Sima Pumacocha

Sima Pumacocha is a limestone cave located in the Yauyos province of central Peru near the village of Laraos, high in the Andes mountains. It is one of the deepest caves in South America.

Physical and geological setting

At 4300m to 4400m above sealevel, the Pumacocha valley is a typical Andes ‘puna’ – high, treeless, and surrounded by 5000m 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 4375m above sealevel, and are thought to resurge in the Rio Alis valley some 14km to the north at an elevation of about 3300m above sealevel.

Cave description

Following the steeply-dipping limestone beds, Sima Pumacocha generally consists of several vertical shafts connected by short sections of horizontal to steeply-sloping passages. Although closely grouped together, the three main entrances each lead to extensive independent passages before meeting underground. SP1 and SP2 are within 60m of each other but their passages join at about the –300m level, while SP3, a further 30m away, is thought to join the main cave at a depth of –550m based on observations of relative water volumes (SP3 is blocked at –120m). All three entrances lead to big shafts: SP1 has the deepest shaft in the Andes (282m), SP2 leads to the 113m deep Ammonite Shaft, and SP3 is a 120m shaft. Several fossilized ammonites up to 20cm in diameter were observed within Ammonite Shaft’s walls. Below the junction of SP1 and SP2 are several wet shafts between 15m and 75m in depth, leading to a gravelled sump at –638m. All explored passages have been surveyed, with a total length of 1427m.

It was hoped that the strong inward draught indicated a junction with a large underground river, but after losing the draught near the sump some cavers concluded that it might be due to the cave’s own stream.

Cave exploration

Sima Pumacocha was explored and surveyed during three international expeditions.

*In June 2001 an expedition of five cavers ran out of rope and time at the head of The Perfect Storm at about –375m.
*In September 2002 an expedition of nine cavers reached the sump at –638m, and located other cave entrances within the same limestone formation.
* In September 2004 an expedition of fourteen cavers explored the SP1 series and five other caves, including Qaqa Mach'ay.

References

*"The Canadian Caver" nos. 57, 59, 63
*"Pumacocha 2004 Expedition Report" (unpublished)

External links

* [http://members.shaw.ca/pumacocha Pumacocha expedition website]