GEOLOGY OF LIMA, PERU

Lima, the capital and largest city of Peru, is located on the Pacific Coastal Plain on dissected alluvial cones formed by the rapidly flowin g 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 Ande s in Central Peru are relatively young mountains with the oldest expos ed rock being pyroclastics of the upper Jurassic Puente Piedra Group. These are overlain by lower Cretaceous shales and quartzites of the Mo rro-Solar Group, followed by 1,000 m of limestone of the Pamplona and Atocongo formations of lower to middle Cretaceous age. In middle and u pper Cretaceous time, vulcanism resumed and 1,500 m of andesites and p yroclastics were deposited to form the Casma Group which forms the bed rock at the highest stratigraphic level. The unconformably overlying a lluvial 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 subduc tion of the Nazca Plate under the South American Plate and consequent uplift and volcanic activity. The structural geology of the bedrock un derlying the Quaternary sediments in the city of Lima is dominated by the northwest-trending Lima anticline and associated synclines and rev erse faults. This anticline is asymmetrical with dips of 5 to 20 degre es on the west and 35 degrees on the east. The engineering problems re lated to the geology of Lima concern earthquake protective design, fou ndation problems on unstable soils, water supply for a rapidly growing city, solid waste disposal and urban sprawl. Modern engineering pract ices have been introduced, with seismic design in the new building cod es. Water resource planning together with modern infrastructure constr uction takes into account the geological setting of this large and imp ortant city.