THE BADLANDS OF VALLEY DE LA LUNA, LA PAZ, BOLIVIA

Valley of the Moon- Valle de la Luna, is a city park developed in the lower reaches of the La Paz Canyon that deeply gouges into the Andes of northern Bolivia. With a loop of trails with stairs that provides access to the badland weathering conglomerate beds of the Miocene La Paz Formation. This park can be reached from the old center area of La Paz by a 15-to-30-minute taxi ride, depending on the traffic. Entrance to the site is only 3 Bolivianos for nationals and 15 Bolivianos for foreigners. The hiking trail is easy and comes with guard rails. Expect anywhere from 30 minutes to one hour to explore the area. The deeply incised conglomerate beds make vertical towers with narrow slots between them. Some of these gaps make chimney like openings with tens of meters of relief. Just as the surficial shapes of the park is intriguing, the story represented by the composition of these conglomerate beds is part of the history of the Andes.

The tan sedimentary beds are dotted with cobbles and boulders of basement rock that was shed from the high mountains to the east, the Cordillera Real and its main snow-capped summit nearby of Illimani (6,460 meters). Any vantage point over the La Paz canyon quickly provides one with an appreciation of how thick the La Paz Formation conglomerate beds can be. These are alluvial fan deposits, made by rushing mountain streams upon existing their confining canyon and depositing the detritus into a massive apron, layered into a larger basin that runs the base of the mountains. The Miocene La Paz Formation extends southward from Lake Titicaca at least 100 kilometers.

The 1970-90s saw a debate of hypotheses about the nature of Andean uplift and deformation, with French researchers (Noblet and others, 1996;  ) arguing for ongoing crustal deformation because of the nature of converge margins overall being a continual process. In contrast, episodic deformation was founded on field observations of major angular unconformities between various Peruvian rock formations that suggested periods of deposition and other of major structural disruption (Noble and others, 1974). By the late 1990s few researchers remained around Peru then the dialogue was more tilted towards the camp headed by Donald Noble. I did my Ph.D. field work in central Peru starting in 1999, gathering samples for geochronology work with supporting geological mapping that refined this tectonic hypothesis of rapid, major, pulses of deformation separated by periods of little activity, previously termed quiescence. Around Ayacucho in central Peru, there was even evidence of extension where the Andes were being stretched instead of compressed (Wise, 2004; Wise and Noble, 2008). By the early 2000’s and continue to today, to the south in Bolivia major formations and depositional basins underwent study by various groups, but the collections of students guided by the very skilled Dr. Brian Horton in the field of sedimentology, reinvigorated the debate of steady versus episodic deformation of the Andes. Their findings were consistently interpreted to represent major basin histories following syn-tectonic activity, some of which require additional context.

Syn-tectonic tectonic sedimentation happens with every major crustal deformation period. Sedimentation continues following these orogenic periods as the forced-up topography of mountain belts are worn down by erosion. Episodic deformation would further exaggerate this effect, having rapid mountain building, and an extended lag time of readjustment through erosion and deposition. Areas that are actively being eroded do not generally record anything in the rock record to be studied in the sedimentary field. Sedimentary deposition in basins generally always have some component of tectonic adjustment to make what is termed accommodation space for the filling material. Thicker sections generated over shorter periods suggest more active tectonics. Highly active tectonics result in very coarse-grained deposits, such as alluvial fans with lots of boulders, and even rock avalanche deposits. There are many depositional basin configurations that can be developed during tectonism, however, for this discussion the main concern is the relative position of said basins to the area of active rock exhumation or mountain formation. Therefore, for simplification, depositional facies of sedimentary rock formation can broadly be broken into two categories, those that are proximal or near the area of deformation, and those that are distal.

In the depositional record, direct inferences can be made about the nature of the source area providing detritus to the basin. The lithology of the clasts speaks of the type of rock formation being eroded in the source catchment basins of river systems. In some cases, the recorded cobble  compositions varying through the section may document the “unroofing” or exposure sequence of layered formations in the source catchment area. And the bed thickness and grain size progression through the stratigraphic section reveals information about energy in the system. The two main trends to be aware of are sequences where from bottom to top, they become finer or thicker grained, and the actual deposition beds grade from being thinner to thicker, or vice versa. These trends may correspond to rates of basin subsidence, or variations in the source areas for rock deformation and topographic relief development.

Another diagnostic  feature in the sedimentary record happens with the presence of intraformational or internal angular unconformities; these generally make a fanning pattern where the older units are more steeply dipping than the younger ones. These are best developed in the proximal setting or the footwall to a major thrust fault, such as has been demonstrated in the Sevier orogenic belt in Utah. In this context, the depositional units tend to be coarse-grained, and form in braided streams to alluvial fans. Mapping these internal bedding contacts requires experience, and in many cases the tracing of individual beds at the map scale to document convergence of the units. Note that parallel lines just through the concept of vanishing point in perspective drawing means that they will appear to converge. Finding these patterns therefore on tilted to folded layered rocks, which are interacting with topography to make apparent dips, takes exacting field efforts to demonstrate fanning angular unconformities or wedging shapes of the bedded units in what is termed growth sedimentation. Whenever I see a published study interpreting growth sedimentation or fanning angular unconformities without showing them on a map, or just presenting an interpreted geological cross section, I take them with a grain of salt (another sedimentary type of material!). In some cases, these are clear and easy to recognize depending on the setting. In others, where the formations have been deformed, meaning layer thicknesses changed through strain, greater caution is required. While the entire La Paz Formation basin history could use additional studies, for the most part these remain horizontal lying, escaping any compressional folding related to uplift of the adjoining Cordillera Real.

Continued erosion and headward advance of the canyon of La Paz over millions of years may eventually drain Lake Titicaca and remove much of the La Paz Formation. This thick pile of conglomerate in many respects is temporary storage by the mountain belt- as erosion continues the deposits will be eroded and relocated to the east to either layer across the flood plains or be dumped in the Amazon delta. So, enjoy Valle de Luna while it lasts.

REFERENCES

Murray, B.P., Horton, B.K., Matos, R., and Heizler, M.T., 2010, Oligocene–Miocene basin evolution in the northern Altiplano, Bolivia: Implications for evolution of the central Andean backthrust belt and high plateau: Geological Society of America Bulletin, v. 122, p. 1443-1462.

Noble, D.C., McKee, E.H., Farrar, E., and Petersen, U., 1974, Episodic Cenozoic volcanism and tectonism in the Andes of Peru: Earth and Planetary Science Letters, v. 21, p. 213-221.

Ruiz, G., V. Carlotto, Van Heiningen, P.V., and Andriessen, P.A.M. 2009, Steady-state exhumation pattern in the Central Andes, SE Peru: Geological Society, London, Special Publications v. 324, p. 307-316.

Wise, J.M., 2004, Geology of the Ayacucho intermontane basin, central Peru: unpublished Ph.D. thesis, University of Nevada, Reno, 203 p.

Wise, J.M., Noble, D.C., Spell, T.L., and Zanetti, K.A., 2008, Quechua II contraction in the Ayacucho intermontane basin: Evidence for rapid episodic Neogene deformation in the Andes of central Perú: Journal of South American Earth Sciences, v. 26, p. 383-396.

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