Integrated analysis for constraining palaeoclimatic and volcanic influences on clay-mineral assemblages in orogenic basins (Palaeogene Andean foreland, Northwestern Argentina)

Abstract

Variations in clay-mineral assemblages in ancient continental deposits are frequently used to reconstruct past climate changes. In active settings, volcanic events can supply highly labile volcaniclastic material, which can easily be transformed into smectite via diagenesis, which can produce a noticeable footprint in clay-mineral assemblages. Southern Central Andean foreland deposits are appropriate case studies to ascertain whether the climatic signal was preserved in the clay assemblages of their fine-grained sediments as tectonic uplift, volcanism, and sedimentation have been interacting since the Cretaceous. We have studied a 1400-m-thick coarsening-upward Palaeogene succession of the Tin Tin basin (northern Calchaquí Valley, Argentina), applying X-ray diffraction (XRD), electron microscopy, and detailed sedimentary facies analysis with the aim of comparing tendencies in the vertical fluctuations of clay minerals with evidence from sedimentological facies.Illite-muscovite plus smectite account for 78% to 100% of the clay minerals in the fine fraction, with kaolinite and chlorite in subordinate amounts. The vertical variation of sedimentary settings from an overbank/lacustrine domain to fluvial braided plains and an aeolian dune field suggests a gradual increase in aridity upsection. However, smectite abundances do not show a gradual decreasing trend compatible with progressively lower hydrolyzing conditions; their relative abundances vary widely throughout the section, depicting pulse-like, abrupt fluctuations. Despite the absence of field evidence for volcanic influence, several indications of volcanic and volcaniclastic material have been found under scanning electron microscopy (SEM) in levels with high smectite abundances from the middle to the top of the succession. They include quartz crystals showing embayments and skeletal forms, with smectite filling the voids, microcrystalline silica, as well as heulandite crystals in close association with authigenic smectite. The XRD analyses of these levels evidence well-crystallized smectite, which is characteristic of a volcaniclastic origin. Therefore, the increase in smectite abundance in these beds reflects a significant volcaniclastic contribution, which is also evidenced by a centimetre-thick ash layer topward in the sequence. The only smectite-rich level near the base of the Tin Tin section also contains well-crystallized smectite associated with heulandite, thus probably evidencing volcaniclastic input. We infer that most of the smectite in these sediments formed during early diagenesis, probably through the dissolution of labile tuffaceous material. Textural and morphological analysis by SEM is essential to determine whether clay-mineral assemblages could be interpreted in terms of palaeoclimate.