Erosional Terraces Research Articles

Rectangular drainage pattern of a submarine canyon controlled by extensional tectonic structures: Case study of the Afam Incision Surface (Tortonian, Niger Delta, Nigeria)

Submarine canyons are a key element of the growth of continental margin depocentres such as the Niger Delta, the progradation of which is commonly influenced by syn-sedimentary processes of gravity-driven tectonism. In such setting, canyons commonly constitute combined stratigraphic-structural traps because of: (i) their clayey infilling and, (ii) the incision of marine reservoirs. Nevertheless, most of the time, their drainage network and/or the external factors leading to their formation are poorly characterized, which is also the case in other settings when the resolution of geophysical dataset is less accurate. Here we use 3D seismic reflection data from the onshore eastern Niger Delta to study the morphology of a buried canyon system in relation to extensional tectonic structures, which are E-W trending growth-faults and roll-over anticlines. We map the Afam Incision Surface (AIS), which cuts up to 1000 m into subjacent Eocene-Miocene deltaic and continental series. In planform, the AIS is organized in 6 stream orders. The Afam Canyon which has the highest stream order is globally NNW-SSE oriented and is locally controlled by E-W oriented grabens, which give it right-angle changes of direction. It perpendicularly captures 5th and 4th order tributaries, each of which has an asymmetric cross-sectional profile controlled by one major growth-fault. In the southern area, the western tributaries all include an upstream part that drains in the opposite direction to the regional slope, leading to bayonet-shaped paths in planform. Whatever the stream order of the tributary considered; its increase is controlled by growth-faults. Therefore, the AIS planform network has the characteristics of a rectangular pattern. Erosion of the AIS as a submarine canyon system is consistent with its more than one-kilometer depth, preserved knickpoints within the thalweg of the main canyon; and the many erosional terraces boarded by arcuate scarps located along the main canyon and its tributaries. The formation of the AIS is correlated to the 50 m Tor2 sea-level fall at 9.5–9.2 my, and may have been initiated by by-pass currents that acted in tandem with retrogressive erosion. The latter greatly contributed to deepening and widening of the canyon and its tributaries whatever the orientation of the local slope. Erosion of the AIS was favored by the high-density of growth-faults. During the final excavation phases, the channelization of longshore drift currents by the canyon and its tributaries probably contributed to the destabilization of their upper margins.

Storage and distribution of organic carbon in cave sediments: examples from two caves in the northern karst region of Puerto Rico.

The clastic sediments that accumulate in cave settings can be an important storage reservoir for organic carbon. This study reports on grain size, total organic carbon (TOC) concentrations, and total organic carbon:total organic nitrogen (TOC:TON) ratios measured in sediments from two caves in Puerto Rico. El Tallonal Cave (TAL) is a small cave with a flowing stream; the sediments in TAL were collected from a deposit that is being actively eroded. Clara Cave (CAM) is an upper level of the Río Camuy Cave System; the sediments from CAM were newly deposited by an internal river that rose in response to Hurricane Maria. Sediments collected from both caves were poorly sorted and contained no apparent stratigraphic correlation. CAM sediments contained a larger range in TOC concentrations but were overall lower than TOC in the TAL sediments. In TAL, the TOC concentrations were higher in sediments collected from below the erosional terrace. TOC:TON ratios from sediments at both caves were highly variable, highlighting the heterogeneous deposition and storage of organic matter. Despite the observed variation, TOC concentrations in both cave systems could cause retardation of organic contaminants by up to two orders-of-magnitude, implying that deposited sediments influence the fate of organic contaminants in the groundwater; therefore, cave sediments could facilitate long term storage of organic carbon and associated contaminants.

Geomorphology of the Po Fluvial Terraces in Turin Deduced by New Subsoil Data (NW Italy)

Urban geomorphology is a theme of increasing interest over the last decades. The present research about the Turin geomorphology (NW Italy) was carried out based on the drilling of 40 boreholes, of which 34 were designed for the construction of a new sewer collector by SMAT-Società Metropolitana Acque Torino, while other 6 were in the Valentino Park. These investigations allow us to evaluate the detailed morphology of the proglacial plain of the Rivoli-Avigliana end-moraine system (RAES) and facies, thickness and petrographic composition of fluvial sandy gravel forming this plain (Turin Unit). The local discovery of a truncated palaeosol suggests that this unit lies on a significant erosional surface shaped on more ancient fluvial sediments. New radiocarbon dating of woody macrorest above the palaeosol proves the Last Glacial Maximum (LGM) age of the Turin Unit. The same investigations suggest the presence of erosional terraces (Molinette T1 and Murazzi T2), shaped by the Po River in the proglacial sediments, and depositional terraces (Vallere T3 and Parco Stura T4), which are essentially formed by sand. Dating of woody macrorest confirms the widespread presence of RAES Late Pleistocene proglacial sediments and the subsequent entrenched Po fluvial terraces due to a significant Holocene fluvial deviation.

The recent fluvial fan of Lorca (Murcia) and its transition to the Guadalentín River: geological controls during the latest Pleistocene-Holocene

The fluvial fan of Lorca (Murcia) (AFL acronym in Spanish), located in the Guadalentín depression to the SE of the city, is approximately 11.5 km long, 9 km wide and 19 m maximum thick, and is presently cut by the Guadalentín river. Data from 17 excavations in the city and its surroundings revealed the presence of silts and channelized sands and gravels in the proximal fan, and tabular silts and clays in both the mid and distal fan. The age of the unit is latest Pleistocene - Holocene and the age of its excavation early Holocene. Incision of the fan head stopped more than 5000 years ago, when the Guadalentín river reached approximately 13 m below the fan surface at the fan apex, and three main erosional terraces were formed. The fan was formed during the change from glacial to interglacial conditions by subsidence in the basin, enhanced by sedimentary loading and compaction of the pre-fan series. Transition from fan to river is attributed to downthrow of the Guadalentín depression to NE of Lorca. This is demonstrated by fan thickness distribution, fan-base and internal beds tilting, palaeocurrents shift, erosional meander formation, tectonic activity in both Alhama de Murcia fault and Guadalentín river blind fault, and headward erosion in Viznaga rambla. The end of the last endorheism in Lorca in early Holocene, and in the rest of Guadalentín depression during the late Holocene, suggests transpression and transtension effects increased by unloading processes in adjacent areas, and probably flexion by loading in the Mediterranean platform after Pleistocene ice melting.

MIS 5e sea-level history along the Pacific coast of North America

Abstract. The primary last interglacial, marine isotope substage (MIS) 5e records on the Pacific coast of North America, from Washington (USA) to Baja California Sur (Mexico), are found in the deposits of erosional marine terraces. Warmer coasts along the southern Golfo de California host both erosional marine terraces and constructional coral reef terraces. Because the northern part of the region is tectonically active, MIS 5e terrace elevations vary considerably, from a few meters above sea level to as much as 70 m above sea level. The primary paleo-sea-level indicator is the shoreline angle, the junction of the wave-cut platform with the former sea cliff, which forms very close to mean sea level. Most areas on the Pacific coast of North America have experienced uplift since MIS 5e time, but the rate of uplift varies substantially as a function of tectonic setting. Chronology in most places is based on uranium-series ages of the solitary coral Balanophyllia elegans (erosional terraces) or the colonial corals Porites and Pocillopora (constructional reefs). In areas lacking corals, correlation to MIS 5e often can be accomplished using amino acid ratios of fossil mollusks, compared to similar ratios in mollusks that also host dated corals. Uranium-series (U-series) analyses of corals that have experienced largely closed-system histories range from ∼124 to ∼118 ka, in good agreement with ages from MIS 5e reef terraces elsewhere in the world. There is no geomorphic, stratigraphic, or geochronological evidence for more than one high-sea stand during MIS 5e on the Pacific coast of North America. However, in areas of low uplift rate, the outer parts of MIS 5e terraces apparently were re-occupied by the high-sea stand at ∼100 ka (MIS 5c), evident from mixes of coral ages and mixes of molluscan faunas with differing thermal aspects. This sequence of events took place because glacial isostatic adjustment processes acting on North America resulted in regional high-sea stands at ∼100 and ∼80 ka that were higher than is the case in far-field regions, distant from large continental ice sheets. During MIS 5e time, sea surface temperatures (SSTs) off the Pacific coast of North America were higher than is the case at present, evident from extralimital southern species of mollusks found in dated deposits. Apparently, no wholesale shifts in faunal provinces took place, but in MIS 5e time, some species of bivalves and gastropods lived hundreds of kilometers north of their present northern limits, in good agreement with SST estimates derived from foraminiferal records and alkenone-based reconstructions in deep-sea cores. Because many areas of the Pacific coast of North America have been active tectonically for much or all of the Quaternary, many earlier interglacial periods are recorded as uplifted, higher-elevation terraces. In addition, from southern Oregon to northern Baja California, there are U-series-dated corals from marine terraces that formed at ∼80 ka, during MIS 5a. In contrast to MIS 5e, these terrace deposits host molluscan faunas that contain extralimital northern species, indicating cooler SST at the end of MIS 5. Here I present a review and standardized database of MIS 5e sea-level indicators along the Pacific coast of North America and the corresponding dated samples. The database is available in Muhs et al. (2021b; https://doi.org/10.5281/zenodo.5903285).

Morpho-stratigraphic characterization of the southern shelf of Porto Santo Island (Madeira Archipelago): Insights for small-scale instability processes and post-LGM sedimentary architecture

Shelves surrounding volcanic islands represent only a small portion of the entire submarine edifice that can extend to a few thousand meters below sea level. Despite their small areas, shelves show a significant morphologic imprint made by the erosional, depositional, tectonic and volcanic processes when compared to those left on slopes or the submarine bases of islands. Therefore, shelf morphology can be used to improve our knowledge about the island evolution, especially the more recent events. In this study, we use multibeam bathymetry, side scan sonar and seismic reflection profiles of the southern shelf of Porto Santo Island in Madeira Archipelago to map its seabed morphology and seismic stratigraphy from the nearshore to the shelf edge. Based on morphological and seismo-stratigraphic differences, the study area has been divided in three sectors (Western, Central and Eastern). In general, the shelf consists of an erosive rocky surface mostly covered by sediments, and locally outcropping on the seafloor, especially on the Western and Eastern sectors. On these sectors two sets of submerged erosional terraces have been mapped at different depths (35–45 m, 50–75 m). The depth range of the terraces and their absence in the Central sector suggests that these were formed/modified at least by the Marine Isotope Stages (MIS) 5a-5d stillstands or even by older relative sea-level stillstands. Dykes, which are more resistant to erosion than the surrounding outcrops, have orientations that are similar to the volcanic and structural features present on the island (NNE-SSW and NW-SE). The sedimentary cover on the shelf is highly varied both in thickness and internal architecture, mainly due to different onshore sediment supply and available accommodation space. Two main seismic units have been recognized: the lower one (U1) is interpreted as a transgressive deposit, formed during the sea level rise after the Last Glacial Maximum (LGM); the upper one (U2) is interpreted as a modern highstand deposit. The Western sector is practically sediment-starved, except for a local thin sedimentary cover of U2 on the inner and outer shelf. The Central sector shows U2 as a sigmoidal-oblique progradational clinoform characterized by inner prograding geometry on top of U1. The Eastern sector shows U2 as sigmoidal clinoform extending over a large area and having a more aggradational component. The shelf edge shows an overall arcuate shape that strongly matches the coastline configuration, suggesting the occurrence of an old large-scale landslide event (LS1) prior to the shelf formation. Further incisions in the shelf edge of the Central sector are interpreted as resulting from mass-movements, classified as LS2 and LS3. LS2 is at least older than seismic unit U1 and LS3 formed during the late Holocene. The volume of the LS2 event (0.6 km3) suggests that it might have caused a hazardous tsunami if it failed as a single event. The recent LS3 events, due to their small magnitude were probably not hazardous. These observations provided a more comprehensive understanding of the recent evolution of Porto Santo insular shelf in terms of mass-movement processes and post-LGM sedimentary architecture.

Geomorphological evidence of Lake Ladoga Holocene regressions after the Baltic Ice lake drainage (derived from sediment echosounder data)

Abstract. Lake Ladoga has been experiencing frequent water-level fluctuations during the Holocene. Plenty surveys about reconstructions of lake level were published during the last century. They focused on the onshore transgressive sediments and landforms, while a regression below present level was only an assumption. Here for the first time, we present indications of lake-level lowstands by means of sediment echosounder data collected within the PLOT-project . Three types of landforms of coastal and terrestrial origin are detected in the basin. These are coastal bars, erosional terraces and buried erosional valleys. Three paleo-shorelines were determined at depths of 13, 21 and 40-42 m. The first two shorelines are identified by the presence of the coastal bars, while the level of maximum regression is obtained from erosional marks. Reported landforms are assumed to be formed in the Early Holocene following the Baltic Ice Lake drainage.

Late Pleistocene submarine terraces in the Eastern Mediterranean, central Lebanon, Byblos: Revealing their formation time frame through modeling

Important evidence related to sea-level fluctuation, human evolution and dispersal that took place onshore during the Late Pleistocene and Holocene eras, are currently found underwater due to the sea-level rise. In this study, we present submarine relative paleo sea-level indicators found offshore the Lebanese coastline, at large from the prominent ancient city of Byblos. Two different submarine erosional terrace sequences were identified at depths ranging from −40 to −25m (“distant” sequence) and −11 to 0m (“nearshore” sequence) below present sea level, by using a combination of high-resolution geophysical means and terrain data analysis techniques. In the absence of datings, a model that uses relative sea-level curves for different uplift rates and examines the terraces' formation for various cliff retreat rates (derived from literature and on-field GPS measurements) was built. This model indicates the most possible time frames of the submarine terraces’ (STs) formation. The “nearshore” terrace sequence is suggested to have originally been formed during Marine Isotope Stage 5a (MIS5a) sea-level highstand, while it was possibly re-flattened during the first MIS1 sea-level slowdown (8–6 ka BP). The “distant” sequence formation is placed between the MIS4 to MIS3 transition (∼62–50 ka BP), and during MIS3 sea-level highstands within 45–35 ka BP. It was also found that a long-term uplift rate of 0.28–0.37 mm/a and cliff retreat rate of 0.03–0.09 m/a best fit our data and existing onshore relative sea-level indicators. The formation of the STs at this time was further supported by chronologically intercurrent uplifting events that facilitated their formation and preservation. Finally, our model supports recent ice-sheet reconstructions related to higher MIS3 values since sea-level curves that were tested for this scenario (eustatically reaching up to −37/−38m depth), proved more successful in the formation of the “distant” sequence.

Marine terraces of the last interglacial period along the Pacific coast of South America (1° N–40° S)

Abstract. Tectonically active coasts are dynamic environments characterized by the presence of multiple marine terraces formed by the combined effects of wave erosion, tectonic uplift, and sea-level oscillations at glacial-cycle timescales. Well-preserved erosional terraces from the last interglacial sea-level highstand are ideal marker horizons for reconstructing past sea-level positions and calculating vertical displacement rates. We carried out an almost continuous mapping of the last interglacial marine terrace along ∼ 5000 km of the western coast of South America between 1∘ N and 40∘ S. We used quantitatively replicable approaches constrained by published terrace-age estimates to ultimately compare elevations and patterns of uplifted terraces with tectonic and climatic parameters in order to evaluate the controlling mechanisms for the formation and preservation of marine terraces and crustal deformation. Uncertainties were estimated on the basis of measurement errors and the distance from referencing points. Overall, our results indicate a median elevation of 30.1 m, which would imply a median uplift rate of 0.22 m kyr−1 averaged over the past ∼ 125 kyr. The patterns of terrace elevation and uplift rate display high-amplitude (∼ 100–200 m) and long-wavelength (∼ 102 km) structures at the Manta Peninsula (Ecuador), the San Juan de Marcona area (central Peru), and the Arauco Peninsula (south-central Chile). Medium-wavelength structures occur at the Mejillones Peninsula and Topocalma in Chile, while short-wavelength (< 10 km) features are for instance located near Los Vilos, Valparaíso, and Carranza, Chile. We interpret the long-wavelength deformation to be controlled by deep-seated processes at the plate interface such as the subduction of major bathymetric anomalies like the Nazca and Carnegie ridges. In contrast, short-wavelength deformation may be primarily controlled by sources in the upper plate such as crustal faulting, which, however, may also be associated with the subduction of topographically less pronounced bathymetric anomalies. Latitudinal differences in climate additionally control the formation and preservation of marine terraces. Based on our synopsis we propose that increasing wave height and tidal range result in enhanced erosion and morphologically well-defined marine terraces in south-central Chile. Our study emphasizes the importance of using systematic measurements and uniform, quantitative methodologies to characterize and correctly interpret marine terraces at regional scales, especially if they are used to unravel the tectonic and climatic forcing mechanisms of their formation. This database is an integral part of the World Atlas of Last Interglacial Shorelines (WALIS), published online at https://doi.org/10.5281/zenodo.4309748 (Freisleben et al., 2020).

Fold architecture predisposing deep-seated gravitational slope deformations within a flysch sequence in the Northern Apennines (Italy)

This research illustrates the case of a deep-seated gravitational slope deformation (DSGSD) affecting a folded structure that developed during Miocene tectonics in the Northern Apennines of Italy, deforming a heterogeneous turbiditic sequence characterised by pelitic-marly and arenaceous intervals. The investigated area includes the headwater portion of the Bidente River basin that has experienced intensive valley incision during the late Quaternary and the sizeable Poggio Baldi landslide with failure episodes in 1914 and 2010.The aim of this work was to unravel the local morphostructural setting determined by the interplay between the active DSGSD, inherited fold geometry, and fluvial dynamics. Dip data collection and structural analysis were used to reconstruct the fold geometry. A complex type of fault-propagation fold was outlined, with later breakthrough thrusts causing the backlimb rotation. Gravity-driven geomorphological features were inventoried and displayed on two geological cross-sections together with Geological Strength Index measurements, while their distribution was analysed to conceive a conceptual model for the development of the DSGSD. Fold setting and lithological anisotropies were considered as the predisposing factors of the gravitational slope deformation, whereas the multi-step entrenchment of the Bidente River valley, testified by the presence of the remnants of at least three generations of fluvial erosional terraces, was considered as the main cause of its initiation.Finally, the reconstruction of the original slope topography in the Poggio Baldi area and the results of structural analysis performed in the landslide detachment zone through remote sensing techniques allowed for the estimation of the size of the wedge-shaped rock block which detached in 1914, an event that occurred in the wider frame of the DSGSD.

The Unique Sabantuy Chromite Paleoplacer in the Sedimentary Cover of the Eastern European Platform

Abstract—In the section of the sedimentary cover of the East European Platform, the new Sabantuy chromite ore occurrence was revealed. It is localized in Upper Kazanian sandstones of the erosional terrace above the floodplain of Malaya Berkutla Creek in the south-west of the Republic of Bashkortostan. The article presents the first results of geological, lithological, and mineralogical–geochemical studies of the new ore occurrence with signatures of an ancient coastal–marine placer. The Cr2O3 content in chromite sandstones (chromitolites) is 15.9–17.2 wt %. The bulk of the heavy fraction (67%) is composed of sub-angular and sub-rounded Cr-spinel grains 0.15–0.25 mm in size. Their mineralogical features indicate proximity to Cr-spinels from ultramafic rocks of ophiolite complexes.

The Turin fluvial terraces as evidence of the new Holocene setting of the Po River

ABSTRACT A detailed geomorphological mapping of the Po River fluvial terraces into Turin was performed through a field survey. Although Turin is crossed by the Po River, most of the town is built on the extended alpine outwash fans linked to its tributaries (essentially fed by the Dora Riparia Basin), made by 20–60 m thick gravel with sand lenses, or on erosional terraces (T1 and T2) shaped by the Po River into these sediments. Only a small sector of the town is built on a narrow belt formed by thin soft sand deposited by the Po, forming depositional terraces (T3 and T4) entrenched in the alpine fans. This research allowed us to recognize three steps in the geological evolution of the area: the construction of the alpine fans (Last Glacial Maximum), the first flow of the Po River that erodes the tributary sediments (Lateglacial-Holocene) and the deposition by Po River (Holocene).

Морфология погребенных долин ленского золотоносного района и связанные с ней особенности строения аллювиальных россыпей

In this article were summarized data of exploration work conducted in the valleys of the Lena gold-bearing area, starting from the early stages of exploration (1910–1920s) and ending with the 1990s, including large-scale production plans. the drainage system of the area inherits the faults that were formed in the late jurassic – early cretaceous. buried valleys inherit their position during several erosion cycles: in the same reaches the position and width of valleys of different ages remains approximately the same, but in the bottoms of valleys of different ages large transformations are observed. in the formation of the morphology of the buried valley of the Lena gold-bearing area, the leading role belongs to river channel processes. Stable position of the river valleys determines the inherited position of the placers, as a rule, they are located within the contours of the modern valleys or their bottoms, or are slightly shifted to terrace staircases. Inherited are also morphodynamic types of channels, however, in cases of displacement of the valleys to areas with different lithological and tectonic conditions, morphological structure of valleys and alluvial deposits also change. This leads to formation of beads-like widening of the valleys and steps in their longitudinal profiles. Specific examples of such changes are illustrated from the large valleys of the rivers vacha and bodaybo. Three types of buried erosional terraces were considered that alternate along the length of valleys and determine specific position of the placers. a significant influence of the major tributaries to the structure of placers was found that is expressed in the presence of a transverse to the axis of the valley of the gold-bearing layers and sudden changes of elevation values of their beds. the established regularities are relevant to other mining areas and may have great practical importance in prospecting and exploration of mineral deposits.

Geomorphological Analysis Applied to the Evolution of the Quaternary Landscape of the Tormes River (Salamanca, Spain)

This paper presents a geomorphological analysis of the Tormes River during the Quaternary. The Tormes River formed in the center-west of the Iberian Peninsula in the province of Salamanca. It runs along a Cenozoic basin with basement materials and through Varisco, and consists of mainly granitic and metamorphic materials, leaving a wide stream of river terraces, both erosional and depositional, that confirm its evolution throughout the Quaternary. Geomorphological analyses using Geographic Information Systems tools, Digital terrain model high resolution (MDT05, LIDAR), Orthophotos (scale 1:5000), and geological maps (1:50,000 Series Magna) have allowed different morphologies and depositional terraces to be distinguished, namely, 19 levels of erosional terraces and 3 levels of erosion surfaces. Based on these correlations, the levels of terraces in the Tormes River between T1 (+140 m) and T7 (+75–80 m) are located in the Pleistocene, those between T8 (+58–64 m) and T14 (+18–23 m) in the Middle Pleistocene, those between T15 (+12–13 m) and T17 (+6–7 m) in the Upper Pleistocene, and those between T18 (+3 m) and T19 (+1.5 m) in the Holocene. The erosion surfaces are divided into six levels: S6 (+145 m), S5 (+150 m), S4 (+160 m), S3 (+170 m), S2 (+180 m) and S1 (+190 m) located in the Lower Pleistocene, This work performs a geomorphological mapping procedure applied to the evolutionary analysis of the landscape, so that it determines different geomorphological units allowing the relief and morphology of the terrain in past times, establishing a dynamic analysis of the landscapes.

Influence of climate on the late Pleistocene depositional history of the Gulf of Gemlik (Sea of Marmara)

The late Pleistocene climate-induced water level fluctuations in the Gulf of Gemlik have been first documented in detail by analyses of high-resolution seismic reflection profiles that are tied to radiocarbon-dated sediment cores. In this study, synthetic seismogram for seismic-to-core comparison has enabled a precise correlation between seismic units and core sediments, on which basis it was possible to establish an accurate chronology of the sedimentary successions deposited in the gulf. In the seismic profiles, pre-Holocene depositional units reflect distinctive hydrological and sedimentological processes due to the fluctuating water levels in response to high climate variability. Progressive drop in water level, due to a dry climate phase in the Last Glacial Maximum, is marked by a progradation of a lacustrine delta on the northern shelf of the gulf. Post-glacial warming prior to ca. 18 cal ka BP gave rise to meltwater-driven, high-energy currents on the shelf slopes, producing erosional gullies. The later transgressive freshwater stage of the Gemlik “Lake” began as a result of the meltwater discharge to the gulf basin, which is associated with deposition of the retrograded clinoforms on the shelf margin and infilling the gullies along the slopes. The final post-glacial water level rise in the gulf was mediated by a regressive phase, inferred by seaward-dipping clinoforms and deltaic deposition on the shelf margin. Such a climatic shift to a drier period may coincide with the Heinrich event 1. In the Gulf of Gemlik, transition from the Bolling/Allerod to Younger Dryas periods is recognized by deposition of small clinoform packages on the shelf margin, marking a decrease in water level due to low precipitations on the catchment area. The lake level decrease in the gulf during the YD was modulated by a stillstand, producing a – 65-m erosional terrace and a beach-berm elsewhere along the gulf shelves.

Beachrock as sea-level indicator – A case study at the coastline of Oman (Indian Ocean)

Future sea level changes are a threat to coastal communities worldwide. Therefore, reliable methods for the reconstruction of past relative sea-level variations are essential to make predictions of future developments. Beachrocks are a widely distributed indicator of Quaternary paleo-coastlines, with a high potential for preservation. Nevertheless, their use as sea-level indicator faces challenges in the determination of the exact elevation of beachrock formation with respect to paleo sea level. This information is crucial to the establishment of a sea-level index point.Our study area on the northeastern coastline of Oman shows a set of twelve marine terraces that reach a height of 500 m above mean sea level. While the upper terraces are erosional, the lowest three are depositional. On the erosional terraces beachrocks are the only sediment-cover that withstood erosion. On the depositional terraces however, beachrocks occur alongside other sediments, which can be fluvial, alluvial, or marine in origin. The coastal development of the area is of high interest as uplift and tectonic activity are ongoing.Fieldwork and petrographic methods on eight outcrops on three terrace levels led to the identification of eight lithofacies in six facies associations. We used facies analysis to improve the information on paleo sea level as derived by the sole use of geomorphological indicators. Here, we demonstrate that facies analysis (so far applied exclusively on sandy deposits) is applicable also to gravel-dominated systems. The results contribute to knowledge on local changes in the depositional environment and help to reconstruct uplift rates.

Sediment dynamics driven by contour currents and mesoscale eddies along continental slope: A case study of the northern South China Sea

Based upon 3-D seismic reflection data, published drill core studies, oceanographic observations and numerical modelling, this study identifies a small scale contourite depositional system that is locally confined by canyons, slope failures, erosional terraces and erosional slopes on the Jianfeng slope, northern South China Sea (~1500 to ~2300 m water depth). Possible oceanic driving mechanisms for the spatial heterogeneity in sedimentation and erosion within the Quaternary deposits in this complex slope system are investigated. Numerical simulations show that the quasi-steady South China Sea deep water circulation, which affects the study area, is characterised by low velocities (maximum 4 cm/s), with a slight modulation by tides (±1 cm/s). This quasi-steady deep-water thermohaline circulation is able to keep sediment particles in suspension, rather than to erode seafloor sediment. The bottom hydrodynamic regime becomes energetic when mesoscale eddies (horizontal scale of 10 to 100 km) approach.Our modelling study indicates that both surface and bottom mesoscale eddies are essential to account for the spatial heterogeneity in sedimentation pattern along the northern South China Sea continental slope. According to simulation, the eddy front contains the highest flow velocity over a mesoscale eddy cycle (45 days), exceeding the threshold for resuspension of unconsolidated sediment (15 cm/s). As a consequence massive resuspension is produced at various sites, and redistributed by sub-mesoscale (horizontal scale of 1 to 10 km) circulations originated from an eddy-topography interaction. In contrast to the surrounding areas, which are subject to erosional forcing for a relatively long part (>7% of an eddy cycle), both the locally-confined drift and a further upstream large elongated-mounded drift experience little erosion (<2.5% of an eddy cycle), and serve as depositional centres for sediment from remote areas and erosion from adjacent areas. Our study demonstrates a promising new perspective for bridging the scales between short-term sediment dynamics and long-term sedimentation through a comparison of modelled scenarios between normal conditions and energetic events.

Identification of Erosional Terraces on Seamounts: Implications for Interisland Connectivity and Subsidence in the Galápagos Archipelago

Shallow seamounts at ocean island hotspots and in other settings may record emergence histories in the form of submarine erosional terraces. Exposure histories are valuable for constraining paleo-elevations and sea levels in the absence of more traditional markers, such as drowned coral reefs. However, similar features can also be produced through primary volcanic processes, which complicate the use of terraced seamounts as an indicator of paleo-shorelines. In the western Galápagos Archipelago, we utilize newly collected bathymetry along with seafloor observations from human-occupied submersibles to document the location and depth of erosional terraces on seamounts near the islands of Santiago, Santa Cruz, Floreana, Isabela, and Fernandina. We directly observed erosional features on 22 seamounts with terraces. We use these observations and bathymetric analysis to develop a framework to identify terrace-like morphologic features and classify them as either erosional or volcanic in origin. From this framework we identify 79 erosional terraces on 30 seamounts that are presently found at depths of 30 to 300 m. Although intermittent subaerial connectivity between the islands has been hypothesized, the depths of these erosional terraces in the Santiago region are the first direct evidence of paleo-connectivity in the modern archipelago. Collectively, the terraces have non-randomly distributed depths. We suggest that peaks in the distribution of terrace depths likely represent long durations of exposure (i.e., sea-level still or lowstands). By comparing these peaks to those of subsidence adjusted sea-level curves, we identify the average subsidence rate that best reproduces the observed terrace distributions. These rates are 0.2–0.4 m/ka for this portion of the central Galápagos, since the formation of the seamounts, consistent with previous independent estimates. Using these subsidence rates and evidence for erosional terraces at depths up to 300 m, we conclude that all islands in the central archipelago have been intermittently connected starting between 435 and 900 ka. Individual island pairs have likely been repeatedly subaerially connected for short intervals since that time.

Late Pleistocene–Holocene environmental change in eastern Turkey: multi‐proxy palaeoecological data of vegetation and lake‐catchment changes

ABSTRACTThis study presents the first pollen record from Lake Hazar, eastern Turkey, together with previously published multi‐proxy records from core Hz11‐P03. The data provide insight into multi‐centennial‐scale climatic changes during the late Pleistocene–Holocene period. The onset of the Younger Dryas was marked by an increase in the number of herbaceous plants (such as those in the families Poaceae and Asteraceae and the genus Centaurea) and lake level lowering with a −73 m erosional terrace on the lake margin. At the beginning of the early Holocene, the expansion of deciduous Quercus was attributed to significant afforestation due to high humidity that was presumably sufficient for the establishment of the oak forest. An increase in deciduous Quercus was maintained until the middle Holocene. An increase in herbaceous plants, and lower lake levels occurred during the late Holocene as a result of cold and dry climatic conditions. Moreover, a deciduous Quercus delay was not detected in Lake Hazar, indicating that the presence of a warm and humid climate created favourable conditions for oaks to expand at the onset of the early Holocene.

Controls on Neogene carbonate facies and stratigraphic architecture of an isolated carbonate platform – the Caribbean island of Bonaire

Abstract The Neogene carbonate succession on the island of Bonaire (Netherland Antilles) shows complex geometries associated with a sequence of depositional and erosional events which reflects the history of this isolated platform and the interaction between eustasy and tectonics. Three major episodes of carbonate platform deposition are defined which show contrasting depositional styles: 1) aggradational platform (Lower-Middle Miocene) with sediments showing a fining-upward trend from mixed coral rudstone to medium-grained coralgal grain/packstone, partly dolomitized and tilted by tectonic deformation; 2) progradational platform (Upper Miocene-Pliocene) which is formed of several shallowing-upward prograding units mainly composed of reworked red algal grain/packstone, with significant dolomitization, passing upward to shoreline and aeolian deposits formed of cor-algal grain/packstone and large benthic foraminifera grainstone, and 3) flat-topped platform (Pleistocene) with a reefal framework composed of a rich variety of corals in a bioclastic pack/wackestone matrix. These platform episodes exhibit contrasting stacking patterns and are separated by erosional unconformities. Overprinting this depositional succession is a series of Quaternary near-horizontal shoreline erosional terraces and vertical cliffs which have been cut into the island stratigraphy and complicate the stratal field relationships. However, this terrace morphology clearly does not represent depositional episodes, as has been suggested before. The internal architecture of each of the three carbonate platform reflects of the dynamics of sedimentation with allogenic controls. The latter relate to major oceanographic and tectonic events in the region, including changing ocean circulation as a result of the closure of the Panama isthmus, and Caribbean plate dynamics that affected sea-floor and island topography. The Bonaire succession provides a model for understanding and predicting isolated carbonate platform development, as well as architecture, facies and potential diagenetic changes, in an active tectonic setting.