Tectono-sedimentary evolution of the late quaternary record on an isolated high marginal platform at the galician continental margin

Resumo

The exploration of Late Quaternary sediments from continental margins provides valuable information about regional geological evolution, especially on neotectonics and recent sedimentary dynamics. These factors control the recent sedimentary infill; therefore, their study allows inferring the tectono-sedimentary architecture attending to the geological context of the margin, as well as palaeoclimatic, palaeoenvironmental and palaeoceanographic changes throughout recent geological times. This thesis project focuses on the study of the surficial sedimentary record of an isolated and offshore high marginal platform, known as Transitional Zone morphostructural province and located at the Galician Continental Margin (NW Iberian Margin). The main aim of this research is to describe the geomorphological features of the study area and identify the Late Quaternary tectono-sedimentary processes that acted on this high marginal platform in order to infer neotectonic events, as well as the sedimentary evolution attending to climatic and oceanographic changes. These goals were achieved using a broad dataset consisting of extensive multibeam bathymetry, chirp and multi-channel seismic records, sediment cores, seabed samples, and direct observations of the seafloor. The main findings of this thesis documented the Late Quaternary lithostratigraphy of the marginal platform, suggesting that neotectonics, topography and hydrographic dynamics related to climate changes are the principal factors governing the sedimentary infill. The seismic and geomorphological data display structural features that affect the surficial record, suggesting that tectonic activity is still active until recent times in the context of the Cenozoic compressive regimes. Seismic results also show four sedimentary systems controlled by different water masses: (hemi)pelagic, bottom current-controlled (hemi)pelagic, contourite and downslope sedimentary systems, as well as a new type of contourite associated with giant depressions, named as pockmarks-related drift. Facies analysis of sediment cores reveals mainly pelagic, hemipelagic and detrital facies, controlled by the superposition of the Milankovitch cycles at the MIS-scale and Dansgaard-Oeschger cycles at the millennial-scale during the last 172 cal ka BP. The detrital facies are associated with Heinrich sediments, allowing identify the last eleven Heinrich Stadials. The palaeoclimate changes resulted in palaeoproductivity variations in response to glacial and interglacial cycles, decreasing the biogenic fraction during cold stadials. Numerous erosive features have been identified throughout the study area, suggesting the presence of intense bottom-hugging currents acting in the study area. These findings suggest that topographic factors and oceanographic reorganisations related to climate changes cause a constriction of water masses, inducing an intensification of the bottom-current activity with its concomitant erosion capacity, explaining the low sedimentation rates obtained for the last 172 cal ka BP. This fact results in coarser-than-expected grain-size facies in an offshore source-to-sink sedimentary system. Also, this scenario produces continuous oxygenation of the surficial sediments leading to non-steady-state diagenetic conditions. Therefore, the results of this research provide a comprehensive picture of how neotectonics and geomorphology influence the Late Quaternary sedimentary disposition and sedimentary dynamics. Furthermore, this thesis demonstrates the impact of climate changes and bottom-current circulation on the sedimentary evolution of this source-to-sink sedimentary system, which also produces alterations in the primary productivity and diagenetic conditions.