Loss of sand-size feldspar and rock fragments along the South Texas Barrier Island, USA

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To add to the limited quantitative data on the rate loss of sand-size feldspar and rock fragments during transport in the beach zone, the abundance of QFR grain types was determined for 31 samples of beach sand from Padre Island, a barrier island 240 km long whose only source of river sand is from the Rio Grande at its southern end. Feldspar, volcanic rock fragments, carbonate rock fragments, and unidentified rock fragments decrease irregularly and monocrystalline and total quartz increase irregularly northward. The abundance of well rounded and spherical quartz grains show no trend, which indicate that selective transport by roundness and shape is not taking place. The decrease in abundance of feldspar and rock fragments northward in the southern half of Padre Island is the result of loss by wave action (abrasion and grain breakage), whereas the decrease of these grains in the northern half of Padre Island is a combination of abrasion/breakage loss and dilution of grains from a more quartzose population to the north. Silicic volcanic rock fragments are as durable to beach abrasion as feldspar, but carbonate rock fragments are not. South of the mixing zone of Rio Grande sand with sand from a northern source, feldspar and total rock fragments each decreased 1 absolute % for each 19 km of beach transport. This is 19 times faster than the rate loss of feldspar along the Florida, USA, coast and 6 times slower than the loss of feldspar along the Rio de Janiero, Brazil, coast. Additional case studies are needed to isolate the variables that control loss of unstable sand grains during beach transport.