Silurian of the San Juan Precordillera, western Argentina: stratigraphic framework  

Silvio H. Peralta¹, Elba D. Pöthe de Baldis², Laura I. León³ and María E. Pereyra⁴  

¹ Universidad Nacional San Juan, CONICET, Argentina. E–mail: speralta@unsj-cuim.edu.ar

² Universidad Nacional de San Juan, Argentina. E–mail: epothe@unsj-cuim.edu.ar

³ Universidad Nacional San Juan, Argentina. E–mail: lauleon@unsj-cuim.edu.ar

⁴ Universidad Nacional San Juan, Argentina. E–mail: epereyra@unsj-cuim.edu.ar

Key words: Stratigraphy. Silurian. Precordillera. San Juan. Argentina.

Silurian rocks of the Precordillera of San Juan Province, Western Argentina, belong to an almost continuous sedimentary cycle beginning in the Late Ordovician (Hirnantian) and ending in the Early-Middle? Devonian. This cycle is bounded at the base by an erosional surface related to the worldwide Hirnantian glacial event, and at the top by the Chánica tectonic phase. In the Central Precordillera, Silurian rocks are represented by the Tucunuco Group and its equivalent Tambolar Formation, and, in the Eastern Precordillera, by the Don Braulio and Rinconada formations. In the Western Precordillera, at the present time, Silurian rocks are not adequately documented, notwithstanding the reported finding of shelly fauna mentioned in the Calingasta Formation by Xicoy (1963, in Furque and Cuerda, 1979), which has been not further constrained. For this reason, the Western Precordillera is not considered in this paper.

Siliciclastic Silurian units of the San Juan Precordillera are characterized by their coarsening-thickening upward sequence paraconformable boundaries (drowning surfaces), and conformable, regressive, depositional system-tracks controlled by sea-level changes (Peralta, 1990; Astini and Piovano, 1992; Astini and Maretto, 1996). Their fossil contents, ferriferous (ironstones) and phosphate deposits, and predominance of biogenic and HCS structures; and associated bioclastic accumulations are also documented.

In this morpho-structural unit, Silurian deposits are distributed along the western flank of the Sierra Villicum, Sierra Chica de Zonda and Cerro Pedernal (Figure 1). The main section outcrops in Don Braulio Creek, at Villicum Range, where the type sections of Don Braulio and Rinconada formations have been established. Silurian rocks occur in the upper two members of the Don Braulio Formation, in which four members have been recognized by Peralta (1993). The lower two are Ashgillian in age (Hirnantian), passing transitionally upward into the bioturbated mudstones of the Ocher Member, which may include the Ordovician-Silurian boundary, but unfortunately, no diagnostic fossils have been recorded in these deposits. These are unconformably overlain by the Ferriferous Member, composed of oolitic ironstone beds containing palynomorphs (Volkheimer et al., 1981; Pöthe de Baldis, 1997), interbedded with shales yielding graptolites of Llandovery age (Peralta, 1986). This member is unconformably overlain by olistostromes of the Rinconada Formation (Amos, 1954).

The Rinconada Formation exhibits a wider distribution than the Don Braulio Formation, outcropping also on the western flank of Sierra Chica de Zonda and Cerro Pedernal, where the Don Braulio Formation disappears as a result of an erosive event. Hence, the Rinconada Formation unconformably overlies (erosive surface) limestones of the San Juan Formation (Early Ordovician). This unit is composed mainly of olistostromes including calcareous olistoliths from the San Juan Formation as large as 1 km in length (Amos, 1954). Taking into account the sedimentologic and stratigraphic framework of this unit, all the fossils included in the mélange are allochtonous, although they are mainly Silurian in age. For this reason its age has been interpreted as Upper Silurian. However, its age could be as young as Devonian, although the fossil record of this age has not been constrained adequately to date.

Figure 1. Index map showing the distribution of Silurian lithostratigraphic units from the northern part of the Río Jáchal up to the southern part of the Río San Juan, in the Eastern and Central Precordillera in San Juan Province.

From the north (Jáchal area) to the south (San Juan River area) and also to the west, Silurian deposits overlie different Ordovician units (Peralta, 1990; Astini and Maretto, 1996), showing an overall decreasing thickness and progressive facies change. From the Rio Jáchal area to south of the Talacasto area, in the La Dehesa and Los Algarrobos creeks (Figure 1), Silurian deposits are included in the Tucunuco Group (Cuerda, 1969), whose type section is placed on the western flank of the Cerro La Chilca, at the Tucunuco locality. It is divided into two units: the lower La Chilca Formation, and the upper Los Espejos Formation, both of which were defined by Cuerda (1965, 1969). From the La Dehesa Creek to San Juan River area, Silurian deposits decrease in thickness and a progressive change in lithofacies from north to south and also to the west has been recognized, and is represented by the Tambolar Formation. The basal boundaries of both the Tucunuco Group and the Tambolar Formation are represented by an erosional surface at the base of a cherty, pebbly conglomerate. At the top they are paraconformably overlain by the basal phosphate-rich fossiliferous mudstones of the Talacasto Formation (no basal Lochkovian until Emsian).

The La Chilca Formation, from 80 m thick in Jáchal area up to 2 m tick in La Dehesa area, is characterized by predominance of quartzites showing thickening-coarsening upward sequence including HCS structures (Peralta, 1990; Astini and Piovano, 1992), trace fossils of the Cruziana Icnofacies, palynomorphs, and Monograptus priodon (Bronn) indicating a Late Llandovery-Early Wenlock age (Kerlleñevich and Cuerda, 1985). However, in some places, such as Talacasto creek and La Trampa range, in the lower part, this formation is composed of bioturbated shales and siltstones, overlying the basal cherty pebbly conglomerate, bearing graptolites of the N. persculptus, P. accuminatus and A. atavus Zones, including the Ordovician-Silurian boundary (Cuerda et al., 1988). This formation is capped by a laterally continuous bioturbated phosphate and ferruginous level, indicating a drowning surface, which is conformably overlain by the Los Espejos Formation.

The Los Espejos Formation, the thickness of which vary from 450 m in the Jáchal area to 150 m in the La Dehesa area, is made up of a thickening-upward sequence. The lower part is composed mainly of bioturbated mudstones and siltstones, containing abundant phosphate and minor ferruginous concretions, including gradually upward thin-and-fine-grained sandstone beds no more than 10 cm thick, interbedded with pelites. In the middle part sandstone beds became thicker and laterally much more continuous, being traced 10’s of m. Trace fossils, para-autochtonous and allochtonous bioclastic coquinas (Sánchez et al., 1991), HCS, and synsedimentary deformation structures are frequent. In the upper part amalgamated sandstone beds reaching up to 2 m thick are prevalent, including thicker and normally graded bioclastic accumulations, HCS structures 30 to 50 cm high, and frequent synsedimentary deformation. This formation is capped by a thick fine-grained pervasively bioturbated sandstone bed.

The Tambolar Formation (Heim, 1952), 74 m thick in its type section at Tambolar Pass, includes Silurian deposits distributed in the San Juan River area, showing a clear decrease in thickness and facies change towards the west. It is characterized by a thickening-coarsening upward sequence bounded at the base by an erosional surface, and at the top is paraconformably overlain by the Talacasto Formation. This sequence includes HCS structures, trace fossils of the Cruziana Ichnofacies (Peralta and Carter, 1990), and normally graded bioclastic accumulations. In the type section, a bioturbated mudstone succession no more than 1 m thick occurs at the base, yielding palynomophs indicating Late Llandovery-Early Wenlock age which correlate with the Upper Member of the La Chilca Formation in the Talacasto section (Peralta et al., 1997). Hitherto, no graptolite fauna has been found in this unit, and its Late Wenlock-Early Llandovery to Ludlow age is constrained mainly by palynomorphs (Peralta et al., 1997) and brachiopod faunas (Benedetto et al., 1996; Benedetto and Franciosi, 1998). Towards the west, in the Pachaco area and the eastern flank of the Sierra de La Invernada, Silurian deposits are represented by reddish and greenish bioturbated mudstones, no more than 25 m thick, which have been named as the "Pachaco Facies of the Tambolar Formation" (Peralta and León, 1993) and their age is constrained by brachiopods (Benedetto and Franciosi, 1998).

Silurian deposits of the Central Precordillera have evolved in a confined environment, and their facies changes indicate shallowness to the south and deepening to the west. Strata abruptly disappear further to the west, to be only represented as allochthonous, into the Devonian deposits of Punta Negra Formation. The occurrence of the La Chilca Formation in the La Dehesa area, and the record of Late Llandovery-Early Wenlock palynomorphs at the basal part of the Tambolar Formation correlate with the Upper Member of the La Chilca Formation and fit the correlation between the Tucunuco Group and the Tambolar Formation. The thickening-coarsening upward sequence and its unconformably boundaries constitute a valuable tool in the correlation with Silurian sequences of Northwest Argentina and Bolivia.

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Received: February 15, 2003

Accepted: June 15, 2003