Quantitative analysis of geological controls on incised-valley-fill geometry and stratigraphic architecture

Incised-valley systems are common features in coastal and shelf regions. Assessments of factors possibly controlling the geometry of incised valleys commonly either focus on experimental or numerical models, or are based on consideration of individual examples. Classical facies models characterizing the internal fills of incised valleys are largely conceptual, descriptive and qualitative, and are mostly limited to individual examples. Here, novel database-driven quantitative analysis of 151 late-Quaternary incised-valley fills (IVFs) is undertaken, aiming at exploring the relative roles of possible controls in determining the geometry and stratigraphic architecture of IVFs. The resulting knowledge is then applied to further our understanding of the significance of ancient IVFs in the older geological record: a data synthesis of 18 Namurian (Serpukhovian to Bashkirian) IVFs from the United Kingdom and Ireland is performed to characterize the palaeohydrologic characteristics of the formative rivers feeding these incised valleys, and to attempt a refinement of the regional palaeogeographic reconstructions. Results indicate that various factors including relative sea-level change operate to control the geometry of IVFs, notably basin physiography, drainage-basin size, climate, substrate type and tectonics. Upstream controls (e.g., drainage area and dominant vegetation type in the catchment) appear to be more important in determining valley geometry, compared to the characteristics of the receiving basin (e.g., basin physiography, substrates and shoreline hydrodynamics), especially for passive margins. Significant variability in stratigraphic architectures of IVFs is identified that is not accounted for by classical facies and sequence-stratigraphic models. Factors other than relative sea-level fluctuations – such as continental-margin type, drainage area, IVF geometry, basin physiography and shoreline hydrodynamics – are demonstrated to be responsible for the observed variability in the internal architecture of IVFs. Facies architecture recorded in the Namurian IVF fluvial deposits indicate that their formative palaeorivers were likely perennial and characterized by relatively low discharge variability. This study challenges some paradigms embedded in sequence-stratigraphic thinking, notably the notion that the degree of exposure of the shelf dictates the magnitude of vertical incision preserved in sequence boundaries, and the view that magnitude and location of valley incision are primarily determined by the coastal-prism convexity at the highstand shoreline, if the sea-level does not drop below the shelf break. The results also have significant implications for improved understanding of source-to-sink scaling and have important applied significance in subsurface-reservoir prediction and characterization.