Guacha
The 2010 field work done down Bolivia from October 20th to November 16th covered an area of 2948 km2 full of volcanic rocks from Los Lipez at the Altiplano, Southwestern of Boliva. Here 1150 km2 belong to the Guacha caldera and 10 days of this field season were dedicated to know and actually understand this enormous caldera.
Evidence shows the Guacha caldera experienced several episodes of eruption, collapse and resurgence. Some of those features are shown here aimed to solve the history of this volcanic center which geology is beautifully exposed in one of the highest, coldest and driest place in the world.
Stop 1
Stop 1 22°33'47.20"S/ 67°16'48.73"W/4644 m
22-10-2010
Domes and intracaldera units
Pict 2.- Totoral dome on the background, Shan and Jason are standing over tha Guacha ignimbrite, northwestern scarp of Guacha caldera
Totoral dome.- Located at the northeast edge of the caldera the Totoral dome is composed of a sequence of pyroclastics flows, blocks and ashes deposits ~100 m thick.
Pict. 3.- Shan walking toward the andesite outcrop located at the northwestern scarp of the caldera
Negreal del Totoral andesite.- A dark greenish isolated outcrop located to the west of the Totoral dome corresponds to one of the more mafic bodies belonging to the Guacha caldera.
Pict 4.- Outcrop of proximal Guacha ignimbrite next to Laguna Busch. It seems that overlies over an older lava or a welded flow unit?
Guacha Ignimbrite.- A proximal ignimbrite close to the Laguna Busch (picts 4) corresponds to the Guacha ignimbrite overlaying a dark grey porphiritic unit (welded?).
Pict 5.- Beautiful Laguna Busch
Pict 6.- Outcrop of younger (?) breacciated intrusives at the eastern scarp
Pict 7.- Brecciated intrusive dark grey dacite ‘bubbles’ surrounded by light dacite ‘bubbles’
Brecciated Intrusives.- Located at the eastern edge of the caldera arquate scarp a semi circular body of ~2 km diameter is made of brecciated intrusive (picts 6 and 7). To the north is bordered by sequences of sandstones, arcoses, conglomerates and travertines (?).
Pict 8.- East view of the moat (summit of intrusive)
Pict 9.- Camp set on the East side of the caldera, the wind direction is inferred by “Paja Brava” plants alignement. On the horizont the Zapaleri volcano as triple point between Argentina, Chile and Bolivia
During that night (and some after) we experienced how cold and severe the winds are on this region, freezing temperatures, several grades below 0˚C, make freeze our own breathe inside the vents! ... anyway the landscapes worth it!!. On picture 9, Jason vent is experiencing the severe wind suggested by the lineated plants (Pajas Bravas). On the background the Zapaleri volcano, which is the triple point were Argentina, Bolivia and Chile met
Stop 2
Stop 2 22°41'05.07"S/ 67°16'23.5"W/4846 m
23-10-10
Flow units
Pict 10.- Proximal Tara Ignimbrite. A 20 cm thick coarse grain sequence showing cross bedding, clasts supported (almost no fine material). This surge deposit has horizontal continuity and varies upward to a silky flow deposit and a “sillar” or vapor phase
Tara ignimbrite.- Outcrops related to the 3.49 Ma Tara ignimbrite were observed, a proximal one, close to the eastern scarp and a distal one, outside of the caldera edge.
Pict 11.- Distal Tara ignimbrite underneath the Zapaleri volcano
Pict 12.- Distal Tara Ignimbrite over older volcanic basement three surge deposits are seen close to the base
Outside the margins of the caldera a very well exposed sequence of distal Tara ignimbrite was described (actually we were in Argentina here). On top lavas related to the Zapaleri volcano, cover this 60 m thick sequence (pict 11)
Stop 3
Stop 3 22°39'4.39"S/ 67°24'38.07"W/4769m
29-X-2010
Guacha Ignimbrite.- The closer outcrop toward the resurgent dome is the Guacha vitrophyde. It outcrops in the ENE side of the caldera, conforming a very conspicuous unit, since is darker than the distal ignimbrites. (pict 13 and 14).
Pict 5.- Dome overlaying the Guacha vitrophyde
Pict 13.- Outcrop of vitrophyde white ignimbrites maybe correspond to Tara ignimbrite ?
Stop 4
Stop 4 22°37'20.90"S/ 67°39'54.10"W/4548m
24-X-2010
Puripica Chico Ignimbrite: Located in the NW edge of the caldera, the Puripicar Chico ignimbrite outcrops form the well known Piedras del Dali monoliths (where we spent another really cold nights).
Pict 14.- Outcrop of Puripicar Chico ignimbrite
Pict 15.- Outcrop made of block and ash 5-8m above the base of Puripicar Chico ignimbrite
Stop 5
Stop 5 22°43'4.74"S/ 67°27'25.51"W/4802m
4-XI-2010
Resurgent dome.- The “trip” towards the resurgent dome is difficult, since is very remote, very high (~4800 m) and non populated at all, so at least two jeeps are a need to reach its summit. Getting from the northwestern side following just old tracks we reach the point view showed in pict 18. There we can see the intracaldera tuffs pounded in a sequence of at least 150 m. On picture 18 we can see the intracaldera tuffs are slightly dipping towards the NE.
Following our tour we stop closer to the caldera center where a graben-like structure with blocks dipping north and south were recognized. A halo of alteration (?) on the summit of the scarp is also observed in the distance.
Pict 18.- Guacha dome, view from the NW side. Beds of intracaldera tuffs are dipping towards the NE
Pict 19.- Resurgent dome and semi graben (Inside the caldera)