Coastal dune structures from Paraná (Brazil)

Abstract

In the coastal plain of Paraná the sand ridge dunes close to the shoreline were examined relative to their primary structures. Vertical sections, both parallel to and transverse to the average wind direction, were dug and cut through the dune body. The Jardim São Pedro dune provided a detailed view of internal structures. These were compared with the ones presented by the older sand ridge dunes from the Paraná coastal plain. The structures of the sand ridge dunes were compared also with those from other types of dunes described by McKee (1966) from the White Sands National Monument (New-Mexico) and from the Libyan desert. The analysis of cross-bedding indicates that many features are common between the dunes studied by McKee and those from the Paraná coastal plain: ( 1) foreset high-angles, which frequently are 30–33°, not uncommonly up to 36° or even more; ( 2) the almost horizontal character of many bounding surfaces which progressively dip steeper within the dune's linguoid protuberance, as well as on the lee side of the ridge; ( 3) the individual sets of the cross-stratification that tend to be thinner — with the strata flatter — near the top than at the bottom of a vertical section; ( 4) tabular planar sets that are dominant, with sets of simple and trough type of cross-stratification frequent in some places of the dune. Especially characteristic of the coastal dunes are the convex-upward foresets, located in the upper part of the dune's brink where high-angle slipface strata occur. This type of bow, arch-shaped strata accumulated mainly around the dune linguoid protuberances; contorted bedding of different sizes and shapes are very common on the lee sides of sets with bent strata. They result mostly from slumping. Cross-bedding measurements indicate that every part of the dune has its own characteristics. However, the cross-strata dip resultant vector for the whole dune coincides with the average wind direction.