J.R.L. Allen

Morphodynamics of Holocene salt marshes: a review sketch from the Atlantic and Southern North Sea coasts of Europe

Abstract Salt marshes, most now embanked, together with genetically related wetlands and high intertidal flats, make a major environmental contribution to the lowland coasts of Northwest Europe. They occur in many different contexts, but chiefly on open and barrier coasts and in estuaries and embayments, and range greatly in scale, from a modest total that measure hundreds of square kilometres in individual extent, to an enormous number each of an area no greater than tens to a few hundred hectares. These marshes and associated environments are under complex natural controls and experienced from the mid-Holocene onward human exploitation and, increasingly, interference. The main external controls are the sea-level, tidal and sediment-supply regimes. Intrinsic infuences are provided by the halophytic vegetation and sediment autocompaction. Upward sea-level movements and autocompaction combine to provide accomodation space within which marshes build upward. Field data and simulation modelling show that youthful mineralogenic marshes grow up rapidly and can mature within a few hundred years of inception. They consist of a vegetated platform dissected typically by extensive networks of blind-ended, branching tidal creeks and gullies. The flow-resistant surface vegetation, shaping the combined wave-tide boundary layer on the platforms, both traps and binds tidally introduced mineral sediment, but also contributes an organic component of indigenous origin to the deposit. When sea-level becomes stable or falls, however, in response to century-millennial scale fluctuations, the organic sediment component becomes dominant and mineralogenic marshes are transformed into organogenic ones. Organogenic marshes normally display a considerable range of sub-environments which create much spatial variation in the peat facies which accumulate. At an advanced stage, domed raised bogs, rising significantly above the general landscape, may appear on the marshes. Because peat is such a porous and permeable sediment, and there is little or no tidal inundation, organogenic marshes in Northwest Europe typically lack surface channels for internal drainage. The stratigraphic sequences accumulated during the Holocene beneath coastal marshes and high tidal flats typically present an alternation on a vertical scale of decimetres to metres of silts (mineralogenic marshes, high intertidal mudflats) and peats (organogenic highest intertidal-supratidal marshes). Coastal barriers and some channels are represented by local accumulations of sand and/or gravel. The silts and peats form couplets which are generally considered to be related to fluctuations of sea-level about the general upward trend. Field investigations and modelling show that, in areas where marshes are mature, the upward change from an organogenic to a sequence of mineralogenic marshes (transgressive overlaps) is accompanied by the initiation and invasive development of a branching network of tidal creeks. These decay and infill during the reversal of the environmental sequence and the approach, expressed as a series of regressive overlaps, of the next set of peat-forming conditions. The operation of the continuous, progressive, irreversible and asymptotic process of sediment autocompaction exerts a major, secondary control on depositional regimes and marsh behaviour. A variety of local responses are consequently possible in an extensive marsh, even though the marsh may be everywhere in dynamic equilibrium with environmental factors. Autocompaction also strongly shapes the character of Holocene coastal sequences as now perceived, introducing significant stratigraphic distortions and displacements which, for the time being, limit the accuracy of sea-level curves and rates of sea-level change based on dated intercalated peats. Prehistoric humans benefitted from the resources of coastal salt marshes, especially at the times when peat marshes begin to be transgressed. Peat domes not yet fully collapsed may have provided vantage points for seasonal ocupancy or even settlement from which the richer resource of the mineralogenic marshes developing on lower ground could be exploited. Increasing human interference on salt marshes over the last millennium, chiefly through wholesale land-claim and set-back, augmented by rising sea levels and continuing autocompaction, has led to a variety of poorly understood but generally deleterious effects in the lowland coastal zone. These include major changes to tidal and sedimentary regimes.

Studies in fluviatile sedimentation: Bars, bar-complexes and sandstone sheets (low-sinuosity braided streams) in the brownstones (L. devonian), welsh borders

Abstract The fine to very coarse sandstones, gravelly sandstones and intraformational conglomerates of the mid to upper Brownstones are excellently exposed in large fresh road cuttings near Ross-on-Wye in the southern Welsh Borders. Detailed mapping of the cuttings reveals an hierarchically ordered system of mainly erosional bedding contacts which divide the beds into hierarchically structured packets. The smallest packets, involving cross-bedded or plane-bedded sediments or combinations of these, are consistent with deposition from strongly three-dimensional and often large, loosely periodic to non-repetitive bars. A locally developed facies of trough cross-bedded sandstones points to the infrequent occurrence of fields of three-dimensional dunes. The bar- and dune-related units are grouped into large complexes (related to the storeys of other workers), with an internal geometry consistent with lateral accretion (in places clearly symmetrical) combined with forward accretion on shoals (sand flats) within a braided channel, as in the South Saskatchewan River. In their turn, the complexes are combined into laterally extensive, conglomerate-floored sandstone sheets several metres thick. These seem to express the wandering of a braided channel across a mud-draped floodplain. To judge from the sedimentary structures and textures, the thickness of the lateral accretion deposits, and the size of the major scours, the bankfull discharge of the rivers was a few thousand cumecs each.

Earthquake magnitude-frequency, epicentral distance, and soft-sediment deformation in sedimentary basins

Abstract Empirical equations available for earthquake magnitude-frequency and the maximum epicentral distance of liquefied sites versus magnitude are combined with the characteristic instantaneous sediment deposition rate at a station in a sedimentary basin to form a model for the local stratigraphic frequency of earthquake-induced soft-sediment deformation structures. Both the style of sedimentation and the characteristics of the earthquake source combine to determine the pattern of incidence of the structures in a basin. The model is probably most directly applicable to the geodynamic interpretation of fluvial basins, in which substantial volumes of readily liquefied sands accumulate at high, local, short-term rates.

Transverse erosional marks of mud and rock: their physical basis and geological significance

Abstract Transverse erosional marks of interest in the earth sciences are described and classified as individual structures and as assemblages on the basis of their field occurrence and character. A theoretical and experimental investigation is made into the physical nature of the hydrodynamic and mechanical processes involved in the production of transverse erosional marks, with particular reference to the flutes and scallops produced by solution in limestone caves, and to the scour and flute marks generated on mud beds by aqueous streams, notably turbidity currents, through corrasion and fluid-stressing. The association of transverse erosional marks with separated flows is established and the relevance of the properties of these flows to the shape and growth of the marks is demonstrated in detail. Alternative explanations of the size, shape and pattern of erosional marks are considered, and the changes in these properties down a turbidity current path are explored. Unfortunately, no single theory of size, shape and pattern can as yet be glimpsed, though some generalizations are possible. Erosional marks generated on limestone surfaces by solution and on strong mud beds by corrasion depend for character on the defects existing in the bed, the duration of the eroding process, and the flow properties. However, structures produced by the fluid-stressing of weakly cohesive mud beds are found to depend in a quite simple way on flow properties alone. It is shown in both cases how inferences of importance to environmental interpretations can be made from measurable properties of the structures.

Salt-marsh growth and fluctuating sea level: implications of a simulation model for Flandrian coastal stratigraphy and peat-based sea-level curves

Abstract Using a sea-level rise term composed of steady and periodic parts, a zero-dimensional, numerical model for the vertical growth of high tidal mudflats and marshes has been operated under varying conditions of sediment supply. The model simulates the creation of dynamically stable marshes beneath which form stratigraphic sections in which organic beds (silty peat and/or peat marshes) are intercalated with silts (mudflats or minerogenic marshes). Such successions are widely observed from Flandrian deposits in the coastal zone of northwest Europe. Partly because of a strong but variable lag effect, the experimental transgressive and regressive overlaps afford a registration of sea-level movements of limited reliability, in terms of chronology, amplitude of fluctuation and indicative meaning. In the model, implemented for conditions in the Severn Estuary (SW Britain), a regressive overlap arises closer to the maximum of the periodic component of sea level than does the corresponding transgressive overlap to follow. Overlaps arise at variable levels within the tidal frame, but generally well above the position of mean high-water spring tides. The results support the ‘oscillatory’ concept of sea-level movement but provide evidence and explanations for large intrinsic uncertainties in sea-level curves based on radiocarbon-dated peals.

The Severn Estuary in southwest Britain: its retreat under marine transgression, and fine-sediment regime

Abstract The retreat of the macrotidal Severn Estuary inland up the Severn Vale, under the drive of a rising relative sea level, is demonstrated by (1) the erosion of marginal bedrock cliffs, (2) the present-day wide intertidal exposure of mid post-glacial rooted peats formed on early tidal marshes, (3) the transposition into post-settlement deposits and the present-day intertidal zone of prehistoric and early historic cultural debris representing wetland exploitation, and (4) the erosive relationship between modem sand-flat deposits and the older post-glacial silts and clays. A consideration of the sources, stores and pathways of fine sediment suggests, on a balance of evidence, that the system has recently reached its maximum capacity, and has for a substantial period been retreating inland by a “roll-over” process involving the reworking of long-deposited fine sediments into active environments. The export of fine sediment to the continental shelf could now be taking place.

Geological impacts on coastal wetland landscapes: some general effects of sediment autocompaction in the Holocene of northwest Europe

A simulation model is described providing general insights into autocompaction effects in stratigraphic sequences formed on coastal mudflats and marshes over uneven basements. Lithological units and buried landscapes are progressively displaced and distorted, the synthetic sequences resembling bedding geometries described from the northwest European Holocene. Autocompaction also promotes local deposition rates that may greatly exceed the rates at which accommodation space is created. Transgressive silts are associated with especially large deposition rates, because of the high compressibility of underlying peats. The experimental relationships between apparent rates of sea-level rise and bed thickness resemble empirical trends for the Holocene. The latter are, however, reinforced by an ‘intrinsic’, lithology-related effect not included in the model.

Medium-term sedimentation on high intertidal mudflats and salt marshes in the Severn Estuary, SW Britain: the role of wind and tide

Abstract Six sites monitored monthly on salt marshes and mudflats in the middle and outer Severn Estuary gave potentially predictive–retrodictive relationships between the vertical response of the sedimentary surface (erosion/accretion), a factor reflecting tidal heights, and the wind-wave power-supply. The continuing rise of relative sea level in the area, at the rate of a few millimetres annually, is providing accommodation space at a pace low enough to permit the continuing vertical build-up of the salt marshes throughout the estuary. On the mudflats, wind and tidal conditions during the survey maintained an accretionary regime in the middle estuary but an erosional one in the outer part. The Severn Estuary is a system in delicate balance which is likely to respond dramatically to modest changes in the tidal and/or wind regimes.

Simulation models of salt-marsh morphodynamics: some implications for high-intertidal sediment couplets related to sea-level change

Abstract A salt marsh composed of a vegetated platform dissected by creek networks has at any time characters determined by the previous history of the system, because forcing factors constantly change in rate. Results from numerical simulation models of marsh growth are combined with engineering regime theory (channel stability) to form a comprehensive conceptual model which qualitatively describes the three-dimensional character of marshes and the stratigraphic sequences they generate. The morphostratigraphic evolution of marshes is examined for two main patterns of regional relative mean sea-level change. Where the level fluctuates smoothly about an underlying upward trend, there develops a stacked sequence of silt-peat couplets, each of which is symmetrical in facies states but asymmetrical in facies thicknesses. Palaeochannel forms and fills distributed through each couplet show that creek networks expand and densify as sea level rises and the hydraulic duty of the marsh platform increases to its maximum, but shrink and silt-up as sea level temporarily stabilises or falls and the duty declines. Great earthquakes, and also the abandonment of manmade flood defences on aseismic marshes, create catastrophic-episodic increases in relative sea level. Where there is also an underlying upward trend of sea level, stacked silt-peat couplets result that are strongly asymmetrical in both facies states and thickness. The creek networks enlarge rapidly under strong forcing within a short time of the sea-level increase but later, as the hydraulic duty declines, shrink and infill. Under each scenario, the tidal creek networks that functioned during the accumulation of one couplet are unlikely to be inherited by the next, except in the case of channels that also transmit fresh water from the hinterland.

Sedimentology of the Old Red Sandstone (Siluro-Devonian) in the Clee Hills area, Shropshire, England

Abstract The sequence of approximately 1300 m is divided by a major unconformity (Middle Devonian) into the thick Lower Old Red Sandstone (Siluro-Devonian), resting disconformably on Ludlovian (Silurian) marine strata, and the much thinner Upper Old Red Sandstone (Upper Devonian) overlain by the Carboniferous. The Lower Old Red Sandstone commences with littoral sediments (Downton Castle Formation) followed by tidal mud-flat deposits (Temeside Formation) formed after a brief marine transgression. The predominant remainder of the sequence (Ledbury Formation, Ditton Group, Abdon Group, Woodbank Group), characterized by fining-upwards cyclothems, records the establishment during a marine regression of extensive and persistent alluvial plains. Prior to Ditton Group times, detritus came from relatively distant regionally metamorphosed rocks lying to the north or west of the Clee Hills. Subsequently, apparently as the result of river-capture or drainage-reversal consequent on the commencement of the final (mid-Devonian) phase of Caledonian movement, high-level crustal rocks closer at hand (largely Wales) replaced the metamorphics as the sources of sediment, the earlier Lower Old Red Sandstone itself being recycled. To judge from the calcretes preserved in the alluvial formations, the area lay near the Equator and experienced a relatively dry hot climate. The Upper Old Red Sandstone likewise reveals fining-upwards cyclothems. The overlying Carboniferous rocks evidence the renewed marine transgression of the area, after the removal of the effects of the mid-Devonian movements.