ALBERT

Description: Incised valleys are critical stratigraphic features for unraveling long-term Earth-surface processes and depositional history, and commonly exhibit stratigraphic attributes that make them desirable fluid reservoirs. While there is much descriptive documentation on architecture of incised-valley fills and paleoenvironmental regimes, relatively little work has focused on quantitative modeling of the dynamics of incised-valley evolution. Here we use well-constrained observations of incised valleys in experiments and the field to explore controls on valley geometry and develop a simple valley model to quantify the primary dynamics of incised-valley evolution. We document a strong tendency for incised valleys to widen downstream independent of the details of the relative base-level curve or initial surface profile, due primarily to the effects of increased sediment flux. This is in general agreement with measurements of experimental and natural incised valleys, though the degree of widening is less in natural systems due to greater sidewall resistance and smaller water discharge (relative to valley size). A first-order model of incised-valley evolution highlights three important trends: valley aggradation and widening; valley incision and widening; and valley incision and narrowing. The model reproduces the primary geometric responses to valley formation as measured from experiments: valley width and depth both increase in response to increases in (1) local relative base-level fall and (2) the initial offshore water depth. Finally, we generalize the first-order controls on valley geometry via dimensional analysis to show that long-term valley narrowing is not readily produced from relative base-level fall alone.

Description: The distinction between depositional belts associated with the basin axis and flanking piedmont streams is a fundamental attribute of the stratigraphic architecture of intracontinental rift basins. Spatiotemporal distributions of these lithofacies associations are sensitive to a combination of factors, including basin geometry, subsidence rate, and sediment discharge; however, most studies have focused on one or two controls and one depositional component (axial or tributary) of the basin-fill succession. A new perspective on how these depositional belts develop under simple but precisely controlled boundary conditions of steady subsidence, sediment flux, and water discharge is achieved through the creation of an experimental stratigraphic succession. The Experimental EarthScape run in 2006 (XES06) focused on the geomorphic evolution of sedimentary successions within an asymmetrically subsiding basin, analogous to a simple half graben, containing four interacting supply points of sediment and water. Under the imposed conditions, the experimental system self-organized into an axial stream flanked by transverse fans. Imposition of various combinations of longitudinal and lateral sediment flux showed that the locations and widths of the axial and transverse systems were strongly controlled by relative sediment fluxes (“flux steering”), and less influenced by the location of the subsidence maximum and subsidence rate. The axial drainage was dominated by transversely sourced sediment through toe cutting of the transverse fans, except during the highest axial-sediment discharges. Footwall fans persisted even under conditions of very large axial-sediment discharge, aided by topographic inheritance of the steeper transverse depositional slopes.

Description: The stratigraphic architecture of intracontinental rift basins is defined by a dynamic relationship between depositional processes associated with the basin floor and flanking tributary streams. The resulting depositional belts are sensitive to a variety of factors, including basin geometry, subsidence rate, and sediment discharge. The Experimental EarthScape run in 2006 (XES06) examined the development of fluvial morphology and alluvial architecture as a function of subsidence and sediment flux in an experimental basin based on the form of a simple half graben. Sediments from tributary drainages were introduced into the axial stream through toe cutting and realignment of transverse drainage courses to parallel the prevailing axial-flow direction. Transverse sediment contributions to the axial stream were almost equally apportioned over a wide range of sediment discharges tested in the experiments. Sediment tracers showed a larger contribution of footwall-derived sediment into the axial belt, probably due to more frequent and aggressive toe cutting by axial streams. Changes in the axial–transverse deposit boundary to external forcing (by subsidence and sediment discharge), and relatively rapid intrastage stabilization of the depositional belts, resembles the large-scale self-organization observed in moving boundaries that define the morphology of fluviodeltaic systems. Basin sedimentation was matched to subsidence in order to maintain a constant base level, which made the location and width of the axial belt sensitive to the relative sediment fluxes from the transverse systems, rather than the axis of maximum subsidence. The asymmetrical subsidence pattern and the transverse-fan morphology influenced the preservation of sedimentary sequences. Stage-bounding stratigraphic lacunae were well preserved in the hanging-wall succession, providing a reliable record of basin development.