The project area is located within the 100,000 scale Yauyos mapsheet number 25-L which was published in 1996 by the Instituto Geologico Minero y Metallurgico (INGEMMET). The entire mapsheet covers a half degree quadrangle, which equates to just over 3000km2. Les Oldham has mapped several areas within the mapsheet including the area directly over the Pumacocha cave while exploring for base and precious metals.During the course of his mapping Les first recognised the potential for major cave development in this area.
Geological
controls are often the primary elements that dictate a cave’s location and
form. Caves form in limestone, and
the best caves are developed in massive limestone with little or no interbedded
silts, shales or other non-carbonate dominated lithological horizons. Within Peru, the best limestone for cave development is the Upper
Cretaceous Jumasha Formation. The Jumasha limestones comprise a massive thickly
bedded sequence of limestones and minor dolomites. Within the Yauyos mapsheet
approximately 700km2 of Jumasha limestones outcrop, making the area
highly attractive for cave exploration and karstic studies. In the region of
study this lithological unit has been estimated at approximately 400m thickness
(Megard et al., 1996). Directly overlying the Jumasha Formation is another
limestone unit known as the Celendin Formation that was also deposited in the
Upper Cretaceous and has also been estimated as having a thickness of 400m. The Celendin Limestones are not as favorable for cave development due to
common interbedded layers of gypsum, red-brown shales and some sandstone. Nevertheless caves can and do occur in this formation. Below the Jumasha limestone lie two further Cretaceous limestone bearing
formations, namely the Pariatambo and Chulec formations. Together these form an
estimated 330m of potential cave bearing stratigraphy. Jurassic age limestone
also occur to the northeast of the principal area of study yet still within the
Yauyos mapsheet. These are the Lower Jurassic Condorsinga unit of approximately
1000m thickness and the middle Jurassic Chaucha Formation of an estimated 300m
thickness. In total therefore the
region has over 2400m of limestone stratigraphy that has subsequently been
thrusted and folded during a sequence of orogenic events. The deformation is likely to be closely associated to a period of
intrusive activity during the Paleogene and Neogene epochs, which has left the
limestones commonly tightly folded, and in many areas standing near vertical. During this period of deformation it is likely that many of the
predominantly limestone hosted mineral deposits for which this area is famous
for were formed. The principal
mineral deposits of the region all have strong magmatic associations suggesting
direct association with the Cenozoic intrusive activity. See the simplified geological map.
Geology at Pumacocha
The development of the Sima Pumacocha cave system shows two key geological controls. Firstly its location is at the contact between a large Miocene age granodiorite and the intruded Cretaceous age Jumasha limestones. The entire catchment area of the drainage leading into the Pumacocha lake is over the granodiorite. The cave has formed where the lake outflow first meets the limestones. The second key control to the cave is the near vertical dips of the limestone bedding planes that have been thrust up into this position during various episodes of Andean Orogenies.
The presence of considerable cherty horizons which were located underground suggest that the mapped cave to date lies close to the lower contact with the underlying Lower Cretaceous Pariatambo Formation. Numerous ammonites have been observed within the cave at ammonite shaft however no attempt to classify these fossils was undertaken.
Geology
and Geomorphology at Qaqa Mach’ayQaqa Mach’ay lies further to the south within the same Jumasha limestone band as Sima Pumacocha. The steeply dipping limestone beds is a common control similar to Sima Pumacocha as is clearly observed in the vast entrance. The location of the cave is believed to be related to the more recent glaciation rather than geological controls. Cave development is believed to have initiated where glacial overflow passed a large bedding plane fissure, which subsequently has grown to the huge breakdown entrance observed today. The cave retains the last remaining icy remnants of the rapidly retreating glacier clearly marked on topographic maps only 30 years old.
Geology at Puyo
The caves of the Tragadero Puyo region lie around the flanks of a 10km-plus long anticline with a core of Lower Jurassic Condorsinga Limestone. Overlying the Condorsinga Limestones are sandstones and shales of the Middle Jurassic Cercapuqio Formation. The areas of best cave development has occurred at the contact between these two rock units. Caves were located at numerous locations around the anticline with also a few smaller pits located on the central part of the anticline. It is unclear from reviewing topographic and geological maps where the most likely area for resurgences to these systems may lie. Although it is possible that several caves of considerable length may be located through further exploration the depth potential in this area is likely to be limited despite its high elevation.
The Pumacocha cave system lies between two active mining camps. To the south is the San Valentin polymetallic fault controlled orebody and to the north lies the larger mineral district of Yauricocha where many of the small lead/zinc/silver orebodies appear to occupy karstic or solution-collapse cavities. In both camps mineralization is located at or close to the contact between the Neogene granodiorite and the Jumasha limestones. The contact is sporadically mineralised along its length but it is considered that younger porphyritic intrusives are the true mineralisers that have taken advantage of the structural weakness and fault zones developed along the pre-existing contact. Zoned mineralogical systems exist at both San Valentin and Yauricocha, with Lead zinc peripheries and copper towards the central source intrusions. At the Sima Pumacocha cave system it is the same pre-existing contact between the granodiorite and limestones that has controlled the initial cave development.
Additional Comments on Geomorphologic Controls
Previous speleological expeditions to the Andes have commented on the lack of deep and well-developed caves and have attributed this in part to an effect of the excessive altitude (Imperial College, 1975). The argument proposed is that rainwater falling at such altitudes is less acidic since less CO2 has been absorbed during the descent. As to whether this argument is valid or not is not here disputed, although the contribution of acidic waters is clearly a pre-requisite for large-scale cave development. The headwaters of Laguna Pumacocha rose over 30sq km of granodiorite bedrock and extensive glacial deposits. Poor drainage over the granodiorite has resulted in the development of peat bogs which themselves produce acid waters due to the decomposition of organic matter which produces CO2 and therefore carbonic acid. Furthermore the oxidation of numerous pyretic sulphide veins within the granodiorite will also have contributed to the low pH of waters entering Laguna Pumacocha and subsequently Sima Pumacocha.
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