ArticlePublisher preview available

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

Authors:
To read the full-text of this research, you can request a copy directly from the authors.

Abstract and Figures

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.
This content is subject to copyright. Terms and conditions apply.
ORIGINAL
Gamma-ray dose rate surveys help investigating century-scale
beach ridge progradation in the wave-dominated Catumbela
delta (Angola)
Pedro A. Dinis
1
&Alcides C. Pereira
2
&Domingos Quinzeca
3
&Domingos Jombi
3
Received: 15 January 2017 / Accepted: 24 January 2017 /Published online: 15 February 2017
#Springer-Verlag Berlin Heidelberg 2017
Abstract 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 shore-
line, which are a few tens of meters wide and up to approxi-
mately 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 accumula-
tions 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 evo-
lution 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.
Introduction
Shoreline deposits occasionally display abnormally high natu-
ral radioactivity, which is attributed to a concentration of U-
and Th-bearing minerals such as monazite and zircon (Malanca
et al. 1998;Alametal.1999;Coelhoetal.2005;Vassasetal.
2006;El-GamalandSaleh2012;Kaiseretal.2014). As natural
radioactivity may also be linked to the presence of K minerals,
which tend to yield lower densities, gamma-ray spectrometry
can be used as a proxy for the composition of distinct beach
units (Seddeek et al. 2005;Vassasetal.2006;El-Gamaland
Saleh 2012). The abundance of a specific mineral in beach
deposits is controlled by sorting processes (e.g. Komar 2007).
Light minerals are more easily entrained and transported by
coastal currents than heavy minerals. Consequently, coastal
stretches under erosion due to sedimentary deficits or high
wave transport capacities tend to be enriched in heavy minerals,
particularly the densest, compared to accreting beaches where
sediment availability is higher (Frihy and Komar 1993;Frihy
2007;Komar2007). As heavy minerals may contain radionu-
clides, natural radiation data can serve to investigate erosional
and accretionary processes in coastal environments (Greenfield
et al. 1989; Prakash et al. 1991;Vassasetal.2006;El-Gamal
and Saleh 2012).
This research focuses on radioactive beach deposits found
at the northern (downdrift) side of the asymmetric wave-
dominated Catumbela delta in Angola. The shore zone of
the Catumbela delta is characterised by beach ridge plains
(Fig. 1), which have juxtaposed patterns that are not easy to
discern due to their amalgamation, as well as human
*Pedro A. Dinis
pdinis@dct.uc.pt
1
MARE - Marine and Environmental Sciences Centre, Department of
Earth Sciences, University of Coimbra, 3030-790 Coimbra, Portugal
2
CEMMPRE, Department of Earth Sciences, University of Coimbra,
3030-790 Coimbra, Portugal
3
Department of Earth Sciences, University of Coimbra,
3030-790 Coimbra, Portugal
Geo-Mar Lett (2017) 37:489500
DOI 10.1007/s00367-017-0497-3
Content courtesy of Springer Nature, terms of use apply. Rights reserved.
... Beach-ridges close to the sectors of river breaching are frequently wider (Fig. 3C), reaching~50 m, and small wetlands with sparse salty adapted herbaceous plants occur in inter-ridge lowland areas ( Fig. 3A and C). Farther from the channel locations the ridges may merge with more continuous ribbon-shaped sand bodies that are frequently enriched in ultra-dense heavy minerals (Fig. 4A), which probably record major breaks in the pattern of the progradation of the delta front (Dinis et al., 2017). The youngest ridges placed near the present-day channel display asymmetric reliefs with steeper seaward facing beach scarps and gentler landward sides (Fig. 5), and can be up to 1.5 m high. ...
... Geomorphological sketch of the Catumbela. Phases of beach-ridge progradation limited by deposits enriched in heavy minerals recognised inDinis et al. (2017). In Lobito region the phases are deducted from the geomorphological map ofCarvalho (1963). ...
Article
It is presented an investigation on the morpho-sedimentary evolution of one of the largest sub-aerial delta protrusions in west Africa: the Catumbela delta. The planform geometry of this delta is characteristic of an asymmetric wave-dominated accumulation, strongly influenced by a north-directed longshore drift. Its post-glacial Holocene construction passed through morphological transformations linked with the relocation of the main fluvial channels and river mouths. Avulsion processes in the delta plain are responsible for an alternation between two major channel courses: one directed E-W and broadly coincident with today's path and the other placed to the north with a SSE-NNW direction. Delta growth started during a period of deceleration of sea-level rise and high rainfall (c.a. 7 ka BP) and was enhanced after the 4.5 ka. BP sea-level highstand. A deflected delta configuration was promoted by the c.a. 2–3 ka BP climatic crisis, when fluvial sediment supply was reduced and sediment bypassing limited to a minimum, forcing the channel to run parallel to the coastline. This configuration can be regarded as one of the finest worldwide examples of medium-size deflected deltas. The later updrift migration in the position of the river mouth was probably determined by an increase in fluvial supply driven by an upsurge in the seasonal rainfall in the drainage area and, in particular during the last 200 years of Portuguese occupation of the territory, changes in land-use. The updrift migration, however, was irregular, being punctuated by events of downdrift shifts and two channel courses with outlets several km apart may persist during wet periods.
Chapter
Full-text available
Resumo: Os deltas do Catumbela e Cavaco estão geneticamente relacionados com o afluxo sazonal de sedimentos fluviais e sua redistribuição sob acção das ondas, revelando uma sucessão de cordões de praia que testemunham anteriores posições da linha de costa. Procedemos a uma análise geomorfo-lógica dos deltas com base em imagens de detecção remota de diferentes idades e perfis topográficos levantados na faixa costeira. A análise morfológica permite propor um modelo evolutivo que envolve fases de intenso acarreio fluvial e fases de redistribuição sedimentar. As fases fluviais, especialmente evidentes durante a estação das chuvas dos anos mais pluviosos, são responsáveis por progradação localizada nas proximidades da embocadura do canal fluvial. Dada a migração da embocadura dos sistemas fluviais, a mais intensa progradação sedimentar teve lugar em diferentes sectores dos deltas. As fases de redistribuição sedimentar são responsáveis pela erosão dos lóbulos deltaicos gerados nas fases fluviais anteriores com consequente afeiçoamento do traçado da linha de costa. Palavras ‑chave: Delta do Catumbela; Delta do Cavaco; Cordões de praia; Ondulação. Abstract: Recent evolution (1950 – 2010 decades) of the morphology of Cavaco and Catumbela deltas (Benguela, Angola) The deltas of Catumbela and Cavaco are genetically related to seasonal fluvial sediment supply and its redistribution under the action of waves, being characterized by a succession of beach ridges that testify previous positions of the coastline. We carried out a geomorphological analysis of the river deltas based on remote sensing images of different ages and topographic profiles surveyed in the coastal strip. Morphological analysis allows us to propose an evolutionary model that involves phases of intense river supply stages and of sediment redistribution. The fluvial phases, especially evident during the rainy season of rainy years, are responsible for localized progradation near the outlet. Due to the migration of the river mouth, the most intense progradation took place in different sectors of the deltas. The phases of sediment redistribution are responsible for erosion of the delta lobes generated in previous phases, with consequent smoothening of the coastline.
Article
Full-text available
The Nile Delta coast is a dynamic sedimentary environment experiencing erosion, transport and redeposition of sand along the coast. The aim of this study was to investigate the effectiveness of using an integrated approach of natural radioactivity of coastal sediments, heavy mineral distribution and grain size information to differentiate between the eroding and accreting areas of the Nile Delta coast. Also, it is to investigate the relationship between grain size and heavy mineral distributions of these sediments on the spatial distribution of the coastal radioactive materials. The relatively higher profile averages of 226Ra, 214Pb, 214Bi and 210Pb (as 238U series) were 104.37±84.66, 45.60±37.83, 38.43±32.49 and 35.69±24.86 Bq/kg, respectively and 228Ra and 212Pb (as 232Th series) were 54.26±56.66 and 42.18±44.66 Bq/kg, respectively. The highest average value of 40K concentration (404.49±125.81 Bq/kg) was detected in the profile located at 2000.1 m west of Rashid estuary. Microscopic study revealed that the more dominant heavy minerals assemblages consisted of Opaques, Hornblende, Augite, Epidote, Biotite and Chlorite. Also, minor amounts of Garnet, Zircon, Rutile, Tourmaline, Kyanite and Monazite have been identified. Relatively high concentrations of 238U and 232Th series members were found in coincident at sites having higher heavy minerals percentages and detected at erosional beach than the accretion one. Coincidence was recognized between the average concentrations of 40K and the percentages of the light minerals collected at 100 m distances of the profiles under investigation.
Article
Full-text available
Sedimentary processes and the evolution of the wave- and tide-dominated, asymmetric Parnaíba Delta during the lateHolocene were investigated based on geochemical and sedimentological analyses of sediment cores collected in 2010, as well as satellite images and historical maps. This is a rare case of pristine deltas essentially unaffected by human activities worldwide. The lowermost part of the main Parnaíba River distributary exhibits several low-sinuosity bends and several anastomosing bifurcation patterns in the east, whereas three NW–SE-oriented tidal channels drain a large mangrove area in the west. Dating of various materials in sediment cores from the tidal flats, tidal channels and supratidal marshes revealed that the oldest sediment (4,853 to 4,228 cal. years BP) is paleo-mangrove soil from the main river distributary. Present-day mangroves and marshes up to 200 years old exhibit high sedimentation rates reaching 3.4 cm/year. The asymmetry of the delta is explained not only by the wind- and wave-induced westward-directed longshore drift but also by neotectonic processes, as revealed by satellite images. Faulting and eastward tilting may have triggered delta lobe switching from west to east. This would explain the erosional character and unusual updrift orientation of the main rivermouth channel. Consistent with existing knowledge on mangrove ecosystems worldwide, sediment carbon and nitrogen signatures lie in the range of freshwater or marine dissolved organic carbon and C3 terrestrial plants. In the western tidal channels, the low Corg/Ntot ratios (16–21) of young mangrove soil (deposited in the last 16 years) reflect a stronger influence of marine plants compared to older mangroves (1,390–1,525 cal. years BP; ratios of 20–37). Thus, there would have been a greater influence of the Parnaíba River on tidal-channel sedimentology 1,400 to 1,500 years ago, entailing a natural connection between the present-day tidal channels and the river in ancient times, which was abandoned later during delta lobe switching. This is substantiated by historical maps that indeed show this connection between the main distributary and the tidal-channel system.
Article
Sand spits are important coastline features in western Angola, but only limited knowledge on their recent evolution and sediment sources were obtained so far. The present study is focused on the Baía Farta and Lobito sand spits of coastal Benguela that develop to the north (i.e. downdrift) of the Coporolo and Catumbela river outlets. We used grain-size distributions, heavy-mineral suites and clay-mineral assemblages of sediments in the Coporolo-Baía Farta and Catumbela-Lobito coastal stretches to characterize the main depositional units and investigate sediment provenance. From the combined grain-size and mineralogical variability in mud and sand samples it is possible to infer sediment sources and dispersal in the two coastal stretches. Kaolinite is mainly derived from the Angola hinterland, and is particularly common in finer grained floodplain sediments from the Catumbela River. Expansive clays (smectite and illite-smectite mixed layers) are inferred to be mainly sourced by Meso-Cenozoic units of the Benguela Basin, being abundant in coarser grained fluvial deposits and in lagoonal deposits near Baía Farta. Sand supplied by the sedimentary units from Benguela Basin and their basement rocks tend to be enriched in epidote associated with blue-green hornblende. The Coporolo River sand is progressively diluted during the longshore northward transport by sand supplied by coastal units. Conversely, beach deposits in the Catumbela-Lobito coastal stretch are mainly sourced by the Catumbela River. A divergent longshore transport from Catumbela river-mouth occurs at Catumbela delta. Sand spit morphology and evolution reflect the patterns of dispersal of bedload and suspended load in settings of contrasting orography and human influence.
Article
Fractions composed of grains having uniform fall velocity were extracted from beach sands collected along the eastern end of Lake Ontario. Heavy-liquid separations and point counts were performed on these fall-equivalent velocity splits to obtain relative abundances of hornblende, augite, hypersthene, garnet, magnetite, and quartz-density lights. Heavy minerals decrease in abundance in the direction of transport, and the degree to which any particular mineral lags behind another lighter mineral is a simple function of the mineral's effective-density ratio. Our results appear to confirm that heavy minerals are less transportable than fall-equivalent lights, an effect that may result from differential entrainability, transport within different zones of the beach, or both. -from Authors
Article
Researchers studying exogenous processes in developing countries have to deal with the problem of scarcity of rainfall data. With satellite measurements, such as those provided by the Tropical Rainfall Measuring Mission (TRMM), it may be possible to overcome this limitation. In this work we link the TRMM rainfall measurements with two examples of exogenous processes from southwest Angola: landslide processes, focused on the events at the Leba road in early 2011, and flow conditions, based on the water level determined in gauging stations from rivers Cavaco and Catumbela. It is demonstrated that major mass flow movements occur when specific TRMM rainfall thresholds are reached. Regarding the flow conditions, the water level in two gauging stations is strongly conditioned by other factors beside the atmospheric precipitation in their watersheds (retention and release of water from reservoirs, channel obstruction, etc.), but short term oscillations are closely linked with the rainfall in the proximity. TRMM data is found to be very useful for the analysis of specific extreme events or the patterns of behavior of natural systems and, consequently, constitute a valuable tool in natural risks assessment.
Article
Understanding controls on sediment distribution in river deltas is paramount to predicting facies relationships and stratal architecture. Most classifications of deltas have focused on subaerial plan morphology as a simplistic function of river, tide, and wave energy. New work, however, is revealing the importance of subaqueous deposits, and in particular shore-parallel sediment redistribution, in the shaping of delta planform. Delta asymmetry has emerged as an important characteristic reflective of patterns of alongshore sediment dispersal. Asymmetry has been described from onshore and offshore environments from several different types of modern deltas, but aspects of asymmetry have not been fully documented and the degree to which these patterns are recorded in deltaic strata is not yet known. This study is a comprehensive literature review of sediment distributions in modern deltas focusing on studies with high resolution geomorphic, geophysical, and geochronological datasets. We reviewed 27 deltas using over 100 papers primarily from the past 15–20 years. Morphological, facies, and stratigraphic aspects were analyzed across the entire spectrum of deposits from the delta apex to the most distal muds of the prodelta. We define quantifiable indices of asymmetry describing upcurrent vs. downcurrent distribution of sediment volume, sediment area, sediment caliber, and distributary channels. Most deltas are asymmetrical to some degree with respect to one or more of these parameters. Many deltas are increasingly skewed toward the downcurrent side from proximal to distal sub-environments (i.e. delta plain, delta front, prodelta). Some deltas are skewed toward the upcurrent side in one sub-environment and toward the downcurrent side in another sub-environment. Sand is preferentially deposited on the downcurrent side of most deltas, but distributary channels tend to develop toward the upcurrent side. The highest sand:mud ratios are often on the upcurrent side in the lower delta plain, but in the delta front these ratios are highest on the downcurrent side. These complex patterns of asymmetry reflect different combinations of controls including discharge partitioning, lobe abandonment and localized transgression, plume deflection by coastal currents, dominance of longshore current direction, variable subsidence, and other factors. These processes may result in upcurrent–downcurrent variations in clinoform geometry, rates of progradation, and stratal lapping relationships. Asymmetry has multiple aspects, manifestations, and controls – even within a single delta – but the long-term preservation potential of these patterns likely varies, depending on depositional setting and shoreline trajectory. Studies of ancient deltas will be better-informed by recognizing the wide variety of controls on sediment distribution and avoiding the tendency toward a single model of delta asymmetry.
Article
Waves are an important agent in the construction, shaping and destruction of river deltas. Notwithstanding the commonality of waves in oceans and seas, wave influence on deltas varies considerably depending on the coastal morphology and nearshore bathymetry. Although there have been advances in understanding the way waves approaching a delta shape its shoreline, much still remains to be known of the interactions between waves and river deltas. Deltas are built essentially from sediments supplied by rivers. Sand-sized and coarser sediments may also be derived from nearby abandoned delta lobes or from older relict nearshore deposits, transported by wave reworking and longshore currents. Alternatively, delta erosion by waves can also release sediment that is redistributed alongshore or that accumulates offshore. The extent to which bedload is supplied to and sequestered in, or lost by, deltas through waves and longshore transport strongly depends on interactions between waves and fluvial discharge at the river mouth. These interactions and the mutual adjustments they engender are not only important in the overall balance between delta retreat, progradation or aggradation but also in processes such as avulsion and channel switching, as well as in the eventual survival of a delta in the face of sea-level rise. Where waves are important, fluvial liquid discharge high, and sediment supply rich in bedload two important aspects are the blocking of waves and longshore currents by strong river discharge and the formation of bars at the river mouth. Field studies of the complex interactive processes prevailing where river flows encounter waves are, however, non-existent and numerical modelling, though promising, hampered by scale constraints and the difficulty of replicating them and generating mouth bars in the presence of longshore currents. This interaction influences the seaward extent of the delta mouth protuberance and its stability, this protruberance then forming the regional shoreline template to which waves and longshore currents adjust. Longshore currents can redistribute wave-reworked mouth bar deposits emplaced during strong river flow. Transport may be either divergent from the mouth or may be regionally unidirectional but wherein the symmetry of some deltas, probably rare, may be maintained by a strong river blocking effect on transport from the updrift flank. The mouth protruberance may be such as to foster transport reversal (counter-drift) at the delta margins that contributes to sediment sequestering within the delta. These interactions largely contribute in shaping delta shorelines, and together with the abundance of sediment supply and grain size, determine the resultant wave-formed shoreline barrier types, which include spits, more or less closely-spaced beach ridges, and barrier islands and cheniers in situations of punctuated progradation or retreat. Where several distributary mouths occur, pronounced longshore variability in wave processes and wave-induced sediment transport may ensue, resulting in multiple drift cells that assure the retention of sand and coarse-grained sediments within the delta. Waves can also be an important agent in the reworking and retreat of mud-rich deltas that generally conform in morphology to the ‘river-dominated’ (such as the Mississippi) or ‘tide-dominated’ (such as the Ganges-Brahmaputra or Chao Phraya) types, resulting in the episodic formation of sandy cheniers and beach ridges.