Combined 40Ar/39Ar and (U–Th)/He geochronological constraints on long-term landscape evolution of the Second Paraná Plateau and its ruiniform surface features, Paraná, Brazil

Abstract

Regional correlation of dated weathered land surfaces provides the necessary constraints to test long-term continental landscape evolution models, but major challenges remain in properly dating these surfaces. The geomorphological province of Second Paraná Plateau, Paraná State, Brazil, is a high elevation (ca. 800m) land surface characterized by widely distributed deep saprolites and scattered lateritic profiles (e.g., Vila Velha and Serra das Almas). Prolonged exposure to weathering and erosion has promoted the pseudo-karstic and ruiniform features that are characteristic of this landscape. In this study, 40Ar/39Ar laser incremental heating geochronology on 22 grains of supergene Mn oxyhydroxides from lateritic profiles at Vila Velha yielded results ranging from 17.2±0.7 to 9.1±0.7Ma. (U–Th)/He geochronology on 28 goethite grains from the same profile yielded results ranging from 36.4±3.6 to 1.0±0.1Ma, with an age cluster lying within the 17.2±0.7 to 7.9±0.8Ma interval. (U–Th)/He geochronology on 17 goethite grains from the Serra das Almas lateritic profile, located 20km from Vila Velha, yield results ranging from 35.1±3.5 to 14.1±1.4Ma. The combined results for the two sites reveal a common weathering history that started ca. 35Ma, suggesting that the Second Paraná Plateau results from regional fluvial incision and denudation before ~35Ma, followed by a decline in denudation rates and proportionally more intense weathering. Consistent with the laterite profile central ages, weathering was particularly intense during the Miocene (17–8Ma). Denudation intensified after the Pliocene.