Geology at Nahuel Huapi National Park. Patagonia. Argentina
Edited by Austin W.
Synopsis Rocks and geology
At Nahuel Huapi National Park. Patagonia. Argentina
Extracted and condensed from: «Cordillera Neuquina» by Emilio F. González Díaz & Francisco E. Nullo.
Published in Segundo Simposio de Geología Regional Argentina 8-11 Sept. 1976, Vol. II. Chap. 25.
Academia Nacional de Ciencias de Córdoba (1980).
Cordillera Neuquina Geologic Unit. The unit corresponds to a mountain range with an average height of 2.000 m (heighest peak 3.776 m). The oldest rocks correspond to Precambrian? - Lower Paleozoic? basement that consists of metamorphics and eruptive rocks. The Upper Paleozoic? - Mesozoic? is represented by volcanic rocks, orthoquartzites and porphyrites. The Mesozoic begins with vulcanites (mainly andesites and its varieties, interspersed with tuffs) and sediments. The Jurassic, mainly marine sediments, fossiliferous, is followed by several magmatic events which continue into the Tertiary, but with sedimentary intercalations, mainly continental. The vulcanism is andesitic and basaltic. The Quaternary is characterized by glacial sediments and features, and finishes with alluvial deposits and basaltic flows.
During the Cretaceous this region is affected by a diatrophism, beginning with granitic intrusions, followed by a general uplift, which is further intensified by the Cenozoic disturbances (Andean Movements).
Precambrian and Paleozoic
Cushamen Formation (Precambrian? - Paleozoic?). Made up of ectinites (micacites, filites and methaquartzites) with occasional magmatic inclusions, which gradually give place to granitic magmatites of nodular and diffuse injection. The area has been greatly deformed (milonitization).
Huemul Formation (Upper Paleozoic - Pre-Jurassic). Formed by andesitic-quartz porphids as well as lesser quantities of acidic lava which are interspersed in sedimentites.
Found on the Huemul Peninsula and the Blest Arm of Nahuel Huapi lake.
Montes de Oca Formation (Upper Paleozoic / Permian - Pre-Jurassic). Forms the Montes de Oca mountain and believed to form the Cuerno, Cacho and Crespo mountains. It has been strongly diagenized and adopts a nearly vertical position.
Its lower strata are greenish black hornblend-biotitic volcanic rocks (lavas and porphids). The upper strata is made up of pale orthoquartzites, dark limolites, tuffites and minor pyroclastic inclusions.
Los Machis Formation (Upper Cretaceous). Gray colored granitic rocks, with predominant hornblendiferous granodiorite. Lamprofid rocks with dark or greenish color are also also found with them, with disseminated pyrite. Ventana Formation overlies this formation.
Has has been dated by Stipanic and Linares , Toubes and Spikermann  and others at 102 ± 10 to 113 M.y. (Million years) or 115 M.y., under the Tronador basalts. Between Traful and Nahuel Huapí lakes the dates range from 96 ± 5 to 109 ± 5 M.y. These confirm an important Cretaceous magmatic event coincidental with the Peruvian Phase of the Andes Cordillera creation. It makes up the main part of Catedral, Bonete, Lopez and Goye mountains as well as the other mountains of the South Western area of the Park up to Tronador volcano and the area between lakes Traful and Nahuel Huapí.
Nahuel Huapí Group: is divided into the Ventana Formation and the Ñirihuau Formation.
Ventana Formation (Eocene). Essentially volcanic with sporadic marine sediments. Its bottom two thirds are volcanic rocks, wackes and tuffites, while the top third is predominantly riodacites, dacites and basalts as well as basaltic hypabisal bodies with a thickness of 4.000 meters.
The tuffites and wackes contain marine fossils of the genus Arca, Cucullaea, Chione, Chlamys, Venus, Atrina, Panopaea, Pyrula, Struthiolaria, Turritella, Natica, Crepidula, etc., which are also common on the Patagonian Atlantic coast. Also plants -Betuliphyllum, Myrca and Scirpites genus- are found here.
Radimetric dating at Minero stream (Traful) give an Eocene age for these rocks: 45 - 43 M.y.
Ñirihuau Formation (Eocene). Part of the Nahuel Huapí Group, is formed mainly by volcanic sediments. They overlie the Ventana Formation and were deposited in a river or lake basin which existed on the eastern edge of the Patagonian Andes between 41º and 43º Lat. South. Their thickness is about 250 meters.
Made up by wackes, tuffites , as well as volcanic rocks (basalts, andesites, riodacites) and conglomerates.
The tuffites contain carbonized and silicified tree trunks as well as sandstone with ostracods and ignimbrites which lived in sweet water and/or shallow salt water (sea?).
The fossil trees are of the Nothofagus flora of Patagonian Tertiary period such as Alnus, Fagus, Zanthoxylon, Nothofagus, Myrcia, Apocynopyllum, Phyllites, etc. (The forests which now cover the region are Nothofagus trees such as Lenga, Coihue, Ñire, etc.)
Coluco Formation (Upper? - Mid Miocene). Thick grained granite, porphidic, aplitic and quartziferous granites. They lack hornblende, are biotitic with occasional pink colored pyroxene. Differing from the Cretaceous granite which has abundant hornblende, and are pale gray colour.[Ljunger 1930-32].
Feruglio  found intrusions of it on Catedral mountain, dated at 13.3 ± 4 M.y. Toubes and Spikerman  also dated Mid Cretaceous tonalite there at 113 ± 7 M.y. So there are apparently two different intrusions on Catedral, one Cretaceous, the other Tertiary.
Found mainly as granitic outcroppings along Blest Arm of Nahuel Huapí lake. These rocks were emplaced during the Quechua or Pontian Phases of the Andes (Andic) Cordillera movement.
Chenqueniyeu Formation (Upper Pliocene). Made up of basaltic and pyroclastic flows. As in the case of Miocene basalts it is difficult to identify the outflow points, but it is believed that they were fissural eruptions.
They form the main body of the extinct Tronador volcano (pre-glacial period). However Bonorino  considers them to be Miocene basalts whereas Dessanti  considers them Pleistocene. Gónzalez  has dated them as Pliocene ( 2.5 ± 1 and 3.5 ± 0.5 M.y. at Mount Mirador and 3.2 - 2.0 M.y. at Tronador.
Glacial drift (Pleistocene). There are well preserved extensive non stratified (till) deposits. These are the moraines that surround Nahuel Huapí lake. Smaller deposits are found between the Otto and Ventana mountains as well as on the upper Manso river valley.
Stratified glacial deposits are found in many mountain valleys, though they are mostly covered by more recent sediments. They correspond to rear moraines, which have been modified or removed by alluvial sands, pebbles and gravel from rivers and streams.
Holocene basalt (Post-glacial). [Greco - 1975] found them on the small pyroclastic cone of mount Volcánico.
Modern deposits. Various types of sediments, of reduced thickness and differing origins. They are either alluvial sand, clay or gravel covering lower areas, transported by streams and rivers or landslide materials.
Precambrian - Paleozoic. During this period clastic material accumulated in a geosinclinal environment. After which they were invaded and deformed by the granitic magma which forms the metamorphic basement of the region.
Mesozoic. During the Triassic, the southern area was a high altitude region, subject to erosion which accumulated volcanic sediments on the eastern edge. Later, during the Jurassic period, the area suffered a downwards movement and was invaded from the north by the Liasic sea. This was followed by restricted upwards movements. Towards the end of the Lower Cretaceous (Aptian-Albian), an important orogenic process began with the intrusion of the Los Machis Formation granitic masses (associated with the Cretaceous Laramic Phase, which raised the western area of the region, the elevation was greater in the North than in the South).
Tertiary. During the Eocene, the Incaic Phase (First Andic Movement) renewed the volcanic activity in the region with active clastic deposits (from basalts to riodacites) creating the Ventana Formation, which shows (marine sediments) evidence of a shallow sea.
This raised the Western area and lowered the South Eastern area, allowing the Patagonian sea to invade the eastern zone (Rio Chico basin), reaching close to Bariloche.
Another author Cazau  states that there was a marine intrusion from the West, that can be noted at Bariloche on mount Otto, this region does not present interspersions of lava but does have frequent banks of tuffites.
The volcanic activity continued with the creation of the volcanic-sedimentary Ñirihuau Formation complex, which shows an environment of rivers as well as shallow salt water (sea?) and low-lands.
An important diastrofic event interrupts the sedimentation during the Oligocene. It had weak compressive effects in the eastern pre-Andean area, while the deformation was more intense at the Nahuel Huapi region where the strata of the Nahuel Huapi Group were folded and dislocated. Strata which was later subject to intense erosion under humid continental climate conditions that dissected the scenery with deep river valleys.
The compression was followed by distention which gave place to basalt flows.
During the Miocene, another diastrofic event occurred (The First Phase of the Second Andic Movement) with more granite emplacements to the east and probable deformation of the Nahuel Huapi Group, this probably led to the formation of great tectonic depressions, one North of the Limay river (Collon Cura basin) and the other to the South East of Bariloche (Rio Chico basin), separated by a dorsal (Alto Limay).
Over this area the La Pava and Collon Cura (pyroclastic rocks) Formations were deposited, followed by an important uplifting of the region (the Second Phase of the Second Andic Movement) and is related to the basaltic rocks of the Chenqueniyeu Formation which covered the area. Followed during the Upper Pliocene by pedemountain erosive processes, which accumulated gravel and sand at the base of the mountains.
Between the Miocene and the Pliocene, the final (Quechua Phase of the Andic Movement) provided upwards and downwards shifts of up to 200 meters, the process ended with intermittent uplifts, basic fissural effusions and erosion.
Quaternary. The western part of the region was covered with an ice sheet during the Pleistocene. Only the highest summits protrude from the glaciers. Many glaciers advanced down the valleys towards the east. In more recent times the ice melted and retreated to the upper valleys, where they can still be found (albeit very reduced in size).
Condensed from «Segundo Simposio de Geología Regional Argentina». Conducted at the Academia Nacional de Ciencias de Córdoba September 8-11, 1976. Ed. 1980 Ac. Nac. Ciencias de Córdoba. Vol. II. Chapter 25 pages 1099 - 1149. «Cordillera Neuquina» by Emilio F. González and Francisco E. Nullo.
Important Note. Credits: We have extracted, translated, adapted and condensed -to our best knowledge- parts of the paper presented by Mr. González and Mr. Nullo at the 2nd Symposium of Regional Argentine Geology Sept. 1976. Any errors in typo, translation or scientific terms are totally involuntary. We do not take any credit for the research and investigation involved in the contents of this web page, and the full credit goes to those mentioned in «Source» above, and to Mr. González and Mr. Nullo.
This information is given in good faith with the sole purpose of providing the layman with interesting information on Argentina's Patagonian Nahuel Huapi region's geology and nothing herein is to be construed as infringing any copyright from its legal owners.
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