therefore only vestiges of past landslides are found along them (GSM, 2002).

Ophiolitic Complex rocks
Ultrabasic igneous rocks occur in numerous discontinuous elongated bodies, and are generally characterized by tectonic boundaries and highly deformed rocks (Rendón, 2003). This unit includes metagabbros, dunites, and gabbros, ranging in age from Late Jurassic to Early Cretaceous (Toussaint and Restrepo, 1978).

Metagabbros are distributed to the west of the valley (El Picacho), in the center (Nutibara hill), the northeast (Medellín – Bogotá highway), and the southeast (Las Palmas). These rocks are composed of hornblende and feldspar and a present well-developed foliation, trending N80°E / 40°W, N80°E / 55°W, and N65°E/60°W.

Dunites crop out in the east of Medellín and to the north of Bello. They are considered to be the base of an ophiolite sequence overthrusted on the CCPC during the Early Cretaceous (Toussaint and Restrepo, 1978). These rocks are composed by olivine, chromite and magnetite. The olivine is slightly altered to talc and clorite. The weathering profile developed on them ranges from 10 to 40 m in thickness, according to joint system of the rock.

Gabbros crop out in the southern Aburrá valley, in elongate bodies associated with the Romeral Fault System. They contains plagioclase, pyroxene, and amphibole. Weathering profiles reach depths of 40 m.

Volcano – sedimentary rocks

This unit is composed by interbedded volcanic and sedimentary sequences as basalts, andesites, and pyroclastics materials, and is named the Quebradagrande Complex. Their distribution along the Aburrá Valley is closely associated with tectonized and serpentinized ultramafic rocks and gabbros (GSM, 2002). The rocks are strongly fractured, with well developed schistosity textures. Residual profiles are silty, red to brown, and reach 15 m in thickness. The sedimentary sequence consists of marine material intercalated with the volcanic rocks. The sediments comprise cherts, sandstones, conglomerates and mudstones (Rendon, 2003). Similarly, they are also characterized by strong tectonic effects, especially with N-S trend.

Granitoid rocks
Plutonic bodies crop out in many places in the valley. They are divided into the Antioqueño Batholith in the north; the Ovejas Batholith northwest of Bello, the Altavista Stock in the west of the Medellín slopes, and Las Estancias stock to the east of Medellín. These bodies are considered to be Cretaceous in age (Kerr et al., 1997).

Granites and diorites of the Antioqueño Batholith contain feldspar, quartz, biotite and hornblende. The texture is equigranular and hypidiomorphic. The weathering profile developed on these rocks is sandy, with thicknesses of up to 40 m, remaining mass structure up to 5 m. Profiles are characterized by spherical residual corestones embedded within the matrix. The residual soils are vulnerable to landslides along joint surfaces, and are highly susceptible to gully formation (Flórez et al., 1997).

The Ovejas Batholith consists of granodiorites, quartz diorites and diorite. These are typically massive, display granitic texture, and are of medium grain size. Mineralogy consists of plagioclase, quartz, biotite, and hornblende. Sheet erosion and small volumes translational slides occur on Ovejas batholith surfaces (Flórez et al., 1997).

The Altavista Stock is composed of andesite to granodiorite (Rendón, 1999). These rocks are strongly weathered. The residual profile consists of sand and silt, remaining the structure mass up to 10 m.

Slope deposits
This unit includes of all the gravitational deposits located on the Aburrá Valley slopes. The deposit materials are variable depending on their source rocks, and are characterized by angular boulders, ranging from cobbles to boulders in size, embedded in a silty matrix. Little is known about the evolution and absolute age. Pliocene and Pleistocene ages have been obtained by fission track and 14C (Restrepo, 1991; Toro, 1999; Ortiz, 2002).

Alluvial deposits
Alluvial deposits consist of sand and gravel distributed along the Medellín River and its tributaries. They include fan deposits and terraces of which elevation heights reach 8 m. The alluvial plain along the central valley ranges between 2 to 7 km in width. Several fan-shaped deposits have been identified and mapped along the valley, and some are intercalated with mud flows and debris flows. Alluvial terraces in the southern valley, 200 m above the modern Medellín River, has been dated as Late Pliocene (3.06 Ma fission track) by Toro et al. (1997).
The relationship between slope sediments and alluvial deposits is complex. The former are mainly distributed on gentle slopes on both sides of the valley, whereas the latter forms alluvial plains along the Medellín River.

Structural outline
The Aburrá Valley is regarded as a tectonic depression zone elongated NE – SW. However, tectonic movements and their depression are very complicated, because no major fault scarps occur within the valley. Intense tropical weathering processes operating in the valley have largely removed these tectonic features from the landscape.

Two major fault systems with NNW – SSE trend obliquely cross the trend of the Aburrá Valley. These are the Cauca - Romeral Fault System to the west, and the Belmira Fault System to the north (Rendón, 2003). The Cauca–Romeral Fault System is considered to be a strike-slip system, kinematically connected to relative dextral displacement between the Caribbean and South America Plates during the Mesozoic (Toussaint, 1996), and marks the boundary between oceanic crust to the west and continental crust to the east. These faults have affected the basement and recent materials of the Aburrá Valley (Ortiz, 2002; GSM, 2002; Rendón, 2003). The most remarkable structures are grouped as the Cauca - Romeral Fault System, e.g. San Jerónimo Fault, Ancon Fault, La Doctora Fault, and La Miel Fault (GSM, 2002).

The Rodas Fault and the Belmira Fault System represent the most remarkable structure along the northern Aburrá Valley. Two main traces form the Belmira Fault System. Rendón (2003) reported linear valleys, saddles, scarps, isolated hills, and triangular facets as topographic expressions of these structures. Rendón (2003), citing Integral S.A. (1982), reported neotectonic evidence of lateral motion and thrusting in the Belmira faults. The Rodas Fault separates metamorphic rocks to the east and ophiolitic rocks to the weast.

Many other important structures categorized as lineaments occur in the Aburrá Valley. They are recognized by alignment of slope breaks, triangular facets, scarps, and affected drainage. They are mostly oriented parallel to the trend of the valley.

Basement rocks composed of gneisses, amphibolites and other lithotypes are elongated NNW – SSW, and obliquely cross the elongation of the valley. Dominant joint planes formed in the gneisses trend N30°E/60°E. Gneissic layering trend N40°E/60°E and E-W/20°N, changing to N10°E/30°E along the Medellín – Bogotá Highway exposures. The schistosity trends N30°-50°W / 50°-70°SW to N45°-60°E / 80°SE. In the Boquerón sector, these rocks are strongly fractured due to the Romeral Fault System.

Amphibolites contain a well-developed foliation, trending N-S / subvertical, N40°E/50°E, and N60°E / vertical. In the sector of Copacabana and Girardota towns, these rocks contain numerous joints trending N30°W / 30°NE and N30°E / 50°NW with open and clean joint surfaces; and N35°E / vertical and N60°W / 70°NE closed and dry surfaces. Local faults affecting granitoid rocks trend N40°W / 60°E, and joints N55°W / 65°E, N45°W / 15°W (GSM, 2002).

[1] http://www.metropol.gov.co/territorio.htm (April/2004)