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.