Evidence of cyclic climatic changes recorded in clay mineral assemblages from a continental Paleocene-Eocene sequence, northwestern Argentina

Abstract

The continental Paleocene-Eocene sequence investigated in this study belongs to the Salta Group, deposited in an intracontinental rift, the Salta Basin (NW Argentina), that evolved from the lower Cretaceous to the middle Paleogene, and is subdivided into the Pirgua, the Balbuena and the Santa Barbara Subgroups. The Maíz Gordo Formation (200 m thick) is the middle unit of the Santa Bárbara Subgroup, deposited during late post-rift sedimentation. We studied the mineralogy of fine-grained horizons of this formation by X-ray diffraction and Scanning Electron Microscopy (SEM) in order to examine the connection between vertical changes in clay mineralogy in alluvial sediments and paleosols, and global paleoclimatic changes registered during the Paleogene.

Clay Minerals at the Paleocene–Eocene Thermal Maximum: Interpretations, Limits, and Perspectives

“…By contrast, some possibly critical or significant features are evaluated in more detail, in order to interpret clay minerals within a specific context of the PETM. The mineralogical association described in the papers is considered by the authors devoid of significant post-depositional transformations, although a few works address the issue explicitly with reference to possible mineralogical changes by diagenesis [19,22,26,31,33,[37][38][39][40].…”

“…A further consideration concerns minerals formed in the soil environment, which are mainly fine grained, so the possible climatic signal transferred from soils to sediments can be studied in such particular fraction (see for example [8,[48][49][50]). Many papers report the complete mineralogical composition of the clay fraction (several works, see Table S1), often with the additional aid of selected mineral ratios, which are significant for provenance or climatic interpretation [22,24,32,[37][38][39]51,52]. A few works adopt a different approach, focusing on the bulk materials [25,26,53,54], or using the bulk sediment as well as the clay fraction, applying similar interpretation scheme [55,56].…”

“…Unfortunately, the discontinuity and difficulty in dating paleosols reveal only a partial or fragmented picture. Although several studies [37,68,69] are concordant in recognizing clay minerals as viable climate proxies, there remain two main critical features: the possible detrital origin of clays [37,69] and the short time needed for kaolinite formation under extreme climatic and physical-chemical conditions during the PETM (such as soil pH) [68]. These two issues remain unresolved, or the final interpretation needs the additional support of other kinds of data, such as the textural characteristics observed under the scanning electron microscope + energy dispersive system.…”

“…It is worth mentioning that other physical-chemical parameters or non-clay minerals have been used in a few studies as proxies for reconstructing the paleoclimatic conditions or paleoenvironment of clay formation (see Table S1). A few studies consider other clay mineral features independently of their concentrations, such as clay mineral textures [37], crystallographic ordering [22,26,53,55], or stable (O, H) [62] and radiogenic (Os) [53] isotope compositions of clays. The use of chemical indexes of the weathering intensity is implemented [38,52,53,68], whereas biochemical markers are restricted to relatively recent papers only [11,38,54,62].…”

“…The use of chemical indexes of the weathering intensity is implemented [38,52,53,68], whereas biochemical markers are restricted to relatively recent papers only [11,38,54,62]. The support of these tools often agrees with a climate-oriented interpretation of the clay mineral assemblage [26,37,38,52,53,68], but this is not conclusive, as different interpretations can be found [22,26,54,62].…”