Pedro Dinis

Holocene interplay between a dune field and coastal lakes in the Quiaios– Tocha region, central littoral Portugal

Coastal dune fields and dune-body associated lakes were studied through sedimentological, malacological and palynological analyses. Lake deposits and palaeosoil horizons gave new information about phases of aeolian activity and dune building, coastline and sea level, vegetation history and cultural impact. After a period of intense aeolian activity (Late Pleistocene) that limited soil-forming processes, a wet aeolian field, recorded by podzolized sands and interdune wetland deposits, was present in the region. Around 5000 to 4000 yr BP sea level reached the current level and the coastline was situated c. 1 km further inland compared with the present situation. Subsequently lagoon-barrier island systems developed c. 4000 yr BP. Vegetation in the region during this time was a pine/oak forest. Between 4000 and 1600 yr BP the forest was replaced by a semi-natural heathland. Later the heathland was degraded, allowing dune migration and finally formation of a desert-like landscape. This process was probably a consequence of a combination of deteriorating climate during the ‘Little Ice Age’ (LIA) and overgrazing. To counteract sand invasion, reforestation efforts were initiated in the region c. 300 years ago and locally along the coastal margin between 1924 and 1940. Formation of the row of freshwater lakes bordering the dune field at present was contemporaneous with the initial reforestation phase. A close relation between lake formation and accumulation of sand dunes was found. Various types of dunes were identified, truncated dunes, parabolic dunes and transverse/oblique dunes. Dune migration caused an eastward displacement of lakes.

Paleoenvironmental and Paleobiogeographical Implications of a Middle Pleistocene Mollusc Assemblage from the Marine Terraces of Baía Das Pipas, Southwest Angola

Abstract Quaternary raised marine terraces containing the remains of diverse, shallow water marine invertebrate faunas are widespread across the coast of Angola. These deposits and faunas have not been studied in the same detail as contemporaneous features in northwest and southernmost Africa. We analyzed the fossil assemblages and sedimentology of two closely spaced middle Pleistocene marine terrace deposits in Baía das Pipas, southwest Angola. This revealed 46 gastropod and 29 bivalve species, along with scleractinian corals, encrusting bryozoans, polychaete tubes, barnacles, and echinoids. The fauna is characteristic of intertidal and nearshore rocky substrates and sandy soft-bottom habitats. Sedimentological analysis is consistent with faunal data and indicates an upper shoreface paleoenvironment along a gravel coast. This diverse fauna stands out as a rare example of a marine Pleistocene assemblage from over 6,000 km of the West African coast. The assemblage is dominated by extant tropical West African molluscs, including species from the “Senegalese fauna” that colonized northern Africa and beyond during Pleistocene interstadials. Additionally, as along the modern coast of the Namibe Desert, the influence of the cool-water Benguela Current is apparent in the paleofauna by the occurrence of a few temperate species. The distribution and thermal tolerances of extant species identified in the Pipas fauna indicate that this region experienced similar climatic and oceanographic conditions as that of the present during this interstadial. Seasonal temperature varied between ∼20 and 28°C and resulted from upwelling in this tropical setting.

Controlling factors on clay mineral assemblages: insights from facies analysis of Pliocene to Pleistocene coastal margin deposits, Western Portugal

The clay fractions in the Pliocene to Pleistocene coastal margin record are usually dominated by kaolinite and illite, with lesser amounts of vermiculite, 10-14 mixed layer clays and smectite. The high clay mineral crystallinity, the mineralogical relations to facies and depositional setting and some horizontal variations along coeval deposits suggest that clay assemblages are mainly detrital. Illite is more important in inner shelf deposits and particularly in alluvial deposits from eastern locations. The high illite content in eastern alluvial deposits is explained by the input from the neighbouring Iberian Variscan Massif that is rich in mica. The high kaolinite content in prograding sand and gravel coastal plain is partially explained by the availability of this mineral in the drainage areas. Given the arkosic nature of some of these deposits, post-depositional feldspars weathering would also contribute to an increase in kaolinite content. Vermiculite is particularly important close to the surface and to unconformities, in horizons influenced by pedogenetic processes that have more aluminous illite with relatively low crystallinity. In organic rich mud sediments low pH conditions favour post-depositional transformation of illite to vermiculite and mixed layer clays.

Geochemistry and detrital geochronology of stream sediments from East Timor: implications for the origin of source units

Crustal fragments with Asian and Australian affinities that outcrop on the island of Timor were stacked together owing to the collision between the Banda volcanic arc and the Australian continent. Geochemistry of floodplain sediments (11 samples) and detrital zircon geochronology of bedload sediments (5 samples) are used to interpret the geological nature of the source units exhumed in East Timor. The geochemical data revealed source rocks with widely variable proportions of felsic and mafic material. In general, the Banda Terrane units supply higher proportions of mafic material, which tend to have lower zircon productivity, than the Australia Passive Margin or Gondwana sequences. Zircon with ages of 2150–1500 and 365–210 Ma constitute the most common populations in all stream samples. Sampling sites that are not sourced exclusively by the Gondwana Sequence and have Banda Terrane units in their watersheds are characterised by high proportions of Triassic zircon, which are common in the Sula Spur, and discordant grains. It is proposed that a significant component of the zircon found in the allochthonous units of Timor is inherited from Australian-related crustal fragments that drifted from the Sula Spur. These units were carried south as the Banda Arc progressed towards the Australian continent and emplaced in Timor with the Banda Terrane.

Genesis of deformation structures affecting Plio-Pleistocene sediments in the western Portuguese mainland (West Iberia). Implication on the regional neotectonics

A careful analysis of the morphology, geologic setting, stratigraphic distribution and sedimentological context of deformation features in sediments can give valuable information for the interpretation of their genesis, namely if they are of sedimentary or tectonic origin. This approach was used for the study area, located in the western region of the Portuguese mainland (West Iberian margin), where significant neotectonic and seismic activities occur, contrasting with the stabler interior of Iberia. Notwithstanding the regional neotectonics (intended as upper Pliocene to Recent), part of the deformation features observed in the studied deposits (Pliocene to Pleistocene in age) may be attributed to other phenomena but seismic shaking and/or active faulting. Slump beds and flame structures in coarse sand deposits are probably related to sedimentary overloading in high supply delta plain setting. Part of the deformation observed in the sediments that overlie Jurassic marls and limestones was probably induced by karst sink. This process is plausible even in Plio-Pleistocene sediments that overlie low carbonate content basement rocks, such as marl dominated successions. Thus, the abundance of deformation features affecting the studied Plio-Pleistocene sedimentary cover may lead to an overestimation of the regional active tectonics. The neotectonic activity in this area is reconsidered from the interpretation of the triggering mechanisms responsible for the observed deformation.

Gamma-ray dose rate surveys help investigating century-scale beach ridge progradation in the wave-dominated Catumbela delta (Angola)

A strandplain at the downdrift side of the wave-dominated Catumbela delta (Angola) includes distinguishable deposits with very high natural radioactivity (up to 0.44 microSv/hour). In order to establish the geometry of these sedimentary units and understand their genetic processes, dose rate surveys were performed with the portable equipment Rados RDS-40WE. In addition, grain-size distribution, heavy-mineral composition and gamma-ray mass spectra of the high dose rate deposits were analysed. High dose rate values are found in ribbon units aligned parallel to the shoreline, which are a few tens of meters wide and up to approximately 3 km long. These units reflect the concentration of Th-bearing grains in coastal deposits enriched in heavy minerals. An integrated analysis of the high dose rate ribbons in GIS environment with aerial photography and topographic maps suggests that parts of the high dose rate units formed during the last two centuries may be related with the erosion of older shoreline deposits, due to updrift displacements of the Catumbela river outlet and recycling of shoreline accumulations with downdrift deposition. Simple gamma-ray surveys carried out with a portable detector can unravel depositional units characterised by significant enrichment in heavy-mineral grains that are likely to correspond to key events in the evolution of wave-dominated accumulations. The location of such deposits should be taken into account when planning future work using more expensive or time-consuming techniques.

Associations between lithology and land-use in a wine production region (Bairrada region, Portugal)

In this work, we analyze the spatial relationships between land-use and lithology in the wine production region of Bairrada (Portugal) and present a new lithological map (1:50,000) created for the region, highlighting the associations with specific types of land-use. The study was focused on the area of Bairrada that establishes the appellation DOC-VQPRD wines. The region shows a heterogeneous lithology and land-use and it is found that these two variables are not independent. Three different zones with characteristic land-use and geology can be distinguished in Bairrada: a western zone where aeolian sand deposits are associated with coniferous forests; an eastern zone with Precambrian and Paleozoic metasedimentary units associated with broad leaf forests; and a central zone with diversified Mesozoic and Cenozoic units in which most vineyards are placed. There is a clear association between a Jurassic marly and dolomitic unit and an occupation by vineyards, which is particularly obvious when larger vineyards are considered. The vineyards are also common when these Jurassic units are covered by relatively thin siliciclastic sediments.

Clays and Vegetation: Comparing Palaeoclimatic Signatures in the Portuguese Lower Cretaceous

We collected a set of 120 clay samples with precise stratigraphic locations from the Lower Cretaceous coastal outcrops of the Ericeira area (western Portugal). The kaolinite versus illite dominance can be considered as revealing wet or dry climates, respectively, prevailing in the watershed. Swelling clays are associated with seasonally dry climates. However, since hydrodynamics and rejuvenation events also control clay associations, the relative role of climate versus depositional setting is still to be fully deciphered. As the onshore Lower Cretaceous of western Portugal is a key area for studying the Early Cretaceous initial diversification of angiosperms, several well-studied floras are summarized and their environmental interpretations presented.