Shoreface Sediments Research Articles

Assessing shoreface sediment transport at Costinha beach, Aveiro, Portuguese Northwest coast

Coastal zones are dynamic environments where sediment transport significantly influences erosion, deposition, and ecosystem stability. Accurate modelling of the complex interactions between hydrodynamics, sediment transport, and morphological changes in high-energy shoreface areas is crucial for sustainable coastal management. This study aims to assess sediment dynamics and understand the dominant physical drivers of sediment transport in the shoreface by coupling high-resolution numerical modelling with a shoreface sand tracer experiment. Results from the field experiment showed high diffusion and a net onshore and downdrift movement of the tracer in agreement with the observed hydrodynamic data. The numerical modelling strategy, validated with in situ hydrodynamic data and tracer results, demonstrated good performance and highlighted the dominant modes of shoreface sediment transport. While during low energetic wave conditions, the dominant transport mechanism involves the slow-rate bed load sediment movement toward the shore, under moderate energetic wave conditions, longshore wave-induced suspended transport becomes the prevailing mode of transport. The study provides valuable insights to increase the understanding of sediment transport in the shoreface and highlights the importance of coupling numerical modelling with sediment tracer experiments.

Tidal Modulated Shoreface Facies in Eocene Agbada Formation, Niger Delta Basin, Nigeria

Tidal influence shoreface are shoreface deposit generated by wave induced tidal action. The extent of the tidal impact from lower to upper shorefaces depend mostly on the tidal range thereby controlling the impact of different shoreline wave action on sediment distribution across shorefaces. The Tidally influenced shoreface sediment display wave generated structure, however the structure are strongly associated with tidal generated structures and trace fossil consistent with tidal depositional setting. Shoreface successions in Titun well display a wide range of variability, largely restricted to the lower and middle shoreface intervals. This variation was controlled by relative wave dominance and intermittent pulsating nature of associated tidal bed deposition. The successions were subdivided into three end members, although a complete gradation between them exists. High energy shoreface consist almost entirely of erosively based amalgamated tide and wave beds with minimal bioturbation. Intermediate energy shoreface consist of stacked laminated to burrowed intervals, recording repeated erosive storm bed emplacement, followed by opportunistic colonization, and commonly overprinted by resident fairweather burrowing. Weakly storm-affected low energy shorefaces typically show intense bioturbation with minimal preservation of high energy beds. Based on the association of the facies, facies vertical build-up patterns, sedimentary structures and trace fossil content, the T1 sands in Titun well are interpreted as a prograding tidally influence shoreface deposit. This interpretation is supported by presence of tide generated structures and marine trace fossil, due to the prevalence of several tidal imprint as suggested by several mud drapes and flasers, couplets of sands and shale’s in heterolithic sequences, and bidirectional current structures (herringbone cross stratification), a tidally influence shoreface geological model is suggested for deposit.

Antecedent geological control on transgressive delta and shoreline preservation: Examples from the SE African shelf

Using a suite of ultra-high resolution geophysical tools, remote operated vehicle dives, legacy cores, and isolated grab samples, we demonstrate at a regional shelf scale, the influence of antecedent geology (basement topography, shelf gradient and submerged shoreline features) on the evolving transgressive shelf stratigraphy of a subaqueous delta. The unconsolidated uppermost delta occurs as isolated remnants scattered across the low-gradient shelf. Seismic data reveal across-shelf heterogeneity in bedrock elevation, with prominent bedrock highs and depressions and several well-preserved aeolianite palaeo-shoreline complexes at water depths of −100, −60 and − 40 m. Analysis of bathymetric and seismic data demonstrates that these pre-existing shoreline complexes exert an overarching control on the distribution patterns of i) deltaic sediments, where they abut the landward flank of the shoreline form, acting as a barrier to seaward dispersal, and ii) shoreface sediments which remain sequestered on the mid-shelf on the seaward flank of the shoreline complex, hampering the landward translation of the shoreface in step with rising sea levels. Moreover, sediment distribution/accumulation is further constrained where elevated portions of the basement topography provide little to no accommodation, and act as zones of sediment bypass, and adjacent basement depressions accommodate sediment to seaward. The gentle antecedent slope, coupled with the gentle shoreline trajectory mediated transgressive erosion directly to landward. This is reinforced where local inflections in the wave ravinement profile form due to the presence of palaeo-shoreline complexes, aiding in the ultimate preservation of these submerged delta facets in the palaeo-shoreline lee. This study shows that low antecedent gradients, and palaeo-shoreline features, lead to development of transgressive coastal profiles that are predisposed to delta overstepping. The pre-existing basement topography and palaeo-shorelines constrain the positioning and morphology of the delta. We suggest that antecedent conditioning has partitioned accommodation for delta accumulation and moderated wave ravinement associated with transgression since the Last Glacial Maximum (LGM). The geological framework has acted as a recurring primary control on the geomorphic evolution of the submarine delta and shelf.

Geologic Framework, Anthropogenic Impacts, and Hydrodynamics Contribute to Variable Sediment Availability and Shoreface Morphology at the Rockaway Peninsula, NY

Recent field and modeling studies have shown that barrier island resiliency is sensitive to sediment fluxes from the shoreface, making it important to evaluate how shoreface sediment availability varies in coastal systems. To do this, we assessed shoreface geology and morphology along the Rockaway Peninsula, NY, USA. We find that spatial variability in shoreface volume is influenced by sediment accommodation above the Holocene-Pleistocene (H-P) contact, historical barrier island evolution, and natural and engineered morphologic features, suggesting that simply identifying the H-P boundary may not be adequate for defining the shoreface reservoir. Further, sediment flux from the lower shoreface to the beach may be reduced by geologically limited cross-shore sediment distribution and shoreface steepening mediated by human modifications to the shoreline. Finally, the geologic limit of the shoreface is often shallower than a wave-based estimate of shoreface extent, implying that the geologic shoreface extent at our study site can be mobilized over short time scales (years-decades) and that the wave-based shoreface extent may be inaccurate when estimating shoreline response to sea-level rise. Our results demonstrate that the combination of hydrodynamics, humans, and geology on shoreface sediment fluxes impact how barrier islands respond to future changes in sediment supply and climate.

Stormier mid-Holocene southwest Indian Ocean due to poleward trending tropical cyclones

Geological evidence of past storminess is fundamental for contextualizing long-term climate variability and investigating future climate. Unlike the Atlantic and Pacific basins, robust storminess reconstructions do not exist for most of the Indian Ocean, despite the hazard that tropical cyclones pose to the SE African margin. Here we combine seismic stratigraphy with analysis of marine sediment cores to look for regionally representative storm-related sediment deposits (tempestites) intercalated in shoreface sediments from the SW Indian Ocean off South Africa. Tempestites, represented by hummocky seismic units, whose sediments have clear marine geochemical signatures, are found to have been deposited between 7.0 and 4.8 cal kyr BP, when sea level was between 0 and +3 m above present. Deposition and preservation of the tempestites reflect unprecedented tropical cyclone impacts, associated with periods of strongly positive Indian Ocean Dipole anomalies and linked to warmer sea surface temperatures. Future climate projections suggest stronger positive IOD anomalies and further intensification and poleward migration of tropical cyclones, like their mid-Holocene predecessors. Given the rarity of tropical cyclone landfalls in South Africa, this urges a revaluation of hazards in areas along the SE African coast likely to become more vulnerable to landfalling tropical cyclones in the future. Tropical cyclones frequently hit SE Africa in the mid-Holocene during positive Indian Ocean Dipole phases according to analysis of storm-related tempestite deposits in shoreface sediments off South Africa.

From rapid coastal collapse to slow sedimentary recovery: The morphological ups and downs of the modern Po Delta

As is the case with many deltas in the world, the Po Delta (Italy) and its barrier systems have experienced severe modifications over the last century as a consequence of anthropogenic subsidence and river input decrease. The interruption of subsurface fluid withdrawals in the early 1960s created an opportunity to evaluate the effects of the subsequent subsidence decrease along the 11 km of beaches and 45 km of barrier islands of the Po Delta, and improve the understanding of coastal system response to relative sea level rise. To this aim, shoreline and bathymetric multi-decadal dataset were coupled with the local rate of subsidence and classified with a geomorphic - response approach. From 1954-78, a mean shoreline variation of −224 m (−9.34 m y−1) occurred under high rates of subsidence (14–43 mm y−1) with a diffuse process of landward barrier rollover (up to 1124 m), frequently associated with break-up. This phase is evidenced by the diffuse sinking of the shoreface (up to 4 m), the negative upper shoreface sediment budget and the parallel drowning of the cross-shore profiles highlighted by the 1968–2008 bathymetric dataset. Despite the loss of altimetry and shoreline retreat being almost irrecoverable, during the period from 1978 to 1996 and 1996 to 2014 barrier islands experienced a stabilisation in response to a decrease in the subsidence rates down to 6 mm y−1. The mean rates of shoreline changes (−0.25 and + 1.51 m y−1 respectively) are the results of different local behaviour with progressively dominant longshore processes and progradation at the most active fluvial branches. The most recent bathymetric data (2008–2014) confirm this reversal trend, both in the altimetry differences and in the upper shoreface sediment budget. This trend is at odds with signals from many of the world's deltas and represents an important issue for the present and future coastal management and adaptation plans.

Impacts of Engineered Diversions and Natural Avulsions on Delta‐Lobe Stability

AbstractReduced sediment supply and rising sea levels are driving land submergence on deltas worldwide, motivating engineering practices that divert water and sediment to sustain coastal landforms. However, lobe response following channel abandonment by diversions has not been constrained by field‐scale studies. Herein, avulsion and engineered diversion scenarios are explored for the Huanghe delta (China), where three lobes were abandoned in the last 40 yr. Two lobes were completely cut off by diversions, and one naturally by an avulsion. Shoreline retreat rates are strikingly different: ∼400 m/yr for diverted lobes and ∼90 m/yr for avulsed lobe. We hypothesize that this variability is linked to vegetal cover across lobes, and therefore the capacity to buffer hydrodynamic reworking of shoreface sediment. Furthermore, the vegetal cover is related to lobe salinity and elevation, which vary by abandonment style. We offer this as a case study to inform about the efficacy of future delta diversions.

SHOREFACE SEDIMENT AVAILABILITY ON NATURAL AND DEVELOPED COASTS

Sediment availability and how sediment is exchanged within a coastal system are key components of coastal resilience, particularly for barrier islands. However, little is known about shoreface sediment distribution and geology, including to what extent it is linked to coastal change. Ultimately, understanding how natural and human-induced variability combine to influence shoreface sediment availability could provide insight to future coastal resilience to both storms and sea-level rise. To explore this, we used vessels and equipment adapted for very shallow water depths and conducted shoreface geophysical surveys along three barrier islands on the east coast of the United States with varying degrees of human intervention.Recorded Presentation from the vICCE (YouTube Link): https://youtu.be/5qF4EeTKTjg

Embayment morphometrics, granulometry and carbonate mineralogy of sandy beaches in the Maltese Islands

The beach and shoreface sediments in the Maltese Islands have accumulated in under-filled accommodation space conditions like many temperate and tropical island carbonate beach and shelf environments. Nevertheless, Maltese beach sediments show anomalously low biogenic carbonate content compared to other carbonate beaches. Fourteen carbonate sandy beaches in the Maltese Islands were sampled to examine the relationships between geological controls on both coastal lithology and configurations of bays and beaches and, grain characteristics described by granulometry and mineralogy. The provenance of the medium to coarse-grained beach sand is mainly from the erosion of coastal and shelf Cenozoic limestone that produces lithoclasts of low-magnesium calcite, with an additional 20 ± 10% biogenic grains derived from marine carbonate productivity. The relationships between geological controls and beach sediments are used to establish a general model of sedimentation along the Maltese shelf area. Mean beach sand size is controlled by the degree of embayment, decreasing where bays are narrower and longer, whereas sediment provenance affects granulometry by the contribution of biogenic grains to the coarse and fine grain size fractions. The mineralogy of the biogenic sediment fraction in beaches is mainly high-magnesium calcite and aragonite, with the latter decreasing significantly where coastal clay is found within or close to bays.

Interplay of sediment supply and lake-level changes on the margin of an intrabasinal basement high in the Late Miocene Lake Pannon (Mecsek Mts., Hungary)

Lake Pannon, covering the Pannonian Basin (Hungary) during the Late Miocene, had a complex lake bottom with deeper sub-basins and intrabasinal basement highs, sometimes emerging above the lake level as islands and peninsulas. Above structural highs, the basin fill sequence usually commenced with deposition of transgressional, locally sourced coarse-grained deltas. These deposits are overlain by distally-sourced deltaic bodies associated with the prograding delta system that gradually filled up the entire basin. The transition between these two distinct delta systems was studied in a large outcrop on the edge of a former basement high (Mecsek Mts., southern Hungary). The transgressional phase is represented by the deposition of a mass flow dominated fan delta body, fed by local material from a granitic catchment area. An overlying fossil-rich, clayey unit records an episode for which sediment delivery into the basin was curtailed, possibly due to submergence of the granite body. The deposition of these two units took place between 8 and 6.8 Ma. The onset of sedimentation associated with the arrival of the distally sourced regressive delta system (around 6.8 Ma) is represented by deposition of shoreface sediments. This unit is characterized by distinctive bioturbation and storm related, sand filled scour-fills. Resedimented local material that forms mm thick, coarse laminas in the scour fills is indicative of denudation due to tectonic events and implies coupled storm-flood sedimentation. The deposition of the two distinct delta bodies and the interplay between tectonic events and lake-level changes occurred during a relatively short, ca. 1.5 Ma long time interval.

3D stratigraphic forward modeling of an ancient transgressive barrier system: A case study of accuracy and sensitivity

Accuracy of geological modeling and understanding of the controlling parameters in sedimentary systems evolution are challenges in both academy and industry. Uncertainty in the prediction of spatial and temporal facies distributions and difficulties in quantitative analysis of controlling parameters are current challenges. To increase geological model certainty and sedimentary process understanding, computational modeling has evolved exponentially during the last 50 years. In this study, 3D stratigraphic forward modeling technique is used to simulate an ancient transgressive barrier system of southern Brazil, deposited during the Lower Permian in the Paraná Basin, upper part of the Rio Bonito Formation. In the study area, the depositional systems are part of a large transgressive cycle that includes fluvial-estuarine facies at the base and barrier-lagoon deposits (associated with the development of salt marshes) at the top. Throughout the region, offshore strata cover the barrier-lagoon facies. The simulation successfully represented the retrograde evolution of the coastal barrier system, with longshore currents as the main transport mechanism of sediments. The evolution of the barrier system started with upper shoreface deposits overlying the lagoon system facies in the south. Later, lower shoreface sediments covered upper shoreface deposits. Then, shoreline migration continued landward. Finally, offshore transition deposits covered all previous deposits. Simulations and tests of controlling parameters allowed an adjustment to actual thickness and lithologies up to 83%. These simulations allowed for the analysis of model sensitivity, in particular to the influence of parameters that control sedimentary evolution. The main controlling parameter was sediment supply (volume and composition), followed by time step. Initial bathymetry and sediment velocity had less of an impact.

Reconstructing Coastal Sediment Budgets From Beach‐ and Foredune‐Ridge Morphology: A Coupled Field and Modeling Approach

AbstractPreserved beach and foredune ridges may serve as proxies for coastal change, reflecting alterations in sea level, wave energy, or past sediment fluxes. In particular, time‐varying shoreface sediment budgets have been inferred from the relative size of foredune ridges through application of radiocarbon and optically stimulated luminescence dating to these systems over the last decades. However, geochronological control requires extensive field investigation and analysis. Purely field‐based studies might also overlook relationships between the mechanics of sediment delivery to the shoreface and foredune ridges, missing insights about sensitivity to changes in sediment budget. We therefore propose a simple geomorphic model of beach/foredune‐ridge and swale morphology to quantify the magnitude of changes in cross‐shore sediment budget, employing field measurements of ridge volume, ridge spacing, elevation, and shoreline progradation. Model behaviors are constrained by the partitioning of sediment fluxes to the shoreface and foredune ridge and can be used to reproduce several cross‐shore patterns observed in nature. These include regularly spaced ridges (“washboards”), large singular ridges, and wide swales with poorly developed ridges. We evaluate our model against well‐preserved ridge and swale systems at two sites along the Virginia Eastern Shore (USA): Fishing Point, for which historical records provide a detailed history of shoreline progradation and ridge growth, and Parramore Island, for which a relatively more complex morphology developed over a poorly constrained period of prehistoric growth. Our results suggest this new model could be used to infer the sensitivity of field sites across the globe to variations in sediment delivery.

Critical Examination of Shoreface Sediment Volume as an Indicator of Coastal Stability in Poland

Critical Examination of Shoreface Sediment Volume as an Indicator of Coastal Stability in Poland

Lower shoreface seismic stratigraphy and morphology off Fire Island, New York: Evidence for lobate progradation and linear erosion

Under rising sea level conditions, barrier islands are largely ephemeral features, eroded on the seaward side by the transgressing shoreline and reformed by overwash to a more landward position. Locally, however, and over shorter time scales, shorelines can either advance or retreat, even in an overall transgressive environment, and the stratigraphy and morphology of the shoreface can be significantly impacted by the evolution of shoreface-attached bedforms. Fire Island, New York, is a well-studied example of such variability, with a stable-to-accreting shoreline at the western end and a retreating shoreline on the eastern end. In this study, we seek to better understand these differences by investigating the lower-shoreface stratigraphy at both stable/accreting (Fire Island West, or FIW) and retreating (Fire Island East, or FIE) shorefaces, using ultra-high resolution chirp seismic reflection data. Within the barrier/marine sands (the seismic unit between seafloor and shoreface ravinement), we identify six seismic units (WSUs 1–6 from bottom to top) in the FIW survey and two units (ESU1 and ESU2 from bottom to top) in the FIE survey; these units constitute the modern lower shoreface wedge. The barrier shoreface in the FIW survey is dominated by discrete and spatially-confined lobes. Isopach maps indicate that the lobe shifting was an episodic process with westward-migrating depocenters. The prograding shoreface was constructed by this lobate deposition; we speculate that these are related to ebb deposition from ephemeral barrier breaches/inlets. In the FIE survey, ESU2 accounts for the majority accumulation of the barrier shoreface and it is more linear than the lobate structure observed within the FIW survey, possibly derived from eroded shoreface sediments. Portions of this unit are absent however, exposing lower Pleistocene units to the erosive forces.

Early Stage Evolution of Nourished Beach under High-energy, Macro-tidal Environment

Beach planform evolution, profile equilibration and sediment grain size change have been studied during the first 4 months from 4th September to 24th December 2011 after the construction of beach nourishment project at Longfengtou Beach, Haitan Bay. Monthly beach profiles, shoreline surveys, sediment sampling and nearshore wave measurements were carried out after implementation of the 1.3km long nourishment project which was completed on 20th August 2011. This study indicates that: (1) rapid beach profile equilibration occurred in the early stage after the construction of the project. A null point was observed, which is equal to the height of mean high tide, basically kept dynamic stable during the process of profile evolution. Shoreface sediment accumulated beneath the height of this point while erosion happened above it, the slope between the beach berm and the landward edge of low tidal zone became more gradual accompanied with seaward transportation of beach sediment. The velocity of beach slope adjustment in earlier period is faster than later. (2) Beach planform adjustment initiated simultaneously with the combination of the process of profile equilibration and longshore sediment transport. Shoreline retreated with an average distance of 11.1m and maximum of 31.02m from 4th September to 24th December, erosion in the south part was more serious than in the north, and 3 erosion hot spots were found along the coast. (3) Sediment redistributed with cross-shore profile equilibration, it showed a pattern across beach profile as medium sand (0.4-0.5mm) in beach berm, smaller (0.3-0.4mm) in high and middle tidal zone, coarse sand(0.6-1mm) in beach slope transitional zone, fine sand(0.1-0.25mm) in low tidal zone. The sediment grain size change of foreshore was rapidly response to the passage of storm surge.

Facies analysis and depositional model of the Serravallian-age Neurath Sand, Lower Rhine Basin (W Germany)

AbstractThe up to 60 m thick Neurath Sand (Serravallian, late middle Miocene) is one of several marine sands in the Lower Rhine Basin which were deposited as a result of North Sea transgressive activity in Cenozoic times. The shallow-marine Neurath Sand is well exposed in the Garzweiler open-cast mine, which is located in the centre of the Lower Rhine Basin. Detailed examination of three sediment profiles extending from the underlying Frimmersdorf Seam via the Neurath Sand and through to the overlying Garzweiler Seam, integrating both sedimentological and palaeontological data, has enabled the depositional setting of the area to be reconstructed.Six subenvironments are recognised in the Neurath Sand, commencing with the upper shoreface (1) sediments characterised by glauconite-rich sands and an extensive biota (Ophiomorphaichnosp.). These are associated with the silt-rich sands of a transitional subenvironment (2), containingSkolithos linearis, Planolitesichnosp. andTeichichnusichnosp. These silt-rich sands grade up to the upper shoreface subenvironment (1), which is indicative of an initial regressive trend. The overlying intertidal deposits can be subdivided into a lower breaker zone (3), characterised by ridge-and-runnel systems, and the swash zone (4) where the surge and backwash of waves resulted in the deposition of high-energy laminites. The intertidal deposits were capped by aeolian backshore sediments (5). Extensive root traces present in this latter subenvironment reflect the development of the overlying peatland (i.e. Garzweiler Seam). Within the Garzweiler Seam, restricted sand lenses indicate a lagoonal or estuarine depositional environment (6). Regional correlation with adjacent wells establishes that shallow-marine conditions were widespread across the Lower Rhine Basin in middle Serravallian times. The shoreline profile, characterised by both tidal and wave activity and influenced by fluvial input from the adjacent Rhenish Massif, is indicative of the complexity of the coastal depositional setting within the Lower Rhine Basin.

Sandy barrier overstepping and preservation linked to rapid sea level rise and geological setting

In an era of global rising sea level, quantifying future shoreline behaviour is a key societal concern. The potential for dramatic shoreline change via overstepping or barrier disintegration on barrier-lagoon coasts as a result of future rapid sea level rise has been considered by various authors, but documented examples of barrier overstepping generally involve either coarse grain sizes or early diagenesis as beachrock. Where overstepping has been inferred, the overstepped barrier is seldom preserved. Using high resolution seismic data we describe the mid-Holocene overstepping of a sandy coastal barrier system and subsequent geomorphological changes that ultimately transformed it into a strandplain fronted by a muddy shoreface. This complete change in character is interpreted in the context of rapid sea level rise (tentatively linked to the 8.2ka event) during which the former sandy shoreface-barrier was overstepped and decoupled from the contemporary shoreline, leaving the latter sand-starved. Upon overstepping, the wave-influenced shoreline was displaced rapidly to the landward margin of the former lagoon. Preservation of the overstepped uncohesive sandy barrier is attributed to the relatively wave-sheltered location, rapid sea level rise and rapid burial by shoreface sediments. This situation provides an insight into the conditions under which overstepping occurs, and thereby, the future response of barrier-lagoon systems to predicted rapid rates of future sea level rise.

Giant rafted pumice blocks from the most recent eruption of Taupo volcano, New Zealand: Insights from palaeomagnetic and textural data

Giant blocks of pumice lie strewn along a former shoreline of intracaldera Lake Taupo, New Zealand, and are the sole subaerial evidence of the most recent volcanism at the Taupo supervolcano. Geochemically they are identical to material erupted during the complex and multiphase 1.8ka Taupo eruption, which they post-date by one to two decades. The blocks, some of which are >10m long, show complex jointing patterns indicative of both surface chilling and continued interior expansion, as well as heterogeneous vesicularity, with dense rims (mean density 917kg/m3) grading via an intervening transition zone (mean density 844kg/m3) into a more highly vesicular interior (mean density 815kg/m3). Analysis of thermal demagnetisation data indicates significant reorientation of the blocks as they cooled through a series of blocking temperatures. Some parts of block rims cooled to below 580°C well before emplacement on the shore, whereas other parts in the interior and transition zones, which cooled more slowly, acquired different orientations before stranding. Some block interiors cooled after blocks were finally deposited, and record the direction of the 1.8ka field. The blocks are believed to be derived from one or both of a pair of rhyolitic lava domes that developed on the bed of Lake Taupo several decades after the climactic Taupo eruption over the inferred vent area.These, and similar giant rafted pumice blocks in other marine and lacustrine settings raise a number of questions about how volatile-rich felsic magma can be erupted underwater with only limited thermal fragmentation. Furthermore, the prolonged flotation of out-sized fragments of vesiculated magma formed during subaqueous dome-growth contrasts with the rapid sinking of smaller pieces of hot plinian pumice under laboratory conditions. The genesis of pumice forming the blocks is not entirely clear. Most simply the blocks may represent part of a vesiculated carapace of a growing lava dome, broken loose as the dome grew and deformed then rising buoyantly to the surface. Parts of the carapace could also be released by local magma-water explosions. Some textures of the pumice, however, suggest fresher magma released from beneath the carapace. This may suggest that silicic dikes and pillows/pods intruded into a growing mound of silicic hyaloclastite, itself formed by quench fragmentation and thermal granulation of the dike margins. This fragmental cover would have inhibited cooling of a still-hot and actively vesiculating interior, which was then released to float to the surface by gravitational destabilisation and collapse of the growing pile. Following their formation, the large fragments of pumice floated to the lake's surface, where they were blown ashore to become embedded in accumulating transgressive shoreface sediments and continue cooling.

Depo sequence as tool to convert shoreline trajectory from space domain to time domain

Deposystems are complex and governed by discrete depo events with variable intervals of stasis or erosion in between. Since shoreface sediments indicate sea level, depo events of shoreline facies are discrete samples of sea level. Only if these samples are sufficiently regular and frequent will the shoreline trajectory in the space domain accurately reflect the sea-level curve in the time domain. This study presents a method to convert shoreline trajectory in the space domain to sea-level curve in the time domain from artificial miniature deltas. One obtains the depo sequence as function of time and uses it to: convert depo sequence from time-to space domain, correlate depo sequence to shoreline trajectory in the space domain, and convert shoreline trajectory from space-to time domain. For natural deltas one would extract the depo set in the frequency domain, i.e. the probability density function of stasis intervals between depo events from the experimental depo sequence and use it to: convert shoreline trajectory from space-to time domain, and infer a range of possible sea-level curves. This method therewith explicitly includes the uncertainty of the inferred sea-level curve.

Magnetic properties of sediments of the Red River: Effect of sorting on the source‐to‐sink pathway and its implications for environmental reconstruction

AbstractWe conducted a mineral magnetic study of river bank and subaqueous delta sediments from the Red River, in order to examine the role of sedimentary sorting on the variation of sedimentary magnetic properties from source to sink. The magnetic mineralogy mainly consists of magnetite and hematite. Bulk sediment particle‐size variations have a strong influence on magnetic properties, with the frequently used magnetic parameters χfd%, χARM, χARM/χ, and χARM/SIRM exhibiting positive correlations with the <4 µm fraction, while S‐ratios are negatively correlated with this fraction. Compared with river bank sediments and shallow shoreface (<5 m water depth) sediments, sediments from the deeper (>5 m water depth) part of the subaqueous delta have lower χ and SIRM values, a finer ferrimagnetic grain‐size and higher proportions of hematite, consistent with selective loss of coarse ferrimagnetic grains on the source‐to‐sink pathway. We suggest that variations in magnetic properties in response to particle‐size compositions and therefore depositional environment changes should be carefully addressed when magnetic proxies such as χARM/SIRM are used in the study of coastal and marine environmental changes (e.g., sea‐level change). In such cases, the combined use of magnetic properties and geochemical indicators, such as Al/Ti ratio, may provide better results for paleoenvironmental reconstruction.