Implications of polymodal distributions in the grain size parameters of coastal dune sands (Oaxaca, Mexico)
2022
Kasper-Zubillaga, J.J. | Arellano-Torres, E. | Álvarez-Sánchez, L.F. | Carlos Delgado, L. | Martínez-Serrano, R.G. | Baltazar-Jiménez, P.A.E.
This study elucidates the geological and physical factors that produce polymodal grain size distributions in two dunes systems in southeastern Oaxaca, Mexico. The dune systems are characterized by (1) small embryo vegetated, transverse and undeveloped dunes; and (2) tall types identified as barchanoid, parabolic, climbing and falling dunes. We performed a best-fit statistical Weibull distribution model with data obtained by laser granulometry to determine the polymodal trends of the coastal dune sands distribution. Polymodal distributions in small dunes arise from coarse and fine-grained sands mainly derived from the erosion of alluvial deposits composed of granitic and metasedimentary rock fragments (e.g., schists, orthogneiss and paragneiss). The medium- to fine-grained sands result from wind deflation, producing a heavy mineral lag. Conversely, the polymodal grain size distributions in the tall dunes are mainly derived from the physical weathering of granites, andesitic-tuff-lava flow marine sandstones (ATLMS), alluvial deposits, and metasedimentary rock fragments (e.g., schists, orthogneiss and paragneiss) due to the frequent storms and intense winds. We constructed binary plots of grain size parameters to compare the dune sand systems in Oaxaca with some others in Mexico. Based on the sedimentary results and the Weibull distributions, we conclude that the grain sizes of the coastal dune sands from Oaxaca, Mexico, are controlled by the sand composition, wind patterns, hydrodynamic conditions, weathering effects, and physical characteristics, regardless of the dune geomorphology. Evidence of multicycle aeolian transport of sand particles in the Oaxaca coastal dunes is supported by the compositional framework of the sands and the roundness of sand grains.
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