Episodes Of Erosion Research Articles

Determining What's Important about Landscapes

Determining What's Important about Landscapes

Past and future perspectives upon landscape instability in Cumbria, northwest England

Geomorphic and lacustrine evidence is used to identify phases of landscape instability in Cumbria, northwest England, British Isles. The temporal and spatial pattern of erosion is used alongside palaeovegetation data from across the region to elucidate the causes of landscape instability in the late Holocene. The lines of evidence include changes in sediment accumulation rate and provenance within the region’s lakes; the hillslope alluvial fan and gully incision record; the lowland fluvial geomorphic record; and pollen analytical data for the late Holocene vegetation history. The sediment supply driven hillslope and lacustrine records suggest that only the 1,200–800 BP and post 600–500 BP erosion episodes were both catchment-wide and directly affected upland hillslopes, with the earlier phases after 5,200 and 3,000–1,600 BP on a lower scale and more restricted spatially. These episodes coincide with major expansions or changes in anthropogenic activity during Neolithic, Iron Age and Romano-British, Norse and late Medieval times. The projected future trajectories of changes in both climate (UKCIP scenarios) and landuse (CAP reform and ESA status) appear unlikely to increase landscape instability, although a shift to greater incidence of storms and high magnitude flood events clearly could. The past shows that the largest increases in erosion and sediment movement occur in the wake of major intensifications in land pressure that primarily affect previously wooded or protected hillslopes, circumstances that land management strategists should mitigate against.

Sedimentology of rocky shorelines: 4. Coarse gravel lithofacies, molluscan biofacies, and the stratigraphic and eustatic records in the type area of the Pleistocene Hulopoe Gravel, Lanai, Hawaii

During the Pleistocene, the south flank of Lanai was the site of repeated erosion and deposition as a consequence of glacio-eustasy superimposed on tectono-isostasy. Throughout this time, terrigenous and bioclastic gravel was generated along mainly rocky shorelines. Gravelly sediment was deposited in topographically low areas of the shoreface and inner shelf that later were uplifted isostatically. The sedimentary lithofacies and biofacies in these deposits are distinctive, but they represent a deposystem and suite of environments that hitherto have not been studied systematically. These unrecognised circumstances have fuelled controversies about the nature and significance of the Hulopoe Gravel, most notably the claim that it represents a mega-tsunami deposit. The Hulopoe Gravel in its type area displays a lithofacies succession that encompasses a complete disconformity-bounded transgressive–regressive cycle on a sand-poor rocky shoreline. This succession incorporates not only a distinctive vertical assemblage of rocky shoreline litho- and biofacies, but also internal disconformities that record episodes of erosion and non-deposition during shoreline migration. Repetition of the lithofacies succession imparts overall stratigraphic order to the Hulopoe Gravel. Four unconformity-bounded transgressive–regressive cycles correspond to eustatic highstands of Oxygen Isotope Stages 5 and 7, superimposed on continuing tectono-isostatic uplift driven by growth of the island of Hawaii. This study of the Hulopoe Gravel demonstrates that the sedimentary record of mixed volcaniclastic-carbonate depositional systems developed on rocky shorelines of tropical and subtropical oceanic island coasts can be preserved. This record may include depositional products of both transgressive and regressive shorelines. Although tsunami and giant submarine landslides are known to occur in the Hawaiian Islands, deposits and features that result from tsunami inundation above the shoreline, such as might be expected from landslide-related mega-tsunami, have not been recognised in the type area of the Hulopoe Gravel. Rocky palaeoshoreline records on oceanic volcanic islands may yield oceanographic and climatic clues that complement data obtained from other palaeoshoreline types.

Major episodes of the hydrologic history in the region of Hesperia Planum, Mars

The High Resolution Stereo Color camera (HRSC) data over Hesperia Planum and its surroundings reveal important details of geologic episodes and water‐related processes in this region. (1) The Noachian fluvial events of Hesperia Planum depression included accumulation of water and formation of a water/ice reservoir there. Later, the reservoir was depleted in several phases reflecting diminishing amounts of water. Climate changes and/or volcanism were important in these volatile releases. (2) The massive, 0.45–1.5 × 106km3, erosion from the Hesperia depression before the main lava eruption possibly resulted in thick, 0.5–1.5 km, deposits in Hellas Planitia. (3) Measurements of the flooded craters within Hesperia Planum provide the estimates of the thickness of lavas there, about 250–500 m. The final volume of lavas within Hesperia Planum (0.4–0.7 × 106km3) is comparable with the range of some terrestrial igneous provinces such as Columbia River Basalts. (4) Extended magmatism possibly triggered formation of the outflow channel in a few locations after the lava emplacement. During this episode, about 0.04 × 106km3of material (about 4.5–8.9% of the volume eroded in the episode of massive erosion) were removed. The thickness of the composite lava layer exposed on the walls of the outflow channels, a few hundreds of meters, corresponds well to the thickness estimates made by the measurements of the flooded craters. (5) Dispersed viscous flows (debris aprons, flow‐like deposits) reflect the final fluvial events. Viscous flows from the subsurface sources in the Southwestern trough associate with Dao, Niger, and Harmakhis Valles. These flows represent the final volatile discharge from the Hesperia reservoir that mostly was depleted by the earlier events of massive erosion and formation of the outflow channels. Viscous surface flows are mostly associated with Reull Vallis and probably reflect redistribution of volatiles related to the late episodes of evolution of this outflow channel.

Geological overview and cratering model for the Haughton impact structure, Devon Island, Canadian High Arctic

Abstract— The Haughton impact structure has been the focus of systematic, multi-disciplinary field and laboratory research activities over the past several years. Regional geological mapping has refined the sedimentary target stratigraphy and constrained the thickness of the sedimentary sequence at the time of impact to ˜1880 m. New 40Ar–39Ar dates place the impact event at ˜39 Ma, in the late Eocene. Haughton has an apparent crater diameter of ˜23 km, with an estimated rim (final crater) diameter of ˜16 km. The structure lacks a central topographic peak or peak ring, which is unusual for craters of this size. Geological mapping and sampling reveals that a series of different impactites are present at Haughton. The volumetrically dominant crater-fill impact melt breccias contain a calcite-anhydrite-silicate glass groundmass, all of which have been shown to represent impact-generated melt phases. These impactites are, therefore, stratigraphically and genetically equivalent to coherent impact melt rocks present in craters developed in crystalline targets. The crater-fill impactites provided a heat source that drove a post-impact hydrothermal system. During this time, Haughton would have represented a transient, warm, wet microbial oasis. A subsequent episode of erosion, during which time substantial amounts of impactites were removed, was followed by the deposition of intra-crater lacustrine sediments of the Haughton Formation during the Miocene. Present-day intra-crater lakes and ponds preserve a detailed paleoenvironmental record dating back to the last glaciation in the High Arctic. Modern modification of the landscape is dominated by seasonal regional glacial and niveal melting, and local periglacial processes. The impact processing of target materials improved the opportunities for colonization and has provided several present-day habitats suitable for microbial life that otherwise do not exist in the surrounding terrain.

Intra-crater sedimentary deposits at the Haughton impact structure, Devon Island, Canadian High Arctic

Detailed field mapping has revealed the presence of a series of intra-crater sedimentary deposits within the interior of the Haughton impact structure, Devon Island, Canadian High Arctic. Coarse-grained, well-sorted, pale gray lithic sandstones (reworked impact melt breccias) unconformably overlie pristine impact melt breccias and attest to an episode of erosion, during which time significant quantities of impact melt breccias were removed. The reworked impact melt breccias are, in turn, unconformably overlain by paleolacustrine sediments of the Miocene Haughton Formation. Sediments of the Haughton Formation were clearly derived from pre-impact lower Paleozoic target rocks of the Allen Bay Formation, which form the crater rim in the northern, western, and southern regions of the Haughton structure. Collectively, these field relationships indicate that the Haughton Formation was deposited up to several million years after the formation of the Haughton crater and that they do not, therefore, represent an immediate, post-impact crater lake deposit. This is consistent with new isotopic dating of impactites from Haughton that indicate an Eocene age for the impact event (Sherlock et al. 2005). In addition, isolated deposits of post-Miocene intra-crater glacigenic and fluvioglacial sediments were found lying unconformably over remnants of the Haughton Formation, impact melt breccias, and other pre-impact target rock formations. These deposits provide clear evidence for glaciation at the Haughton crater. The wealth and complexity of geological and climatological information preserved as intra-crater deposits at Haughton suggests that craters on Mars with intra-crater sedimentary records might present us with similar opportunities, but also possibly significant challenges.

Earliest Triassic Claystone Breccias and Soil-Erosion Crisis

Abstract Chemostratigraphic methods (δ13Corg) for pinpointing the Permian–Triassic boundary in nonmarine rocks of Antarctica, Australia, and South Africa have drawn attention to unusual claystone breccias at this horizon of the greatest extinction in the history of life on Earth. These rocks differ from other breccias in having a high proportion of clasts with birefringence microfabrics (sepic plasmic fabrics) characteristic of soils, and can be called sepic pedoliths in the terminology of soil science. At many localities, earliest Triassic sepic pedoliths are thin (5–15 cm) beds, but some fill shallow paleochannels, and pinch out laterally. Other non-marine breccias at the Permian–Triassic boundary are dominated by fragments of coal, pedogenic carbonate nodules, and deeply weathered volcanic rock fragments. Earliest Triassic sepic pedoliths record an unusually severe and widespread episode of soil erosion associated with forest dieback at the Permian–Triassic boundary. Comparable sepic pedoliths are found in debris flows after clear-cutting of forests in western Oregon today. Studies of these modern deposits show that sepic soil peds do not withstand weathering for more than a few months. Such clayey clasts are held together mainly by roots and fungi, which decay rapidly in soils and streams. These modern analogs demonstrate that terminal Permian soil erosion was rapid and profound. Sepic pedoliths are also known at other times of mass extinction, such as the Triassic–Jurassic boundary, but are not yet known before the Devonian advent of trees and forested clayey soils.

Variação morfodinâmica diária da praia do Cassino, RS, durante os verões de 2002/2003 no setor do terminal turístico

High frequency morphodynamic surveys permit to analyze, in detail, beach morphology changes. With the purpose to investigate daily beach profiles variability, surveys were carried during the summers of 2002 and 2003 at Cassino beach. Bar migration rate of 4,8m per day was found and berm development in 3 days, as well as high erosion in the first days of a storm followed by accretion in the final days of the event. This accretion was verified in other points toward the south of the monitored point; nevertheless, the material deposited was in form of fluid mud, highlighting the importance of a specific wave power band in the reworking of the mud deposits. The results found here show the existence of small changes in the sediment stock, not detected in previous works, which only related erosion episodes to meteorological systems.

Volcaniclastic debris flows at La Fossa Volcano (Vulcano Island, southern Italy): Insights for erosion behaviour of loose pyroclastic material on steep slopes

This paper describes a qualitative model of erosion of loose volcaniclastic material on steep slopes of La Fossa cone (Vulcano Island, Italy) that generates small volume debris flows. The loose volcaniclastic cover rests on an almost impermeable substratum, which comprises a fine-grained hydromagmatic tuff. Morphologically, three sectors were distinguished: 1) a water supply zone (slope > 32°) where rain water is enriched in mud and concentrated in the initiation zone; 2) an initiation zone (or source area sensu stricto slope with ∼30°) where debris flows form and; 3) a transport and deposition zone where debris flows deposited levees and terminal lobes (slope ∼30°). Main episodes of erosion of loose volcaniclastic cover and debris flow generation occur in response to thunderstorms. The erosion of tuff from superficial water added a significant amount of fines (mud fraction) to the initiating flows. Debris flow stoppage on steep slopes was interpreted as the result of a loss of mass (estimated between 35% and 53% of total mass) to form levees. From field and laboratory data the density of debris flows was estimated at about 1950 kg/m 3. Morphological data, matrix texture, mass number ( N m) and clay content of debris flow deposits suggest that they were deposited by saturated granular slurries.

Generation, migration, accumulation, and dissipation of oil in Northern Iraq

ABSTRACT Most of the known oil accumulations of Northern Iraq probably originated by upward migration from earlier, deeper accumulations which were initially housed in stratigraphic or long-established structural traps, and which are now largely depleted. The earlier concentrations had their source in basinal sediments, into which the porous, primary-reservoir limestones pass at modest distances east of the present fields. Development of the region favored lateral migration from different basinal areas of Upper Jurassic and Lower-Middle Cretaceous time into different areas of primary accumulation. Important factors affecting primary accumulation included: (1) early emergence and porosity improvement of the reservoir limestones, followed by burial under seal-capable sediments; (2) the timely imposition of heavy and increasing depositional loads on the source sediments, and the progressive marginward advance of such loads; (3) progressive steepening of gradients trending upward from source to accumulation area; (4) limitation of the reservoir formations on the up-dip margin by truncation or by porosity trap conditions. In late Tertiary time, large-scale folding caused adjustments within the primary reservoirs, and associated fracturing permitted eventual escape to higher limestone reservoirs, or to dissipation at surface. The sulfurous, non-commercial crudes of Miocene and Upper Cretaceous reservoirs in the Qaiyarah area are thought to stem from basinal radiolarian Upper Jurassic sediments, which lie down dip, a few tens of miles east of these fields. Upper Cretaceous oils of Ain Zalah and Butmah drained upward from primary accumulations in Middle Cretaceous limestones, which were filled from basinal sediments of Lower Cretaceous age situated in a localized trough a few miles northeast of these structures. The huge Kirkuk accumulation, now housed in Eocene-Oligocene limestones, ascended from a precedent accumulation in porous Middle-Lower Cretaceous limestones, which drew its oil from globigerinal-radiolarian shales and limestones of the contemporaneous basin, a short distance east of the present field limits. Eocene-Oligocene globigerinal sediments, considered by some the obvious source material for Kirkuk oil, seemingly provided little or no part of the present accumulation. The reservoir formation may have been filled from these sources, to lose its oil by surface dissipation during the erosional episode preceding Lower Fars deposition. Upper Cretaceous basinal sediments probably contributed nothing to known oil field accumulations, though they may have subscribed to the spectacular impregnations of some exposed, Upper Cretaceous reef-type limestones. Neither Miocene nor pre-Upper Jurassic sediments have played any discernible role in providing oil to any producing field. Indigenous oils are thought to be negligible in the limestone-reservoir formations considered.

Predicting Storm-Induced Beach Erosion in Caribbean Small Islands

Storm episodes often cause extensive beach erosion that has severe economic impacts in most Caribbean Small Islands (CSIs). To better understand beach erosion hazard and manage its impacts, research has been directed at utilizing existing coastal data, geographic information systems (GIS), and other technologies to develop a prototype beach analysis and management system (BAMS) for CSIs. This article presents the results of phase II of the research effort, which includes the development of tools for predicting episodic beach erosion and potential for erosion damage. The Southeast Peninsula, St. Kitts, is used as a case study to develop these tools and demonstrate system functionality.

Radiocarbon Dates from Soil Profiles in the Teotihuacán Valley, Mexico: Indicators of Geomorphological Processes

Radiocarbon dates largely obtained from bulk soil samples in 24 soil profiles in the Teotihuacán Valley, Mexico, are reported insofar as they represent a first step towards developing a sequence of soil formation, erosion, vegetation change, and human impact during the Holocene. Limitations of 14C dating in the area are considered, particularly the absence of charcoal in sediments and poor preservation of pollen. A broad temporal scheme is proposed to guide future research in which 4 periods are defined: ∼5000–2000 BP (relative stability with short, intermittent episodes of erosion); ∼2000–1500 BP (erosion-sedimentation, deforestation, and intensive agriculture); ∼1500–1000 BP (relative stability, depopulation, and partial recovery of the landscape); and ∼1000–500 BP (erosion-sedimentation, deforestation, and intensive agriculture).

Biophysical Model of Intertidal Beach Crab Burrowing:Application and Significance

The biophysical principles and mechanisms involved in intertidal beach-crab burrowing have been studied. These answer some important and fundamental questions, such as how and what basic burrow patterns evolve in dynamic environmental conditions through a delicate balance struck between physical and biological (ethological) processes, and how does a burrow form faithfully record its environmental history in a sedimentary sequence? The study is primarily based on field and experimental data collected on the amphibious brachynuran crab family Ocypodidae (four species of Ocypodeand one species of Ilyoplax) from modern intertidal beach sectors on the Bay of Bengal coast, India. The model introduces a new concept of crab burrow cycle that includes three phases and involves three genetic categories of burrow forms—occupied, abandoned, and collapsed. The first phase (subaerial) begins with the excavation of simple (I or J types) occupied burrow populations and terminates with the development of plugged burrow apertures and sub-surface trapped air-column systems. Pressure equilibrium is reached between the burrow-base capillary water, air columns and sediment plugs. Formation of an air column is a mechanical means of supporting hollow burrow from collapse and creating an underwater quasi-terrestrial condition of living. The second phase (subaqueous) starts with tidal inundation and results in the final burrow architecture (Y, U, multibranched Y, and multiform U). The continued effects of external dynamic pressure fields on the plugs increases the internal air-column pressure causing pressure disequilibrium and lowering of capillary water levels unfavorable to the crabs for gill respiration. The burrower neutralizes the extra pressure to achieve previous pressure equilibrium and capillary water levels by excavating secondary branch(es) advantageously on the landward side of the main shaft and dumping the materials at the burrow-base. This mechanism, besides adding morphological complexities, ensures continued pressure equilibrium, the failure of which in any stage results in burrow collapse. The third phase (subaerial) creates a permanent pressure imbalance as the tidal water quickly recedes and protects the burrower inside as the air column bursts to set the crab free to start another burrow cycle afresh. The model has application potential in varied post-Jurassic coastal intertidal environmental settings occupied by quasiterrestrial crabs. It provides insight into various aspects of ichnology (taxonomy and recognition of ancient crab burrows; characterization and variability of the Psilonichnusichnofacies and intertidal ichnocoenoses; burrowing life habits of crabs and modalities of burrow preservation), environment (influence of bathymetry, tide and wave action, substrate properties, storm-induced episodic erosion and deposition on burrow architecture; environmental zonation of burrows; paleotidal range and marine transgression-regression events) and geomorphology (recognition of shoreline features). Being the first biophysical model of intertidal crab burrows, it contributes original data to the theoretical background of ichnology.

Timing and nature of the deepening of the Tasmanian Gateway

Tectonic changes that produced a deep Tasmanian Gateway between Australia and Antarctica are widely invoked as the major mechanism for Antarctic cryosphere growth and Antarctic Circumpolar Current (ACC) development during the Eocene/Oligocene (E/O) transition (∼34–33 Ma). Ocean Drilling Program (ODP) Leg 189 recovered near‐continuous marine sedimentary records across the E/O transition interval at four sites around Tasmania. These records are largely barren of calcareous microfossils but contain a rich record of siliceous‐ and organic‐walled marine microfossils. In this study we integrate micropaleontological, sedimentological, geochemical, and paleomagnetic data from Site 1172 (East Tasman Plateau) to identify four distinct phases (A–D) in the E/O Tasmanian Gateway deepening that are correlative among ODP Leg 189 sites. Phase A, prior to ∼35.5 Ma: minor initial deepening characterized by a shallow marine prodeltaic setting with initial condensation episodes. Phase B, ∼35.5–33.5 Ma: increased deepening marked by the onset of major glauconitic deposition and inception of energetic bottom‐water currents. Phase C, ∼33.5–30.2 Ma: further deepening to bathyal depths, with episodic erosion by increasingly energetic bottom‐water currents. Phase D, <30.2 Ma: establishment of stable, open‐ocean, warm‐temperate, oligotrophic settings characterized by siliceous‐carbonate ooze deposition. Our combined evidence indicates that this early Oligocene Tasmanian Gateway deepening initially produced an eastward flow of relatively warm surface waters from the Australo‐Antarctic Gulf into the southwestern Pacific Ocean. This “proto‐Leeuwin” current fundamentally differs from previous regional reconstructions of eastward flowing cool water (e.g., a “proto‐ACC”) during the early Oligocene and thereby represents an important new constraint for reconstructing regional‐ to global‐scale dynamics for this major global change event.

Depositional and tectonic setting of the Miocene Wood Mountain Formation, southern Saskatchewan

Coarse gravels of the Miocene Wood Mountain Formation that cap the Wood Mountain upland in southern Saskatchewan are an erosional remant of a much more expansive sheet of gravel and sand. The formation consists of gravel and sand deposited as a thin sheet on a regional planation surface that extends approximately along strike for at least 160 km. This sheet, with a maximum thickness of 31 m, is the deposit of low-sinuosity, braided-river channels that flowed to the north-northeast. The Wood Mountain Formation represents an initial episode of erosion followed by low-accommodation deposition, where the net aggradation rate was likely low. The sediments of the Wood Mountain Formation are associated with change in tectonic style in western North America from one of compression to one of extension. The gravels were deposited above a regional erosion surface associated with isostatic uplift of the central Montana Uplift.

Syngenetic permafrost growth: cryostratigraphic observations from the CRREL tunnel near Fairbanks, Alaska

AbstractSyngenetic permafrost forms when alluvial, aeolian and/or colluvial sediment accumulates under cold‐climate conditions. Observations from within the CRREL permafrost tunnel near Fairbanks, Alaska, indicate that layered, lenticular‐layered and micro‐lenticular cryogenic structures are characteristic of this type of permafrost. In contrast, reticulate cryogenic structures indicate local thaw modification. During the growth of syngenetic permafrost, episodes of thermokarst erosion may operate preferentially along ice wedges leading to the development of gullies and tunnels in the near‐surface sediments. The local thaw unconformities that result are inferred by the recognition of thermokarst‐cave ice (‘pool’ ice), and various soil and ice pseudomorphs. These may be regarded as further characteristics of syngenetic permafrost growth. Copyright © 2004 John Wiley & Sons, Ltd.

Heterogeneous mixed-layer clays from the Cretaceous Greensand, Isle of Wight, southern England

AbstractThe sea-cliffs of the Isle of Wight were deposited during a period of overall sea-level rise starting in the Barremian (Lower Cretaceous) and continuing into the Aptian and Albian. They consist of fluvial, coastal and lagoonal sediments including greensands and clays. Numerous episodes of erosion, deposition and faunal colonization reflect condensation and abandonment of surfaces with firmgrounds and hardgrounds. This study focused mainly on shallow marine cycles where variations in clay mineralogy would not be expected, because overall system composition, sediment source, and thermal history are similar for all the samples in the studied section. Instead we found a wide variety of clay assemblages even in single samples within a 200 m interval.In this interval, distinct clay mineral assemblages were found and can be described as consisting of Al-rich, Fe-richand intermediate Fe and Al compositions withrespect to 2:1 and 1:1 layers in mixed-layer arrangements. Nearly pure glauconite-nontronite clays exist in the <2 µm fraction only when the bulk rock is free of K- and plagioclase feldspar. Conditions favorable to glauconite-nontronite formation are interpreted to result from a hiatus in volcanoclastic sedimentation, thus providing a stable substrate for glauconitization.The Fe-bearing mixed-layer clay assemblages consist of glauconite, nontronite and berthierine-like layers in various proportions with several mixed-layer clays often coexisting in the same sample. In different samples, Al-richand Fe-Mg-rich mixed-layer clays are similar in their content and distribution of 1:1 and 2:1 layers. This suggests that the original clay assemblages were similar and later diagenesis affected certain horizons resulting in substitution of Al by Fe + Mg while preserving the original layer structure and arrangement.Structural formulae for the berthierine-like phase and berthierine-like layers in these mixed-layer clays show their layer cation composition is intermediate between odinite and standard berthierine. The total sum of octahedral cations varies from 5.26 to 5.55 whereas the amount of Fe2+ cations varies from 2.12 to 2.22 per O10(OH)8. A feature of the berthierine-like phase as well as of berthierine-like layers is that they are di-trioctahedral and Fe2+ and Fe3+ are the prevalent cations. Moreover, in these berthierine-like components, the amount of Fe2+ is greater than that of Mg (in contrast to odinite) and Fe3+ cations prevail over Al (in contrast to berthierine). The presence of authigenic ferrous Fe clays and the relationship between glauconite-nontronite and bulk mineralogy has implications for sedimentological processes and geochemical conditions during and shortly after deposition.

Annual textural banding in Holocene estuarine silts, Severn Estuary Levels (SW Britain): patterns, cause and implications

A high-resolution textural study has been made of laminated and banded estuarine silts exposed intertidally at representative localities and horizons in the Holocene deposits of the Severn Estuary Levels. The laminae, on a submillimetre to millimetre scale, are sharp-based, graded couplets formed of a lower silty part overlain by a finer-textured clayey element. The centimetre-to decimetre-scale banding is formed of laminae in alternating, gradually intergrading sets of relatively coarse and relative fine-grained examples. At outcrop in the field, the banding is recognizable because the coarse sets prove to be recessive to varying degrees under the influence of weathering and current action. Independent evidence at two localities points toward an annual origin for the banding; at a third it arose during part of what appears to have been a relatively short period. Quantified physical arguments suggest that the textural banding is a response of suspended fine sediment to marked seasonal changes in sea temperature and windiness. The banded silts occur in four distinct stratigraphical contexts and record high deposition rates (order 0.01-0.1 m/yr). Because physical factors determine their textures, the silts potentially afford insights in all contexts into aspects of changing Holocene climatic conditions. In one context, the thickness of the bands points to high (order 0.01-0.1 m/yr) but comparatively short-lived (order lOs-lOOs yrs) rates of relative water-level rise. In the others, however, the banding has no implications for sea-level behaviour, and simply records gross environmental disequilibrium, for example, the recovery of mudflats/marshes after an erosional episode. Similarly, because on account of their rapid accumulation the banded silts prserve animal and human tracks and trackways especially well, they provide an archive of animal and human behaviour in the area during the Holocene.

Patterns, rates and possible causes of saltmarsh erosion in the Greater Thames area (UK)

The estuary-dominated coast of the Greater Thames in England has experienced rapid lateral erosion and internal dissection of saltmarshes. This paper provides an overview of saltmarsh development in this area, and re-examines the role of environmental and human forcing factors. It draws on documentary evidence, including historical maps, survey data and time-series data of forcing factors. Lateral marsh retreat began in the 19th century in the Medway and Blackwater Estuaries, followed by other estuaries in the Greater Thames region at the beginning of the 20th century. The outer estuaries and the wider parts of the inner estuaries especially have experienced erosion. Erosion has been modest in wave-sheltered areas, e.g., the Colne and the inner Crouch. In the 1960s and, more widely, the 1970s, a phase of rapid erosion took place, with erosion rates of up to ca. 16 ha year −1 per site, notably along the open coast of Dengie and Foulness, and in the Blackwater and Thames Estuaries. At all sites, vertical sediment accretion was well able to keep up with sea level rise over the past century. Evidence indicates that there may have been several causes for the erosion of saltmarshes. These are notably land claim and embankment construction (increasing the tidal range and current velocities) and a continuous rise of, especially, high and extreme water levels. The latest episode of rapid erosion in the 1970s is largely attributed to changes in the wind/wave climate. For example, erosion at wave-exposed sites coincided with a peak in high magnitude waves combined with a high incidence of southeasterly waves. The study shows that many factors, including natural forcing factors and human activities, have to be taken into account when explaining saltmarsh development.

Lower Cretaceous Alisitos Formation at Punta San Isidro: Coastal sedimentation and volcanism

The Lower Cretaceous (Aptian-Albian) Alisitos Formation is well exposed at Punta San Isidro and adjacent sea cliffs on the Pacific shores of Baja California near Eréndira, Mexico. Continuous coastal outcrops define a local stratigraphic succession less than 100 m in total thickness, including repetitious tuff, sandstone and conglomerate units interbedded with discontinuous limestone beds rarely more than 2.5 m thick. The limestone beds are biostromal units that include scattered oysters and/or corals, as well as units dominated by the rudistid bivalve, Caprinuloidea perfecta. Two distinctive conglomerate units are composed of andesite cobbles colonized by encrusting oysters in a quasi rocky-shore setting. One sandstone unit includes abundant fossil wood with tree limbs up to 55 cm long and 5 cm in diameter. Proximal volcanic activity is indicated by a series of dikes that cut through pyroclastic beds and lead to a 10-m thick andesitic flow that caps the succession at Punta San Isidro. Compared with thicker intervals of the Alisitos Formation elsewhere in Baja California that are dominated by andesitic flows and offshore limestone, the Punta San Isidro sequence offers a window on a back-reef environment adjacent to a paleoshore that received pyroclastic lahars from a terrestrial origin or mass flows from shallow submarine explosions. Recovery of marine life and the renewal of a carbonate substrate followed successive episodes of volcanism and massive erosion along an active coastline. This scenario is very different from depositional processes of the Upper Cretaceous (Campanian-Maastrichtian) Rosario Formation that cannibalized and subsequently formed a regional unconformity against tilted Alisitos strata with substantial topographic relief in the Eréndira region.