Luiz J. Tomazelli

Beach ridges, foredunes or transgressive dunefields? Definitions and an examination of the Torres to Tramandaí barrier system, Southern Brazil

Many prograded barriers and some dunefields in the world have been termed ‘beach ridge’ plains, but the actual genesis of the ‘ridges’ is often unknown. Use of the terms, berms, beach ridges and foredunes is also confusing in the literature because their definitions are highly variable and are commonly used interchangeably. Thus, the formation and definition of sand berms, beach ridges and foredunes is briefly reviewed. Beach ridges are re-defined as entirely wave formed deposits which are most commonly formed during high wave conditions and/or elevated water levels (e.g. storm surges). Foredunes are formed by aeolian sand deposition in vegetation on the backshore. Some dunefields in Brazil have been called beach ridge plains when they are, in fact, foredune plains, transgressive dunefields, or complex barriers (i.e. barriers comprising two types of dunes). The Holocene barrier extending from Torres to Tramandai in southern Brazil has been regarded as a beach ridge plain. The landforms of this Holocene barrier comprise wide, relatively linear, widely spaced (400–600m), shore parallel ridges on the landward half, and more closely spaced (80–400m), lobate and crescentic, discrete ridges on the seaward half. Low, rolling dunefields, sand sheets, nebkha fields and deflation plains occur between the ridges. The barrier is re-interpreted as a prograded, transgressive dunefield

Barrier evolution and placer formation at Bujuru southern Brazil

Abstract This paper demonstrates the link between the evolution of a coastal barrier in southern Brazil during the Late Holocene and the formation of a large volume of eolian disseminated heavy mineral deposits. Our data set is based on an earlier heavy mineral prospecting campaign (1991) and on 10 new shallow vibrocores, 2–5 m long. The model presented has three main steps of barrier evolution. The first step is the recycling of coastal plain deposits during the Postglacial Marine Transgression, which ended at 5.6 ka when heavy minerals were incorporated into beach and washover facies of a transgressive barrier. The second step is the shoreward retreat of the barrier, under a slow and small sea-level fall, during the last 5.6 ka. This second step eroded and recycled sediments from the Pleistocene substrate, which acted as an extra source of heavy minerals. Heavy minerals were concentrated in backshore deposits by wave action during barrier recession. The third step is the erosion and transport of backshore sands by onshore winds into an inter-barrier depression in the form of transgressive dune deposits. These deposits contain an average of 4.66±1.02% disseminated heavy minerals (1494 samples). This eolian placer deposit has started to form 1 ka ago and is still under formation.

Long- and Short-Term Coastal Erosion in Southern Brazil

ABSTRACT Rio Grande do Sul is the southernmost state in Brazil. Open sandy beaches dominate the 630-km long shoreline that is 76% still undeveloped. Less than 5% of the states population (totalling 9.7 million people) live in coastal cities. However, the coastal population is growing faster than the states average since 1990. Although intense erosion is widely accepted along the beaches of Conceição lighthouse and Hermenegildo, the extent of erosion along the Rio Grande do Sul shoreline is still a controversial issue. Discussions arise from the contrasting results presented by studies addressing coastal erosion in Rio Grande do Sul. Recent DGPS monitoring indicates that about 80% of the Rio Grande do Sul shoreline is eroding; wave refraction studies indicate that it is mainly stable, and long-term coastal evolution modelling reveals a predominantly prograding shore for the last 5 ka. This work critically evaluates published data on long- and short-term causes of coastal erosion in Rio Grande do Sul, in an attempt to highlight the unanswered questions that could minimize the debate. The analysis includes sea-level rise, concentration of wave energy due to large-scale coastal topography, sand deficit as the long-term causes of erosion, storm surges, concentration of wave energy due to small-scale submerged features, interference in the longshore sediment transport, and human activities as the short-term causes. Discrepancies in shoreline change results are a matter of the temporal scale in question and what are the causes that play a significant role in it. For coastal management purposes short-time events represent a far greater hazard than long-term trends. It is therefore reasonable to state that in order to support decision-making mechanisms in Rio Grande do Sul a better understanding of the relationship of storms, sand budget, and beach erosion is necessary.

A critical evaluation of coastal erosion in Rio Grande do Sul, Southern Brazil

Evidences of coastal erosion in Rio Grande do Sul have been obtained by three methods: (a) analysis of the long-term morphodynamics and stratigraphy of coastal barriers, (b) annual shoreline mapping using the Differential Global Positioning System (DGPS), and (c) local beach profile measurements. The first method reflects coastal erosion as continuity of the geological evolution in the last 5 ka, taking place mainly along the southern half of gentle coastal projections. The second method represents a shorter temporal scale and indicates that approximately 80% of the coast is eroding. Beach profiling has been measured in very few places that are distant from each other since the early 1990s; consequently, their results reflect local and very short time shoreline behavior. A critical evaluation of published data addressing coastal erosion in Rio Grande do Sul strongly suggests that short and long term negative balance on the sediment budget is the main cause of erosion along this coastline.

Geologia do Sistema Lagunar Holocênico do Litoral Norte do Rio Grande do Sul, Brasil

O sistema lagunar holocenico do Litoral Norte do Rio Grande do Sul compreende um conjunto de ambientes e sub-ambientes deposicionais que incluem: corpos aquaticos costeiros (lagos e lagunas), sistemas aluviais (rios meandrantes e canais inter-lagunares), sistemas deltaicos (deltas fluvio-lagunares e deltas de “mare lagunar”) e sistemas paludiais. O presente estudo busca analisar os processos de sedimentacao, os padroes morfologicos e de distribuicao dos sedimentos bem como a historia evolutiva do sistema ao longo do tempo. Ele mostra que quatro mecanismos principais tem sido responsaveis pela evolucao do sistema durante o Holoceno: (1) as variacoes do nivel de base que seguiram as flutuacoes do nivel do mar, (2) o progressivo avanco da vegetacao marginal aos corpos de agua, (3) o aporte de sedimentos clasticos trazidos pelos rios e (4) a migracao das dunas eolicas para dentro dos corpos de agua.

Sedimentary facies and depositional environments related to Gondwana glaciation in Batovi and Suspiro regions, Rio Grande Do Sul, Brazil

Abstract Sedimentary facies were produced by Late Paleozoic Gondwana ice sheets in the Batovi and Suspiro regions, Rio Grande do Sul State, southern Brazil. The glaciogenic sediments, assigned to the Itarare Group of Parana Basin, are the result of processes related to the subglacial, supraglacial and proglacial (ice-contact, glaciofluvial and glaciolacustrine) environments of the glacial depositional system. Several features developed by the action of glaciers were identified in the studied outcrops: striated and grooved pavements; striated, polished and faceted stones; finely laminated rhythmites bearing dropstones (varvites); erratic clasts and deposits exhibiting deformational structures such as folds, normal and reverse faults, and beds with abnormal dips. The available evidence suggests that the glacial episodes took place in a terrestrial (continental) context, far from direct marine influence.

Aeolian Deposition and Barrier Stratigraphy of the Transition Region between a Regressive and a Transgressive Barrier: an example from Southern Brazil

ABSTRACT Dillenburg, S.R., Barboza, E.G., Tomazelli, L.J., Rosa, M.L.C.C., Maciel, G.S., 2013. Aeolian Deposition and Barrier Stratigraphy of the Transition Region between a Regressive and a Transgressive Barrier: an example from Southern Brazil. The transition region between regressive and transgressive barriers on the northern and central coastal sectors of Rio Grande do Sul (RS) in southern Brazil is represented by a 170 km long stretch of coast where the barrier shows a general aggradational/stationary (a/s) behavior on a long term time scale. In this study, approximately 50 km of this transition region was analyzed at four sites: Tramandaí, Jardim do Éden, Cidreira and Dunas Altas. All barriers began forming around 7.8 - 7.2 ka. The northernmost site (Tramandaí) has a typical a/s stratigraphy, and barrier morphology is dominated by transgressive dunefields. Ten km to the south, at Jardim do Éden, the barrier shows a transgressive stratigraphy, and barrier morphology is also dominated by transgressive dunefields. At Tramandaí the maximum barrier height is around 4 m, with a maximum of 6 m, while at Jardim do Éden it averages 6 m, with a maximum of 11 m. Further to the south, 10 km from Jardim do Éden, the barrier at Cidreira shows a dominant transgressive stratigraphy developed in the last 7 ka, but with evidence of a relatively small regression (~600 m). Barrier height averages 10 m, reaching 20 m due to the existence here of a large transgressive dunefield. And finally, at around 30 km to the south of Cidreira, the a/s barrier of Dunas Altas occurs. Barrier morphology here is dominated by transgressive dunefields, with a surface height of around 8 m, but fronted by the largest and highest foredunes (around 8–10 m) in Rio Grande do Sul. The Dunas Altas coastline has regressed around 600 m in the last 7.8 ka. The Holocene barrier analyzed in this study, along a 50 km segment shows a general a/s behavior in the last 7.8 ka. The long time of a relatively stationary condition allowed the barrier to grow vertically by accumulation of aeolian deposits of at least four phases of transgressive dunefield development.