Cristina Gama

A Precise and Efficient Methodology to Analyse the Shoreline Displacement Rate

Abstract The shoreline change rate is one of the most significant parameters in analysing sandy shore behaviour with time. This parameter can be monitored by means of low- and high-resolution survey methods, depending on the objectives of the monitoring programme. Survey efficiency is also very important for achieving high resolution in both space and time. Another important aspect is the precision and significance of the obtained results, not only from the survey method itself but also from the comparative analysis used to process the data from several surveys. The survey method and the processing algorithms are the basis of shoreline analysis. This paper presents an evaluation of two proposed high-resolution methods that are simultaneously highly accurate and very efficient. These methods are based on a global positioning system (GPS) in differential mode for surveying and on novel algorithms for assessing the spatial change rate of the shoreline. The most significant difference of the two presented survey systems is the physical support: whereas one uses a land vehicle (motor-quad) to delineate the shoreline in wide straight coastal stretches, the other considers an on-foot simplified version to survey small, more irregular stretches. The analysis of the error associated with the proposed methodologies is thoroughly described in this paper. In both modes of operation, system-inherent errors are within the centimetre level—in general lower than 0.05 m. Operation-specific errors can remain within the centimetre level, but if instrument handling is careless in the on-foot survey method, they can reach decimetre levels. If successive monitoring surveys are not carried out under similar field morphological conditions, when the frontal dune baseline is adopted as a shoreline indicator, rough errors can be introduced. Two case studies of the application of these methods, evaluating the shoreline evolution of two distinct coastal regions in Portugal, are presented.

Provenance of upper Triassic sandstone, southwest Iberia (Alentejo and Algarve basins): tracing variability in the sources

Laser ablation ICP-MS U–Pb analyses have been conducted on detrital zircon of Upper Triassic sandstone from the Alentejo and Algarve basins in southwest Iberia. The predominance of Neoproterozoic, Devonian, Paleoproterozoic and Carboniferous detrital zircon ages confirms previous studies that indicate the locus of the sediment source of the late Triassic Alentejo Basin in the pre-Mesozoic basement of the South Portuguese and Ossa-Morena zones. Suitable sources for the Upper Triassic Algarve sandstone are the Upper Devonian–Lower Carboniferous of the South Portuguese Zone (Phyllite–Quartzite and Tercenas formations) and the Meguma Terrane (present-day in Nova Scotia). Spatial variations of the sediment sources of both Upper Triassic basins suggest a more complex history of drainage than previously documented involving other source rocks located outside present-day Iberia. The two Triassic basins were isolated from each other with the detrital transport being controlled by two independent drainage systems. This study is important for the reconstruction of the late Triassic paleogeography in a place where, later, the opening of the Central Atlantic Ocean took place separating Europe from North America.

Use of sand beds of variable permeability in beach profile engineering

Here we present a technique based on theoretical grounds for dealing with beachface dynamics in response to wave forcing. Previous work has provided a relationship involving mean sand grain size, wave height, and beachface slope for a broad range of Iribarren number. The key aspect of beach morphodynamics was shown to be the permeability of the sand bed, which may be correlated to sand grain size and sphericity, and bed porosity, through the Kozeny‐Carman equation. Therefore, we show how beach nourishment aiming at beach profile recovering must be carried out with the use of sand beds of appropriate mean grain size. The theory also illuminates beach dynamics, namely the reshaping of sandy beachfaces in response to changes in wave height.

Detrital provenance of the Upper Triassic siliciclastic rocks from southwest Iberia: a review

BackgroundIn southwestern Iberia the Upper Triassic successions of Lusitanian, Alentejo and Algarve basins records the fragmentation of Pangaea in Permian–Triassic during which the paleogeography of Iberia was dominated by a series of coalescing, alluvial–deltaic wedges and axial braided rivers.PurposeIn this study, we discuss the potential sediment sources of the Lusitanian, Alentejo and Algarve basins based on detrital zircon-age spectra, suggesting that Iberia occupied a central position Iberia in Pangaea during late Triassic.MethodsConventional sedimentary petrography and paleocurrent measurements of previous works was combined with recently published detrital zircon U–Pb geochronology of the Upper Triassic siliciclastic rocks of southwest Iberia to shed light on the detrital provenance record.ResultsZircon age populations found in the Upper Triassic strata of the Lusitanian, Alentejo and Algarve basins is dominated by Neoproterozoic (33–76%) and Paleoproterozoic (12–15%) grains. The most important differences are the dominance of Devonian–Carboniferous (33%) zircon in the Alentejo basin and the greater representativeness of Permian–Carboniferous (6%) zircon in the Lusitanian basin.ConclusionThe deposition in these Upper Triassic basins of Portugal is marked by variability in sedimentary sources, involving the denudation and local-scale directions of sediment transport from the Iberian basement with possible additional supplies derived from outside present-day Iberia. The Upper Triassic successions evolved separately with the detrital transport being probably controlled by local drainage systems, and occupying a central position in Pangaea just before the opening of the Central Atlantic Ocean.ResúmenEn esta revisión, combinamos la petrografía sedimentaria convencional y las mediciones de paleocorrentes obtenidas de trabajos previos de las rocas sedimentarias siliciclásticas del Triásico Superior del suroeste de Iberia, con datos de geocronología U-Pb de circón detrítico recientemente publicados por nosotros. El objetivo del trabajo es mejorar el conocimiento sobre la procedencia de las rocas sedimentarias siliciclásticas del Triásico Superior de las cuencas Lusitaniana, del Alentejo e del Algarve. En las poblaciones de circón detrítico de las cuencas del Triásico Superior del suroeste de Iberia predominan las edades del Neoproterozóico (33-76%) y Paleoproterozóico (12-15%). Las diferencias más importantes son: el predominio de granos del Devónico-Carbonífero (33%) en la cuenca del Alentejo y la mayor representatividad de granos del Permiano-Carbonífero (6%) en la cuenca Lusitaniana. La deposición en estas cuencas del Triásico Superior de Portugal se caracteriza por la variabilidad de las fuentes sedimentarias, que probablemente implicó la denudación y dirección local del transporte sedimentario desde el zócalo ibérico (zonas Centro Ibérica, de Ossa-Morena, Sul Portuguesa y do Pulo do Lobo) con posibles suministros adicionales procedentes de regiones actualmente localizadas fuera de Iberia (Terreno Meguma, Nueva Escocia). Las sucesiones del Triásico Superior evolucionaron por separado, siendo el transporte de sedimentos probablemente controlado por sistemas de drenaje locales.

Sedimentary Provenance of Neogene Strata From the Southwestern Portuguese Coast (Sines Cape): Detrital Zircon U–Pb Geochronology

In this preliminary study of provenance analysis, Neogene sandstones and conglomerates of the Alvalade Basin were sampled from sea cliffs in the vicinity of the Sines Cape (southwestern Portuguese coast). Detrital zircons were extracted by conventional methods of particle size separation including magnetism and heavy liquids. Detrital zircons were dated using the U–Pb method by laser ablation with inductively coupled plasma mass spectrometry (LA–ICP–MS). The U–Pb geochronology results indicate the following as potential sources for the samples: (1) Carboniferous greywackes of the Mira Formation (South Portuguese Zone) or the Upper Triassic sandstones of the Silves Formation (Alentejo Basin) for the zircons with pre-Permian ages; and (2) syenites from the Sines Massif for the zircons with Late Cretaceous ages. One sample includes a significant population of detrital zircons of Permian age whose source is unidentified as there are no source rocks of such age in the Alvalade Basin.