Abstract

Intertidal sands in the Minas Basin are mineralogically immature, and are generally medium grained and moderately well sorted. Quartz grains show a wide range of roundness: mean roundness is subangular, but rounded to well rounded grains are also present. The sands are derived mainly from coastal erosion of Pleistocene tills and outwash and Triassic sandstones.Scanning electron microscope examination of the surface textures of more than 40 samples of source materials and tidal sands reveals that although many of the textures are relict, tidal sands show high, smoothed surfaces with non-oriented V's, and low areas with 'patchy growths', neither of which are present on source sands. Rounding of grains is undoubtedly taking place in the tidal environments, but much of the roundness is inherited from Triassic source material, and new surface textures are developed most rapidly on grains already partly rounded.Average distances of grain transport in the layer of sand moving on the bar surface is of the order of 1 m per tidal cycle, and allowing for reworking of the entire mass of sand in the bars, this results in a long term transport rate of only about 10 m per year. For the distance of transport, sediment transport by tides is probably more efficient at rounding sand grains than transport by rivers, but less efficient than transport by waves or wind. Geologically long periods of reworking by tides would be necessary to produce texturally supermature sands.

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