Palaeoenvironmental changes in the Lower Cretaceous Coyhaique Group, southern Chile (Aysén-Río Mayo Basin): A multi-stratigraphical approach

This thesis presents the most detailed historical, stratigraphical, and paleoenvironmental analysis of the Aysén-Río Mayo Basin, and its sedimentary fill, i.e., the Coyhaique Group. The Aysén-Río Mayo Basin was a marginal basin developed in southern South America (currently Chile and Argentina) between the Late Jurassic-Early Cretaceous (Tithonian-Aptian). The study is based on the analysis of lithofacies, microfacies and fossil associations from thirty-six stratigraphic logs described in the Aysén Region of southern Chile (45°S). In the study area, the Coyhaique Group consists of a transgressive-regressive succession settled between the Valanginian-Aptian. Facies models of these rocks reveal mixed calcareous-volcaniclastic deposits, settled in volcanic apron- and carbonate ramp settings during the early marine transgression (Toqui Formation; Valanginian). These mixed deposits were covered by carbonaceous mudstone (“black shales”) during the enhanced marine transgression (Katterfeld Formation; Valanginian-Hauterivian). Mudstone represents oxygen-poor shelf to slope marine settings; the onset of these lithologies may have been coeval to a humid global interval in the Valanginian, i.e., principally during the Weissert Event. A mid-Hauterivian high-productivity event in the basin is likely associated with coastal upwelling; it is here reported for the first time. Fine-grained deposits were gradually covered by heterolithic and sand-dominated sediments, which represent the progradation of prodelta and tidal-influenced delta systems during the regression (Apeleg Formation; Hauterivian-Aptian). The geotectonic evolution of the Aysén-Río Mayo Basin was influenced by the break-up of the Gondwana supercontinent, and here to the onset of subduction of the western proto-South American margin and the formation of back-arc and intraplate extensional basins. Rifting-linked rhyolitic volcanism was widespread, partly superimposed to arc activity. For the period of basin development (Tithonian-Aptian) cool time-spans alternated with warm intervals, thus representing arid-humid cycles. The Aysén-Río Mayo Basin shares a similar depositional evolution with the Rocas Verdes Basin to the south. The basin was separated from the continental Cañadón Asfalto and Golfo San Jorge basins by tectonic highs.