Incised valleys of end-member fluvial systems in the gulf of cadiz shelf: constraining the role of controlling factors

Abstract

Incised valleys are common stratigraphic features beneath modern coastal plains and continental shelves, whose incision and sedimentary fill might provide critical crucial insights into earth-surface processes, depositional histories, and their controlling factors. The formation and evolution of these valleys result from the dynamic interplay of various controlling elements, including changes in relative sea level, tectonics, sediment supply, hydrodynamic processes, pre-existing tectonic features, and antecedent geology. This study aims to enhance our understanding of the geological processes governing incised valley systems, emphasizing their significance across different temporal scales. The northern margin of the Gulf of Cadiz provides an ideal setting to examine the interaction between sediment supply and sea-level changes. Characterized by a hierarchical stratigraphic architecture, this region reflects the complex influence of late Quaternary glacio-eustatic changes and tectonic forces. The Gulf of Cadiz features locally high fluvial supplies within a moderately energetic oceanographic regime, presenting an intermediate environment between low-energy delta-forming and high-energy ravinement-forming settings. A network of cross-shelf paleovalleys off the Gilão-Almargem Estuary, situated in the eastern Algarve shelf, has been studied to discern the driving controls shaping ancient fluvial systems. The investigation focuses on primary glacio-eustatic controls, such as antecedent geology, low fluvial supply, and changing hydrodynamic regimes, along with secondary controls on valley genesis and evolution. Antecedent geology strongly influences valley formation, limiting the development of wide incised valleys and determining landward incision near a well-defined break of slope. The postglacial infilling is predominantly estuarine, suggesting a dendritic system with barriers interrupted by tidal inlets and channels in a mixed estuarine environment with low fluvial supply. On the Guadiana Shelf, the study of two N-S incised valleys systems separated by a few kilometres, revealing a compound nature with up to five repeated incision phases, suggesting a major glacio-eustatic origin likely linked to 100 ka sea-level cycles. Significant differences in internal architecture and morphology between both valleys suggests varying sediment supply conditions and hydrodynamic controls, emphasizing the role of variable fluvial supplies and antecedent morphology in influencing the distribution of valley infill facies. The postglacial sea-level rise after the Last Glacial Maximum provided ideal conditions to study the transgressive sedimentary response to sudden shelf flooding driven by different rates of sea-level rise. Analysis of four postglacial transgressive units linked to the mouth of the Guadiana River reveals a complex deltaic evolution in response to varying rates of sea-level rise and climatic changes. Delta progradation and preservation episodes were observed during periods of high sediment flow associated with significant climatic events. The investigated incised valleys and transgressive deposits exhibit complex stratigraphic architectures influenced by glacio-eustatic cycles, underlying geology, and sediment supply dynamics, resulting in distinct morphologies, infilling patterns, and responses to postglacial transgression. The stratigraphic architecture of postglacial transgressive units off the Guadiana River mouth reveals significant variability in driving factors, highlighting the intricate interplay between sea-level rise, sediment fluxes, and climatic conditions. This research significantly contributes to understanding geological and sedimentary processes in the Gulf of Cadiz, with direct applications in the oil and gas industry and insights into biogeographic and paleoclimate processes.