CONICET | Buscador de Institutos y Recursos Humanos

The Río Chico Group, integrated from bottom to top by the Las Violetas, Peñas Coloradas, Las Flores and Koluel Kaike formations, is an epi-pyroclastic continental succession deposited during the early Paleogene in Central Patagonia (~46° LS) that was only affected by early diagenesis during burial. While deposition was associated to regional volcanic activity from the source area, weathering of the deposits were close related to the worldwide greenhouse conditions recorded during the Paleocene-middle Eocene lapse in Patagonia (Raigemborn et al., 2009; Krause et al., 2010; Raigemborn et al., 2014). Within this climatic scenery kaolin minerals developed. In order to confirm the origin of these minerals, determine probable morphological variations that suggest different degrees of weathering, and correlate these conditions with the paleoclimate context, samples of the Río Chico Group were analyzed by X-ray diffraction (XRD), scanning electron microscopy (SEM) and X-ray energy dispersive spectroscopy (EDS). Kaolin minerals of the Río Chico Group are constituted by kaolinite, halloysite and kaolinite/smectite mixed-layers, which occur with very variable abundances (5‒95%) together with smectite (5‒95%) and less proportions of illite/smectite mixed-layer (< 30%), illite (< 26%) and chlorite (< 10%). The major relative percentages of kaolin minerals occur in edaphized tuff and tuffaceous facies of the group. Halloysite is mainly restricted to lower‒middle part of the group (top of the Las Violetas and Peñas Coloradas formations, and bottom of the Las Flores Formation). Kaolinite is concentrated at the middle‒upper part of the group (from the Las Flores Formation to the middle part of the Koluel Kaike Formation), and kaolinite/smectite mixed-layers are to the top of the group (top of the Koluel Kaike Formation). On X-ray diffractograms, kaolinite of the Río Chico Group displays narrow and sharp peaks, indicating its good crystallinity (Fig. 1A). Less narrow but well-defined peaks are also present suggesting a poorer degree of crystallization. Besides, irregular kaolinite/smectite mixed-layers were identified by the presence of broad serrated peaks at 21.91, 11.7 and 8.6 Å (Fig. 1A) following Bertolino et al. (1991). Halloysite could not be distinguished from kaolinite on the basis of their XRD patterns, but they are clearly distinguished by SEM. Using SEM imaging, it could be revealed that for example kaolin minerals are in close relation with smectite and/or silica phases such as volcanic glass, pumice fragments and opal. Kaolinite crystals have a book-like, vermiform texture and occur as regular face-to-face stacks or as irregular stacks. Pseudohexagonal plates of kaolinite are also present (Fig. 1B). Halloysite appears as spherical bundles and with a fibrous texture which arranged in a spongy fibrous-mesh texture together with kaolinite (Fig. 1C). Also, halloysite shows lath, tubular, acicular or microtubular morphology showing in some cases a morphological evolution (Fig. 1B), which finishes with vermicular or platy kaolinite. Kaolinite and halloysite were reaffirmed as kaolin minerals by EDS, as they show high peaks of Si and Al, and small peaks of Fe (Fig.1D). However, where kaolin mineral occurs as irregular plates, small peaks of Mg and K are identified, suggesting the presence of irregular kaolinite/smectite mixed-layers. The detailed micromorphological analysis and the semi-quantitatively estimated composition of the kaolin minerals of the Río Chico Group, allows us to establish that the origin of these minerals is mainly authigenic and pedogenic, and that they chiefly came derived from the transformation of smectite and volcanic materials. Such in situ reactions take place at general in tropical soils at warm and humid conditions, which are in correlation with previous climate interpretations based on biotic and non-biotic proxies for the Río Chico Group. Under these conditions, the circulation of acid interstitial waters and the chemical weathering of volcanic glass and smectite allow dissolution/precipitation events with the formation of kaolin minerals. Thus, kaolinite/smectite mixed-layers represent an intermediate component in the transformation from smectite to kaolinite. Its presence suggests relatively less intense leaching of base cations and chemical weathering, and lower precipitation than the occurrence of halloysite and kaolinite, and also, attests the presence of a short dry season in a moist tropical climate. On the other hand, the mentioned evolution in the morphology of the halloysite occurs due to an increase in weathering intensity, and the transformation from metastable halloysite to the more stable and less soluble kaolinite could take place as weathering advances. Besides, the occurrence of kaolinite suggests more oxidizing conditions than halloysite. Thus, the presence of kaolin minerals in the Río Chico Group attests strong chemical weathering conditions which are in correlation with a general warm and humid climate that took place in central Patagonia during the early Paleogene. However, the distribution of these minerals throughout the group indicated an increase in chemical weathering conditions from the lower‒middle part (halloysite + kaolinite) to the middle‒upper part (kaolinite), meanwhile towards the top of the group a relatively less intense weathering conditions can be interpreted as kaolinite is replace by kaolinite/smectite mixed-layers. Similarly, upsection a less humid climate, probably with alternation of dry and humid conditions, can be established. Consequently, kaolin minerals of the basal to middle?upper part of the Río Chico Group could record the global greenhouse conditions in Central Patagonia, meanwhile the most upper part of the group could record the first stage in the global transition from greenhouse to icehouse conditions.