Siliciclastic Sand Research Articles

Holocene and Reworked Pleistocene Sediments in Mud Depocenters of the Inner Continental Shelf of Sao Paulo Bight (Southeast Brazil)

Mud depocenters are found across most siliciclastic continental shelves, occurring in various bathymetric ranges and typically dating to the Holocene. This study analyzes the sedimentary characteristics and formation processes of mud depocenters in the central sector of the inner continental shelf of the São Paulo Bight. A total of 1700 km of high-resolution seismic profiles, four gravity cores, and 1346 surface sediment samples were analyzed. The sedimentary analysis involved determining grain size parameters, organic matter content, carbonates content, and radiocarbon dating. Seafloor sedimentary mapping shows the predominance of very fine siliciclastic sand, together with three mud depocenters located shallower than the 30-meter isobath. The northern depocenter comprises one sedimentary unit (SU-NZ-01), while the central depocenter consists of two sedimentary units (SU-CZ-01 and SU-CZ-02) and the southern depocenter is made up of two other sedimentary units (SU-SZ-01 and SU-SZ-02). Units SU-SZ-02, SU-CZ-02, and SU-NZ-01 were deposited on Precambrian rock surfaces during the Holocene transgression and include reworked sediments from nearby Upper Pleistocene coastal plains, characterized by a transparent seismic pattern. Above these, units SU-SZ-01 and SU-CZ-01 developed following the Holocene Transgression Maximum, associated with local mud input from the Peruíbe River mouth and the Santos Estuary, respectively. The paleorelief of Precambrian rocks favored the formation of these depocenters by creating wave shadow zones and trapping mud within the paleovalleys.

Late Holocene geochemical signature of inter reef mixed sediments on the northeastern Brazilian outer shelf

The northeastern outer shelf in Brazil is a complex ecotone that holds significant information on Holocene sedimentary processes and evolution. X-ray diffraction (XRD), X-ray fluorescence (XRF), and scanning electron microscopy/energy dispersive spectrometry (SEM/EDS) were employed to investigate bioclastic sedimentary facies and patches of relict siliciclastic sands. Radiocarbon dating (AMS) indicates that the carbonate sediments and corals formed in the past millennium. The sediment samples were divided into two groups using a multivariate analysis of XRD and XRF. The first group consists of siliciclastic materials, characterized by Si, Al, Fe, K, Zr, and Ti, and has low levels of carbonate and organic matter. The second, bioclastic and mixed facies, has a high carbonate content and exhibits elevated concentrations of Ca, Sr, and Mg, indicating authigenic processes. The EDS data revealed that the shells predominantly consisted of Cl, Cu, Al, Fe, and Ca. The terrigenous sites are detached from the coastal sources and are mainly located within the reef areas of the outer shelf. The erosion processes in these areas were inefficient to eliminate the siliciclastic sediments. This deposit trapped in inter reef sediments holds the key to understanding the timing and interactions between past coastal processes, shelf valley incision, and reef evolution during the Late Holocene transgression.

Grain Size Distribution and Provenance of Holocene Sand from the Sava River (Zagreb, Croatia)

This study involves an investigation into the grain size distribution and provenance of the sand deposited near Zagreb (Croatia) in the riverbed of the regionally important, almost 1000 km long Sava River, which connects several SE European countries. Recent research in the study area has mainly focused on the deposits forming the Zagreb alluvial aquifer system, rather on the Sava River sediment deposited in its riverbed, which is the focus of this study. The grain size distribution results obtained by dry sieving and laser granulometry showed a predominately fine and medium sand deposition at riverbanks and sand point bars. Medium sand increased downstream towards the east, within the artificially more channelized riverbed in the urban area. Fine sand prevailed 50 km further downstream in a more meandering low-relief area, near the city of Sisak and Lonjsko Polje Nature Park. Provenance analysis showed predominately carbonate sand in the western part of the city of Zagreb, originating from distant (Alpine) and local (Medvednica Mt. and Samobor Hills) sources. More siliciclastic sand was deposited in the Sava riverbed in the middle and eastern parts of Zagreb, originating mainly from the Medvednica Mt. The prevailing siliciclastic sand further downstream of the Sava River is probably sourced from the Kupa River tributary. Although various studies of the Zagreb alluvial aquifer system have been conducted so far, this study represents a novelty in its investigation into the grain size distribution of the Sava riverbed sand itself, setting the foundations for investigations in the future.

Assessing sediment quality at tropical mangrove areas for using as reference sites in ecotoxicological bioassays

The sediment of five mangrove in Pernambuco/Brazil was investigated to find a reference site for toxicity bioassays. Sediment characteristics, metal levels, and toxic effects on the nauplius of the copepod Tisbe biminiensis were studied. The sediment was composed by terrigenous muds and siliciclastic sands with medium to high organic matter contents. The FeAs association show most of the high concentrations and positive correlation among its constituents in the south. In the north, the Ca-Sr-La association is higher and exhibits positive correlation among its components that usually found in carbonate rocks. Very intense toxic effects were observed, mainly in the south, with >70 % reductions in development. As and Hg were positively correlated with mortality and decrease in development. The sediment quality at studied mangroves prevent their use as a reference site in bioassays. These could be linked to the direct/perennial contribution of geogenic sources associated with anthropogenic environmental impacts.

Evolution of Coniacian facies and environments in the Iberian basin: a longshore current controlling siliciclastic sand distribution on a carbonate platform

The Coniacian carbonate sediments of the Iberian basin were deposited on a homoclinal ramp that grades upwards into a distally steepened ramp, with a major shoreline siliciclastic fringe. Twenty-three facies have been identified and grouped into three main depositional environments: outer, mid, and inner ramp. The last include barrier (shoal), lagoon, carbonate tidal-flat and shoreface sub-environments. The more prominent biogenic components show a mixture of sunlight-dependent phototrophic organisms (mainly large benthic foraminifera) and nutrient-dependent heterotrophic organisms (mainly rudists), with a remarkable rare occurrence of corals. Nutrients supplied from the emergent mainland probably promoted the development of heterozoan organisms. The vertical evolution of the ramp shows: a basal transgressive stage with facies retrogradation; a maximum flooding stage, and a regressive stage with aggradation and progradation of a distally steepened ramp. The presence and distribution of siliciclastics are problematic, since sands coming into the basin are likely to be rapidly and widely redistributed along the basin, taking into account the common storm, wave, and tidal processes preserved by the sedimentary facies. The presence of a clockwise NW-flowing longshore current is postulated to account for this distribution, which was likely induced by both dominant external currents around Iberia and wind-driven currents. These clockwise gyres facilitated the invertebrate dispersion into this enclosed basin and the local presence of upwelling. This could have been another source of episodic nutrient-rich waters from the deep ramp, which may have favored heterozoan development even in the more proximal and relatively shallower-water facies.

Israelite Plain, southwestern Australia, a siliciclastic, late Quaternary Coorong analogue, without dolomite

The Israelite Plain, a narrow terrace at the western edge of the Great Australian Bight, is characterised by a series of schizohaline, ephemeral lakes leeward of a marginal marine eolianite dune complex that has many similarities to the Coorong Coastal Plain (CCP). The late Pleistocene and Holocene sediments are a mixture of dominantly quartzose sand and lesser carbonate components. Paleolagoon to shallow marine MIS 5e, mollusc-rich quartzose deposits are capped by calcrete or karst and contain a biota similar to the well-documented, coeval Glanville Formation on the CCP. Holocene sands with minor carbonate are also lagoonal and resemble many modern Coorong Lagoon deposits. Both deposits are interpreted to reflect marine accumulation during sea-level highstands. Lakes today range from dry to water-filled with most floored by calcareous quartzose sand. Microbial influenced sedimentary structures in the form of pustular to laminated mats veneer lake margin bedrock and sediment. Carbonate minerals are mostly calcite with abundant accessory magnesium-calcite, aragonite and traces of magnesite, monohydrocalcite and halite. Gypsum is minor but ubiquitous and floors one of the smaller, marginal-marine lakes. Although having a similar evaporative climate and evidence of groundwater resurgence like Coorong lakes, there is no significant synsedimentary dolomite. This aspect is interpreted to reflect the narrow width of the plain, and the Mg-poor source of groundwater from the nearby karst Nullarbor Plain to the north. The Israelite Plain, together with other coeval, young successions marginal to the Southern Ocean under a semiarid climate now illustrate the depositional variations and early diagenetic differences that can occur in similar environments adjacent to a cool ocean. KEY POINTS The Israelite Plain is a narrow, marginal marine terrace covered by Quaternary, lagoonal, eolian to lacustrine, calcareous siliciclastic sands, similar to several other calcareous terraces along the southern Australia coast. The interpreted sediment ages are late Pleistocene stage MIS 5e with the deposit sporadically overlain by similar Holocene deposits. Most 14C age dates are suspicious because of interpreted diagenesis or MIS 5e age, whereas U/Th age dates appear correct or altered by diagenesis. Pleistocene biota is dominated by the bivalve Anadara trapezia, whereas Holocene deposits contain a varied array of cooler water molluscs. The carbonates are similar to those on the Coorong Coastal Plain but do not contain any dolomite likely because of climate and lack of Mg-groundwater flow.

Evidence of upwelling on the Brazilian continental shelf of Rio Grande do Norte

This study evaluates Foraminiferal assemblages and their relation with grain size, calcium carbonate content, organic matter, and mineralogy of sediment samples collected at the sediment-water interface along a transect on the northern continental shelf of Rio Grande do Norte State, Brazil, adjacent to the city of Areia Branca. The sedimentary sequence of this shelf is represented by lithostratigraphic units of a marine regressive sequence dominated by four facies: Siliciclastic sand, Silicibioclastic Sand, carbonate mud, and Biosiliciclastic sand. The carbonate content ranged from 5.83% to 85% and the organic matter content from 1.16% to 27.05%. Mineralogical characters separated the predominant siliciclastic content (37% to 92%) from the bioclastic content (8% to 63%). We have identified 14 species out of 50 species of Foraminifera, associated to particular depths and sediment types as follows: (1)deeper-water sediments in the middle shelf contain Bolivina striatula, Bulimina marginata, Triloculina trigonula, Pyrgo ringens, Textularia gramen (2)the shallowest sediments in the inner shelf contain Ammonia tepida, Buccella peruviana, Miliolinella subrotunda, and Quinqueloculina patagonica, (3)the central parts of the transect, also in the inner shelf, provide habitats for Quinqueloculina lamarckiana, Textularia earlandi, Buliminella elegantissima, Discorbis peruvianus, and Pyrgo nasuta. The distribution of Uvigerina striata and Buccella peruviana is probably related to colder water temperatures and possibly the occurrence of an upwelling phenomenon for the deepest parts of the area rather than the sedimentological features discussed here.

Controls on carbonate-siliciclastic relationships in quaternary deposits of the Midyan coast of the Gulf of Aqaba, Saudi Arabia

Quaternary deposits of the Midyan coast of Saudi Arabia include coral-bearing limestones interbedded with sheet-flow siliciclastic sands and gravels, interpreted as braid-delta fan deposits. The limestones and their contained faunas provide evidence of periodic local stabilization of gravel surfaces and effective starvation of siliciclastic supply. Some indicate growth and deposition in waters a metre or so deep whereas others were substantially deeper. An additional group reflects initial deposition in shallow waters with resedimentation downslope by storm or other ‘delta-front’ processes. The surfaces of the fans provided morphological and environmental analogues to fore- and back-reef situations in which corals, calcareous coralline algae, molluscs, and echinoderms flourished, but in this area did not form independent reefs. The dominant controls on the changes from siliciclastic- to carbonate-dominated deposition were not transitional, and primarily related to channel and fan-lobe migration rather than tectonic or eustatic sea level changes. Similar variations in the distribution of siliciclastic sediments may control carbonate occurrence in other mixed sequences.

Rare preservation of Triassic pedorelicts with biogenic traces from a hot semi-arid upland palaeoenvironment at Portishead, SW England

Pedorelicts occur sporadically in coastal exposures of Middle to Late Triassic continental sediments, at Portishead, SW England. The clasts include aeolian and bedrock constituents, calcrete textures, vesicles and other pedogenic features reflecting a hot semi-arid palaeoenvironment. Interpreted as being derived from an upland soil, they are classified in palaeosol terms as Calcic Protosol. In the most significant exposure, cryptic tubular trace fossils in branching networks occur within these pedorelict clasts in a localised fluvial deposit. This extremely rare palaeoecological archive offers a glimpse into continental upland life in a Pangaean desert landscape. It is documented for its own merit, to support continuing studies of its palaeobiology and to prompt investigation for comparable deposits.The pedorelicts occur within the Mercia Mudstone Marginal Facies (MMMF), close to its basal unconformity with underlying Paleozoic strata, at the margin of the Somerset Basin. The soil from which they are derived is interpreted to have developed in an upland regolith over Tournaisian bedrock of interbedded limestone and siltstone. These weathered and, with aeolian dust containing calcite and iron minerals and siliciclastic sand, formed a structured soil. This was aided by intermittent light and moderate rainfall that promoted mainly vadose pedogenic calcretization with displacive calcite crystallisation and siliciclastic grain breakage. In places, the soil was vesicular, as an ‘Av’ horizon, and in others, it was bioturbated with the development of calcite-lined tubules and unlined tunnels. At intervals of perhaps 104 to 105 years, catastrophic deluges eroded the regolith and transported clasts downslope towards the basin to form onlapping coarse clastic beds typical of the MMMF. The soil structure disintegrated but fragments that became pedorelicts were segregated in a fault-controlled palaeo-valley, possibly partly as a debris-flow, to be deposited as localised conglomeratic lenses with fluvial sand. After burial by further sedimentation, this deposit underwent diagenetic calcite cementation and baryte mineralisation before its present exposure by coastal erosion.

Water column and bottom gradients on the continental shelf eastward of the Amazon River mouth and implications for mesophotic reef occurrence

Continental margin sedimentation represents the terrestrial-marine geological transition. Coarser coastal sediments usually transition to fine-grained sediments across the shelf as the energy level decreases. As the distance from the continental source of terrigenous sediments increases, the mid-shelf mud belt transitions to a seaward organic/carbonatic sedimentation domain. However, this general pattern might change depending on the terrigenous sediment load. This is the case for the Amazon continental shelf, where the Amazon River empties and builds up the large Amazon River plume (ARP). The ARP promotes an inverse pattern with muddy nearshore prodelta sediments and siliciclastic sands offshore. This study investigates water and bottom characteristics to the east of the Amazon River mouth to understand the complex interplay that leads to simultaneous massive river plume spread and mesophotic reef development. Three cross-self profiles were performed, including physical-chemical water column measurements and bottom sample collection. The profiles were approximately 150 km apart, comprising a longitudinal gradient off of the ARP. The results reassure that there is enough light for mesophotic reefs to grow at the outer shelf and that at the mid-shelf, large-scale sand movement is likely to be the major limiting control on reef accretion. Also, mesophotic conditions (i.e., low light levels) occur in shallower waters, where a trend of reef expansion was identified, but high-energy levels and the lack of hard substrates hinder reef expansion.

Shallow‐Water Tsunami Deposits: Evidence From Sediment Cores and Numerical Wave Propagation of the 1601 CE Lake Lucerne Event

AbstractThe 1601 CE earthquake (Mw ca. 5.9) in “Unterwalden,” Central Switzerland, triggered multiple subaqueous mass movements and a subaerial rockfall that generated tsunami waves with run‐up heights of up to 4 m and several hundred meters of inundation along the coastal lowlands of Lake Lucerne. In the shallow Lucerne Bay, historical chronicles reported an oscillation of the water with an initial amplitude of ∼1–2 m and a period of 10 min, which continued for several days after the event with decreasing amplitude. Here, we investigate the lake‐tsunami process chain from subaqueous mass movement‐generated tsunami to wave propagation and ultimately to sediment resuspension, transport, and deposition in the shallow‐water environment. The effects of the historical tsunami on Lucerne Bay are reconstructed using sediment‐core analysis and numerical simulation of wave propagation. A 60‐cm‐thick event deposit was recovered along a sediment‐core transect in the shallow waters and radiocarbon dated to 1306–1442 cal CE. The event deposit has a sharp basal contact with carbonate shell fragments followed upwards a normally graded succession of siliciclastic sand to silt with a high proportion of horizontally bedded wooden particles. The numerically simulated tsunami waves are characterized by a water‐surface displacement of up to 1.5 m and generate bed shear‐stresses that are likely capable of remobilizing large amounts of sediments in the Lucerne Bay area. Our study successfully links the sedimentology of event deposits with physical principles of sediment mobilization derived from numerical wave‐forward modeling, providing a tool to improve the identification and interpretation of potential tsunami deposits.

Study of shelly shore ridge formation using wave flume experiments: Morphological evolution and depositional patterns

Coarse shore/berm ridges comprising mollusk shells are peculiar morphological features occurring in some beaches globally, generally wave-built, generated during storms and high tides. Studies regarding the formation of these shelly shore ridges using wave flume experiments (physical modeling) do not provide a detailed analysis of their morphological evolution and depositional features. A bioclastic beach profile with a constant dip, formed by mollusk shells (whole and fragmented ones, DSV-50 = 4.5 mm) of bivalves and gastropods, was subjected to a storm wave condition simulated in a two-dimensional flume with a scale reduction factor of 1/20. A decimeter-scale shelly shore ridge was successfully formed by swash bar emergence. The seaward surface was steeper than the landward slope. The bar was progressively built by swash currents that accumulated shells along the shoreline. According to Dean's function, the equilibrium beach profile displays a good fit for coefficient A, as related to the settling velocity of the bioclasts. The power of m is lesser in the equilibrium beach than that in beaches composed of siliciclastic sand. Three patterns of shell organization and pore framework represent the back, top, and frontal sectors of the ridge. The ridge front includes poorly organized shell particles of dense packing. The crest comprises bioclast beds of uniformly obliquely oriented, densely packed particles. Back ridge sediments exhibit loosely packed moderate shell organization. The morphological evolution of the bioclastic ridge formed in the flume closely resembled bioclastic beach sediments. The depositional pattern in the flume ridge was directly related to the energy of breaking waves and associated swash-current energy.

Siliciclastic and bioclastic contouritic sands: Textural and geochemical characterisation

The purpose of this study is to differentiate and characterise contouritic sands in two different locations with variable sediment compositions (siliciclastic and bioclastic) based on a multiproxy approach that includes the analysis of sedimentary texture, semi-quantitative geochemistry, microfacies and ichnological information, as well as a Principal Component Analysis (PCA) applied to geochemical and sedimentary data on sediment cores. The integration of sedimentological analyses and the PCA permits also to the differentiation between fine-grained deep-water deposits such as hemipelagites, muddy contourites and hyperpycnites. A depositional model is proposed here, based on geochemical and sedimentological characteristics of the Holocene-highstand Mozambique Channel upper slope sands, and glacial-lowstand Corsica Contourite Depositional System middle slope sands. The upper continental slope of the Mozambican margin is characterised by siliciclastic sandy contourites, muddy contourites and muddy hyperpycnites. Mozambique siliciclastic sandy contourites constitute large accumulations of well-sorted very fine to coarse sand with evidences of strong winnowing and reworking under high-energy conditions. The sedimentary facies represents highstand contourite sands and shows a reversely-graded trend. The contourite drift on the Pianosa ridge (eastern flank of the Corsica Trough) consists of bioclastic sandy and muddy contourites and hemipelagites. Bioclastic sandy contourites are made up of shallow-marine winnowed bioclasts with a reversely- and normally-graded trend and represent lowstand contourite sands. The PCA in the two environments —showing a distinctive geochemical signal— allows for differentiation of the contourite deposits. In siliciclastic sands, reworking is marked by an accumulation of Si, Zr, and Sr in fine- to medium-grained sands. In bioclastic sands, reworking is less evident but it is characterised by accumulations of Ca and Sr. The reworking and winnowing bottom current effects are also observed at the microfacies scale. Both types of contourite deposits show evidences of intermittent depositional conditions depending on the ichnodiversity, distribution and abundance of trace fossils. This work is useful to discriminate similar fine-grained deposits in different continental margins which would contribute to a better understanding of sedimentary deposits and processes in deep-marine environments.

Basaltic phreatomagmatic fissure at 71 Gulch Part 2: unusual pyroclasts from sediment magma mingling and melting

The behavior of magma as it encounters unconsolidated sediment can be studied in fortuitous exposures of incised volcanic systems to help determine the conditions that control the transport, arrest, or mingling of that magma in the sediment. The Pliocene subaqueous basaltic fissure at 71 Gulch, Idaho, USA contains unusual light-colored glassy mingled clasts that were produced through the incorporation of domains of siliciclastic sands and silts into basalt. These clasts contain textures reflective of melting and mechanical mixing of these sediments with the rising basalt along intrusions at depths > 30 m beneath the pre-eruptive surface and transport into and through a shallow subsurface debris-filled vent. Mingled clasts comprised colorless glass with locally high silica contents and visible swirls of sideromelane along with crystals derived from both the host sediment and the 71 Gulch basalt. The unique combination of textures, minerals, plus major and trace element geochemistry within the geologic context of 71 Gulch points to a two-phase mixing that did not contribute directly to, nor impede, the phreatomagmatic explosions that erupted the mingled clasts.

Improved drag coefficient and settling velocity for carbonate sands

Sediment transport calculations are used globally in the numerical models that coastal managers, scientists and engineers use to assess and forecast coastal change. Most of the existing sediment transport equations were defined based on experimental results using siliciclastic sands. Yet these equations are applied to all types of sand, including carbonate sands that have different characteristics and therefore, settling behaviour. A rigorous management of the transport of carbonate sand is essential for the present and future management of sedimentary features in coral reefs such as sandy beaches or reef islands. Here we present a new approach to estimating the drag coefficient of carbonate sands that considers both friction and pressure. Based on our new method, the calculated drag coefficients explain the great variability in settling velocities of carbonate sand observed in nature (from 0.025 m/s to 0.364 m/s in our database). Using our formula, we demonstrate that even small differences in the settling velocity obtained with the new drag coefficient can lead to substantial changes in sediment transport and call for an update of numerical models.

Late Miocene wood recovered in Bengal–Nicobar submarine fan sediments by IODP Expedition 362

Abstract. Drilling and coring during IODP Expedition 362 in the eastern Indian Ocean encountered probably the largest wood fragment ever recovered in scientific ocean drilling. The wood is Late Miocene in age and buried beneath ∼800 m of siliciclastic mud and sand of the Bengal–Nicobar Fan. The wood is well preserved. Possible origins include the hinterland to the north, with sediment transported as part of the submarine fan sedimentary processes, or the Sunda subduction zone to the east, potentially as a megathrust tsunami deposit.

Issues with integrating carbonate sand texture data generated by different analytical approaches: A comparison of standard sieve and laser-diffraction methods

Palaeoenvironmental reconstructive work relies on sediment-texture information as a proxy for particle-transport dynamics. As the integration of data from multiple sources is often sought for studies at the regional scale, a body of work exists that addresses compatibility levels of different analytical approaches. Past insights are derived predominantly from siliciclastic deposits. This study instead addresses standard sieve and laser-diffraction methods of texture analysis on a suite of carbonate beach sands from San Salvador Island, Bahamas. While the near-equant grain shapes of siliciclastic sands generate comparable results between methods, our findings show that laser diffraction tends to overestimate the coarser fractions in carbonate sands, given indiscriminate measurement of elongate particle axes. Peloidal grains that have width-to-length ratios on the order of 0.5 can pass through much smaller mesh sizes when sieved than diffraction would suggest. The resulting deviation in analytical outputs significantly influences measures of skewness, sorting, and other derivative metrics, which vary by up to several Wentworth classes between procedures. Meaningful integration of texture data obtained by different methods is ill-advised when dealing with carbonate beach sands. Similar problems are documented for muddy deposits, wherein platy clay minerals favor size-overestimation by laser diffraction and a size underestimation by settling tube technique. An improved understanding of methodological biases resulting from application of different particle-size methods is needed to help facilitate data-reconciliation efforts. This study provides a glimpse of issues facing the integration of carbonate sand-texture information, troublesome given prevalence of irregular grain shapes.

Pliocene short-term climate changes preserved in continental shallow lacustrine-palustrine carbonates: Western Opache Formation, Atacama Desert, Chile

AbstractMultiple climate proxies indicate episodic changes in moisture levels within an ∼1 Ma duration (early–mid Pliocene) interval. Limestones within the Opache Formation, Calama Basin, Atacama Desert region, Chile, contain evidence for wetter and drier periods on short time scales. Proxies include carbonate lithological changes, paleontology (stromatolites, oncolites, gastropods, ostracods and diatoms), O and C stable isotopes, geochemistry, and mineralogical changes (aragonite, calcite, Mg-calcite, dolomite and gypsum) throughout a 30 m stratigraphic section. Stromatolite fossil cyanobacteria dark and light laminations and mesohaline to hypersaline diatom species suggest Pliocene annual seasonality. Short-term changes between wetter and drier conditions indicate that at least this part of the Atacama region was experiencing relatively rapid early–mid Pliocene climate instability. The predominance of limestone in the Opache Formation, in contrast to the 1500 m of Oligocene-Miocene siliciclastic conglomerates and sandstones, interpreted as arid climate alluvium, that underlie it, indicates a shift from arid or hyperarid climate to a semi-arid climate. Semi-arid conditions promoted limestone deposition in a shallow lacustrine-palustrine environment. In this setting, events such as storms with associated surface water flow, erosion, siliciclastic sand, gravel, and intraclast deposition, coupled with significant biological activity, represent sedimentation during more humid periods in a shallow lacustrine depositional environment. In contrast, limestone characterized by mudcracks, Navicula diatoms, and vadose syndepositional cementation, reflect periods of enhanced evaporation, water shallowing, and episodic desiccation, characteristic of a palustrine depositional system. These facies shifts, in conjunction with geochemical and isotopic proxy evidence, yield a sedimentary record of wetter and drier climate shifts.

The Santo Antônio Bank: a high-resolution seismic study of a deflected ebb-tidal delta located at the entrance of a large tropical bay, eastern Brazil

The Santo Antonio Bank (SAB) is a large sand accumulation on the inner continental shelf of Salvador City, next to Todos os Santos Bay (TSB) entrance, eastern Brazilian coast. Since the sixteenth century, chroniclers report this feature as a navigational hazard. Previous studies have tried to determine the origin and volume of sands stored within this feature without success. In the present study, high-resolution shallow seismic surveys were used to investigate the architecture, origin, and evolution of the SAB. One hundred seventy kilometers of high resolution shallow seismic lines was acquired during December 2014 and February 2015 using a Meridata® sparker system operating on frequencies between 0.3 and 1.5 kHz, with a maximum 200 J of energy. Three major stratigraphic units were identified: the bedrock, a transgressive unit, and a highstand unit. The bedrock comprises sedimentary rocks of the Northern Camamu Sedimentary Basin and high-grade metamorphic rocks of the Precambrian basement. The transgressive unit is interpreted as the result of deposition in an estuarine environment infilling incised valleys. The highstand unit represents the SAB itself. A well-preserved proto-bank exists at the core of the SAB marking the early stages of bank development. This proto-bank drowned in place possibly during the 8.2-ka event. Present-day SAB and tidal prism volumes show that the bank reached a maturity phase of development. The volume of siliciclastic sands stored in the SAB is 5.98 × 108 m3. The SAB is potentially a major source of sands for beach nourishment for the urban beaches of the city of Salvador and metropolitan area, thus increasing the resilience of the city to future sea-level rises.

Petrography, micromorphology and geochemistry of Mio-Pleistocene palustrine limestones in Kuwait, Arabian Gulf

Elongated lenses of palustrine limestone interbedded with Mio-Pleistocene fluvial sands were exposed within a sand quarry in the north of Kuwait. Detailed field investigations, thorough petrographic and nanoscopic examinations and geochemical analysis revealed that they were originally precipitated as carbonate microbialite in isolated freshwater ponds scattered within a fluvial floodplain. These lacustrine microbialites were subjected to intermittent drying and wetting conditions and eventually altered to pseudobrecciated nodular palustrine limestone through several pedogenic and early diagenetic processes. The pedogenesis is exhibited by cracking, nodulization, mottling, pseudomicrokarst and root traces. The desiccation macrocracks are filled with siliciclastic sands derived from short-lived flash floods. Diagenesis is represented by aggradational neomorphism of micrite, cementation of cracks and voids by sparry calcite and precipitation of biogenic needle-fibre calcite. Concurrently, the cracks filled with siliciclastic sands have been calcretized, forming an embedded interapalustrine calcrete. Mottling postdates the early diagenesis and is manifested by pseudoclotted texture and abundant black spots and dendrites, which may have been developed by the marmorization of iron and manganese ions due to the seasonal fluctuation of groundwater. The δ13C and δ18O values are mostly comparable with those of closed freshwater lakes; however, the high negative values of δ18O may be attributed to diagenetic calcite. The occurrence of this palustrine limestone within a fluvial siliciclastic sequence that lacks clearly developed calcrete profiles may present an exceptional setting for palustrine limestone.