Julian D. Orford

The use of fractals and pseudofractals in the analysis of two-dimensional outlines: Review and further exploration

Abstract The need for quantitative measures of particle form are reviewed with special reference to sedimentology. The concept of fractals as, self-similar, “space-filling” curves is introduced, and it is shown how they can be used to characterize closed loop outlines. This is done by way of the Richardson Plot of log stride (a variable length stepped off around the perimeter) vs log perimeter (the estimate of perimeter outline corresponding to an appropriate stride). The points obtained may be fitted by a regression line the inverse slope of which (— b ) gives the “fractal dimension” (Hausdorff-Besicovitch dimension, D ) from D = b − 1. Many outlines of sedimentary particles, especially where these are highly irregular, give points on a Richardson Plot which are best fitted by two or even three linear segments. The large-scale stride segment is termed the “structural fractal”, the segment corresponding to fine detail from small strides is the “textural fractal”. The uses and significance of such a division are discussed. The uses of this basic method are discussed and some recent developments in the application of perimeter stepping methods are outlined.

Coarse clastic barrier beaches: A discussion of the distinctive dynamic and morphosedimentary characteristics

Abstract Coarse clastic barriers are common on mid- and high-latitude coasts. They possess a morphosedimentary and dynamic distinctiveness which sets them apart from sandy fineclastic barrier forms. The reflective nature of the seaward barrier favors the development of zero mode, sub-harmonic edge waves particularly during long period swells (10–20 s), manifest in the formation of high level cusps. In some circumstances the pattern of recent overwashing of the barrier is related to cusps. Lack of distinct tidal passes, due partly to the high seepage potential of coarse barriers, means that very little sediment is transported seaward. Thus the barriers roll steadily onshore, and sections show a variety of washover facies, related to the volume of overwash surges.

MORPHODYNAMIC EVOLUTION, SELF-ORGANISATION, AND INSTABILITY OF COARSE-CLASTIC BARRIERS ON PARAGLACIAL COASTS

Abstract Beaches and barriers on many mid- to high-latitude coasts comprise mixtures of fine and coarse clastic materials forming a distinctive morphodynamic environment. In many cases, the sediments are derived primarily from limited glacigenic deposits and the coasts are considered paraglacial . Over relatively long time scales (decades to centuries), coarse-clastic barriers on such coasts show evidence of self-organisation through large-scale morphological evolution and facies differentiation. This process involves gradual reworking, partitioning, and textural sorting of material toward transport minima. Long intervals of slow evolution are punctuated by episodes of rapid reorganisation, involving breakdown of stable barrier structures and facies patterns, remixing of sediment, and accelerated migration of transgressive systems. Drift-aligned systems develop longshore cell structure, sometimes leading to breaching and segmentation, and may evolve toward progressively greater swash alignment under appropriate circumstances. Swash-aligned systems may experience catastrophic transformation when appropriate environmental triggers lead to threshold exceedance in the morphodynamics of the shore system. Adjacent barriers may show quite different behaviour, depending on the antecedent states of individual coastal cells. While appropriate parameterisations and sediment budget formulations allow us to model the long-term evolution of some barrier structures, the non-linear dynamics that appear to dominate large-scale behaviour may limit predictability. The identification of stability threshold criteria remains an important research priority.

Coarse clastic barrier environments: Evolution and implications for quaternary sea level interpretation

Abstract This paper reviews recent work, particularly from eastern Canada, on the dynamics and evolution of coarse clastic barriers. It is argued that an understanding of Quaternary coastal changes on paraglacial (glacially-influenced) coasts can only be achieved with a knowledge of gravel barrier morphodynamics and sedimentation. Gravel barriers are distinctive coastal forms in which morphological inheritance and sediment sorting provide major behavioral controls. Spatially-adjacent gravel barriers may display different depositional tendencies: prograde seawards; migrate landward; or aggrade vertically. Such inconsistent spatial behaviour must be considered in terms of an erosional front, advancing landward under stationary or rising sea level. Redistribution of sediment across or along the erosional front supplies sediment for the development of gravel barriers, which in turn provides environments for fine-grained deposition which is favourable for further barrier migration. An example of an erosional front is given from Chezzetcook Inlet, Nova Scotia. Three stages are recognized in the development of gravel barriers, starting with the initiation of small drift aligned forms at the leading edge of the erosional front as sea level rises. This is followed by a period of consolidation and stabilization when drift-aligned forms may become swash-aligned. Conditions for this transition are discussed. Under rising sea levels it is difficult to explain the conditions leading to the formation of multiple parallel swash-aligned ridges, although three hypotheses are advanced, all of which may operate in different locations. The third stage is associated with the breakdown of barriers, as sediment supply fails and/or sea level rises. Drift-aligned forms are inherently unstable and show signs of pre-breakdown stress in the form of reworking and remobilization of proximal sediments into cells. Swash-aligned barriers may be breached, revert to drift-alignment, become segmented or be overstepped. Some gravel is re-incoporated in barriers farther landward while other material is passed seaward of the erosional front and becomes abandoned on the shoreface. Gravel barriers may exert a major influence on estuary and back barrier environments, especially where they control the depositional record. The implications for the reconstruction and interpretation of Quaternary environments are summarized.

Organisational controls, typologies and time scales of paraglacial gravel-dominated coastal systems

Abstract The fundamental controls on the initiation and development of gravel-dominated deposits (beaches and barriers) on paraglacial coasts are particle size and shape, sediment supply, storm wave activity (primarily runup), relative sea-level (RSL) change, and terrestrial basement structure (primarily as it affects accommodation space). This paper examines the stochastic basis for barrier organisation as shown by variation in gravel barrier architecture. We recognise punctuated self-organisation of barrier development that is disrupted by short phases of barrier instability. The latter results from positive feedback causing barrier breakdown when sediment supply is exhausted. We examine published typologies for gravel barriers and advocate a consolidated perspective using rate of RSL change and sediment supply. We also consider the temporal variation in controls on barrier development. These are examined in terms of a simple behavioural model (BARCH) for prograding gravel barrier architecture and its sensitivity to such controls. The nature of macroscale (10 2 –10 3 years) gravel barrier development, including inherited characteristics that influence barrier genesis, as well as forcing from changing RSL, sediment supply, headland control and barrier inertia, is examined in the context of long-surviving barriers along the southern England coastline.

Gravel-barrier migration and overstepping

Abstract Despite the widespread occurrence of gravel barriers on mid- to high-latitude coasts, understanding of their stability and relationship to gravel deposits on the inner continental shelf remains rudimentary. On sand-dominated coasts subject to rising relative sea level, barrier overstepping has been postulated as a mechanism for preservation of beach deposits on the shelf, but direct observation of overstepping events has been lacking. Observations at a site on the Atlantic coast of Canada reveal migration of a gravel barrier at 8 m a −1 landward over back-barrier mud, following abandonment of part of the barrier volume on the shoreface. This provides clear evidence for overstepping and suggests that it may play a role in the formation of inner-shelf gravel deposits. Integrity of the migrating (stretching) barrier depends on continued sediment supply from the headland source to a connecting ridge, which must become progressively longer as the landward displacement of the barrier increases.

Barrier and lagoon coast evolution under differing relative sea-level regimes: examples from Ireland and Nova Scotia

Abstract Although the coasts of southern Ireland and eastern Nova Scotia possess many basic similarities, especially in terms of geology, glacial history, sediment character, resistance to erosion and wave and tide regimes, there is a fundamental contrast in Holocene sea-level history. In Ireland, recent (the past 4000 years) sea-level rise has been limited to less than 1 mm/year. By contrast, the Atlantic coast of Nova Scotia has been experiencing sea-level rises up to three to five times greater over the same period. Under these markedly differing relative sea-level rise regimes, barrier and lagoon evolution has varied considerably. In Ireland, barrier-lagoon form is controlled by local basement expression, particularly through the emergence of headlands (creating largely closed sediment systems) and the requirement to maintain cross-shore drainage. It is this latter factor that leads to a simple four-fold categorization of the Irish barriers and lagoons, from freshwater seepage to open tidal environments. The Nova Scotian examples are associated with rapidly moving erosional fronts, with local basement control relegated to a subordinate role at the expense of rapid changes in sediment supply. Barriers can both grow and decay under these conditions (subject to certain critical, but as yet ill-defined, thresholds), simultaneously forcing development and destruction of adjoining lagoons. These major differences also find expression in sedimentary sequences.

Late-Holocene (post-4000 years BP) coastal dune development in Northumberland, northeast England

The recent environmental history of coastal dune systems in Northumberland, northeast England, has been examined using geomorphological, stratigraphical and sedimentological techniques linked to radiocarbon and infrared-stimulated luminescence (IRSL) dating. Stratigraphies were determined from 22 vibracores and three sections, and dune chronology was based on 28 14 C dates, from peat and soil organic horizons, and 26 IRSL dates on K-feldspar grains from within sand layers. Almost all dune systems are associated with regressive shorelines consequent upon a fall in relative sea level (RSL) from its Holocene peak, and indicate RSL functioned as a macroscale control on dune development. Where dunes are anchored on terrestrial sediment, dune expansion may have been either transgressive or regressive in nature. Where near-shore marine sediments form the dune substrate, a regressive (prograding) dune model seems most likely. Most dune building occurred during the ‘Little Ice Age’ (LIA), probably in association with specific climatic and morphosedimentary conditions, principally periods of easterly circulation, a greater frequency of severe North Sea storms, RSL fall, and sediment and accommodation space availability. Dune development in Holocene cool intervals earlier than the LIA was of limited spatial extent, suggesting some differences in prevailing conditions at those times.

Crestal Overtop and Washover Sedimentation on a Fringing Sandy Gravel Barrier Coast, Carnsore Point, Southeast Ireland

ABSTRACT Storm sedimentation on a fringing. sandy gravel barrier system in southeast Ireland is examined. Recent wave overtopping and overwashing during severe onshore storms have resulted in a distinctive pattern of crest and backbarrier sedimentation. This is not easily matched with available models of washover sedimentation drawn from the barrier islands of the eastern United States. Beach crest overtopping occurs at heights up to +6 m MHWS and is related to the development of upper beach swash ramps that are plastered against storm-wave-eroded dune scarps located at the barrier crest. Ramp sediments comprise coarse basal beach gravels overlain by coarsening-upward units and finally topped by distinctive seawarddipping fine grit/coarse sand couplets. In places where ramps reach the dune cre t, overtopping swash has deposited a fine marine gravel veneer (

Prograded Holocene beach-ridges with superimposed dunes in north-east Ireland: mechanisms and timescales of fine and coarse beach sediment decoupling and deposition

Abstract Two depositional models to account for Holocene gravel-dominated beach ridges covered by dunes, occurring on the northern coast of Ireland, are considered in the light of infrared-stimulated luminescence ages of sand units within beach ridges, and 14 C ages from organic horizons in dunes. A new chronostratigraphy obtained from prograded beach ridges with covering dunes at Murlough, north-east Ireland, supports a model of mesoscale alternating sediment decoupling (ASD) on the upper beach, rather than macroscale sequential sediment sourcing to account for prograded beach ridges and covering dunes. The ASD model specifies storm or fair-weather sand beach ridges forming at high-tide positions (on an annual basis at minimum), which acted as deflationary sources for landward foredune development. Only a limited number of such late-Holocene beach ridges survive in the observed prograded series. Beach ridges only survive when capped by storm-generated gravel beaches that are deposited on a mesoscale time spacing of 50–130 years. The morphodynamic shift from a dissipative beach face for dune formation to a reflective beach face for gravel capping appears to be controlled by the beach sand volume falling to a level where reflective conditions can prevail. Sediment volume entering the beach is thought to have fluctuated as a function of a forced regression associated with the falling sea level from the mid-Holocene highstand (ca. 6000 cal. yr BP) identified in north-east Ireland. The prograded beach ridges dated at ca. 3000 to 2000 cal. yr BP indicate that the Holocene highstand’s regressive phase may have lasted longer than previously specified.