SPATIAL DISTRIBUTION AND PLANFORM CHARACTERISTICS OF CLAY DUNES (LOMAS) IN THE RIO GRANDE DELTA ACROSS TEXAS, USA AND TAMAULIPAS, MEXICO

Clay dunes are aeolian landforms often found in arid and semi-arid lake basins and lagoon flats, containing a high amount of clay (15–20%) relative to typical desert dunes. These water bodies’ intermittent drying and wetting cycles eventually expose fine sediments transported by wind to form crescent-shaped elevated (1-8 m) landforms near tidal flats. Clay dunes have been documented globally in Australia, Africa, and China, and are especially abundant in the windy and arid coast of the Rio Grande Delta (RGD), spinning southernmost Texas, USA, and Northern Tamaulipas, Mexico. Locally known as Lomas and first identified in Texas in 1909, these clay dunes are adjacent to hypersaline wind tidal flats (Esteros) and bays (Bahia). Lomas are considered ecologically significant, supporting vegetation and habitats for endangered species such as the Ocelot. Yet, Lomas’ spatial distribution and planform characteristics are poorly constrained, hindering the effectiveness of conservation efforts. To address this problem, the first comprehensive map of Lomas in the RGD was produced using satellite imagery and digital elevation models via ArcGIS, which resulted in the identification of 654 Lomas. Long and short axes of best-fit eclipses to the polygons of Lomas are used as proxies to measure the dune crestline and lee face orientations. Sediment samples were also collected from an accessible Lomas for grain size analysis. Preliminary results show that Lomas consists mainly of silt (>60%), with ~25% sand, are evenly distributed in the RGD with a mean area of 0.21±0.24 km² (1σ) , and do not appear to have a spatial trend. The median crestline orientation is ~232^o, nearly perpendicular to the prevailing southeasterly winds (~135^o), while the median lee face orientation is ~293^o, almost paralleling the wind direction. Despite the above intuitive findings, a subgroup of Lomas’ orientations (~29%, n = 188) shows no correlation with the local wind direction. A working hypothesis is that the shapes of this subgroup are impacted by the adjacent abandoned channels of the RGD (Resacas), as wind velocity is reduced by the topography of the channel levees up to 10 meters high, forcing sediment depositions. The findings of this study show the importance of aeolian sediment transport processes in arid deltas and hope to inspire future studies on the interaction between aeolian and fluvial-deltaic landforms.