Geologic History

Geologic History

The Cordillera Blanca owes its magnificent relief to a combination of uplift and erosion, both of which continue at high rates today. Prior to uplift the region was the locus of sediment accumulation and lithification, which ultimately generated the thick sedimentary strata that can be found folded and faulted along the flanks of the Cordillera.

The sedimentary rocks were originally deposited in shallow marine, riverine, lake, and swamp environments during the late Mesozoic Period (160-65 million years ago). The oldest strata belong to the Chicama Formation, a faulted and folded mudstone and sandstone complex that is up to 1 km thick (Wilson et al., 1967). The next younger stratigraphic unit, the Chimu Formation, comprises up to 400 m of sandstone, mudstone and layers of anthracite coal that are the focus of mining operations along the road to Chavin, on the eastern side of the Cordillera; these strata are also exposed in the region around Caraz. The coal beds were deposited in swamps during the early Cretaceous, long before the region became uplifted. Stratigraphically superimposed on the Chimu formation is the Santa Formation, which is a locally fossiliferous-rich limestone up to 350 m thick, and a calcareous mudstone that is locally brown to red in color reflecting an abundance of iron oxide minerals. The youngest sedimentary strata include sandy and locally red mudstones, quartzite, fossiliferous limestone, dolomite, and conglomerate with interbedded volcanic rocks collectively comprising a thickness of well over 1 km. These stratigraphic units belong to the Carhuaz, Farrat, Pariahuanca, Chúlec, Pariatambo, Jumasha, Celendín, and Huaylas Formations, from oldest to youngest (Wilson et al., 1967). The increase in abundance of conglomerate and volcanic rocks in the youngest strata mark a profound change in the tectonic setting of western South America about 65 million years ago, and the initiation of regional uplift.

Uplift began as a result of collision between the Nazca and South American tectonic plates. The Nazca plate is forming in the eastern Pacific and moves in an eastward direction at a rate of about 7 cm/year. In contrast, the South American plate is being driven to the west at a rate of 10 cm/year by the formation of new oceanic crust in the South Atlantic Ocean. Because of its 5-10% higher density, the Nazca plate subducted beneath the South American plate to depths at which it has begun to melt. The resultant rising plume of magma has broken through the surface in many regions and is responsible for the numerous active volcanoes along the western edge of central South America. The thick columns of black rock exposed along the road to Huaraz from the coastal town of Pativilca are deposits of lava and pyroclastic material that cooled rapidly about 50 million years ago. This rock is known as the Volcánico Calipuy and it underlies much of the eastern two-thirds of the Cordillera Negra. Other volcanic rocks in the region include the Yungay Formation, which is found in isolated regions along the Rio Santa valley near Laguna Conococha, and near the villages of Carhuaz, Yungay, and Caraz. This rock was formed between about 5 and 2 million years ago from volcanic flows and ash.

In addition to volcanism, magma, if allowed to cool at depth in this tectonic setting, generates granodiorite, a rock intermediate in composition between the dense, iron-rich rocks that form the oceanic crust and the relatively low density and iron-poor rocks, known as granite, which typically underlie the stable central and eastern portion of South America. The axis of the Cordillera Blanca is underlain by the Cordillera Blanca Batholith, an enormous mass of granodiorite, which formed as recently as 10 million years ago many kilometers below the overlying sedimentary strata.

Collision between the Nazca and South American plates resulted in uplift of the western edge of South America by more than 5 km. This has caused the strata of sedimentary rocks to become folded and faulted. Erosion by rivers and ice has exposed the central pluton of granodiorite, and in many regions the marine strata can be found nearly vertical draping off the edge of uplifted granodiorite block that forms the central axis of the Cordillera. Continued uplift of the granodiorite block has produced an active range-front fault known as the Cordillera Blanca Normal Fault, which has faulted glacial moraines as young as 10,000 years old. Studies of the strata that underlie the Cordillera Blanca Normal Fault reveal a recurrence interval of fault movement of about 2500 years, the age of the most recent event of between 750 and 2500 years, and a long-term uplift rate of the Cordillera of about 1 mm/year. This active uplift has been associated with considerable earthquake activity, the most famous of which was the large-magnitude earthquake of 31 May, 1970, which caused a massive rock fall off the north peak of Huascarán and an enormous and devastating avalanche of rock, mud, and ice which buried Ranrahirca and much of the original village of Yungay, killing some 18,000.

Glaciation and modern Hydrology

Although today the Cordillera Blanca is the most extensively ice-covered region in the tropics with 25% of the world’s tropical ice, as recently as 10,000 years ago the Cordillera was home to glaciers and ice caps many times larger than the modern glaciers that give the Cordillera its name. Studies of the age of moraines in various drainages along the western side of the Cordillera reveal at least eight phases of glacier expansion in the last several hundred thousand years. Some of the best evidence for the earliest of these glaciations can be found along the deeply incised valleys present above the villages of Huaraz, Olleros and Recuay. The earliest phase of glaciation occurred at least 140,000 years ago and reached elevations as low as 3300 m. Boulders on moraines from this glaciation are deeply weathered and commonly possess unusual weathering features on the boulders that mantle their surfaces. Subsequent glaciations occurred about 20,000-30,000, 14,000-15,000, and 10,500-11,500 years ago; these glacier advances reached elevations as low as 3400, 3550, and 3650 m asl, respectively. In the last several thousand years glaciers have expanded and contracted four times, but these advances did not extended below 4200 m asl and generally were limited to elevations above 4500 m asl. Many of the terminal moraines deposited during these advances have acted as dams and impound hundreds of small lakes in the Cordillera. The most recent ice advance occurred during the Little Ice Age, a period of global cooling between 1300 and 1850 AD.

Ice retreat from moraines deposited during the Little Ice Age has been rapid and has accelerated in the last several decades due, primarily, to global warming. Aerial photographs, taken in the early 1960s, clearly reveal that there has been an ~30% reduction in ice cover in recent decades. As global warming continues we can expect the continued formation moraine dammed lakes, and the loss of ice cover in the Cordillera. This will dramatically alter the appearance of the Cordillera Blanca, and will have a devastating effect on the water supply of the Callejon de Huaylas. Glaciers provide a reservoir of water that maintains discharge through the Río Santa during the dry season; without them the Río Santa would run ~40% lower during the months June-August. The continued loss of glacier cover has fueled plans to construct dams in several tributaries to the Río Santa. Such dams would enable glacial meltwater to be stored during the wet season and released during the dry season to provide discharge for agricultural needs and to drive the turbines of the hydroelectric plant in the Canon del Pato, located at the northern end of the Callejon de Huaylas. With the exception of the drainage control tunnel at Laguna Parón, no such dams have yet been built. One of the obvious hazards of building dams along the west side of the Cordillera is the proximity of the Cordillera Blanca Normal Fault to most potential dam sites. Faulting and earthquake activity could weaken or destroy dam structures, thus generating potentially enormous floods, which could have disastrous effects on the many towns and villages located along the Río Santa. Small examples of the kinds of floods that could be generated are the alluviones, which have occurred repeatedly in past centuries.

Alluviones are the floods generated by the failure of the moraine dams. These tend to occur in response to earthquake shaking and have destroyed large parts of villages in the Callejon de Huaylas. One of the most famous of these occurred in December 1941 in Quebrada Cojup, directly above Huaraz. Earthquake shaking caused a moraine dam to be breached and the instantaneously release of a moderate-size glacial lake. Flood waters descended on the town of Huaraz moving enormous boulders along its bed and destroying much of the Village. In response to the ever-present threat that moraine dams will fail catastrophically, engineers at Hydroandina have undertaken a long standing project to replace the earthen dams with artificial outlets.

Reference: Wilson, J., Reyes, L., and Garayar, J., 1967, Geología de los cuadrangulos de Mollebamba, Tayabamba, Huaylas, Pomabamba, Carhuaz y Huari, Servicio de Geología y Minería Boletin, 95 p.

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