Abstract
One of John Southard's enduring contributions to sedimentary geology is to have shown the power of carefully designed physical experiments in illuminating the origin of sedimentary features. Here we show how this approach can be applied at the basin scale. Physical (analog) models of sedimentary basin filling can bridge the gap between numerical- and outcrop-based models because boundary conditions are controlled and a detailed chronostratigraphic record can be generated. Furthermore, in the alluvial case it is possible to generate a multi-storied stratigraphy that is rarely encountered in outcrop studies, yet has the detailed resolution that seismic reflection profiles lack. In this study, we used the Experimental Earthscape Facility ("Jurassic Tank") at the University of Minnesota to examine the effects of differential subsidence rate and geometry on alluvial stratigraphic architecture. An intuitive link exists between tectonically-induced subsidence and the architecture of alluvial deposits: river channels should be attracted to the zone of maximum subsidence, leading to a concentration of alluvial channel fill deposits in this zone. Furthermore, it has been inferred that high subsidence rates lead to the formation of isolated channel sand bodies encased in overbank deposits, whereas low subsidence rates lead to amalgamated, sheet-like sand bodies. It is difficult, however, to demonstrate the precise connections between subsidence, the behavior of river systems, and the geometry of the basin fill. Outcrop studies shed light on the geometric consequences of differential tilt and inferred variations in subsidence rate and geometry, yet they lack precise chronostratigraphic control and exposure that are requisite if the kinematics of the system are to be understood. Numerical models incorporate kinematic and partially dynamic processes that begin to provide insight into the problem. At this time, however, these models are relatively poorly calibrated. Our experiments suggest that the relationship between subsidence and alluvial architecture is not as straightforward as outlined above. Vertical changes in facies architecture mainly accompany shoreline transgressions and regressions, which are, in turn, driven by changes in subsidence rate, sediment supply, and/or base level.