Argentine Precordillera: Sedimentary and Plate Tectonic History of a Laurentian Crustal Fragment in South America

Abstract

The Argentine Precordillera (Precordillera) is part of a larger terrane which is exotic to the western margin of Gondwana. This larger terrane is characterized by a Grenvillian-type basement and the presence of Laurentian carbonate-platform rocks of Cambrian-Ordovician age. The original position of the terrane is interpreted to be the Ouachita embayment of southeastern Laurentia. Terminal Neoproterozoic continental breakup and the formation of the Appalachian-Ouachita margin during the late Precambrian-Early Cambrian established a passive margin to the present east of the Precordillera. This passive margin is the original continental margin of Laurentia, which was located outboard of the Precordillera. Continuous crustal extension during the Cambrian along the Ouachita margin created a marginal plateau that accommodated the Argentine Precordillera carbonate platform. This marginal plateau to Laurentia was separated from mainland Laurentia by a deep graben system, today represented by the offshore Ouachita facies of the southeastern United States. The Cambrian through earliest Middle Ordovician mark the climax of the Laurentian episode because the Argentine Precordillera sediments and fauna are indistinguishible from Laurentia. In the carbonate platform succession of the Argentine Precordillera, the La Laja Formation marks the transition from rift-related redbeds to carbonate sedimentation. The La Laja Formation was deposited on a platform seaward of the nearshore siliciclastic trap but inboard of a (only partly exposed) carbonate belt which might have marked the platform margin. Several third-order cycles developed; differential rates of sea-level rise were the driving forces for cycle formation. Near the base of the La Laja Formation, a sandy interval is separated from the older rocks by a presumed hiatus. This hiatus might represent the Hawke Bay regression event of the eastern margin of Laurentia. Upper Cambrian limestones and dolostones of the Zonda and La Flecha Formations record the evolution of vast peritidal flats. The spatial and biostratigraphical relations between both formations are not clear and await further research. At present, it seems that the lower, more calcareous facies of the La Flecha Formation in the Guandacol subbasin is coeval to the dolomite facies of the Zonda Formation in the San Juan subbasin. The last important terrigenous input is noticed in Marjuman sediments of the La Flecha Formation, which marks a major shift in coastal onlap toward the hinterland. The Tremadocian La Silla Formation reflects a somewhat deeper environment, although open-marine rocks are absent. However, extensive tidal flats are also absent. The rocks are mud-dominated limestones and dolostones with locally abundant oolites. Small-scale cycles are locally present. The general arrangement of facies is best explained by a tidal-flat island model. The La Silla Formation shows the most uniform facies development of all successions of the carbonate platform in the Argentine Precordillera. The very uniform facies arrangement can be explained either by a rimmed-shelf model or the model of a carbonate ramp. The San Juan Formation was mainly deposited during the Arenig and consists of open-marine limestones. Two important reef accumulations are present near the base and near the top of the formation. The San Juan Formation represents sediments of a carbonate ramp including inner ramp, mid-ramp, and deep-ramp environments.