M. Colin Marvin's research while affiliated with Stanford University and other places

The recognition of meandering rivers in Earth's prevegetation stratigraphic record is hindered by an absence of depositional models specifically tailored to unvegetated single‐channel rivers. As a result, their abundance on the early Earth is currently unknown. Here, sedimentological studies of two modern meandering river reaches with unvegetated banks were conducted. Both reaches are located in arid basins of Nevada, USA. Stratigraphy was analysed in conjunction with satellite imagery and high‐resolution topographic data to understand how channels and floodplains record past flow conditions. Analyses of point‐bar and channel‐fill deposits showed that lateral accretion sets are not heterolithic but, rather, composed of mixed sand and sand‐sized mud aggregates (clumps of clay and silt) interpreted to have formed through flocculation. Levées are present but subdued. Notably, downstream‐migrating outer‐bank‐attached bars are prominent architectural features that may be common in unvegetated meandering rivers in arid landscapes elsewhere. The identification of such depositional elements in the stratigraphic record may enhance recognition of ancient unvegetated meandering rivers and improve reconstructions of palaeohydrology on early Earth and Mars.

Windblown dunes are common features in our solar system, forming on planetary surfaces that span wide ranges in gravity and both atmospheric and sediment properties. The patterns formed by their crests, which are readily visible from orbital images, can record information about recent changes in boundary conditions, such as shifts in wind regime or varying sediment availability. Here, we demonstrate that the density of dune interactions (where neighboring crestlines are close to each other) within a dune field is an indicator of such changes. Using orbiter-based images of 46 dune fields on Earth and Mars, we compiled a database of pattern parameters including dune spacing, crestline orientation, and interaction density. Combined with sediment fluxes derived from ERA5-Land data and a martian global circulation model, we also compiled dune turnover time scales (the time it takes for a dune to migrate one dune length) for each investigated dune field. First, we show that dune fields undergoing changes in boundary conditions display higher than expected dimensionless interaction indices. Second, dune fields with longer turnover times display a wider range in interaction indices on both Earth and Mars because they are more likely to be observed while still adjusting to recent changes in boundary conditions. Thus, a dune field’s interaction index offers a novel tool to detect and possibly quantify recent environmental change on planetary surfaces.

Ephemeral channels incise into the piedmonts (both alluvial fans and pediments) of the northeastern Sonoran Desert, USA. Located around metropolitan Phoenix, this tectonically quiescent region experienced only aggradation in endorheic structural basins throughout the Pliocene. A wave of aggradation then followed Salt and Gila river integration at the start of the Pleistocene. Aggradation of piedmont base levels continued throughout the rest of the Quaternary. This paper explores two hypotheses to explain piedmont incision despite rising base levels. The classic explanation is that incision is part of the evolution of desert mountain ranges as they decrease in size. A new alternative we propose here involves a lateral shift in base level from Pliocene endorheic basin playas to positions kilometers closer to range fronts in response to river integration. We present a thought exercise of modeling a pediment longitudinal profile as a 1D diffusive system, and we also analyze incision into alluvial fans of the Sierra Estrella range. While our 1D modeling results for pediments are consistent with both explanations for range-front incision, Sierra Estrella bajada incision is best explained by the sudden relocation of the base level to the toe of desert piedmonts.