Sedimentology and Paleontology of the Santa Rosi ta Formation (Tilcara and Casa Colorada Members, Late Cambrian) in the Iruya region, Salta province.

Acta geológica lilloana 21(2): 2009

Abstract

edimentology and Paleontology of the Santa Rosita Formation (Tilcara and Casa Colorada Members, Late Cambrian) in the Iruya region, Salta province. A 45 m thick section of shales and subordinated sa ndstones of the Sa nta Rosita Formati on (Tilcara and Casa Colorada Members) crops out in the San Isidro river at Iruya, Salta Province, Argentina. The succession is represented by five sedimenta ry facies, which are dominated by laminated, fine-g rained sediments, as we ll as current and wave-generated s tr uctu res. These facies reflect the vertical passage of restricted tide-domined estuarine environments to openmarine settings affected by waves. Although such en vironments usually show high levels of bio- turbation, the section studied herein contains well preserved primary structures. The lack of infaunal activity is attributed to low-oxygen conditions in the substrate (Facies A and B) and a high sedimentation rate (Facies C to D). The presence of quiet, low-oxygen conditions and low-rate sedimentation processes (Facies A) facilit ated the development of a particular trilobite fauna, which was especially adapted to live in disoxic environments. The agnostoids Lotagnos-tus (Semagnostus) cf. zuninoi (Harrington and L eanza), Trilobagnostus sp., Micragnostus cf. vilonii Harrington and Leanza, and the olenids Parabolina (Neoparabolina) frequens argentina (Kayser), Angelina hyeronimi (Kayser), Parabolinella cf. coelatifrons Harrington and Lea nza, and Plicatolina scalpta Harrington and Le anza indicate a latest Cambrian a ge (latest Furongian, Parabolina frequens argentina Biozone) and perm it correlation with other localit ies of the Cordillera Oriental.