ALBERT

Description: A bstract :  Subsidence patterns resulting from differential sediment loading on a mobile substrate (e.g., salt) are an important process for the development of accommodation and stratigraphic architectures in intraslope minibasins. Numerous studies of minibasin systems have focused on either the tectonic processes involved in salt deformation or the stratigraphic interpretation of the sedimentary fill of minibasins. This study focuses on the coevolution of depositional and tectonic processes to investigate the response of substrate movement to minibasin sedimentation. Using a silicone polymer to model a viscous mobile substrate, a series of 2D experiments were conducted to explore the effects of variation in 1) sediment supply rate, 2) depositional style (intermittent sediment supply), and 3) the thickness of the deformable substrate on subsidence patterns and minibasin stratigraphic development. Experimental results indicate that larger initial thickness of salt substrate as well as lower sedimentation rates result in greater amounts of subsidence for a given amount of deposit. Furthermore, in the experiments with intermittent sediment supply, increasing subsidence rate was observed as sedimentation continued, while decreasing subsidence rate occurred once sedimentation ceased. These accelerations and decelerations in subsidence were attenuated as the total thickness of the minibasin deposit increased and the thickness of the remaining salt decreased. Lower sediment supply rate led to a narrower but deeper minibasin formation. The increase in overall time allowed the salt substrate to have a greater response to the overburden. In contrast, the linked depositional and tectonic processes caused higher sediment supply rate to increase the planform size of the minibasin. Based on the experimental results, a new model of autostratigraphic minibasin evolution is suggested: 1) differential loading causes initial subsidence, 2) ponded (basin infilling) architecture occurs during a period of acceleration in subsidence, and 3) perched (spilling) architecture occurs over the duration of final subsidence deceleration.

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: 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.