Geology/Quebrada de Juan Pobre

The Most Relevant Geological Settings of Quebrada de Juan Pobre, Sierra Chica de Zonda (Province of San Juan)
The Quebrada de Juan Pobre (Fig. 1) is located  at  the  Sierra  Chica de Zonda  (Rivadavia Department, Province of San  Juan ,Argentina). The whole area exhibits a succession of carbonatic rocks whose age ranges from the Lower Cambrian to the Ordovician. Those rocks are in contact with Tertiary sequences due to a fault which is located at the western slope of the Sierra Chica de Zonda. The wide interval of time involves approximately 540 million years, since life started complexifying.

The existence of a unique geological and palaeontological record for South America in the Sierra Chica de Zonda and surroundings has encouraged many prestigious research works, some of which are mentioned in this contribution. The features described for this area  make it a preferential place to learn  about geology and  palaeontology for natural sciences students.

Students of the Historical Geology Course of the Universidad Nacional de La Plata, having a class before field work with Dr. Carlos Cingolani at the Camping Municipal Rivadavia (photo belongs to the author).

Introduction
The geological  history  of Precordillera,  where the  Quebrada de  Juan  Pobre is  located, is the  result of a  broad and  complex geotectonic, sedimentary, palaeontological and paleoclimatic  event  succession, analyzed in the last  decades by the current  paradigm of  Earth  Sciences known as  Global  Tectonic. There are a couple of theories which try to understand the geological history of Precordillera, and it is important to point out that this work offers just a brief explanation of one of them. The Geological History of Precordillera: About 540 million years from the present the Iapetus Ocean was formed and in the same process a paleocontinent known as Laurentia (which included the North American territory) was separating from the Gondwana paleocontinent (which included the Argentinean territory). In this continental breakup process a passive margin was originated in the eastern border of the United States of America (like the current east American coast). After this phase, during Cambrian times, extension occurred within the Laurentic crust (which is similar to what is taking place in the Rift Valley, Africa). That process of crustal extension generated the  Ouachita Rift as well as its  related oceanic crust. As a result of these phenomena a basin was created and then sedimentary rocks were formed. Those rocks are found in the Precordillera Argentina and its counterpart Laurentia. In relation to the rifting process, a piece of Laurentian crust (known as Precordillera Terrane or Cuyania Terrane) split from its homonymous paleocontinent and started its drifting towards Gondwana. Drifting continued  during the rest of the Cambrian  System and part of the  Ordovician  System,  lastly, during the Middle  Ordovician the Precordillera Terrane collided against and amalgamated to the Gondwana west margin.

Figure 2: Evolution model for the Cuyania Terrane which occurred during Upper Neoproterozoic and Lower Paleozoic (Naipauer et al., 2010).

Since those times the area has been experimenting several deformative and erosional processes  including the compressive neotectonics of the Andean Orogeny, responsible for the widely known seismological activity in this broken foreland basin (Fig. 3).

Figure 3: Cross section showing the seismicity through 31° S for the 1973 to 2010 period according to NEIC (Badi, 2010).

The Three Sectors of Precordillera
There are three areas which have  been recognized in the Precordillera  Argentina exhibiting a distinctive  lithological record and structural style. Researchers (Baldi and Chebli, 1969, Ortiz and Zambrano, 1981) divided this geological province in Eastern Precordillera, Central Precordillera and Western Precordillera. The Quebrada de Juan Pobre is included in the Eastern Precordillera (Fig. 4).

Figure 4: Geological provinces of the central – western area of Argentina (Keller, 1999).

Concerning the outcrop of Cambrian rocks, Borrello (1969) mentioned two typical areas for the Province of San Juan: Sierra Chica de Zonda and the Sierra de Villicúm. Both of them have a preferential eastward slope and a westward vergency (Fig. 5), which are first-order features of the Central Precordillera.

Figure 5: Cross section of rocks exposures in front of the Camping Municipal Rivadavia (Bordonaro, 1980).

Sierra Chica de Zonda, Eastern Precordillera
A basement which is about 1100 to 1200 million years old –like its counterpart in Laurentia- underlying the Cambrian rocks is inferred. The Cambrian sedimentary sequence is mainly composed by  limestones, with  subordinates shales and sandstones in the lower  section. These units bear an  abundant fauna of  trilobites, brachiopods  and molluscs which were inhabitants  of the marine carbonatic platform during  the Lower Cambrian to the Upper  Ordovician Systems. The sedimentary units of Sierra Chica de Zonda are known as Marquesado Group, composed of two formations: La Laja Formation, divided into five members which are from base to top: El Estero quarcites and shales, Soldano limestones and marls, Bernardino Rivadavia black limestones, Juan Pobre oolitic limestones (Baldis and Bordonaro, 1984), and Las Torres black limestones and argillaceous limestones (Gómez and Astini, 2006). Zonda Formation, a dolomitic unit which passes transitionally through the Cambrian – Ordovician boundary. The Marquesado Group exhibits its most thick outcrops to the north of Sierra Chica de Zonda where the oldest sequences are exposed as a result of a failure (Fig. 6) resulting from the Andean neotectonic. The base of the Marquesado Group is not exposed because it is truncated by the previously mentioned fault.

Figure 6: Geologic map of the Sierra Chica de Zonda and surroundings – Zonda fault.

Appropriate geological mapping works were made in different sectors at the Sierra Chica de Zonda, then an integrated geological section for the Cambrian sedimentary inner platform rocks could be confected (Fig. 7 and 8).

Figure 7: Stratigraphic chart. In de column “Plataforma interna” are mentioned the members and formations belonging to the Sierra Chica de Zonda.

Figure 8: Outer, middle and inner Precordillera Argentina platform (Bordonaro, 2003).

Cambrian and Lower Ordovician Fossil Record of the Precordillera
The Precordillera Terrane has peculiar features for South America which are related with its  lithology as well as with the Cambrian to Lower Ordovician  palaeontological content. Such an indistinguishable fossil record gives strong evidence favouring the idea of a Lower Paleozoic connection between the Cuyania Terrane and Laurentia. It is important to point out the discovery of individuals belonging to the Olenellus Fauna (tipicaly  Laurentic forms) carried out by Dr. Borrello (1962) at the Sierra Chica de Zonda, where major a unique evidence was added to the South American geology and palaeontology (Figs. 9 and 10).

Figure 9: On the left the Olenellus found by Dr. Borrelo, on the right an illustration of a whole individual.

Addenda
Trilobites and brachiopods were proliferous  marine  animals which lived in the  Precordillera Terrane, therefore they are commonest  fossil individuals in the area. Trilobites are extinct animals named after their trilobular external body pattern. They lived at the Paleozoic Era from the beginning of Cambrian (540 million years), then they had a climax in the Ordovician System, and finally all of them extinguished in the Permo – Triassic boundary (250 million years). Trilobites have a copious fossil record and have been deeply studied so that they are probably the best known fossil animals. About brachiopods (from Greek brakhý: "short" and podós: "feet”), they comprise more than 2000 fossil and 300 living species of benthonic animals which have an upper and lower valve hinged at the rear end, they live attached to hard surfaces by means of a pedicle or burrowing into soft substrates where they dig using their valves while they construct galleries.