Geology of Lima, Peru

Abstract

Lima, the capital and largest city of Peru, is located on the Pacific Coastal Plain on dissected alluvial cones formed by the rapidly flowing Rimac, Chillon and Lurin rivers. The Western Cordillera of the Andes Mountains rises abruptly east of the city and reaches altitudes of 6,100 m (20,000 ft) only 130 km (80 mi) from the Pacific Ocean. The Andes in Central Peru are relatively young mountains with the oldest exposed rock being pyroclastics of the upper Jurassic Puente Piedra Group. These are overlain by lower Cretaceous shales and quartzites of the Morro-Solar Group, followed by 1,000 m of limestone of the Pamplona and Atocongo formations of lower to middle Cretaceous age. In middle and upper Cretaceous time, vulcanism resumed and 1,500 m of andesites and pyroclastics were deposited to form the Casma Group which forms the bedrock at the highest stratigraphic level. The unconformably overlying alluvial cones and beach sediments formed during the rapid rise of the Andes have been dissected by rejuvenated down-cutting to as much as 20 m (62 ft) at Lima. The geology of the Lima region reflects the subduction of the Nazca Plate under the South American Plate and consequent uplift and volcanic activity. The structural geology of the bedrock underlying the Quaternary sediments in the city of Lima is dominated by the northwest-trending Lima anticline and associated synclines and reverse faults. This anticline is asymmetrical with dips of 5 to 20 degrees on the west and 35 degrees on the east. The engineering problems related to the geology of Lima concern earthquake protective design, foundation problems on unstable soils, water supply for a rapidly growing city, solid waste disposal and urban sprawl. Modern engineering practices have been introduced, with seismic design in the new building codes. Water resource planning together with modern infrastructure construction takes into account the geological setting of this large and important city.