Abstract The Neogene Ica Formation is a major sedimentary deposit in central Amazonia (Brazil) that is critical to the understanding and modeling of the evolution of the Amazon Basin. However, its chronostratigraphy is poorly defined, mostly because it is not fossiliferous and it does not contain volcanic ash levels. This study is the first attempt at the absolute dating of the sediments and overlying soils of the Ica Formation by electron paramagnetic resonance (EPR) of radiation-induced defects in kaolinite (RIDs). Four profiles of unconsolidated sediments overlaid by lateritic soils from the Barcelos region (Amazonia, Brazil) were studied. The concentration of RIDs was assessed from EPR spectra through a simplified procedure on a selected set of samples, excluding those showing too strong of interference from the signal of trace vanadyl complexes. It is shown that in such sedimentary material, both the microscopic distribution of thorium (together with uranium) and the origin of kaolinite, either detrital or authigenic, are critical for kaolinite dating. Considering an argued authigenic origin and two scenarios of Th distribution, it appears that most of the present chronologic data support the stratigraphic correlation with the Madre de Dios sedimentary Formation in Peru previously proposed in the literature for the Ica Formation (9.5–2.5 Ma). This study also shows a rejuvenation process of kaolinite both in the uppermost soil and at depth of the profiles. This rejuvenation occurred during paleoclimatic periods already revealed in other weathering covers from the Amazon Basin and possibly resulted from downward weathering in the topsoil and water table or lateral weathering at depth.

中文翻译：

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从新近纪 Içá 地层和上覆红土中测定高岭石年代，巴西亚马逊河中部：地层相关性的约束

摘要 新近纪 Ica 组是亚马逊河中部（巴西）的一个主要沉积矿床，对于理解和模拟亚马逊盆地的演化至关重要。然而，它的年代地层定义不明确，主要是因为它没有化石成分，也不包含火山灰层。这项研究是首次尝试通过电子顺磁共振 (EPR) 对高岭石 (RID) 中的辐射诱导缺陷进行 Ica 组沉积物和上覆土壤的绝对年代测定。对 Barcelos 地区（巴西亚马逊地区）的红土土壤覆盖的松散沉积物的四个剖面进行了研究。RID 的浓度是通过 EPR 光谱通过简化程序对选定的一组样品进行评估的，排除那些对痕量氧钒配合物信号干扰太强的。结果表明，在这种沉积材料中，钍（连同铀）的微观分布和高岭石的来源，无论是碎屑还是自生，对于高岭石测年都至关重要。考虑到有争议的自生起源和 Th 分布的两种情况，目前大多数年代学数据似乎支持与秘鲁 Madre de Dios 沉积地层的地层相关性，该地层相关性先前在文献中提出的 Ica 组（9.5-2.5 Ma）。该研究还显示了高岭石在最上层土壤和剖面深处的再生过程。