Sedimentary facies, depositional evolution of the Upper Cambrian–Lower Ordovician Santa Rosita formation in northwest Argentina

Abstract

The Upper Cambrian–Tremadocian Santa Rosita Formation, the lowermost unit of the Santa Victoria Group in northwest Argentina, records part of the infill of a retroarc basin. Sedimentologic, ichnologic, and sequence stratigraphic evidence indicates that this unit represents a complex mosaic of siliciclastic facies that reflects the vertical passage of restricted fluvial and tide-dominated estuarine environments to open-marine settings affected by waves. Twelve facies grouped in two facies associations were recognized. The base of the Santa Rosita Formation represents a sequence boundary formed from a relative sea level fall responsible for the incision of fluvio-estuarine valleys in the underlying Mesón Group, which most likely occurs close to the Middle–Late Cambrian transition. Valley filling, represented by the Tilcara Member, started with deposition of conglomerate and sandstone of the lowstand systems tract in braided fluvial systems restricted to the valley axis. The bulk of valley sedimentation, however, is recorded by estuarine deposits that accumulated during a subsequent transgression and were in turn replaced upward by transgressive marine facies (transgressive systems tract). The maximum flooding is represented by lower offshore, greenish gray shales of the Casa Colorada Member. These strata are overlain by upper offshore to offshore transition deposits that from progradational parasequence sets, which represent a highstand systems tract. Fine-grained deposits of the Casa Colorada Member are replaced upward by forced regressive shoreface sandstone of the Alfarcito Member. A major transgression, recorded at the base of the Rupasca Member, probably occurred during the Early Tremadocian and led to deposition of anoxic black shales in shelf areas. The Rupasca Member is characterized by a series of transgressive–regressive events during the Early Tremadocian and early Late Tremadocian. Deposition culminates in a regressive event recorded by the shoreface sandstone of the Humacha Member during the late Late Tremadocian.