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 (Qs) and water discharge (Qw) 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.
Bibliographical noteFunding Information:
Support for our research was provided by the St. Anthony Falls Laboratory Industrial Consortium (BHP Billiton, Chevron, ConocoPhilips, ExxonMobil, Japan National Oil Company, and Shell) and by the Science and Technology Center Program of the National Science Foundation via the National Center for Earth-Surface Dynamics under agreement EAR-0120914.
We thank Dick Christopher, Jim Mullin, and Chris Ellis for help designing and conducting experiments discussed in this manuscript. Further we thank David Mohrig for fruitful discussions that aided the crafting of this manuscript. We wish to thank our associate editor Peter Burgess and an anonymous reviewer for constructive reviews. Support for our research was provided by the St. Anthony Falls Laboratory Industrial Consortium (BHP Billiton, Chevron, ConocoPhilips, ExxonMobil, Japan National Oil Company, and Shell) and by the Science and Technology Center Program of the National Science Foundation via the National Center for Earth-Surface Dynamics under agreement EAR-0120914. Additionally we thank Penny Patterson (ExxonMobil) and Sanjeev Gupta (Imperial College London) for advice in designing the experiment. A link to the supplemental movie is available from JSR’s Data Archive: http://sepm.org/pages.aspx?pageid5229.