Discriminating palaeoclimatic and volcanic influences on clay-mineral assemblages in Andean Palaeogene foreland basins from Northwestern Argentina through integrated analysis

M. DO CAMPO; DEL PAPA CECILIA; NIETO, F; F. HONGN; PETRINOVIC, I.

Congreso; 18th International Sedimentological Congress; 2010

Clay minerals are common products of earth-surface processes, such as weathering and authigenesis, controlled by the complex interaction of source-area lithology, continental morphology, depositional environments and paleoclimate. Variations in the clay-mineral assemblages in ancient continental deposits are frequently employed to reconstruct past climate changes, based upon the premise that the other variables play a secondary role. However, in active settings, volcanic events can supply highly labile volcaniclastic material, which can easily be transformed into smectite via diagenesis, which could produce a noticeable footprint in clay-mineral assemblages. Southern Central Andean foreland deposits represent an appropriate case study to ascertain the preservation of climatic signal, as tectonic uplift, volcanism and sedimentation have been interacting since Creta - ceous times. In this context we have undertaken the study of a 1400-metre-thick coarsening-upward Palaeogene succession corresponding to the Tin Tin basin (Calchaquí Valley, Argentina). We have applied an integrated approach including x-ray diffraction (XRD), SEM and TEM analyses, along with detailed sedimentary facies analysis. The aim of this survey is to compare tendencies outcoming from vertical fluctuations in clay-minerals as - semblages with evidence coming from sedimentological analysis. In the analyzed levels, 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 overbank/lacustrine domain to fluvial braided plains and an aeolian dune field suggests a gradual increase in aridity upsection. However, smectite abundance 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. Smectite depicts under SEM a rose-like texture typical of authigenic origin; furthermore, smectites with such morphology were found in close association with heulandite. Despite the absence of field evidence of volcanic influence, other indications of volcaniclastic material have been found in levels with high smectite abundances from the middle to the top of the succession. They include quartz crystals exhibiting embayments and skeletal forms, with smectite filling the voids and microcrystalline silica. Furthermore, the XRD analyses of these levels evidence well-crystallized smectite, characteristic of a volcaniclastic origin. Therefore, the increase in smectite abundance in these beds probably reflects a significant volcaniclastic contribution, which is also evidenced towards the top of the sequence by a centimetre-thick ash layer. The only smectite-rich level located near the base of Tin Tin section can be interpreted in the same way since it contains well- crystallized smectite associated with heulandite. We infer that most of the smectite in these sediments formed during early diagenesis, probably through the dissolution of labile tuffaceous material. However, SEM images also reveal the occurrence of mainly smectitic rounded pelitic lithoclasts in several levels from the middle of Tin Tin section. This implies an additional subor - dinate origin for smectite from reworked fines (including palaeosol levels) from the floodplain. This survey de - mostrates that textural and morphological analysis by SEM is essential to recognise diagenetic reactions, like smectite authigenesis, or to identify reworked fines, and thus decide if clay mineral assemblages could be inter - preted in terms of paleoclimate. 305