Wave-cut Terraces Research Articles

Onshore–offshore correlation of central Lake Erie glacial deposits

Dive observations, echogram transects, core sampling, and a seismic profile revealed that the lake bed of north-central Lake Erie is an extensive terrace cut by storm waves and currents. The terrace is an erosional unconformity on which Late Wisconsinan (Port Bruce and Mackinaw) glacial units crop out. Beds of massive diamictons, and glaciolacustrine sediments containing parallel reflections, crop out alternately from west to east, resulting from an oscillatory ice retreat. These beds correlate with the Port Stanley Drift (Port Bruce phase) and Wentworth Drift (Mackinaw phase) exposed in nearby shore bluffs and onshore moraines. The Port Bruce glacier and earlier readvances formed ice tongues and ice shelves in the central basin. Diamicton layers, some with debris flows, constitute the Port Stanley Till (offshore units M and O). A glaciolacustrine unit N was deposited during Port Bruce glacier recessions. Glaciolacustrine unit P lies between Port Bruce unit O and the Mackinaw Wentworth Till, unit Q. A subsequent glaciolacustrine unit R overlaps unit Q. The onshore Galt and Moffat moraines, composed of Wentworth Till, correlate with ridges of the Norfolk moraine unit Q which extend across Lake Erie between the base of Long Point, Ontario, and Erie, Pennsylvania. The onshore Paris moraine appears to have been eroded on the wave-cut terrace and is evident offshore only near the south shore of Lake Erie. Laminated unit S, younger than unit R, occurs in the western part of central Erie basin, and correlates with overflow of Lake Algonquin from the Huron basin.

Etudes géochimiques et geomorphologie lacustre du bassin du lac Yakhi en Mongolie orientale

This study presents the landscape evolution of Lake Yakhi basin in eastern Mongolia where the lake plays an important role in the surface water resources. The multiple paleoshoreline features indicate wave-cut terraces, whereas the modern-day shorelines show that Lake Yakhi is at rapid northward regression. The paleo- and modern-day shorelines imply that the paleo-Lake Yakhi may have covered an extensive area with different hydrological conditions in the past. Lake Yakhi shows a decreasing trend in an area with a loss of ~62.2 km2 or 63.9% of the total area during the past five decades. The hydrological dynamics of Lake Yakhi during the last 50 years show its shift into a playa lake coinciding with the local climate changes in warming since 1987 and drying since 1992. In the lake's margin, mafic igneous provenance indicates a derivation from weathered granite and gneiss terrains on the oceanic island arc. However, in the lake's center, felsic igneous and quartzose sedimentary provenances show a derivation from the pre-existing sedimentary terrain on the active continental margin. Lake Yakhi sediments show the granite (felsic) and granite basalt (felsic to mafic) source rocks with mostly shale and arkose. The A-CN-K ternary plot shows that granites, granodiorites, and tonalities dominated in the lake sediments may have been transported from the source rocks enriched in plagioclase and feldspars. The weathering trend in the source area shows a low degree of chemical weathering and the chemical maturity shows a climatic shift from semiarid to arid climatic conditions in the Lake Yakhi basin.

Antecedent topography controls preservation of latest Pleistocene-Holocene transgression record and clinoform development: the case of the São Francisco delta (eastern Brazil)

The present study used shallow high-resolution seismic surveys to characterize the architecture and stratigraphic evolution of the last depositional sequence of the Sao Francisco Delta (SFDS). The sequence was accumulated within a bathymetric low (BL) on the shelf located in front of the Sao Francisco river. This antecedent topography provided additional accommodation of tens of meters in comparison with the rest of the shelf. The BL and its sediment-trapping effect allowed the accumulation of a complete stratigraphic record of the eustatic sea-level rise since the Last Glacial Maximum (LGM). The SFDS comprises six stratigraphic units. Because cores and radiocarbon dates are not available for this study area, the depth of occurrence of the top and base of each unit and their lateral relationships with wave-cut terraces carved into the BL walls were used in an effort to tentatively tie the mapped units to known eustatic sea-level curves available in the literature. Results suggest that major episodes of accumulation occurred during intervals of reduced rates in sea-level rise. The lower five units mapped accumulated in a large bay-estuarine protected environment, which resulted from the inundation of the BL. Only the topmost unit (the present clinoform) accumulated in an open ocean environment when sea level reached approximately its present position after 8 ka and the whole shelf was flooded. The most important delta backstepping event was probably associated with Meltwater Pulse 1B, which followed the Younger Dryas and produced one of the most significant stratal surfaces of the SFDS. Among the existing wave-dominated deltas in the Brazilian coast only, the Sao Francisco was able to construct a well-developed sigmoidal muddy clinoform in an otherwise very shallow shelf subjected to high wave-energy levels. This was only possible because of the existence of a bathymetric low in front of the river mouth, which illustrates the role that antecedent topography might play in providing larger amount of available accommodation space called in this paper of additional accommodation.

The polygenetic Kahouanne Seamounts in the northern Lesser Antilles island arc: Evidence for large-scale volcanic island subsidence

Remotely operated vehicle (ROV) explorations of the Kahouanne Seamounts, located 25 km SE of Montserrat in the northern Lesser Antilles island arc, have discovered the occurrence of honeycomb-scalloped erosional features on volcanic and limestone outcrops at depths of up to 600 m below sea level (mbsl). These features, combined with the flat-topped morphology of the seamounts, the occurrence of shallow-water carbonates (rhodoliths, benthic foraminifera), and the presence of oxidized, highly-vesicular volcanic fragments, suggest that the seamounts were once subaerial islands and have subsequently subsided to their present depth. The subsidence is likely to have been in response to a combination of 1) graben development southeast of the island of Montserrat where faulting and extension are driven by the accommodation of slip convergence associated with oblique subduction along the Lesser Antilles volcanic arc and 2) regional subsidence inferred from studies of nearby carbonate platforms. 40Ar/39Ar dating of basalt/andesite lavas from the central seamount indicates active volcanism around 5.4 Ma. Shallow-water carbonates recovered from an apparent wave-cut terrace at ~600 mbsl in the same area, were deposited approximately 3 Ma, based on Sr-isotope stratigraphy, indicating significant subsidence of the complex since that time. The total subsidence is in line with displacements in the Kahouanne valley graben and regional subsidence rates inferred from carbonate platform depths around the islands of Guadeloupe and Martinique, implying that intra-arc subsidence has been a significant process shaping the present bathymetry and topography of this island arc.

Tectonic, diapiric and sedimentary chaotic rocks of the Rakhine coast, western Myanmar

The western margin of Myanmar is the northern extension the active Sunda (India-Eurasia) subduction zone. Coastal regions and offshore islands have remarkable exposures of chaotic rock terranes along wave-cut terraces that allow characteristics of tectonic, sedimentary and diapiric mélanges to be recognized. Tectonic shear zones (tectonic mélanges) contain fragments of Cretaceous ophiolites (chrome-spinel-bearing peridotites and radiolarian cherts) that are in contact with thrust packets of Eocene turbidite units (broken formations). The turbidites contain shale-rich beds that have been sheared during soft-sediment deformation (sedimentary broken formations) and are sandwiched between undeformed thick sandy beds. These are mass transport deposits (MTDs) that most likely formed during deposition of the initial detritus of the Himalayan orogenic zone, probably trench slope basins on the accretionary prism. The ophiolitic and turbiditic thrust slices have been exhumed and are currently being intruded by active mud volcanoes that bring fragments of units up from depth to the surface, forming diapiric mélanges. These diapiric mélange bodies contain only small fragments (<50 cm) that are randomly oriented and do not exhibit shear fabrics. Because the region lacks superimposed deformation characteristic of most orogenic belts, the origins of all three rock bodies can easily be distinguished.

Anatomy of a late Quaternary carbonate island: Constraints on timing and magnitude of sea-level fluctuations, West Caicos, Turks and Caicos Islands, BWI

The data set of the island of West Caicos consists of a combination of high-resolution lidar and digital imagery, radiometric data, and amino acid racemization (AAR) D/L values as well as extraordinary preservation of sedimentary and bio-constructed deposits on the island. Together with a well-established regional stratigraphic framework from the neighboring Bahamian island chain and detailed field mapping, the West Caicos data set provides an opportunity to advance our understanding of the link between carbonate stratigraphy and the controlling parameters of sea level, sediment supply, tectonics, and climate. The exceptionally well-preserved stratigraphic record of the mid–late Pleistocene from West Caicos allows recognition of marine isotope stages (MIS) 9/11, 5e, and Holocene. The west coast of the island provides a continuous 8.4 km exposure of the MIS 5e record that enhances our understanding of the intra-MIS 5e bipartite succession in this region and supports a sea-level lowering that occurred during the last interglacial (LIG).West Caicos accreted as a series of eolian, foreshore–shoreface, barrier-reef, and fringing-reef facies. Shoreline positions for all but the MIS 7 and MIS 5a/c deposits are now captured in the sub-meter geodetic-quality topobathy lidar survey. The Star Town unit of MIS 9/11 age forms the southern margin of the island as well as the central karst-modified spine, with uranium–thorium (UTh) coral ages of between 368 and 365 thousand years ago (ka) and a D/L Glu average of >0.60. Early MIS 5e strata include the Railroad Ridge unit in the center of the island, which was previously identified as one of the older units. The Railroad Ridge unit is a north–south linear dune ridge that has a D/L Glu average of 0.52 and indicates the onset of sediment sources from the east associated with a flooded central Caicos Platform. Approximately synchronously, the early MIS 5e South Reef unit (+3.75–4 m sea-level elevation) nucleated on the western coast of West Caicos, attached to the middle Pleistocene Star Town unit. The South Reef corals yield an average coral UTh age of 126.5 ka, with an average D/L Glu from skeletal matrix of 0.46. This transitional barrier/fringing reef is unconformably overlain by the late MIS 5e Boat Cove unit. Corals in the basal transgressive phase of this upper MIS 5e Boat Cove unit yield an average 120.6 ka UTh and average D/L Glu of 0.45. Shallow-marine, foreshore, and eolian facies of the late MIS 5e rest on a wave-cut terrace that cuts the early MIS 5e South Reef unit by as much as 3 m down to a +0.5 m above present sea-level elevation. The upper MIS 5e Boat Cove unit displays a maximum of +4.5 m elevation above present-day sea level, measured from the middle of the foreshore. This estimate may be a minimum value as the exposures on the west coast are part of a forced regressive foreshore sequence. Last stages of the upper MIS 5e deposition are seen in a series of regressive dune-beach ridge deposits referred to as the Northeast Ridges unit, with sea-level positions ranging from +4.5 m early in the central part of West Caicos, to +0.5 m on the far northeast coast, eventually building down below present-day sea level. This sea-level history fits well with previously documented records in the Bahamas and globally, and the extensive erosion supports a short-lived (between 126 and 120 ka) intra-MIS 5e relative-sea-level fall. Holocene deposition on West Caicos is restricted to northern and eastern margins of the island as a combination of strandplain and dune ridge mixed skeletal–oolitic units.Using the high-resolution digital elevation model (DEM) and linked digital geologic map makes it possible to compare areas of the different Pleistocene and Holocene units quantitatively and to constrain volumetric assessments and accumulation rates on the island for different units. The peak sediment volume production/accumulation rate—between 6.5 and 8 × 106 m3 or between 2.2 and 2.7 × 106 m3 per ka—is observed in the upper MIS 5e unit. This rate is 5 times greater than the 0.5 × 106 m3 per ka estimated for the Holocene ooid dune-ridge/strandplain complex, suggesting that the combination of higher sea level, greater wave and wind energy, and warmer ocean temperatures may have fostered the higher rate of chemically precipitated sediment supply.

Quaternary sealevel changes and coastal evolution of the Island of Trindade, Brazil

The present work presented the first dated paleosealevel indicators and new paleogeographic reconstructions of the Island of Trindade. All evidence for Holocene sealevels point to the elevations higher than the present in the age interval between 5.06 ka and 0.55 ka. The altitude and ages of the reconstructions are in good agreement with hindcast model curves and the empirical sealevel envelope for the mainland eastern and northeastern Brazilian coast. However, the position of paleo vermetid-reef indicates that sealevel was lower than eustatic elevation, suggesting an effect of island subsidence. In contrast to the Brazilian mainland coastal zone, where conspicuous evidences of Pleistocene highstands were recorded, no evidence of sealevels predating the Holocene has ever been observed on the island. This could be attributed to intense coastal erosion and log-term island subsidence.A volcanic edifice rising more than 5500 m above the ocean floor was built by magmatic activity that extended over the late Pliocene and early Pleistocene. After the cessation of volcanic build up, the island started to shrink due to subaerial and marine erosion. During Pleistocene sealevel lowstands, alluvial fans were formed beyond the present limits of emerged areas. They were enclosed within and preserved from erosion by the Middle to Late Pleistocene lava flows and pyroclastic deposits. During Late Pleistocene sealevel highstand, intense coastal erosion removed the distal parts of the alluvial fans. It is very likely that at that time the entire coast of the island experienced intense erosion and subsidence as deduced from the absence of Pleistocene coastal deposits, which are widespread along mainland Brazilian coast.The east coast of the island was dominated by high sea cliffs sculpted into volcanic rocks. A volcanic cone was subsequently formed, when the sealevel was lower than present during the Late Pleistocene to early Holocene. During the mid to late Holocene sealevel highstand the volcanic cone was partially eroded and a bay formed, where vermetid-reefs grow and sand beach deposits prograded until the bay was infilled. During the sealevel maximum, wave-cut terraces were formed and sea cliffs were active. During the lowering of the sealevel, cliffs became inactive and vermetid-reefs were eroded. Beach erosion and aeolian deflation presently prevails, indicating a low sediment supply.

Geochemistry of О, Н, C, S, and Sr isotopes in the water and sediments of the Aral basin

The paper presents original authors' data on the O, H, C, S, and Sr isotopic composition of water and sediments from the basins into which the Aral Sea split after its catastrophic shoaling: Chernyshev Bay (CB), the basin of the Great Aral in the north, Lake Tshchebas (LT), and Minor Sea (MS). The data indicate that the δ18О, δD, δ13C, and δ34S of the water correlate with the mineralization (S) of the basins (as of 2014): for CB, S = 135.6‰, δ18О = 4.8 ± 0.1‰, δD = 5 ± 2‰, δ13C (dissolved inorganic carbon, DIC) = 3.5 ± 0.1‰, δ34S = 14.5‰; for LT, S = 83.8‰, δ18О = 2.0 ± 0.1‰, δD =–13.5 ± 1.5‰, δ13C = 2.0 ± 0.1‰, δ34S = 14.2‰; and for MS, S = 9.2‰, δ18О =–2.0 ± 0.1‰, δD =–29 ± 1‰, δ13C =–0.5 ± 0.5‰, δ34S = 13.1‰. The oxygen and hydrogen isotopic composition of the groundwaters are similar to those in MS and principally different from the artesian waters fed by atmospheric precipitation. The mineralization, δ13С, and δ34S of the groundwaters broadly vary, reflecting interaction with the host rocks. The average δ13С values of the shell and detrital carbonates sampled at the modern dried off zones of the basins are similar: 0.8 ± 0.8‰ for CB, 0.8 ± 1.4‰ for LT, and –0.4 ± 0.3‰ for MS. The oxygen isotopic composition of the carbonates varies much more broadly, and the average values are as follows: 34.2 ± 0.2‰ for CB, 32.0 ± 2.2‰ for LT, and 28.2 ± 0.9‰ for MS. These values correlate with the δ18O of the water of the corresponding basins. The carbonate cement of the Late Eocene sandstone of the Chengan Formation, which makes up the wave-cut terrace at CB, has anomalously low δ13С up to –38.5‰, suggesting origin near a submarine methane seep. The δ34S of the mirabilite and gypsum (11.0 to 16.6‰) from the bottom sediments and young dried off zone also decrease from CB to MS in response to increasing content of sulfates brought by the Syr-Darya River (δ34S = 9.1 to 9.9‰) and weakening sulfate reduction. The 87Sr/86Sr ratio in the water and carbonates of the Aral basins do not differ, within the analytical error, and is 0.70914 ± 0.00003 on average. This value indicate that the dominant Sr source of the Aral Sea is Mesozoic–Cenozoic carbonate rocks. The Rb–Sr systems of the silicate component of the bottom silt (which is likely dominated by eolian sediments) of MS and LT plot on the Т = 160 ± 5 Ma, I0 = 0.7091 ± 0.0001, pseudochron. The Rb–Sr systems of CB are less ordered, and the silt is likely a mixture of eolian and alluvial sediments.

Uplifted Late Holocene shorelines along the coasts of the Calabrian Arc: geodynamic and seismotectonic implications

Late Holocene (~6.5 ka) shorelines represented by tidal notches, beach deposits, wave-cut terraces and intertidal organic rims are raised from few decimetres up to 5.5 m above the present sea level in the southern part of the Calabrian Arc, southern Italy. At five localities (Capo Vaticano and Scilla in southern Calabria and Taormina, Schiso, Capo Milazzo in north-eastern Sicily), the uplifted paleo-shorelines form a distinct vertical sequence where the older shorelines rest invariably above the younger ones. Such arrangement documents the occurrence of abrupt uplift events that, within the limits imposed by existing age controls, we attribute to ancient earthquakes. A comprehensive appraisal of published studies has allowed to draw an inventory with a total of possibly sixteen earthquakes which, based on the amount of shoreline displacement (~0.5-2 m) and the length of coastal section involved in uplift, were likely to be of strong size. It appears that the amount of uplift decreased with time during the Late Holocene at all sites but Capo Vaticano, where it remained almost stationary. The co-seismic events appear grouped within four temporal clusters, during which uplift occurred at most of the five coastal sectors investigated here. These clusters spanned time intervals whose duration, although difficult to bracket with precision, is of few hundred years, and are separated by longer (~0.5-1.5 ka) periods of apparent tectonic quiescence. The sources of co-seismic uplifts are still undefined, and should be searched between normal faults in the stretched Calabrian upper crust, or lower crustal thrust faults related to the Ionian subduction.

Reconstruction of the geology and structure of Lake Rotomahana and its hydrothermal systems from high-resolution multibeam mapping and seismic surveys: Effects of the 1886 Tarawera Rift eruption

Present-day Lake Rotomahana is one of the two focal points of the most destructive eruption in New Zealand's historical record, i.e., that of Mt. Tarawera on 10 June 1886, with devastating loss of life and presumed destruction of the iconic Pink and White Terraces that adorned the margins of the lake. Basaltic dikes are considered to have ascended near surface in the area, intruding into hydrothermally altered and water-saturated ground beneath the existing lake. The consequential hydrothermal and phreatomagmatic eruptions ejected 0.5325km3 of material from the lakefloor and below, plastering the nearby landscape for several kilometers with mud and other debris. The eruption buried the natural outlet of the lake, with the bottom of the craters becoming filled by water within months and completely concealed from view within years; today Lake Rotomahana has depths up to 118m.High-resolution (0.5m) bathymetric mapping, when combined with a 2-D seismic reflection survey, enables us to ‘see’ details of the maar craters on the lakefloor, including those parts subsequently buried by sediment. The large Rotomahana Crater described by workers immediately after the eruption measures ~2.5km in diameter near its southwestern end, and excavated ground to 155m below present-day lake level. The vent system, as revealed by the present study, forms an array of right-stepping (dextral) craters, with the main crater being host to two sub-craters Rotomahana West Crater and Rotomahana East Crater today buried beneath the lakefloor, and which are in-filled by 36 and 37m of sediment, respectively. Subordinate craters along the same 057° Tarawera Rift trace include Hochstetter Crater (11m of infill), Waingongongongo Crater (14m) and Rotomakariri Crater (26m). These craters host a total 0.0268km3 of sediment. Other features highlighted by the bathymetric data include; craters not filled by sediment, sediment fan deltas, volcanic ridges and dikes, submerged wave-cut terraces formed during times of lower lake levels and gas pockmarks, all either related to the 1886 eruptive episode or post-eruption hydrothermal and erosional processes.Application of results from bubble plume, CO2 flux, magnetic and heat flux surveys of Lake Rotomahana to this study, when combined with regional earthquake relocation analysis and broadband magnetotelluric data, suggest an explanation in terms of a magmatic heat source located south of Waimangu and a corresponding convective water/heat transport system extending thence to beneath the western end of the lake. A holistic approach has provided a coherent context for the eruption and its effect on the historical Pink and White Terraces hydrothermal system that appears to have been the eastern-most extension of a larger system that lay beneath the Waimangu area before the eruption. The newly named Pink Terraces hydrothermal system (~1.5km2) is a continuation of the historical hydrothermal activity that was concentrated on the western shores of the old lake, and together with the formation of the new, post-1886 Patiti hydrothermal system (~1km2) located SW of Patiti Island, mark the two distinct areas of hydrothermal activity in the lake today.

TerraceM: A MATLAB® tool to analyze marine and lacustrine terraces using high-resolution topography

High-resolution topographic data greatly facilitate the remote identification of geomorphic features, furnishing valuable information concerning surface processes and characterization of reference markers for quantifying tectonic deformation. Marine terraces have been used as long baseline geodetic markers of relative past sea-level positions, reflecting the interplay between vertical crustal movements and sea-level oscillations. Uplift rates may be determined from the terrace age and the elevation of its shoreline angle, a geomorphic feature that can be correlated with past sea-levels positions. A precise definition of the shoreline angle in time and space is essential to obtain reliable uplift rates with coherent spatial correlation. To improve our ability to rapidly assess and map shoreline angles at regional and local scales, we have developed TerraceM, a MATLAB® graphical user interface that allows the shoreline angle and its associated error to be estimated using high-resolution topography. TerraceM uses topographic swath profiles oriented orthogonally to the terrace riser. Four functions are included to analyze the swath profiles and extract the shoreline angle, from both staircase sequences of multiple terraces and rough coasts characterized by eroded remnants of emerged terrace surfaces. The former are measured by outlining the paleocliffs and paleoplatforms and finding their intersection by extrapolating linear regressions, whereas the latter are assessed by automatically detecting peaks of sea-stack tops and back-projecting them to the modern sea cliff. In the absence of rigorous absolute age determinations of marine terraces, their geomorphic age may be estimated using previously published diffusion models. Postprocessing functions are included to obtain first-order statistics of shoreline-angle elevations and their spatial distribution. TerraceM has the ability to process series of profiles from several sites in an efficient and structured workflow. Results may be exported in Google Earth and ESRI shapefile formats. The precision and accuracy of the method have been estimated from a case study at Santa Cruz, California, by comparing TerraceM results with published field measurements. The repeatability was evaluated using multiple measurements made by inexperienced users. TerraceM will improve the efficiency and precision of estimating shoreline-angle elevations in wave-cut terraces in both marine and lacustrine environments.

Evidence for progressive phosphorus limitation over long-term ecosystem development: Examination of a biogeochemical paradigm

Aims To test predictions of ecosystem theory for changes in P cycling over primary succession, we determined soil phosphorus (P) in labile, primary mineral, organic, and occluded forms along a chronosequence of five wave cut terraces known as the “Ecological Staircase”. The Ecological Staircase terraces (T1-T5) transition naturally from fertile native coastal forests in California, USA, to diminutive pygmy vegetation over the span of > 500,000 years of pedogenesis.

Late Pleistocene–Holocene sedimentary records of climate and lake-level changes in Lake Hazar, eastern Anatolia, Turkey

The sedimentary record of Lake Hazar has been documented by detailed seismo- and chronostratigraphic analyses using high-resolution seismic profiles and sediment cores. In this study, two main seismic stratigraphic units can be confidently correlated with sediment cores using lithology and physical properties. As a result of radiocarbon AMS dating, these sedimentary units presumably cover the period from the late Pleistocene to Holocene. The Holocene sequence of the lake infills can be subdivided into five subunits, each subject to distinctive hydrological and sedimentological processes due to fluctuating lake level. The elevations of these deposits including coastal paleo-terraces provide history of the lake level changes. In Lake Hazar, the maximum lowstand lake level occurred during the transition from the Younger Dryas to Holocene, when the −73 m wave-cut terrace was formed as the main shelf-crossing unconformity surface. The beginning of the Holocene was associated with a abrupt lake level rise and resultant deepening. Analysis of the Lake Hazar terraces has the potential to provide complementary data on the chronology of past lake level and lake volume changes, and their link to past climate change. The continued rising lake level during beginning of the Holocene was modulated by stillstands at −63 m and −56 m, as indicated by broad coastal paleo-terraces across the northern shelf. The main shelf-margin sedimentary system for the Holocene period is composed of three distinct buried deltaic-complexes that were presumably sourced from the Kürkçayı River, the largest input into Lake Hazar. The 3rd millennium crisis at around 3 14C ka BP is recorded in the seismic and core data when the cold and dry climatic condition gave rise to a significant lake level drop.

Late Holocene sedimentary changes in shallow water settings: the case of the Sele river offshore in the Salerno Gulf (south-eastern Tyrrhenian Sea, Italy)

A complementary data set consisting of sedimentological, petrophysical, geochronologic and seismo-stratigraphic analysis was used to detect the sedimentary changes in the inner shelf record in a mostly stable region, over the last 3 ky cal BP, that is since the sea level attained its present day position. This study has been carried out within the VULCOST project on the Salerno Gulf (south-eastern Tyrrhenian Sea, Italy), to learn more about the impact of minor morpho-climatic changes and anthropogenic disturbances on the shelf sedimentary record. Some decimeter-thick sandy beds deposited on the inner shelf, off the Sele river mouth, in the time span corresponding to the Little Ice Age (LIA), despite the fact that, at the time, the coastline at that time was farther inland with respect to its present day position. Since the beginning of the 20th century, mainly fine grained sediment settled, possibly owing to the restoration of mild climatic conditions, changes in the rivers’ regime and land-use. The analysis of the marine cores, collected off the coast, also reveals a litho-stratigraphic mismatch among coeval intervals, possibly driven by the different response of the catchments to the morpho-climatic variations. High resolution seismic data shows that the sediment bypass area, shaped by wave action, is much deeper than the estimated limit, if computed on the base of fair-weather wave length and sedimentologic analysis. Indeed, toplap terminations of reflectors are recognizable as deep as 25 m in the southern sector of the Gulf and suggest an intense reworking of the seabed over long periods, much deeper than the outer limit of the beach. This evidence is also supported by the state of pyroclastic layers in the core record, which proves sediment reworking in the marine setting, down to 24 m of depth. The high resolution sedimentologic and stratigraphic reconstructions point to a non linear relation between depth, distance from the coast and sand deposition over the time. This suggests a complex picture of coastal dynamics within the same basin, with a remarkable outcome regarding the wave-cut terraces as proxies of sea level stands.

Lake levels in the Erie Basin of the Laurentian Great Lakes

Water levels in the Lake Erie basin are inferred from glacial lake times to present. An era of early to middle Holocene lowstands is defined below outlets by a submerged paleo-beach, and truncated reflectors in glaciolacustrine sediment beneath a mud-covered wave-cut terrace. Also, the glacial clay surface above the paleo-shore level has elevated shear strength because of porewater drainage during subaerial exposure. Below the paleo-shore where exposure did not occur, clay strength remained normal. Sedimentation rates were reduced during the lowstands. The distortion of once-level shore zone indicators by differential glacial rebound was removed by computing original elevations of the indicators using an empirical model of rebound based on observations of upwarped former lake shorelines. Erie water-level history was inferred from a plot of the original elevations of lake-level constraints and outlets versus age. The lake history was validated by reference to ~83 water-level indicators, not used as constraints. During the deglaciation, lake-crossing moraines were likely eroded by fluvial drainage into low-level Lake Ypsilanti and a subsequent unnamed low lake to produce the Lorain Valley and Pennsylvania Channel. Once inflow from the upper Great Lakes basins was directed to Ottawa Valley about 10,400 (12,270 cal BP), Erie water levels descended in a dry, evaporative climate to a closed lowstand during which ostracode δ18O increased ~2‰ above present values. Lake level began to rise 6,000 to 7,000 (6,830 to 7,860 cal) BP in response to increased atmospheric moisture and later, to northern inflow as the Nipissing Transgression returned upper Great Lakes drainage to Lake Erie by about 5,200 (6,000 cal) BP. At that time, the lake overflowed the uplifted Lyell–Johnson Sill north (downstream) of the present Niagara Falls at higher-than-present levels. After recession of the Falls breached this sill about ~3,500 (~3,770 cal) BP, Lake Erie fell 3–4 m to its present Fort Erie–Buffalo Sill. The extended low-water phase with its isolated sub-basins could have restricted migration of aquatic fauna. The early to middle Holocene closed-basin response highlights the sensitivity of Lake Erie to climatic reductions in its water budget.

Taking the measure of a landscape: Comparing a simulated and natural landscape in the Virginia Coastal Plain

A landform evolution model is used to investigate the historical evolution of a fluvial landscape along the Potomac River in Virginia, USA. The landscape has developed on three terraces whose ages span 3.5 Ma. The simulation model specifies the temporal evolution of base level control by the river as having a high-frequency component of the response of the Potomac River to sea level fluctuations superimposed on a long-term epeirogenic uplift. The wave-cut benches are assumed to form instantaneously during sea level highstands. The region is underlain by relatively soft coastal plain sediments with high intrinsic erodibility. The survival of portions of these terrace surfaces, up to 3.5 Ma, is attributable to a protective cover of vegetation. The vegetation influence is parameterized as a critical shear stress to fluvial erosion whose magnitude decreases with increasing contributing area. The simulation model replicates the general pattern of dissection of the natural landscape, with decreasing degrees of dissection of the younger terrace surfaces. Channel incision and relief increase in headwater areas are most pronounced during the relatively brief periods of river lowstands. Imposition of the wave-cut terraces onto the simulated landscape triggers a strong incisional response. By qualitative and quantitative measures the model replicates, in a general way, the landform evolution and present morphology of the target region.

Evolución de lagos proglaciales embalsados por hielo en Última Esperanza, Chile: Implicancias de la explosión volcánica tardiglacial R1 del volcán Reclús, Zona Volcánica Austral Andina.

Newly described outcrops, excavations and sediment cores from the region of Ultima Esperanza, Magallanes, contain tephra derived from the large late-glacial explosive R1 eruption of the Reclus volcano in the Andean Austral Volcanic Zone. New radiocarbon dates associated to these deposits refine previous estimates of the age, to 14.9 cal kyrs BP (12,670±240 14C yrs BP), and volume, to >5 km3, of this tephra. The geographic and stratigraphic distribution of R1 also place constraints on the evolution of the ice-dammed proglacial lake that existed east of the cordillera in this area between the termination of the Last Glacial Maximum (LGM) and the Holocene. This proglacial lake generated wavecut terraces, and also caves, such as the Cueva de Milodon, along the highest prominent terrace. The current elevation of these terraces depends on the total amount of post-glacial isostatic rebound, which is unknown. Due to differential rebound, the highest prominent lake terraces decrease in height from west-to-east, from ~170 m a.s.l. on Peninsula Antonio Varas west of Seno Ultima Esperanza, to ~150 m a.s.l. around Lago Sofia, and down to ~125 m a.s.l. along their easternmost margin. The presence of thick deposits of R1 tephra in some of the caves around Lago Sofia implies that the proglacial lake had already dropped below its highest level prior to the time of this eruption, and, in fact, even earlier, prior to 16.1 cal kyrs BP (13,560±180 14C yrs BP), when land mammals first occupied these caves. The depositional environment of R1 in a core from Dumestre bog suggests that the lake level was in fact 70 m a.s.l. until 12.8 cal kyrs BP (10,695±40 14C yrs BP). However, a 14.2 cal kyrs BP (12,125±85 14C yrs BP) Mylodon pelvis from a nearby site, located at only ~7 m a.s.l., suggests that the lake could have emptied, for at least a brief period, to this low level at this time. This latter datum, combined with the lack of any prominent terraces between the highest ones (170-125 m a.s.l.) and much lower ones (at only 30 m a.s.l. on Peninsula Antonio Varas and 20 m a.s.l. along the coast north and south of Puerto Natales), suggests abrupt changes in the lake level after the R1 eruption. The likely mechanism for producing these changes in Ultima Esperanza was the catastrophic failure and subsequent re-sealing of an ice dam in Paso Kirke, the only below sea-level pathway west to the Pacific north of Fjordo Obstruccion. The final stage of lake drainage, from the lower terrace level (20-30 m a.s.l.) occurred at 10.3 cal kyrs BP.

Geomorphologic indicators of sea level and lowstand paleo-shelf exposure on early–middle Miocene sequence boundaries

Relationships between clinoformal sequence boundaries and relative sea level change, in particular locations of lowstand shorelines and amounts of paleo-shelf exposure, represent a key topic in sequence stratigraphy. This paper uses 3D seismic data to analyze morphologies of two sequence boundaries in the early–middle Miocene section of the Northern Carnarvon Basin of the Australian Northwest Shelf. A step-like discontinuity is recognized on the outer paleo-shelf of sequence boundary DLS4 based on reflection truncation and seismic facies change. Vertical offset at this step varies from ~7 to 65m, increasing along strike from SW to NE. The trace of this discontinuity comprises both linear and arcuate segments in coherence time slices. Isolated mounds and small platforms also occur on this sequence boundary, basinward of the discontinuity. Underlying sub-sequence boundary DLS3.1 displays a smaller step-like discontinuity, together with U- and V-shaped incisions as well as numerous, isolated depressions, the latter occurring within two large arcuate, concave-basinward features on coherence slices. We discuss several possible origins for these discontinuities. We conclude that step-like discontinuities on DLS4 and DLS3.1 represent buried wave-cut terraces or sea cliffs. Erosion is presumed to have occurred at or near ambient sea level. The incisions of DLS3.1 are karst. Together, these interpretations imply significant lowstand paleo-shelf exposure of early–middle Miocene sequence boundaries.

Late glacial to Holocene sea-level changes in the Sea of Marmara: new evidence from high-resolution seismics and core studies

The late glacial to Holocene sedimentary record of the northern shelf of the Sea of Marmara (SoM) has been documented by detailed seismo-, chrono-, and biostratigraphic analyses using sub-bottom (Chirp) profiles and sediment cores. During MIS 3 and the main part of MIS 2 (60–15 14C ka b.p.), disconnection from the Mediterranean and Black seas together with a dry climate resulted in a regression in the SoM, when the Sea was transformed into a brackish lake. The river incisions below 105 m water depth along the northern shelf took place during the last glacial maximum, when the lake level was modulated by stillstands at −98 and −93 m. The post-glacial freshwater transgressive stage of the Marmara ‘Lake’ occurred between 15 and 13.5 14C ka b.p., leading to a rise in water level to −85 m by 13.0 14C ka b.p., as evidenced by broad wave-cut terraces along the northern shelf. Since 12 14C ka b.p., high-frequency sea-level fluctuations have been identified at the SoM entrance to the Strait of Istanbul (SoI). Thus, wave-cut terraces have been recorded at water depths of −76 and −71 m that, according to an age model for core MD04-2750, have ages of 11.5 and 10.5 14C ka b.p., respectively. Ancient shoreline at −65 m along the northern shelf presumably formed soon after the Younger Dryas (YD) at ca. 10.1 14C ka b.p. Moreover, there is compelling evidence of Holocene outflow from the Sea of Marmara to the Black Sea. At the SoM entrance to the SoI, the existence of bioherms on the reflector surface together with abundant Brizalina spathulata and Protoglobulimina pupoides in a core suggests a return to higher salinities due to strong Mediterranean water incursion into the SoM at ∼8.8 14C ka b.p. This finding is consistent with earlier suggestions that, after the YD, the Black Sea was flooded by outflow from the SoM as a result of global sea-level rise.

Large-scale sedimentary bedforms and sediment dynamics on a glaciated tectonic continental shelf: Examples from the Pacific margin of Canada

The Pacific margin of Canada has been subjected to tectonism, dramatic sea level change and vigorous storm and tidal energy since glacial times resulting in a complex seafloor. Extensive multibeam mapping of this shelf has provided an opportunity to understand how these processes have impacted sedimentology and morphology. Bathymetric restriction of the tidally dominated flow between the inland seas and the open Pacific has resulted in the development of very large subaqueous dune fields and terrace moats. For example, in the southern Strait of Georgia nearly symmetrical dunes with wavelengths between 100 and 300 m, dune heights up to 28 m, cover the seafloor in 170–210 m water depth. In northern Hecate Strait a 72 km 2 area of large 2D dunes occurs at the transition with Dixon Entrance which opens to the Pacific Ocean and steep (>10°) wave-cut terraces and drowned spits, a result of sea level changes during the Holocene, are now being undercut to generate moats 7 m deep, in a narrowing shelf trough. Currents, with velocities ranging between 0.2 and 2.2 m s −1, are dominated by semi-diurnal tidal streams that are continually modified by wind and estuarine circulation. There appears to be a clear association of grain size, water depth and flow velocity controlling the size of the subaqueous dunes.