Abstract
The modal composition of sandstones has long been used as a tool for palaeotectonic and palaeoenvironmental analyses. Herein it is demonstrated that caution must be used when beach sand composition is utilized to assess tectonic provenances at both the local and continental scales. The occurrence of quartz arenites and compositionally mature sands along the coast of Brazil corresponds well to their passive margin location. However, modern sands from some beach locations in coastal Uruguay are less compositionally mature compared to the quartz arenites found in coastal Brazil. Along the beaches of Las Vegas and Parque del Plata, the composition of the beach and dune sands is impacted by the transport of continental outwash sediments derived from the Palaeozoic and Precambrian rocks in the interior to the beach by a low-gradient stream, Arroyo Solis Chico, and the erosion of Pleistocene-age formations that crop out on the beach and along an erosional scrap. The mean beach composition ratio of monocrystalline quartz to feldspar to rock fragments plus composite quartz grains is 67.5/8.0/24.5 compared to the ratio along the coast of Brazil at 86/4/10. The beach and coastal dunes environment studied is representative of about 35% of coastal Uruguay. At the smaller scale, changes in the textural properties of the beach and dune sands (grain-size distribution and roundness) are caused by the addition of the sediment derived from erosion and stream transport. Porosity values show little influence and range between 0.35 and 0.36, but hydraulic conductivity is influenced to a greater degree, ranging from 18.5 to 31 m/day. Within one kilometre down-drift of the river, the mean grain diameter and the mean hydraulic conductivity increase to the same values found in the up-gradient area before influence of the sediment flux, due to the offshore transport of the finer sediment fraction during down-gradient movement along the beach. The Arroyo Solis Chico area demonstrates that abrupt changes in composition and hydraulic properties can occur over short distances in sands deposited in passive margin environments. The potential for this local variability in sand composition and hydraulic properties should be considered in the analyses of ancient marine sandstones deposited in beach and coastal dunes environments.