SPATIAL DISTRIBUTION, TEXTURAL CHARACTERIZATION, AND PRELIMINARY CHRONOLOGY OF THE STABILIZED LUNETTE DUNES (LOMAS) IN THE RIO GRANDE DELTA, TEXAS, USA AND TAMAULIPAS, MEXICO

The Rio Grande Delta (RGD), located in south Texas, USA, and northern Tamaulipas, Mexico, contains hundreds of eolian landforms, locally called Lomas. These features are crescent-shaped elevated (1–8 m) landforms that were first identified in Texas in 1909 and are found downwind adjacent to hypersaline tidal flats (Esteros) and bays (Bahia) between the abandoned RGD channels. Previous studies have interpreted these landforms as ‘clay dunes’; however, detailed grain-size data and a mechanistic explanation for the formation remain lacking. To address these knowledge gaps, we produced the first comprehensive map of Lomas in the RGD using satellite imagery and digital elevation models, analyzed with ArcGIS Pro, which identified over 700 Lomas. Results show that the Lomas are evenly distributed in the RGD with a mean area of 0.21±0.24 km² (±1σ). 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. Leveraging this remotely sensed map, four accessible Lomas were sampled for grain size, including a detailed ~4 m vertical profile at 10-cm intervals, and used for ¹⁴C and Optically Stimulated Luminescence dating. Preliminary results show that contrary to past interpretations, Lomas contain little to no clay (0–6%); they are primarily composed of silt (>60%) with up to 25% sand and a median grain size of 50–70 microns. Material collected from the Lomas shows depositional age dates clustering between 2–4 ka. Given this new information, we suggest that the Lomas are better classified as Lunettes—lake-adjacent dunes found in other arid and semi-arid regions worldwide rather than ‘clay dunes.’ Field observations indicate that, at present, these landforms are stabilized by vegetation and undergo reworking by gully erosion. We hypothesize that these dunes form due to complex eolian sediment transport patterns dictated by shifts in sediment supply and availability resulting from RGD channel avulsions and rising sea levels during the late Holocene. The outcomes of this study highlight the importance of the interaction between eolian and deltaic processes in arid regions.