Microtidal Settings Research Articles

Hierarchies of stratigraphic discontinuity surfaces in siliciclastic, carbonate and mixed siliciclastic‐bioclastic tidalites: Implications for fluid migration in reservoir quality assessment

AbstractThe hierarchies of the stratigraphic discontinuity surfaces observed in ancient tidalites are qualitatively assessed, aiming to evaluate their role as possible preferential conduits for fluid migration. Three outcrop examples are presented from microtidal settings of southern Italy: (i) siliciclastic tidalites consisting of quartz‐rich cross‐stratified sandstones generated by strong two‐directional tidal currents flowing along a tidal strait; (ii) carbonate tidalites, which accumulated in a Cretaceous lagoon and tidal flat where peritidal cycles formed vertically‐stacked sequences of biopeloidal and fenestral packstones, wackestones and bindstones during repeated phases of Milankovitch‐scale sea‐level changes; (iii) mixed, siliciclastic‐bioclastic tidalites, deposited in a bay and recording offshore‐transition, to shoreface wave‐dominated and tide‐influenced environments. Observations made during this study suggest that fluid movement can be controlled by the presence of main bounding surfaces that occur at different dimensions, from large (hectometre)‐scale, to medium (decametre)‐scale, to smaller (metre)‐scales. These surfaces produced either by depositional or erosional processes, are characterised by different features and geometries in siliciclastic, carbonate and mixed siliciclastic‐bioclastic tidalites arguably revealing complex internal pathways for fluid flows. These results suggest that fluids propagating along the main discontinuities follow a dominant sub‐horizontal direction of propagation, associated with minor sub‐vertical movements, due to local internal surface geometries and interconnections and a general lack of fractures. This surface‐based approach to the study of fluid‐flow transmission within stratified rocks represents a conceptual attempt to predict fluid mobility and reservoir potential in tidalite‐bearing siliciclastic, carbonate and mixed reservoir rocks.

Gravelly Beach Deposits as a Proxy for Relative Sea-Level Changes in Microtidal Wave-Dominated Shoreline Systems: Examples from the Hinterland of the Taranto Gulf (Middle Pleistocene, Basilicata, Southern Italy)

The hinterland of the Taranto Gulf in Basilicata (Southern Italy) provides a great opportunity for the study of coarse-grained coastal systems belonging to a staircase of Quaternary terraced marine-deposits. Among gravelly successions, beach deposits abound in the stratigraphic record, offering exceptional outcrops useful for providing detailed information on their facies features. In this paper, we describe sedimentary facies, textural variations, and the depositional architecture of these deposits in order to: (1) demonstrate that the area is an excellent training ground for the study of gravelly beaches in microtidal settings; (2) discuss the use of beach deposits as a proxy for even small relative sea-level variations.

Climatic control on the formation of marine-notches in microtidal settings: New data from the northwestern Mediterranean Sea

The genesis and distribution of marine notches around the microtidal Mediterranean basin has been widely debated in recent years. Here we provide new climate and geomorphological insights into the factors controlling notch formation based on the bathymetric distribution of marine notches found in Marseille Bay (NW Mediterranean). In this area, the notches exist (i) either near present Mean Sea Level (MSL); or (ii) at ∼35 cm below the MSL, but with no notch present at higher elevations on the same profile. We investigate the genesis of this unusual notch distribution using bio-geomorphological surveys, numerical modelling of nearshore hydrodynamics and palaeo-climate data. This analysis shows that the submerged notch only occurs in coastal sectors characterized by minimal or negligible hydrodynamics. Comparison with the millennial sea-level evolution shows that the present elevation of the submerged notch closely matches the sea-level stabilization that occurred during the Late Antique Little Ice Age (LALIA, ∼1400 to ∼1290 BP). During this period, the notch formed in sheltered areas of the coast, despite minor wave mechanical action and bioerosion, because relative sea-level stability concentrated erosion in the same portion of the cliff for ∼400 years. The increased rates of sea-level rise over the last 1500 years hampered the formation of a younger notch in sheltered sectors of the coast. By contrast, changes in sea-level rise rates did not affect notch formation at exposed sites where the mechanical action of waves coupled with intense bioerosion were the major control on notch formation. These data further confirm that the preservation of a fossil submerged notch is not only ascribable to co-seismic subsidence but also to climatic factors. This has implications for palaeo-seismic assessments of the Mediterranean region.

A comprehensive study of the river plume in a microtidal setting

On the basis of observations and modelling of the plume generated by the Misa River (AN, Italy), we performed a comprehensive study, which integrated different sources of information (field data, numerical simulations, etc.), of the generation and transport mechanisms of river plumes flowing into microtidal environments. First, we analysed images simultaneously acquired by both two shore-based stations and satellite to determine plume fronts and extensions. Then, we correlated such information with the estuarine forcing to recognize the plume generation and transport mechanisms. Being real-life events influenced by a combination of factors, we run numerical simulations to separately study each force and its influence on the plume evolution. We also performed simulations of two real-life cases, to compare the modelled and observed results. We identified the river discharge and the wind as the main generation and transport mechanisms, respectively. Moreover, waves could stir, suspend, and drag plume sediments, even if results showed that a river discharge associated with a return period smaller than 1 year produced a plume denser than 5-year return period waves. The transport mechanisms were responsible for the alongshore extension of the plume. The tide, even if secondarily, affected the plume evolution, depending on its phase shift to the river discharge peak. Particle Tracking Velocimetry from videos acquired by a shore-based station provided the surface velocity field in the final river stretch. This and the contributions by wind and waves were correlated with the plume extension through a power law.

Palaeostrait tectonosedimentary facies during late Cenozoic microplate rifting and dispersal in the western Mediterranean

Abstract Palaeostraits are common features formed during the extension and fragmentation of active continental margins, when back-arc basins are formed and microplates develop through upper-plate extension. In this contribution, we focus on narrow seaways and straits associated with successive pulses of late Cenozoic rifting along the southwestern Eurasian continental margin, and the ensuing formation and dispersal of microplates. The incomplete and largely non-cylindrical Africa–Eurasia continental collision in the Mediterranean region provides snapshots of the various phases of microplate fragmentation and dispersal, punctuated by the presence of narrow seaways/straits. These peculiar physiographical conditions induced current amplification, and led to the accumulation of characteristic large-scale, cross-stratified deposits and tidal facies in an otherwise generally microtidal setting. Although not univocally related to rifting, the identification of such distinctive sedimentary facies within deformed orogenic belts may ultimately help in terrane analysis and in the discrimination of individual microplates, which were then amalgamated. Recognition and dating of palaeostrait tectonosedimentary facies can thus constrain the presence and the onset of extension in the geological record.

Shoreline variability and coastal vulnerability: Mossel Bay, South Africa

Coastal erosion may cause significant damage to property and infrastructure with far reaching socio-economic consequences. Assessing the site-specific shoreline dynamics is fundamental to understand the morphodynamic behaviour of a particular coastal area, as well as the associated coastal hazards. However, changes in shoreline position, even when significant, are not necessarily associated with increased coastal hazards. In this contribution we investigate the impact of short-term changes in shoreline position within a crenulated embayment of Mossel Bay. The 30 km-long embayment, located in the Western Cape region of South Africa, lies in a high-energy wave-dominated, micro-tidal setting. Mossel Bay is heavily populated and experiences an influx of tourists year-round. Much of the coastal community and infrastructure lies within 25–40 m of the foredune toe.Georeferenced Landsat 7/8 and Sentinel 2A scenes are used to manually digitise shoreline position in ArcMap, using the “wet/dry” line as a shoreline position proxy. The Digital Shoreline Analysis System was then used to generate shoreline change statistical metrics. Wave conditions were modelled using SWAN wave model, implemented using a nested grid approach with a high-resolution (10 m) inshore grid, and a lower resolution (50 m) offshore regional grid. The nearshore wave field during mean and storm conditions was obtained along the 15 m isobaths along the entire embayment.The embayment's orientation in relation to the prevailing swell direction results in significant alongshore variability in nearshore wave conditions; wave heights increase towards the east along the embayment. This variability in wave forcing is reflected by the changes in shoreline position in both long and short-term, computed using the end-point rate method. However, the areas of higher shoreline change are not those experiencing the worst detrimental effects.Over the long-term, the present-day Mossel Bay embayment is relatively stable, with no significant signs of extensive accretion or erosion. However, rapid migration the shoreline is documented on a seasonal scale (short-term) with significant change proximal to river mouths, areas influenced by megacusps, and regions where the highly dynamic shoreline behaviour is constrained by rocky platforms and unable to freely adjust to variations in forcing. Thus, Mossel Bay is divided into three sub-cells in terms of coastal processes and coastal vulnerability with hazards associated with the location of such infrastructure rather than the specific patterns of shoreline change.

Sediment transport and deposition in flood-tidal deltas: Insight from the Texas Coast

While tidal environments are ubiquitous around the planet, limited work on these systems, particularly those that are in microtidal settings, hinders our understanding of tidal paleo-environments. In this study, we analyze the modern sediment dynamics of the San Luis Pass area, a microtidal system located on the Texas Gulf Coast. The focus of this work is to characterize sediment distribution and establish a quantitative relationship between sedimentary structure geometry, water depth, sediment grain size, and current velocities. A total of 160 seafloor sediment samples were collected in the modern San Luis Pass tidal delta between July 2017 and August 2018; this time span coincided with the landfall of Hurricane Harvey in the area in August 2017. Ripple geometry was measured in 46 locations along the flood-tidal delta. There is a rapid transition in grain size, from proximal (fine sand) to distal (silt and very fine sand) sediments at about 4.3 km from the mouth of the tidal inlet. This change is likely due to a decrease in current velocities away from the inlet entrance, resulting in a change in sediment transport from a combined bedload/suspension to suspension. Post Hurricane Harvey sediments are finer grained compared to those collected before the hurricane landfall, most likely due to high runoff during the storm event. In addition, increased water depths were measured after the hurricane landfall, indicating a significant erosion in the San Luis Pass area. Current ripples were only present in the proximal (sandier) area. We found a positive correlation between ripple height/ripple wavelength and water depth. However, the correlation is much stronger for ripple wavelength, even when compared to other published ripple measurements. We, therefore, propose to use ripple wavelength as a more reliable means to estimate paleo-water depths instead of ripple height. Nevertheless, more measurements from modern environments are necessary to establish a more robust geometric relation from ripples in a variety of paleoenvironments.

MORPHOMETRY OF BLUE LAND CRAB (CARDISOMA GUANHUMI) BURROWS: ICHNOLOGICAL CONTEXT AND PALEOENVIRONMENTAL IMPLICATIONS

ABSTRACT A large brachyuran, the blue land crab (Cardisoma guanhumiLatreille, 1825) is a keystone species and a prolific burrower in a variety of low-latitude supratidal habitats; however, few studies have addressed its ichnological aspects. Here we present a detailed morphological assessment of C. guanhumi burrows and make comparisons to Macanopsis plataniformis, a trace fossil thought to be constructed by a similar brachyuran. Using nine burrow casts from a mangrove wetland on San Salvador Island, Bahamas, 29 dimensionless morphometric parameters were compared. Results of Bray-Curtis tests show high overall similarity index (0.85) between casts, with complexity, tortuosity, and opening inclination angle having moderate to high similarity (> 0.8). End chamber angle of the blue land crab has highest similarity both within the modern morphometric parameter dataset (> 0.9) and to that of M. plataniformis (0.88). The large terminal chamber is particularly important because of its high preservation potential, as a repository for organic remains, and as a reliable water table (∼ sea-level) indicator. Cluster analysis suggests size-independent Macanopsis-resembling burrow morphology among different extant taxa can be discriminated via quantitative neoichnological methods. Our findings demonstrate how a quantitative assessment of modern burrows and their likely counterparts from the sedimentary record can help constrain tracemaker identity and aid paleoenvironmental reconstruction of low-energy microtidal settings.

Beyond marsh drowning: The many faces of marsh loss (and gain)

The morphodynamics of tidal marshes is investigated using a novel model (MarshMorpho2D) that couples 2D tidal dynamics, wave dynamics, mass-conserving sediment transport, edge erosion, pond dynamics, and marsh upland migration. By only changing tidal range, sediment supply, and RSLR rate, the model reproduces a great variety of spatial patterns in marsh loss. For all tidal ranges, an increase in RSLR triggers channel widening, whereas a decrease in sediment supply triggers tidal flat deepening and increases wave edge erosion. Marsh loss by pond runaway expansion dominates for intermediate RSLR rates, especially for small tidal ranges and for small sediment supplies. Drowning only takes place in the microtidal setting for high RSLR rates and extremely low sediment supply. Drowning is restricted to the landward portion of the marsh, and lacks the variety of spatial scales associated with pond runaway. For a large tidal range, the amount of marsh loss by drowning and pond runaway is negligible, thus increasing the relative importance of edge erosion and channel widening. Upland migration deepens the channels, which release sediment and thus help sustaining the existing marsh. Nonetheless, marsh upland expansion compensates for existing marsh loss only for a large tidal range or for a high sediment supply, and thus microtidal marshes are likely to experience net loss. Marsh loss is highly heterogeneous, and is not well represented by the dichotomy between marsh survival and marsh loss based on vertical accretion thresholds. Marsh loss can take place by a variety of mechanisms, hindering the ability to generalize predictions about marsh loss.

The Walpole-Nornalup Inlets System, Western Australia: A case study of a potential estuarine geopark

Unlike other estuaries Nationally in Australia, the Walpole-Nornalup Inlets is unique complex twin-basin ria estuary in the most humid part of Western Australia. The estuary fronts the oceanographically-dynamic Southern Ocean and, with the high annual rainfall, it provides a range of estuarine landforms, estuarine peripheral wetlands, a dynamic sand barrier that records climate changes and, with its microtidal setting, it provides examples of complex riverine-to-marine dynamics such as intra-basinal gyring. A range of geological to estuarine features that are of geoheritage significance and available for exploration and explanation as geotrails include: (1) the Precambrian geology, (2) the stratigraphy of the Cainozoic Werillup Formation, (3) Cainozoic weathering, sedimentation, and climate history, with a very wet climate to produce erosionally-resistant quartz grain lags, (4) Cainozoic to Quaternary formation of a rock tombolo, (5) the complex estuarine shorelines and history, and (6) complex estuarine processes and history. As an ensemble of geological and other natural history features, Walpole-Nornalup Inlets system also provides a case study of a systematic approach, using the Geoheritage Tool-kit, of identifying and evaluating different natural values. This forms the foundation for to baseline monitoring (for environmental management) and tourism to explore through geological time the natural history of this geologically and biologically rich location.

Tidal amplitude decreases in response to estuarine shrinkage: Tokyo Bay during the Holocene

Tidal changes in Tokyo Bay during the last 10,000 years were investigated by combining a numerical model with newly compiled model bathymetries for the present day, the early 20th century, and every 1000 years from 4000 to 10,000 years ago. Over this time period, sea level has changed by more than 30 m, and the coastline has shifted 50–60 km. Tides during the early 20th century and 4000–9000 years ago were larger than those at present; from 9000 to 4000 years ago, the bay setting was mesotidal, in contrast to the modern microtidal setting. The maximum tidal level at the head of the bay was 72% higher 7000 years ago than at present, and M2 tidal currents at the bay mouth from 10,000 to 9000 years ago were three times stronger than those at present. When tides were compared at a fixed location, the greatest tidal enhancement was found to have occurred 9000 years ago. The weakening of tides over the last 9000 years can be explained by introducing the concept of estuarine shrinkage together with quarter-wavelength resonance theory. Tidal weakening occurred in two phases: during the first phase, 9000–7000 years ago, it was due to the rise in sea level (type 2 shrinkage), and during the second phase, a decrease in the length of the bay was responsible (type 1 shrinkage). These findings can be applied to the assessment of both past tidal changes and potential future tidal changes in other elongated estuaries with lengths of several tens of kilometers.

Morphodynamic interactions of continental shelf, beach and dunes: the Cabopino dune system in southern Mediterranean Spain

AbstractThe complex morphodynamic interactions between nearshore, shoreface and dune systems are usually simplified by studying these zones and their associated processes in isolation. However, the established relationships between each of them suggests that an integrated approach is required to examine the genesis, evolution and adaptation of the entire morphodynamic system. The Cabopino dune system in the southern Spanish Mediterranean Sea provides a clear example of a linked morphodynamic system in which a relatively large dune system has been generated and grown through the supply of sediments from an adjacent littoral supply environment. Here, we present a conceptual model of how the nearshore has provided suitable conditions for beach and dune development. We purport that synchronization of sediment activation in the marine and aeolian sections of the system have played a major role in this microtidal setting in which temporal aspects are not only tied to storm action, but to large sedimentary features moving alongshore. © 2019 John Wiley & Sons, Ltd.

Oligo-Miocene coal in a microtidal environment reworked under Quaternary periglacial conditions (Western Falkland Islands/Isla Gran Malvina) – Coal formation and natural sand processing

The Western Falkland Islands are characterized by landforms and sediments of Quaternary age originating from the interplay of periglacial and marine processes. These have left their imprints on siliciclastic sediments and subbituminous coal, peatland, and podzolic soil. Our depositional study focuses on interpreting a stratigraphic sequence which contains white quartz sand with an interbedded coal seam by using petrography (coal petrography, carbon isotope analysis, sedimentary petrography, clay mineralogy, granulometric, morphometric studies), and geochemistry. The study of the coal-sand couple sheds some light on two contrasting sedimentary geological settings. Firstly, it broadens the knowledge of the regional geology at the southern tip of South America by examining how coal seams may be emplaced in a periglacial-microtidal environment. Secondly, the study is also relevant for applied geology by investigating how sand is processed in a coal-bearing paralic regime. The coal-sand couple evolved during 6 discrete stages. Stage I: The detrital parent material was derived from the Siluro-Carboniferous Gran Malvina Group. Stage II: From the Oligocene through the late Miocene under a warm tropical climate organic matter was accumulated in a paralic environment giving rise to subbituminous coal. Stage III: During the Pleistocene alternating erosion and deposition in a wave-dominated coastal-marine regime gave rise to the build-up of coal interbedded with sand. The hydrodynamic regime reduced the grain size to the level of medium to fine sand and enhanced its sorting coefficient causing the separation of minerals according to their specific gravity (mechanical processing). Stage IV: The coal seam has been reworked by the last glacial progradation. Stage V: Weathering by wind and waterworn sand grains into semispherical to spherical forms and the content of heavy minerals increased along backshore and in dune belt environments. Stage VI: Organic acids derived from the coal promoted chemical leaching and significantly contributed to the decrease of the labile constituents of arenites (feldspar, carbonate minerals, and calcareous tests of faunal remains). Chemical alteration was grinded to a halt by another sea level rise, which gradually destroyed the coal layer. The depositional environment most effective for the processing of sand is located in the microtidal setting. In the meso- to macrotidal hydrodynamic regime the chemical leaching becomes the only efficacious tool to improve the quality of the footwall rocks. Due to the predominance of argillaceous sediments there, the quality of clay is enhanced and kaolin deposits may come into existence.

Facies evolution and depositional model of an arid microtidal coast: example from the coastal plain at the mouth of Wadi Al-Hamd, Red Sea, Saudi Arabia

This study describes the subsurface sedimentary facies of a modern Red Sea coastal plain at the mouth of Wadi Al-Hamd, northern Saudi Arabia, in an attempt to infer the influence of aridity and limited tidal range on facies characteristics. The study provides criteria to delineate the fluviomarine transition in a setting where tidal signatures and other marine influences are weak. Six manually collected cores (1.4 to 1.75 m long) from the channels at the wadi mouth, beach ridge, intertidal flat, strandplain, and supratidal flat enabled the identification of 23 sedimentary facies. The sediments displayed rapid and tremendous variability in facies both within and among cores. They were deposited generally under low energy conditions occasionally punctuated by short-lived high energy events. The sediments in the channel exhibit features characterizing deposition under an arid and semiarid climate, such as rapid and intermittent discharge and short-lived rapidly abandoned channels. The sediments and depositional settings on both sides of the wadi mouth are different. A low gradient intertidal flat influenced by wave and tidal processes develops to the south, whereas a relatively high energy wave dominated strandplain occurs to the north. The deposition in the supratidal flat was accomplished by a short-lived high energy washover event. The facies stacking pattern indicates that the depositional systems are in a state of transgression, with a regime of gradually increasing accommodation possibly during the rising of relative sea level and diminishing of sedimentation. The turnaround from regressive to transgressive stage that coincides with the fluviomarine transition in channel fill deposits is placed at a level of increased soft sediment deformation because the tidal influence is weak in this microtidal setting. Though the plane view shows a deltaic shoreline, the depositional system is interpreted as estuarine. The findings of this study can be applied to similar recent and ancient settings.

The formation of beaches on shore platforms in microtidal environments

ABSTRACTBeaches are common features of many rocky shorelines and can be considered to be constrained by the underlying geology. In mesotidal to macrotidal areas the slope of the substrate and sediment supply are the primary factors in constraining the size and development of beaches on shore platforms. In microtidal settings it is not known if these factors are wholly responsible for determining the presence of beaches on shore platforms, nor the contribution of other factors such as hydrodynamics. The microtidal coast of Victoria, Australia, is surveyed in this study in order to quantify the morphological boundary conditions that constrain beach development on semi‐horizontal shore platforms. An ample sediment supply indicates that the underlying geology is controlling the presence and absence of beaches. Where beaches occur they always overlie a rock ramp which is the steepest part of the platform. The two most important morphological constraints were platform width and height both of which significantly correlated with beach volume. An elevational threshold exists at just over +1.0 m where beaches cannot accumulate. Below this threshold, platform width appears to be the principle constraining factor in beach accumulation. An evolutionary model is inferred which suggests that dissipation of wave energy associated with platform widening plays an important role in allowing beaches to accumulate. The model suggests beaches on platforms will be particularly sensitive to sea level rise. Copyright © 2014 John Wiley & Sons, Ltd.

Formation and migration of transverse bars along a tidal sandy coast deduced from multi-temporal Lidar datasets

A field of long-crested transverse bars was monitored from a 2.5-year series of topographic Lidar surveys in the vicinity of a tidal inlet on the macrotidal (mean spring tidal range = 7 to 12 m) west coast of Cotentin (Normandy, France). The bar field, the alongshore extent of which is about 1.8 km, is composed of a total of 8 bars with lengths varying from 320 m to 1300 m and mean heights comprised between 0.5 m and 2.5 m. Bar cross-sections are variable between bars and for a single bar, and also over time. The surveys show a consistent northward migration of the bars at a mean rate of about 2 m/month, but the rate is larger in winter than in summer. The Lidar observations show that the tidal inlet, located at the southern limit of the bar field where the bars start forming, comprises a large sediment platform that acts as a source of sand for the bars. The ebb jet debouching from the inlet is deflected northward by the ambient strong shore-parallel tidal currents in this large tide-range setting, and this may be the primary mechanism leading to the emplacement of the bars. Smaller wave-formed swash bars that further feed the development of these large transverse bars have also been observed. Monitoring of bar migration in the course of six consecutive spring tides with fair-weather conditions showed that strong spring tidal currents are sufficient to drive bar mobility in the absence of waves. Storm wave resuspension of sand is thus expected to enhance bar mobility rates, as shown by the higher rates of winter bar migration compared to the summer rates. The ebb jet explains the slower bar migration rates at the vicinity of the inlet, these rates increasing with distance northward of the inlet as the tidal currents become unimpeded. The main difference between these macrotidal transverse bars and their counterparts in microtidal settings resides in these strong tidal currents that are the essential driver of bar migration, unlike the wave-driven migration of microtidal bars. The large tidal range, in conjunction with storm wave activity, also induces longshore and seasonal variability in bar morphology. The transverse bars of Normandy appear to be inscribed in a sand circulation system involving the west Cotentin coast, the large ebb tidal delta from which they are formed, and the central Cotentin embayment where they are ultimately incorporated into the nearshore sand pool. Longer-term field hydrodynamic monitoring and modelling will be required in order to further elucidate the mode of formation of these transverse bars.

The morphodynamics of shore platforms in a micro-tidal setting: Interactions between waves and morphology

The interactions between waves and morphology on shore platforms were investigated at four platforms located at Kaikoura Peninsula, New Zealand and the Otway coast, Victoria, Australia. Waves were measured over six tidal peaks of a spring tidal sequence at each site during a six month period (February 2007 – July 2007). This paper reports the results of an investigation into the research question: how are waves modified by platform morphology in a micro-tidal setting? Through the investigation of wave interactions on shore platforms it was found that complex hydrodynamic patterns formed on the shore platforms studied. Through direct measurement of waves on the platforms it was found that wave attenuation was not dependent on platform width, but rather influenced by platform gradient and depth of water on the platform. Spectral analysis was used to assess how wave energy was transformed across the platforms. At three of the four platforms investigated, energy gradients formed across the platform with wave energy recorded within the infragravity range.

Coastal karren features in temperate microtidal settings: spatial organization and temporal evolution

Basin pools are the diagnostic feature of Coastal Karren landscape at temperate settings. According to the size and connectivity parameters four morphological zones are identified along limestone coastal profiles. Each zone reflects the balance between the effects of physical and chemical weathering-erosion agents. Broadly, marine abrasion, bioerosion and biological driven solution show a larger influence seaward, whereas non-biological driven solution enhances its participation landward

The Late Holocene sediment infilling and beach barrier dynamics of the Thau lagoon (Gulf of Lions, Mediterranean sea, SE France)

Abstract A study combining very high resolution seismic and sediment core data has been carried out on the Thau lagoon (Mediterranean coast, microtidal setting, SE France) in order to understand more clearly the dynamics and Holocene chronology of its closure through the different stages of its filling. One main seismic unit (U2) has been defined into the infill, above the rocky basement (U0) and a composite unit U1, which is interpreted as remnants of Pleistocene fluvial terraces or/and to early marine Holocene deposits. Unit U2, that reaches locally 9 m in thickness, rests conformably on U1 in the central part of the lagoon and onlaps U0 or U1 close to the edge of the lagoon. It is divided in two sub-units, U2-1 and U2-2. U2-2 rests paraconformably on U2-1 in the central part of the lagoon where the infill is the thickest, while a marked erosional unconformity is observed between U2-1 and U2-2 on topographic highs of the basement and on the seaward edge of the lagoon. A total of seven elementary sequences have been observed in U2-1 and U2-2. According to core data, U2 consists in a series of mud-dominated sequences, with shell fragments dispersed at the base. The vertical distribution of the fauna into U2-1 and U2-2 reveals a lagoonal environment. However in U2-1, marine species are more abundant in the south of the lagoon. 14C AMS dating provides three ages: ~ 6000 cal yr B.P. in the lower part of U2-1 on CAL1, ~ 5400 cal yr B.P. just above the boundary between U2-1 and U2-2 on CAL4, ~ 3000 cal yr B.P. in the middle of U2-2 on CAL4. A scenario to explain the lagoon infill stratigraphy and geometry is proposed. The beginning of the lagoon infill occurred with the initiation of the barrier construction, as soon as the sea-level rise slowed down significantly, i.e. between 7000–6000 yr B.P. The sediment-fill began into the back-barrier system, with a high rate of sedimentation for U2-1, according to the radiocarbon data. At 5400 yr B.P., the barrier is assumed to be totally closed leading to the deposition of the fully lagoonal U2-2 succession. The unconformity between U2-1 and U2-2 is interpreted as the result of a rapid landward retreat of the barrier. This severe retreat could be related to climate forcing and/or brutal change in sediment discharge driven by the Rhône River, which is the main sediment source of the longshore drift. In this scenario, the last sub-unit, U2-2, represents most of the late Holocene infill. In this framework, the elementary sequences observed in U2-2 could be related to high frequency climate changes of about 1000–1500 years periodicity according to 14C dates.

Recovery estimates for the Río Cruces after the May 1960 Chilean earthquake

The May 22nd 1960 Chilean earthquake was the largest earthquake on record with a magnitude 9.5. The event caused extensive subsidence and uplift over wide areas (at least 130,000 km 2) with uplift on the offshore South American Plate margin and downwarping in the coastal and landward areas which ranged in magnitude from 1–2 m. The subsidence deepened coastal rivers creating extensive shallow banks (now wetlands) in many rivers including the Río Cruces. The amount of subsidence was well-documented with field measurements recorded soon after the event. This study using sediment accumulation histories predicts the recovery time of the Río Cruces to pre-1960 conditions, an important factor for assessing the fidelity of seismic recorders in the geological record. Sediment cores ( n = 139) from the subtidal areas of the Río Cruces were lithologically logged and selectively analyzed using high-resolution particle-size distribution (PSD) plots, microfossil analysis (thecamoebians) and 210Pb, 137Cs dating, documenting the post-1960 thickness of sediment. The analysis differentiated four main facies which include: 1) Floodplain Soil Horizon, 2) Tsunami or Flood Unit, 3) Post-Earthquake Adjustment, and 4) River Sedimentation. The post-1960 river sediment is clearly demarcated in the cores by highly-peaked PSDs (4–5 phi), abundant thecamoebian tests and high diversities compared to the underlying facies. Species compositions are typical of lacustrine or slow-moving river systems ( Cucurbitella tricuspis, Difflugia oblonga, Centropyxis constricta, Centropyxis aculeata, Lagenodifflugia vas, and Difflugia protaeiformis). Based on this core data, the shallow banks along the Río Cruces will likely shoal in less than 100 yrs (< 50 yrs from present). Between 1960 and 2008 the water depths of the shallow banks were reduced by more than half, to an average water depth of less than 1 m due to 60 to 80 cm (1.25–1.68 cm/yr) of sedimentation over that period. Time estimates for recovery to a floodplain are estimated to be in the 100–200 yr range due to uncertainty in compaction rates of the organic rich sediment. These estimated recovery times in a microtidal setting (< 1 m) are longer than those measured in the macrotidal (∼ 9 m) setting of Portage, Alaska (10–20 yrs) and more similar to the estimated rates of 100–150 yrs for the mesotidal (∼ 3 m) Willapa Bay, Washington at the Cascadia subduction zone. Based on these estimates, the submerged mid-lower reaches of the Río Cruces are at the limit of being a good seismic recorder and it is possible that this setting would under represent large events if they had recurrence intervals less than several centuries.