ALBERT

Description: A bstract :  Shorelines move in response to the balance of geodynamic processes acting on sedimentary basins; thus the stratigraphic record of shoreline migration is an important tool for reconstructing climate, tectonic, and eustatic histories from ancient deposits. Here we test whether subsidence geometry influences shoreline migration in response to sea-level change by comparing two physical experiments conducted in the Experimental EarthScape (XES) basin. The experiments had similar sediment supply, subsidence rate, and sinusoidal sea-level cycles, but one experiment had a fore-tilted subsidence profile, where subsidence rates increased with distance from the sediment source (similar to a passive-margin setting) and the other had a back-tilted subsidence profile, where maximum subsidence was close to the sediment source with subsidence rates decreasing downstream. In the recent back-tilted experiment, decreasing subsidence rates downstream resulted in a tendency for shoreline regressions to self-amplify during base-level fall, whereas increasing subsidence rates upstream caused a rapid shoreline retreat during base-level rise, causing amplified shoreline fluctuations during sea-level cycles compared to the previous fore-tilted experiment. These results indicate that the spatial pattern of subsidence in a basin has a significant effect on shoreline migration in response to eustatic cycles. Shorelines in back-tilted basins are substantially more sensitive to changes in relative sea level than comparable coastlines in passive-margin settings, all else being equal.

Description: :— Net deposition is necessarily accompanied by overall loss of sediment mass from the transport system; on long time scales the deposition compensates for subsidence and creates the stratigraphic record. Although its spatial pattern changes and it can be locally stopped or reversed, depositional mass loss is a fundamental driving influence on the morphology and behavior of depositional systems. Mass extraction to deposition leads directly to facies changes as sediment flux declines; indirectly it usually leads to down-channel sediment fining as deposition preferentially removes coarser particles. Laboratory experiments illustrate the effects of systematic mass extraction on fluvial channel stacking and deposit grain size, and show how a mass-balance framework allows consistent comparison of facies despite major shifts in depocenter location. Mass-balance analysis of experimental fluvial and turbidite systems, and a turbidite mini-basin from the Gulf of Mexico, show a change from channel-dominated to lobe-dominated deposits at about 80% total mass extraction. Experimental and field studies also show a close connection between rate of mass loss and rate of downstream grain-size fining, as predicted by a mechanistic theory based on mass balance. These are first steps towards a framework for quantitative basin analysis based on mass extraction. A mass-balance framework allows for consistent, quantitative comparison across basins of varying scale and shape. Mass-balance frameworks can account for overall and/or generic mass-balance effects, and help separate them from local effects. One way to use such generalized models effectively is to view the results as reference cases of down-transport change resulting from the basic interplay of spatial mass-extraction pattern and sediment supply. Where such reference cases do not provide detailed predictions of specific field cases, they can still provide a baseline to separate local, case-specific features from generic system behavior. The approach is analogous to the way the geoid serves as a reference case against which to measure gravity anomalies.

Description: A bstract :  Models of stratigraphic architecture make testable predictions regarding the subsurface spatial density and connectivity of channel sandstone bodies in subsiding basins. Here we test one of these predictions: that lateral gradients of subsidence rate in alluvial basins tend to draw channels to local subsidence maxima and thus increase the subsurface stacking density of channel sand bodies in the vicinity of subsidence maxima. Here we define channel steering as any change in channel course due to lateral gradients in subsidence, focusing on the attraction of channels to regions of high subsidence. We examine the hypothesis that steering is controlled by the tilting ratio : the ratio of the rate of lateral tilting to that of lateral channel mobility, with steering effects expected to increase as the tilting ratio increases. We present measurements of channel steering from experiments in which we varied the tilting ratio over four stages. The experiments used a relay-ramp geometry with laterally variable uplift and subsidence. Initially, with a small value of the tilting ratio, we did not detect noticeable channel steering. Through reductions in input sediment discharge ( Q s ) and water discharge ( Q w ) we decreased channel mobility in later stages while keeping the subsidence regime the same. This resulted in systematic increases in the tilting ratio and in observable steering towards regions of high subsidence. Interestingly, the increase in tilting rate relative to channel mobility also resulted in a preference for channel occupation over uplift regions as channels were trapped by incision into the rising surface. We also develop theory to predict when the strength and duration of pulsed tilting events are sufficient to steer channels. As with the theory for steady subsidence, the new theory suggests that pulsed events must be strong enough and long-lived enough to produce comparable cross-basin to down-basin transport slopes. An experimental stage with pulsed tectonics supports this theory. Finally, we document autogenic shoreline transgressions in the relay zone during deformation. These transgressions produce downstream to upstream facies translation of the sand–coal boundary in the preserved stratigraphy and illustrate a mechanism by which transgressions can develop without external cause.

Description: A BSTRACT :  Flow expansions are a fundamental mechanism of sediment deposition. Here we define flow expansions as self-formed alluvial expanding deposits developing over a larger experimental delta that 1) are characterized by unchannelized flow, comprising a single sheet flow that covers the entire surface of the expansion, and (2) do not interact laterally with still fluid and thus are not related to jets. The flow-spreading mechanism is transverse topographic curvature rather than mixing with ambient fluid; hence we term these expansions "topographic expansions" to distinguish them from jet-related flow expansions. In topographic expansions, transverse bed convexity drives lateral flow, allowing topographic expansions to maintain much higher, and more variable, opening angles than those of jet-related expansions. The characteristic depositional body produced by the expansions is a relatively thin, tabular sand body with a flat base and slightly convex top, comparable to some "sheet flood" deposits in the stratigraphic record. Most of the deposition takes place as the topographic expansion develops, followed by a longer interval of stable sediment bypass, where all measured flow expansions show similar geometries. This stable-bypass phase is typically terminated by the flow abandoning the deposit or by channelization of the deposit itself, after which the cycle can begin again. One of the research questions raised by this study is the mechanism by which topographic expansions are able to maintain themselves in an unchannelized condition at flow aspect ratios that should be unstable according to current theories of bar and channel instability.