A paraglacial coastal gravel structure: Connell’s Bank, NW Ireland

A paraglacial coastal gravel structure: Connell’s Bank, NW Ireland 121 A paraglacial coastal gravel structure: Connell’s Bank, NW Ireland Jasper Knight†, Helene Burningham‡ † School of Geography, Archaeology & Environmental Studies, University of the Witwatersrand, Private Bag 3, Wits 2050, Johannesburg, South Africa [email protected] ‡ Department of Geography, University College London, Gower Street, London, WC1E 6BT, UK [email protected] www.cerf-jcr.org ABSTRACT Knight, J. and Burningham, H., 2014. A paraglacial coastal gravel structure: Connell’s Bank, NW Ireland. In: Green, A.N. and Cooper, J.A.G. (eds.), Proceedings 13th International Coastal Symposium (Durban, South Africa), Journal of Coastal Research, Special Issue No. 70, pp. 121-126, ISSN 0749-0208. www.JCRonline.org Coastal gravel structures have been well documented worldwide and are formed dominantly by onshore wave transport of gravel, mainly during storm events. They are commonly observed along paraglacial coastlines where their origins are more ambiguous because of the effects of antecedent patterns of sediment supply, glacioisostatic sea-level change, and contemporary coastal processes. This paper describes the properties and polygenic origin of Connell’s Bank, a small paraglacial gravel structure on the Atlantic-facing coast of NW Ireland. This feature has been shown on historical maps, air photos and satellite imagery since c. 1850 but its outline has varied depending on seasonal migration, expansion and contraction of a sand veneer. It has also acted as a major control on tidal channel position within the estuary, and thus on sensitivity of the estuary system to ocean forcing. In detail, the bank surface is composed of cobbles sourced from outside of the immediate catchment. These surface cobbles are strongly winnowed, forming a lag deposit, whereas below the surface, cobbles exist within a granule and shell matrix. A significant proportion of surface cobbles shows evidence for recent ventifaction by blown sand at low tide. The paraglacial evolution of Connell’s Bank since the last glaciation comprises the following stages: (1) deposition of coarse glacigenic sediment as a moraine or proximal outwash fan during the late Pleistocene lowstand; (2) reworking of sediments onshore during early Holocene sea-level rise; (3) surface winnowing during mid to late Holocene tides and storms; and (4) surface cobble modification by contemporary wind abrasion. ADDITIONAL INDEX WORDS: Paraglacial, glacial sediment, ventifacts, erosional lag, gravel barrier, landscape palimpsest. INTRODUCTION A range of coastal gravel structures including barriers, bars, banks and spits has been well documented worldwide. Most gravel structures are found where coastline orientation changes abruptly. They are usually elongate structures with a steep shoreface, attain a height of several metres above sea level, and are often characterised by systematic changes in clast size along the length of the structure (Carter, 1983; Orford et al., 2002). Most of the documented examples of gravel structures are formed by onshore transport of clasts during storm events and associated with strong storm wave setup and in response to long-term changes in sea level (Forbes et al., 1991; Carter and Orford, 1993; Orford and Anthony, 2011). As such, most gravel structures are assumed to form episodically under contemporary conditions, and to attain a maximum threshold height with respect to incoming storm waves and/or sea-level position (Orford et al., 1995). However, coarse clastic (gravel) structures that are documented from glaciated coasts are more ambiguous and probably polygenic in origin (Carter and Orford, 1988). This is because glacier retreat often leaves gravel-rich moraines and drumlins in coastal lowland settings where eustatic and glacioisostatic sea-level rise can ____________________ DOI: 10.2112/SI65-021.1 received 2 December 2013; accepted 21 February 2014. © Coastal Education & Research Foundation 2013 rework these sediments, leaving a winnowed lag of glacigenic clasts from which the finer matrix has been washed away (Carter et al., 1990; Forbes and Syvitski, 1994; Greenwood and Orford, 2007; Hayes et al., 2010; Hoffmann et al., 2010). It is notable that many coastal dune fields across northwest Europe are anchored on gravel ridges (e.g. Orford et al., 2003) which are likely to have a similar history. Such gravel landforms are commonly overstepped by postglacial sea-level rise, forming a drowned or welded barrier, and winnowing may result in formation of sandy beaches, saltmarsh or lagoonal sediments adjacent to a residual moraine core (Carter and Orford, 1988). The term paraglacial has been applied to such coastal landforms (Forbes and Syvitski, 1994) because their morphology and post-depositional evolution have been strongly influenced by glaciation (cf. Church and Ryder, 1972). Although examples of coarse clastic paraglacial structures, in particular barriers, have been described from eastern Canada and USA (Forbes et al., 1995a; FitzGerald and van Heteren, 1999), few have been described from other glaciated coastal lowlands or drowned continental shelves (Ruz, 1989; Johnston, 2001). Here, we describe one element of a paraglacial coastal landscape in northwest Ireland – a cobble-mantled bank – where a combination of high glacigenic sediment supply and postglacial sea-level rise has resulted in a complex history of sediment reworking, morphological evolution, and polygenic physical attributes of these elements in today’s landscape. Journal of Coastal Research, Special Issue No. 70, 2014 126 Knight and Burningham Figure 1. Location of Connell’s Bank in the Loughros More estuary, northwest Ireland. LOCATION AND GEOLOGIC SETTING The west coast of County Donegal, northwest Ireland, is dominated by a headland–embayment system which reflects a combination of relatively hard granite and metasediment rock types, and fault zones which have acted as lines of weakness for fluvial and glacial erosion. The coastline is macrotidal and with high wave energy and onshore winds from the adjacent Atlantic. Typical sedimentary landforms include sand dunes, saltmarsh, tidal sand flats and ebb deltas primarily associated with estuarine settings that lie within glacially eroded valleys or larger coastal embayments (Figure 1). Although sand dune, sandy beach and ebb-channel dynamics have been described in detail from some of these west coast estuaries (Burningham, 2002, 2008), there has been little consideration of the glacigenic controls on postglacial estuary evolution (Shaw and Carter, 1994). Here, we describe the properties and origins of Connell’s Bank, a gravel structure located within Loughros More Bay, the estuary of the underfit Owenea and Owentocker rivers (Figure 1). The gravel bank exerts an important control on ebb channel dynamics in Loughros More estuary, particularly in terms of preventing the northward migration of the mid-estuary ebb channel (Burningham, 2008). This feature (450 x 300 m dimensions) has a relative relief of < 1.5 m and is fully exposed within the lower intertidal zone (Figures 2, 3). During spring tides, the uppermost part of the bank is not covered by water. The area of Connell’s Bank is underlain by quartz monzodiorite that corresponds to one part of the zonal differentiation within the Ardara granite pluton (Stevenson et al., 2008). The quartz monzodiorite outcrops on the adjacent Derryness headland. Coarse crystalline tonalite from the same pluton outcrops on low-lying areas adjacent to this headland. The core of the pluton, located 1.5 km to the east, is composed of granodiorite. During the late Pleistocene glaciation in west Donegal, ice flowed generally westwards through embayments and on to the Atlantic continental shelf. Evidence for this comes from the presence of glacial erosional and depositional landforms, including moraines, found onshore and offshore (Knight, 2009, 2011, 2012; Ó Cofaigh et al., 2012). Glacial abrasion helped break down the coarse-grained granite and quartzite bedrock of west Donegal into sand-sized particles. This sand was then largely reworked onshore from the continental shelf and into adjacent estuaries and embayments, particularly during postglacial sealevel rise. Most of the present coastal landforms appear to have been built as a consequence of a stable sea-level position attained around 7 kyr BP (Carter and Wilson, 1993). There is very little preserved evidence for coastal events prior to this period. METHODS Historic map, air photo and satellite data from different dates between c. 1850 and 2012 were imported into and georeferenced within a GIS in order to examine centennial and shorter time-scale changes in the area and position of Connell’s Bank, and the boundary between sand and exposed cobble substrates, which can be clearly identified from these data sources. Episodic field surveys between 2007 and 2013 mapped the sand–cobble boundary in more detail using a handheld GPS, and differential GPS surveys in 2005 and 2008 captured the topographic context, in particular with reference to the vertical tidal frame (Figure 4). These spatial data were compared quantitatively within the GIS. Field surveys also examined the surface and immediate subsurface lithology and structure of the bank itself. The lithology and Journal of Coastal Research, Special Issue No. 70, 2014 A paraglacial coastal gravel structure: Connell’s Bank, NW Ireland 125 Figure 2. Change in the surface character and planform of Connell’s Bank from map, aerial photograph and field surveys, 1850–2013. properties of representative surface cobbles were examined in twenty 1 x 1 m quadrats. These quadrats were identified using a stratified sampling method in which a north-south and east-west grid was marked across the bank, with a line spacing of approximately 20 m, and quadrats were placed at grid intersections. Thirty randomly selected clasts were sampled from the bank surface within each quadrat (600 clasts in total). Clast lithology; clast shape based on the scheme of Zingg (1935) using a, b, c axial measurements; relative degree of weathering (qualitative scale from 0 [not weathered] to 5 [highly weathered]); and angularity/roundness using the Powers (1953) scheme (quantified where rounded has a value of 1 and angular a value of 4) were noted and averaged for the quadrat as a whole. As each quadrat was located with the GPS, this yielded consideration of spatial patterns of clast features. Summary (averaged) data are presented here. Test pits (< 60 cm deep) were excavated in the bank top in order to examine its subsurface structure. RESULTS AND INTERPRETATION Spatial Analysis From spatial mapping of the sand–cobble boundary around Connell’s Bank over the last 160 years, it is evident that the landform has not changed significantly in size and shape over this time period (Figure 2). However, over the shorter timescales of field mapping, it is notable that the landward (easterly) side of the bank remains relatively fixed but that the seaward boundary migrates over tens of metres, which reflects seasonal to interannual variations in wave intensity and to a lesser extent variations in ebb tide velocity. Sand accumulates as an onlapping wedge against both the seaward and landward sides of Connell’s Bank during flood and ebb tides respectively, or in response to small movements of the ebb channel (Figure 3). As such, the bank acts as a sediment capture point. This outermost part of the estuary is ebb-dominant, shown by the orientation of asymmetric ripples and the onlap of a low-density, therefore quickly-deposited, sand wedge against the landward side of the bank. The relative permanence of Connell’s Bank, irrespective of northwest Ireland’s exposure to Atlantic storms and large waves (Dawson et al., 2004), suggests that it is a relict feature that is largely insensitive to today’s coastal climate. A further point is that, although the broader Loughros More estuary contains extensive sands at a range of elevations, the high mobility of the sand cover around Connell’s Bank suggests that this location is the focus of a specific set of processes that are conducive to sand mobilisation and transport. Journal of Coastal Research, Special Issue No. 70, 2014 126 Knight and Burningham Falcarragh Limestone (3.1%). It is notable that the majority of clasts are not derived from the immediate catchment but from sources outside of the watershed, in particular to the south and southeast (quartzite, appinite). Aggregated data on clast shape (n=600) show that 44.2% are tabular, 34% are equant, 12.6% are prolate and 9.1% are bladed. There are no significant differences between these percentage splits between different quadrats. The index of relative weathering shows that 14.3% of all clasts show little or no weathering and 5.8% of all clasts are highly weathered, but there is some spatial variability in this pattern whereby both these weathering end-members are more common at slightly lower elevation sites on the bank, and in more proximal positions. This may suggest that clasts may be turning over as a consequence of undercutting wave action. With respect to clast angularity, angular clasts only make up 1% of all clasts observed, and are only observed in those quadrats where clasts are also variably weathered. Most clasts (53%) are subrounded. In total, 18% of all clasts observed show evidence for ventifaction, with 106 clasts showing a surface polish caused by the rubbing of wind-blown sand grains, and two clasts showing pits caused by saltating sand grains. The quadrats in which over 30% of clasts are polished are located at lowest elevations closest to the low tide channel. It is notable that no clasts show clear evidence for glacial transport, such as faceting and striations, which may reflect overprinting by more recent marine and aeolian processes (Figure 3). Excavation into the cobble surface to around 60 cm depth shows vague planar stratification of openwork well-rounded cobbles with a poorly sorted coarse sand to granule matrix. The cobbles are a similar size, shape and lithology to those forming the surface lag. Intact marine shells are present within the matrix, in particular cockles (Cerastoderma edule) and periwinkles (Littorina littorea), indicating that the surface layer of the bank has been reworked and redeposited in a full marine setting. As the present lagged surface is, according to the spatial data presented in Figure 2, relatively geomorphically stable, we suggest that sediment reworking and development of this lagged surface layer took place during the mid-Holocene sea-level transgression, and that this is marked by a basal erosional unconformity (transgressive surface). The timing of this sea-level rise is uncertain as a consequence of uncertainty in the rate of glacioisostatic adjustment (Brooks et al., 2008) but took place between around 6.5–5 kyr BP (Shaw and Carter, 1994). Since this time period, sea level has been relatively stable and the estuary has evolved through sediment infilling and development of fringing sand dunes and saltmarsh (Burningham, 2002). This is very similar to the gravel barrier-overstepping model presented by Forbes et al. (1991). (i) (ii) (iii) (iv) DISCUSSION Connell’s Bank as a Glacigenic Feature Figure 3. Photographs showing (i) view south over Connell’s Bank into the estuary; (ii) view north showing the raised cobble bank (foreground); (iii) view south showing the cobble/sand boundary; and (iv) ventifacted cobble on the bank surface. Clast Analysis When data from all 20 quadrats are aggregated together, clasts (n=600) are dominantly quartzite (64.5%), with subordinate appinite (10.8%), tonalite (6.2%), granodiorite (4.3%) and Although no diagnostic evidence for glacial abrasion is seen on Connell’s Bank cobbles, the embayment geomorphic setting, positive relief morphology, erratic lithologies and wider glacial context suggest that Connell’s Bank formed as a glacigenic feature, most probably as a moraine during late Pleistocene ice retreat, and formed by a valley glacier extending northwards from the Glengesh peninsula (e.g. Dury, 1957; Knight, 2012). It is also likely that during deglaciation and in the early Holocene, rivers were more vigorous than they are at present, and transported glacigenic materials into the present-day estuary valley. This may be considered as a paraglacial response to deglaciation (Knight and Harrison, 2009). Journal of Coastal Research, Special Issue No. 70, 2014 A paraglacial coastal gravel structure: Connell’s Bank, NW Ireland 125 Paraglacial context of Connell’s Bank Paraglacial coasts are generally characterised by high sediment supply and significant changes in sea level that affect coastal evolution over millennial timescales following ice retreat (Forbes and Syvitski, 1994; Forbes et al., 1995b). As such, paraglacial coasts evolve in response to antecedent factors that in turn give rise to a palimpsest of relict, reactivated and contemporary landforms. Within Loughros More Bay are located examples of all these landform types, including bedrock shore platforms, sand and boulder (storm) beaches, coastal sand dunes, saltmarsh and ventifacts (Burningham, 2002, 2008; Knight, 2008), as well as Connell’s Bank itself. This suite of landforms results from a range of processes operating on different spatial and temporal scales, and are also set against a typical postglacial history of estuary infilling and thus long-term reduction of tidal prism and sediment accommodation space. This fits with similar themes in recent literature. For example, Hein et al. (2012) described the paraglacial and ‘post-paraglacial’ evolution of barrier islands in Maine, USA, in which recent morphodynamic behaviour is both much reduced in magnitude and is spatially constrained by landforms and structures of the paraglacial past. The coastline of western Ireland also shows this general behaviour (Shaw and Carter, 1994; Delaney and Devoy, 1995). Recent morphodynamics of Connell’s Bank The physical properties and range of morphological features found in association with Connell’s Bank confirm that it has a polygenic origin, formed as a glacigenic moraine which is lithologically sourced from outside of the catchment, and subsequently modified by postglacial marine reworking by tides and waves, and with aeolian ornamentation of surficial clasts. The presence of Connell’s Bank as a positive-relief obstruction within the estuary has helped control the position of the flanking ebb channel, at least over the time scale of available map data, because it has acted as an obstruction to channel migration (Burningham, 2008) (Figure 4). Furthermore, as a consequence of this channel control, it allowed for the coeval eastward migration of sand dunes located on the northerly estuary margin, which closed off Sheskinmore Lough (Barrett-Mold and Burningham, 2010). Thus, it could be argued that this paraglacial control on ebb channel configuration has subsequently influenced Holocene morphodynamic behaviour of the whole estuary and surrounding coastal landscapes and ecosystems. It is notable that at low tide, when the landward part of the estuary is exposed, wind processes are dominant with streamers driven by strong offshore winds transporting saltating sand grains across the bank from east to west. This is a geomorphicallysignificant process, as seen by the presence of well-developed ventifaction of surface cobbles. These clasts commonly show small (mm-scale) pits and larger smooth and abraded surfaces typical of dreikanter-type ventifacts. This fits with regional evidence for geomorphically-significant wind-driven sand transport, forming ventifacts of different types and scales under the contemporary wind regime (Knight and Burningham, 2001; Knight, 2005, 2008). There is no particular clustering of ventifacted clasts, although some have been abraded on several sides, suggesting that clasts may have moved position, probably by wind-wave undercutting. CONCLUSIONS Despite Ireland having a strong glacial imprint, paraglacial landscape responses in the coastal zone are poorly documented. Connell’s Bank is a glacigenic structure that has exerted a Figure 4. Detailed field surveys of sand/cobble boundary at Connell’s Bank, and representative topographic transects A–C. significant impact on subsequent paraglacial coastal evolution. The permanence of this structure over mapping timescales shows that it is largely relict with a surface cobble lag that has protected underlying sediments from contemporary wave disturbance. Such features are common throughout the western Ireland coast, suggesting that the present-day coastal landscapes are a palimpsest of past and contemporary geologic and climate-driven processes, and of recent human activity. 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