Sima Pumacocha - Physical and Geological Setting

Physical and Geological Setting

At 4,300 m to 4,400 m above sea level, the Pumacocha valley is a typical Andes ‘puna’ – high, treeless, and surrounded by 5,000 m peaks. The valley is traversed by a dirt road serving active and abandoned mines nearby, and is dotted with stone huts used by locals tending flocks of sheep, llamas and alpacas. At the head of the valley is Laguna Pumacocha, a small lake which along with its catchment area sits atop Miocene age granodiorite. Where the lake's outfall stream meets near-vertically bedded Cretaceous age Jumasha limestones, it has carved a short, shallow canyon containing several abandoned and one active sinkpoint where the entire stream disappears underground. Underlying the limestone is the Lower Cretaceous Pariatambo Formation.

A small concrete canal, intended to keep the lake outfall on the surface by diverting it around the canyon, is in poor shape and normally does not function at all. When local repairs occur, the active sinkpoint (SP1) becomes enterable, but considerable water then leaks into the abandoned sinks (SP2 and SP3).

On a regional scale, the long, irregular band of limestone containing Sima Pumacocha and several other caves runs roughly south-southeast to north-northwest. The Pumacocha waters sink at an elevation of 4,375 m above sea level, and are thought to resurge in the Rio Alis valley some 14km to the north at an elevation of about 3,300 m above sea level.