S. Peralta

Early Gondwanan connection for the Argentine Precordillera terrane

Abstract The Precordillera of Argentina is widely accepted as an exotic terrane of Laurentian (North American) affinity. Newly acquired U/Pb ages on individual detrital zircons from Lower Cambrian and Upper Ordovician quartz sandstone beds in the Argentine Precordillera indicate a Gondwanan provenance not associated with any known part of Laurentia. Accordingly, the Precordillera terrane is likely underlain by basement rock of Gondwanan affinity. In addition, detrital zircons from the Upper Ordovician sandstone bed provide no evidence for a Mid Ordovician position against the inboard Famatina arc. These results demand critical re-evaluation of widely held assumptions regarding the paleogeography of the Argentine Precordillera.

The Early Paleozoic history of the Cuyania (greater Precordillera) terrane of western Argentina: evidence from geochronology of detrital zircons from Middle Cambrian sandstones

U-Pb geochronology of large detrital zircons populations is a powerful tool for interpreting sandstone provenance. Here, it is applied to three Middle Cambrian sandstones from the Precordillera of Argentina with the purpose of using the provenance interpretations to test paleogeographic and paleotectonic models proposed for the Cuyania or Precordillera terrane. Two samples from the La Laja Formation have distinctive detrital zircon age distributions. All zircon grains fall within unimodal populations of 1688-1200 Ma in one sample and 1559- 1316 Ma in the other. Of these grains, 23% and 65%, respectively, are within the age range of the North American magmatic gap (1610-1490 Ma), indicating a non-Laurentian provenance. A very different sample was taken from a sandstone interval in a large olistolith within the Estancia San Isidro Formation. Its zircon population is dominated by a single, prominent 615-511 Ma age cluster, which is indicative of a provenance in a Brasiliano orogenic belt. The absence of zircons with Grenvillian ages (1200 to 950 Ma) is difficult to reconcile with paleogeographic and geotectonic models in which Cuyania rifted from Laurentia in Cambrian or Ordovician time. The data are most consistent with models in which Cuyania rifted from the southern margin of West Gondwana. Given a Cambrian association with Gondwana and a post-Ordovician arrival at its present position in Gondwana, the Cuyania terrane must have migrated along the southern and western margins of Gondwana during the Ordovician Period.

GRAPTOLITES FROM THE UPPER LLANDOVERY OF THE TALACASTO AREA, CENTRAL PRECORDILLERA, SAN JUAN, ARGENTINA

Graptolites are the most important biostratigraphic tool in the study of the Silurian of the Argentine Precordillera. Since, however, the graptolites are concentrated primarily in the Lower Llandovery and Ludlow of the region, the ages of parts of some stratigraphic units are poorly known. The recovery of a small Upper Llandovery graptolite assemblage in the lower one-third of the La Chilca Formation of the Talacasto area helps in resolving this problem, and that assemblage is the focus of this study. Brachiopods, trilobites, bivalves, and ostracodes are present in the same stratigraphic intervals, but are less common and less studied and, thus, of less biostratigraphic value. Silurian sedimentary rocks are widely distributed in the Central Precordillera of San Juan Province (Fig. 1) (Baldis and Chebli, 1969). They are composed of siliciclastic marine deposits, including those crossing the Ordovician-Silurian boundary in the lowermost part of the sequence (Cuerda et al., 1983, 1988; Melendi and Volkheimer, 1982). Figure 1 —Location map and partial geology of the Central Precordillera of San Juan in the Talacasto region (after Baldis et al., 1984). The key graptolite collection locality of the Quebrada Ancha is shown by star number 2. The Los Banos and Salto Macho collection localities are shown by (smaller) stars 1 and 3, respectively The Central Precordillera includes the classical, widespread, and well-exposed Silurian deposits of the Tucunuco Group (Cuerda, 1965, 1969) containing assemblages of graptolites, brachiopods, and trilobites. Outcrops of the Tucunuco Group range geographically from the tributaries of the Jachal River in the north to the La Dehesa Range in the south, a belt 140 km long. The structure is a system of blocks tilted to the west, bounded between north-south oriented reverse faults. The Tucunuco Group includes the La Chilca Formation (up to 200 m thick; Late Asghill …

Sea level fluctuations and forced regressions in the Silurian basin in the Precordillera of Western Argentina

Abstract Silurian strata, as part of an Upper Ordovician to Lower Devonian siliciclastic succession, are widely distributed in the Central Precordillera of Western Argentina, but they are scarce in the Eastern Precordillera, and are not known with certainty in the Western Precordillera. The Silurian rocks are bounded at the base by an erosional unconformity that cuts Ordovician units of different age and are paraconformably overlain by Lower Devonian strata. In the Central Precordillera, the Silurian succession is represented by the Tucunuco Group, composed by the La Chilca Formation (Hirnantian to lower Wenlock) and the Los Espejos Formation (middle Wenlock to Lochkovian). These formations, which are separated by a paraconformity that coincides with a global regression, represent two regressive sequences and are interpreted as records of episodic subsidence related to an extensional regime that controlled basin geometry and infilling. In the Central Precordillera, in the San Juan River area, the Silurian is represented by the Tambolar Formation. From this area, evidence of deposition in extensional basins includes: (1) thickening-coarsening upwards successions of strata in each sequence, (2) occurrence of slumps in the middle and upper part of the Los Espejos Formation, (3) the geometry of infilling, and (4) marked north to south and east to west facies changes. Additional evidence of extensional tectonics includes the absence of Silurian strata in Western Precordillera, and tectonic boundaries separating the Central Precordillera from the Western Precordillera, and the Eastern Precordillera from the Pampeanas Ranges. It is apparent that the Silurian strata of Central Precordillera were not deposited in a foreland basin as generally proposed, but in extensional basins.