Impact of relative sea-level changes since the last deglaciation on the formation of a composite paraglacial barrier

Marine Geology 400 (2018) 76–93

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Impact of relative sea-level changes since the last deglaciation on the formation of a composite paraglacial barrier ⁎

T

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Julie Billya, , Nicolas Robina, , Christopher J. Heinb, Duncan M. FitzGeraldc, Raphaël Certaina a

Université de Perpignan Via Domitia, CEFREM UMR-CNRS 5110, 52 Avenue Paul Alduy, 66860 Perpignan Cedex, France Department of Physical Sciences, Virginia Institute of Marine Science, College of William & Mary, Gloucester Point, VA 23062, USA c Department of Earth and Environment, Boston University, 675 Commonwealth Avenue, Boston, MA 02215, USA b

A R T I C LE I N FO

A B S T R A C T

Editor: E. Anthony

Comprehensive onshore-offshore surficial and sub-surface mapping of a composite barrier (combination of prograded, aggraded, and/or transgressive segments) have provided a better understanding of the (i) mechanisms responsible for the formation and development of coastal barrier systems, (ii) relationships and interactions among individual parts of those systems, and (ii) the overall stratigraphic framework of subaerial and subaqueous segments of the barriers. Here, we investigate these facets of barrier evolution through integration of stratigraphic data from subaqueous high-resolution seismic and subaerial ground-penetrating radar, sedimentology (terrestrial cores and seafloor surface samples), and merged topographic and bathymetric mapping of the Miquelon-Langlade Barrier (northwest Atlantic Ocean, south of Newfoundland). This barrier system has two open coasts and evolved in a paraglacial setting, influenced by the reworking of glaciogenic sediment (glacial moraines) in a regime of complex sea-level changes. The barrier stratigraphic sequence is placed within the context of a shifting period from shoreline transgression to one of regression; the resulting sedimentary units reflect the isolated position of the Saint-Pierre-and-Miquelon Archipelago distal from continental influence. Seismic profiles reveal the position of the lowstand shoreline, located 20–25 m below modern sea level, further refining the existing lowstand model of southern Newfoundland. Continuous onshore-offshore subsurface geophysical mapping of the barrier allows for the identification of the relative positioning of distinct sedimentary units interpreted as subaerial barrier (beaches, dunes, spit), shoals, and shoreface deposits, and allows for estimation of the total barrier sediment volume (235 × 106 m3) and its relative subaqueous (90%) and subaerial (10%) components. Moreover, it reveals the three distinct morphological units comprising the Holocene barrier: (i) central, regressive, swash-aligned beach-ridge plains developed atop both thin (westward-prograding) and thick (eastward-prograding) shoreface deposits, (ii) drift-aligned, elongating spits located in the northwest and northeast of the island, and (iii) a transgressive barrier located adjacent to the northwest spit, pinned on its landward side to parabolic sand dunes, and currently experiencing erosion and limited overwash. Finally, this study places evolution of this system in the framework of paraglacial barrier evolutionary typology.

Keywords: Seismic profiling Ground-penetrating radar data Paraglacial barrier Gulf of Saint Lawrence Stratigraphic framework Regressive-transgressive sequences

1. Introduction Coastal systems—consisting of emerged landforms such as barriers, spits, beach ridges, beaches, dunes etc., in addition to their seaward terminations (shoreface deposits)—evolved in response to changes in the ratio between the rate of sediment accumulation and the rate of accommodation creation (space available for sediments to fill, a function of relative sea-level (RSL) variations and shoreface morphology) (e.g. Carter, 1988; Roy et al., 1994; Short, 1999; Timmons et al., 2010). Three main morpho-sequences are identified when the rate of sediment accumulation exceeds, is less than, and equals than rate of

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accommodation creation, respectively (e.g. Galloway and Hobday, 1983; Davis and FitzGerald, 2004; Costas and FitzGerald, 2011; Otvos and Carter, 2013): (i) regressive (prograded) barriers defined by a seaward extension of the barrier either under normal (sediment supply dominated; (e.g. Rodriguez and Meyer, 2006; Barusseau et al., 2010)) or forced (RSL fall dominated; e.g. Tamura et al. (2008); Sanjaume and Tolgensbakk (2009)) regression as beach-ridge foredune-ridge, strandplain, chenier, or prograded barrier systems; (ii) transgressive (retrograded) barriers defined by landward shoreline translation in response to either erosion or overwash-induced rollover (e.g. Mellett et al., 2012; Lima et al., 2013; Otvos and Carter, 2013); and (iii) stationary

Corresponding authors. E-mail addresses: [email protected] (J. Billy), [email protected] (N. Robin).

https://doi.org/10.1016/j.margeo.2018.03.009 Received 29 November 2017; Received in revised form 30 March 2018; Accepted 31 March 2018 Available online 03 April 2018 0025-3227/ © 2018 Elsevier B.V. All rights reserved.

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Fig. 1. Location map of the study area (the Miquelon-Langlade barrier) and data collected as part of this study. All data are projected in WGS 84 Zone 21 N. The Saint-Pierre-et-Miquelon Archipelago is located south of Newfoundland (inset), in the Gulf of Saint Lawrence. The barrier is composed in 4 sections (see Fig. 3): 1) at the north-west: les Buttereaux; 2) at the North-east: a mainland-attached recurved spit; 3) central northern beach-ridges plains; and 4) southern, small beach-ridge plains. The map locates marine sediment samples (black points) and seismic profiles along the west and east sides of the Miquelon-Langlade Barrier (gray dotted lines). Colour lines correspond to profiles shown in Figs. 4, 5, and 7. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

histories of post-glacial sea-level change, paraglacial coastal systems may record in their subaerial and subaqueous sedimentary archives evidence of transitions between shoreline transgression and regression. Examples of such paraglacial sedimentary archives include the Gulf of Saint Lawrence (Boyd et al., 1987; Orford et al., 1991a; Forbes and Syvitski, 1994), the Gulf of Maine (van Heteren et al., 1998; Hein, Fitzgerald, Buynevich, et al., 2014), the Baltic coast (Hoffmann et al., 2005; Harff and Meyer, 2011), and the coast of Alaska (Hayes and Ruby, 1994). Here, we present the results of continuous onshore-offshore geophysical mapping of a paraglacial, composite barrier located south of Newfoundland and develop an evolutionary model of its formation following the last deglaciation. The morphology and internal architecture of the subaerial segments of the barrier are derived from published data (Billy et al., 2014, 2015) and are supplemented with new seafloor sediment samples and seismic surveys collected along the barrier's two open coasts. The proposed stages of barrier formation correlate with the overall stratigraphic framework available from this combined onshore-offshore mapping and are placed in the context of the paraglacial environment in which the system developed. Finally, the importance of this kind of comprehensive study is discussed and results are compared to existing stratigraphic models and paraglacial barrier evolutionary models. Moreover, this study provides an example of the utility of developing onshore-offshore geophysical datasets to: (i) investigate barrier stratigraphic complexity, (ii) highlight correlations between sedimentary units and contacts to better establish the developmental framework of the barrier, and (iii) to estimate barrier sediment volumes and quantity of sediment deposits needed to form this coastal feature.

(aggraded) barriers characterized by vertical growth through the stacking of sand layers (e.g. Simms et al., 2006). However, single coastal systems are commonly much more complex than these end members and often defined by a combination of prograded, aggraded, transgressive segments; these are termed composite or hybrid barriers (Otvos, 1982, 2012). Investigations of such systems contribute to improved knowledge of the factors influencing coastal-system development, variations through time, and interactions or connections with pre-existing coastal features (underlying geology and antecedent topography). Stratigraphy holds the potential to record the history of coastal system formation and evolution, as well as the sedimentation processes, time-varying sediment delivery rates (Vespremeanu-Stroe et al., 2016), role of inherited topography (FitzGerald and Heteren, 1999), and changes in RSL (Costas and FitzGerald, 2011; Billy et al., 2015; Costas et al., 2016), wave energy (Allard et al., 2008; Hein et al., 2013), tidal regimes (Hayes, 1979; Chaumillon et al., 2013), and climate (Billeaud et al., 2009; Costas and FitzGerald, 2011). Only few studies (Timmons et al., 2010; Oliver et al., 2017) utilize both onshore and offshore datasets to explore the stratigraphic record as preserved on both sides of the modern shoreline. However, the combination of both shallow seismic (offshore; e.g. Certain et al. (2005); Mellett et al. (2012); Billy et al. (2013); Aleman et al. (2014)) and ground-penetrating radar (onshore; Rodriguez and Meyer (2006); Hede et al. (2013); Lima et al. (2013)) technologies presents a powerful tool with which to investigate the evolutionary history of, in particular, composite barrier systems. Paraglacial coastal systems are those whose characteristics reflect: (i) glacio-isostatic and eustatic sea-level changes, (ii) reworking of generally glacigenic sediment representing a range of grain sizes, and (iii) a clear influence of bedrock or inherited geomorphology (Church and Ryder, 1972; Forbes and Syvitski, 1994; Ballantyne, 2002; Hein, Fitzgerald, Buynevich, et al., 2014). Given their commonly complex 77

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Fig. 2. Relative sea-level trends for southern Newfoundland. (A) RSL change at Saint George's Bay (see location, Fig. 1A) following retreat of the Laurentide Ice Sheet (modified from), highlighting a period of emergence (FSST: Falling Stage System Tract; +40 to −25 m at 13.7–10.5 ka), stability (LST: Lowstand System Tract at approximatively −25 m between 10.5 and 8.0 ka) then subsidence (TST: Transgressive System Tract, −25 to −3 m at 8.0 to 3.0 ka; and HST: Highstand System Tract −3 to 0 m over the last 3.0 ka). LU, MU and UU are the name of seismic units, and associated colour bars highlight the estimated time period over which each unit was deposited on the shoreface. (B) RSL trends over the last < 3000 years at the Saint-Pierre-et-Miquelon Archipelago (Billy et al., 2015), Placentia Bay (Daly et al., 2007) and Port-au Port Peninsula (St George's Bay) (Brookes et al., 1985; Wright and Van de Plassche, 2001); see locations, Fig. 1A. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

2. Study area

whereas the eastern and northeastern shores are largely protected by nearby Newfoundland.

2.1. General setting 2.2. Previous work on the emerged barrier

The Miquelon-Langlade Barrier is a mixed sand-and-gravel composite coastal barrier located 50 km south of Newfoundland, Canada (Fig. 1). This paraglacial system formed from the reworking of glacial deposits by waves, tides, and wind during subsequent changes in RSL (Billy et al., 2014, 2015). Following ice retreat (13–12 ka BP, Fig. 2) and crust variation in Canada, mainly due to postglacial rebound (Dyke and Peltier, 2000; Tarasov and Peltier, 2004), these RSL variations southwest Newfoundland encompassed several periods of change initially emergent then subsiding (type B sea-level curve; Quinlan and Beaumont (1981)). Four distinct periods are distinguished (e.g., Daly, 2002; Shaw et al., 2006): 1) a period of rapid crustal rebound and falling RSL (+40 to −25 m) at 13.7–10.5 ka; 2) a period of relative sealevel stability at ca. −25 m between 10.5 and 8.0 ka; 3) a period of RSL rise between −25 and −3 m at 8.0–3.0 ka; and 4) slow RSL rise during the last 3000 years (Brookes and Stevens, 1985; Forbes et al., 1993; Forbes and Syvitski, 1994; Shaw et al., 1997; Bell et al., 2003; Daly et al., 2007). During the period of ice retreat, a large volume of sediment (till and outwash) was deposited on the shelf and coastal zone (Syvitski, 1989; Forbes et al., 1993; Billy et al., 2014). The paraglacial coastal systems of southwest Newfoundland share similar regional stratigraphy. Basal units are composed of glacial deposits associated with ice-sheet contact, include (1) till and (2) glacio-fluvial deposits, and by (3) thick (20–45 m) glaciomarine mud (Syvitski, 1989; Forbes et al., 1993; Forbes and Syvitski, 1994; Bell et al., 2003). Post-glacial deposits are characterized by mud and deltaic deposits (sand to gravel, flat tops and steep foreset beds), which prograded offshore during the period of falling RSL (falling-state systems tract; FSST) associated with post-glacial isostatic rebound (Shaw and Forbes, 1992). The thickness of each of these units is spatially variable, but the presence of each is generally a consistent feature along the southwest coast of Newfoundland. At the Saint-Pierre-et-Miquelon archipelago, moraines (generally 3–5 m thick; Figs. 1 and 3E; Robin (2007); Billy et al. (2015)) border Miquelon and Langlade Islands (forms locally a land relief up to 10 m thick along the west coast of Langlade Island). They are composed of a combination of very fine (< 0.05 mm) to coarse (pebbles up to tens of centimeters) sediments (80–60% sand; and 20–40% gravel (0.8–2.5 mm); Billy (2014)). Erosion and reworking of these sediments by coastal processes led to the development of the modern sand-andgravel barriers, beaches, spits, beach-ridge systems, and shoreface deposits that comprise and surround the Saint-Pierre-et-Miquelon Archipelago (Robin, 2007; Billy, 2014). The wave regime of the region is dominated at present by regular, high-energy Atlantic swells from the west to south; 10% of waves are > 5 m in height and 3% are > 7 m in height (maximum recorded height is 8.4 m). The western and southwestern shores of the archipelago receive the dominant wave energy,

The emergent part of Miquelon-Langlade Barrier (Figs. 1, 3) is dominated by a 12-km long, 50–2500-m wide, Y-shaped isthmus with a dominant north-south orientation (Billy et al., 2014). Longshore currents drive convergence of sediment transport both towards the narrow central region, along the west side of the barrier, and towards the inlet of the Grand Barachois lagoon at the northeast (Robin, 2007; see location Fig. 1). The northwestern section of the barrier (Fig. 3A) consists of a narrow (50–200 m wide) and high (up to 15–20 m) dune system, called ‘Les Buttereaux’. This area formed during the early development of the subaerial barrier (Billy, 2014), and is termed the “proto-barrier” in this study. The northeastern section is composed of a sandy, mainland-attached recurved spit that terminates at its southern end at the active Goulet tidal inlet (Fig. 3B). This inlet connects the coastal ocean to the 12 km2 Grand Barachois Lagoon. The inlet is characterized by a well-developed flood-tidal delta in the lagoon and a smaller ebb-tidal delta on the ocean side. South of the lagoon, the central and southern sections of the barrier consist of a well-developed sand-and-gravel beach-ridge plain and are fronted by a modern foredune ridge (Fig. 3C, D). The historical map of Fortin (1782) shows an active tidal inlet connecting the eastern and western coasts of the barrier. It closed by the late 18th century (Aubert de la Rüe, 1951), due to elongation and convergence of the northern and southern sections of the isthmus (Robin, 2007). Previous work has primarily concentrated on the emergent part of the barrier (Robin, 2007; Billy, 2014), notably the 5 km2 central beachridge plain, which has been mapped and studied using a combination of ground-penetrating radar (Mala ProEx GPR system with a 100, 250 and 500 MHz antennae coupled with a survey wheel and a Magellan-Ashtech RTK-GPS), optically stimulated luminescence (OSL) dating, and sediment cores and surface samples (Billy et al., 2014, 2015). The plain is formed from two distinct, wave-constructed beach and dune-ridge systems which have built along a bi-directionally prograding coast; four ridges sets with concave shape and two ridges sets with fan-shaped define the eastward- and westward-prograding systems, respectively (Billy et al., 2014). Each ridge set marks a successive phase of plain progradation, and records local changes in sediment delivery and regional variations in RSL rise rates over the last 3000 years. Progradation of the eastward-building system occurred at a rate of 65 ± 45 years per ridge (75 ± 50 m per 100 years) for the two older ridge sets (21 ridges), and decreased over the past ca. 600 years to 100 ± 35 years per ridge (30 ± 10 m of progradation per 100 years) (Billy et al., 2015). The internal architecture of the subaerial isthmus system is described in detail by Billy et al. (2014). Beach ridges overlie a deeper 78

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Fig. 3. Photographs of different sections of the Miquelon-Langlade Barrier: A. Northwest section showing the Buttereaux erosional aeolian sand dune system; B. Northeast section illustrating the Goulet Inlet and a mainland-attached recurved spit; C. Central sand-and-gravel beach ridge plain with its eastern- (fan-shaped) and western- (curved shaped) prograding systems; D. The southern part of the barrier is composed of a pond, beach-ridges, and aeolian dune systems; and E. Moraines in erosion along the Miquelon shore.

both side of the barrier.

Holocene sedimentary unit (BU) that disrupted incoming wave energy and altered the planform morphology of the prograding plain. Individual dune/beach ridges commonly display a well-developed (3.0–7.5 m thick) sigmoidal internal configuration with seaward-dipping internal reflections (Table 1). At a finer scale, lens-shaped forms and overtopping features can be identified at the top of the wave-built ridges (Billy et al., 2014). Ridges formed along the most exposed westward-facing coast have internal reflectors that are 1–2° steeper than those formed along the more sheltered eastward-facing coast. Wave-built ridges are overlain by peat or aeolian sand deposits, which can form dune ridges. The topography of the beach-ridge plain, and the interface between wave-built facies and overlying aeolian deposits increase in elevation by ca. 2.4 m in a seaward direction (towards the modern shoreline). Mapping of the elevation of this interface, combined with a series of 13 OSL ages from across the plain, have been used to develop the first late Holocene (last 2400 years) sea-level curve for the Saint-Pierre-et-Miquelon Archipelago (Fig. 2) (Billy et al., 2015). Despite these advances, previous studies have been limited to the onshore Miquelon-Langlade system, and specifically the beach-ridge plain. Here, we extend this database to the nearshore and shallow shelf, mapping sedimentary contacts, and placing development of various segments of this complex system into context of past relative sea-level changes and underlying geology.

3.1. Very high resolution seismic data Architecture of the shoreface was investigated from very high-resolution (VHR) seismic data and was acquired using a 4–12 kHz bifrequency INNOMAR seismic sounder, fixed at 6 kHz. This sounder is specifically adapted to shallow-water environments (e.g., Aleman et al., 2014; Nutz et al., 2014), with a maximum theoretical resolution of 5 cm. Approximately 330 km of seismic-reflection data (Fig. 1), 115 km and 215 km along the west and east sides of the barrier respectively, were acquired during three field campaigns in 2011 and 2012 aboard Le petit Saint-Pierre (DTAM's ship: Direction des territoires de l'alimentation et de la mer). Along the west side of the barrier, surveys extended offshore to the limit of sedimentary cover (up to 2.5 km of the coast and 25 m water depth). Along the east side of the barrier, surveys extended offshore up to 12 km from the coast, to the flank of a deep (bottom 110 m below mean sea level) channel which separates the archipelago from Newfoundland. Seismic profiles were processed (tide correction, wave filter, and amplitude correction) using ISE V.2.92 software and the key contacts were digitized (seafloor, contact between unconsolidated sediment and underlying bedrock) using Kingdom Suite (V.8.7.1; IHS Global Inc.) software. The sound velocity in the water and the sand were taken at 1550 m/s and 1800 m/s, respectively, and the maximum depth of penetration in unconsolidated sediment was 10 m below the sea floor. Seismic data interpretation (reflection geometries and unit configuration) follows the principles of seismic stratigraphy, allowing for the identification of seismic units (after Mitchum et al., 1977), and

3. Methods Investigation of the subaqueous part of Miquelon-Langlade barrier includes seismic data and seafloor sediment samples collected along the 79

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Maximum flooding surface; change in sediment supply and slow RSL Rise rate HST: High stand deposits (emerged M-L Barrier)

Rise: −3 to 0 m Rate: 1.3 mm/yrs

The sedimentology of seafloor deposits (the uppermost unconsolidated sedimentary unit) was investigated from 223 surface samples (Fig. 1) collected with a Shippek grab sampler. A total of 212 samples were collected during a survey in 2004 and 2005 (Robin, 2007) and 11 additional samples were collected during seismic surveys in 2012 and 2013. Sixty-four and 159 surface samples were collected along the west and east sides of the barrier, respectively (Fig. 1). All sediment samples were dry sieved using a nested column that ranged in size from gravel (10 mm) to coarse silt (50 μm) and grain-size statistics (modes, asymmetry, and spatial distribution) were calculated. Sediment reworking is considered to be limited by the depth of closure (10–15 m depth), especially given that currents are < 0.2 m/s on the offshore (recorded at 17 m depth at 3 km seaward of the barrier shoreline Robin (2007)). In the absence of sediments cores, the authors assume that seafloor sediment surface statistics characterize the uppermost seismic units. 4. Seismic units The shoreface proximal to the Miquelon-Langlade Barrier is composed of bedrock or glaciogenic and/or paraglacial (derived from the reworking of primarily glacial deposits) gravel (granules to cobbles) and sand deposits (Fig. 1). Sandy units are characterized on both sides of the island by a homogenous fine sand cover (grain size mode at 0.125 mm; Fig. 1), and are more developed along the eastern side of the barrier. Seismic data penetrated up to 10 m in sedimentary deposits; however proximal to the shore, deeper units are not imaged due to a loss in signal amplitude. Bedrock is highly visible in seismic profiles and easily distinguishable from sedimentary deposits due to its rugged morphology with numerous outcrops (e.g., the 10 m high peak imaged on Fig. 4). Moreover, photographs of the seafloor (obtained in 2011 by IFREMER, Goulletquer et al. (2011)) confirm this unit is consolidated bedrock and not a till deposit. Four seismic units are identified in profiles along the east side of the barrier and overlying bedrock (Figs. 4, 5 and 6): (1) a deep unit, DU, (2) LU (lower unit), (3) MU (middle unit), and (4) the uppermost surficial unit, UU. On the west side of the isthmus, only a single seismic unit, corresponding with unit UU, is identified. Upper and lower boundaries of seismic units are defined by discontinuities (strong reflection surfaces), named D0-D3. Stratal terminations and seismic facies (reflection patterns, descriptions, and interpretations) that characterize the Miquelon-Langlade barrier shoreface are summarized in Table 1.

38

40.5

South-eastward reflections (Slope: 0.3–0.5%)

North-eastward reflections (Slope: ~0.7%) Transparent

MU

UU

15

40 Transparent LU

4.1. DU: deep unit The basal discontinuity (D0) corresponds to the top of the bedrock. It contains large spatial irregularities and is characterized by both a basin, with a central depression estimated to be at least 25 m below modern sea level (Fig. 5), and seafloor surface outcrops (Figs. 1 and 4). Above D0, seismic unit DU (deep unit) is typified by a lack of internal reflections. To the east, and in more distal locations, DU dips eastward towards the Green Islands Channel, where it reaches depths of up to 150 m. The upper boundary of DU is a smooth erosional surface (D1). D1 is interpreted to represent the long term (> 2500 years) wave

UU west

East

– Transparent or some eastward reflections (Slope: 0.7%) DU

D3: Sub-horizontal

– – Bedrock

D2: Sub-horizontal

3000–0

?

Rise: −10 to −3 m

5000 (?) - 3000

8000–5000 (?) ? Rise: −20 to −10 m

10,500–8000

LST: Low stand period Erosional surface; maximum regressive surface TST: Transgressive sequence Transgressive ravinement surface; change in sediment supply and slow RSL Rise rate TST: Transgressive sequence D1: Polygenic, sub-horizontal

Fall: +40 to −25 m Top of bedrock (basin, outcrop) FSST: Regressive sequence D0: irregular

Stable: −25 to −20 m

– – 13,700–10,500

Chronological framework (estimated in BP) Associated RSL depth (m msl) and change trend Interpretation Discontinuity

3.2. Seafloor sediment samples

Facies/reflections

Estimated volume (106m3)

through correlation with seafloor sediment samples. Sediment cores and direct chronology are not available for the shoreface. As such, the approximate ages of individual seismic units and bounding surfaces are estimated through correlation with younger, onshore units (Billy et al., 2014, 2015) and the regional post-glacial and late Holocene relative sea-level curves (Fig. 2) to obtain a relative chronology of deposition of marine deposits and formation of key morphologic features in relation to RSL variations during this period.

Seismic units

Table 1 Synthesis of seismic units (discontinuities, facies, and volumes) identified offshore of the Miquelon-Langlade barrier, and interpretations, estimated chronology, and associated changes in relative sea level during the period of formation of each unit.

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Fig. 4. Merged onshore-offshore subsurface stratigraphy of the Miquelon-Langlade Barrier (west-east transect #1; location Fig. 1) illustrating the subaerial (beach ridges) and subaqueous (shoreface) parts of the system. Processed (top) and interpreted (bottom) seismic and radar profiles illustrate four major units overlaying the bedrock: DU (Deep Unit), LU (Lower Unit), MU (Middle Unit) and UU (Upper Unit). Beach ridges (SW and SE) are the landward extension of the upper unit UU. Lower boundary of units and the top of bedrock is not visible across the whole transect, thus their assumed positions are marked in dotted associated with a question mark and degraded colour. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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Fig. 5. Processed (top) and interpreted (bottom) seismic-reflection profiles illustrating two south-north longitudinal sections (AA′ and BB′; location Fig. 1) along the east coast of the barrier. Three seismic units overlay the bedrock (in gray) and show the depositional history of transgressive deposits: LU (Lower Unit in red), MU (Middle Unit in purple) and UU (Upper Unit in orange). White areas with question marks are undetermined. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

center of the barrier (Figs. 5A, 7).

reworking of DU during sea-level lowstand between 10.5 and 8.0 ka (Fig. 2). The spatial extent and thickness of DU are poorly resolved proximal to the coast due to rapid signal attenuation caused by the thick overlying sediment cover. As such, the full volume of DU within the study area cannot be estimated. Surface sediment samples from DU are composed of very fine to fine sand (a single mode at 0.125 mm). This unit was not imaged on the west side of the archipelago, but may nonetheless be present, especially to the west, proximal to the Hermitage Channel (ca. 20 km westward).

4.4. UU: upper unit 4.4.1. Unit UU along the eastern shore: UUEast Seismic unit UUEast (upper unit) overlays MU and contains deposits which are > 4 m thick (Figs. 4, 5) along the coast, and a well-defined lobate shape towards its center (Fig. 6D). UU East is a 17.5 km2 area and contains an estimated 40.5 × 106 m3 of sediment. On distal parts of the lobe, reflections dip to the northeast (0.4°; Fig. 5B). Proximal to the shore and at the middle of the lobate shape, this seismic unit contains high-amplitude internal reflections, across UUEast and the top of MU (Fig. 5A). Proximal to the shore, cut-and-fill structures of 1.0–1.5 m thick are observed as high amplitude reflections visible up to 1.0 km seaward (Fig. 7). More distally (1.5 km from the coast), two incisions (A and B) are recorded and identified by 5–6 m thick cut-and-fill structures (Figs. 5A, 7). Together, these structures are interpreted as paleo-channels associated with a northward-migrating channel (Fig. 7).

4.2. LU: lower unit Seismic unit LU (lower unit) is acoustically transparent and overlies either bedrock or unit DU. LU is characterized by a gently eastwarddipping, fan-shaped, 24 km2 deposit. It reaches up to 4 m in thickness proximal the shore and thins seaward, extending to the east into −20 m water depth (Figs. 6B, 4). Surface exposures of unit LU are composed of fine sand (mode of surficial sediment samples: 0.125 mm) and its volume is estimated at 40 × 106 m3. The upper boundary of LU, D2, dips gently and smoothly seaward at a slope of ca. 0.1°.

4.4.2. Unit UU along the western shore: UUWest Unit UUWest overlies bedrock along its entire length (Fig. 4). It is acoustically transparent (Fig. 4) and 1-2 m thick (Billy et al., 2013). The northern part of UUWest contains high-relief (2 m high) rocky outcrops within 100–200 m of the shore. Here, the sand reservoir extends to 1.0–1.5 km offshore (to the 13 m depth contour) and a shoreface (upper contact of the unit) slope 0.6°. The central and south parts of UUWest extend to 2.0–2.5 km offshore (corresponding to a maximum depth contour of −17 m) and a shoreface slope of 0.35–0.40°. The overall volume of this unit is estimated at 15 × 106 m3 and its surface exposure is composed of fine to very fine sand deposits (mode: 0.125 mm).

4.3. MU: middle unit Seismic unit MU (middle unit) overlies LU. It is characterized by acoustic transparency close to the shore (Figs. 4, 5A) and southeastdipping internal reflections throughout distal sections. Along northsouth profiles (Fig. 5B) this unit is marked by high-amplitude reflections dipping 0.2–0.3°, reflecting paleo sediment transport directions and progradation. MU covers a lobate area of 22 km2, with a NW-SE orientation and direction of progradation. It reaches up to 3 m in thickness and thins seaward (Figs. 4, 5 and 6C). MU is composed of fine sand (grain size mode at 0.125 mm; Fig.1) and its volume is estimated at 38 × 106 m3. The upper boundary of MU, D3, is smooth, sub-horizontal, and contains local cut-and-fill (erosional) structures near the 82

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Fig. 6. Location and thickness of seismic units: 1) DU: Deep unit; 2) LU: lower unit (LU) 0–4 m thick; 3) middle unit (MU) 0–3 m thick; and 4) upper unit (UU) 0–4 m thick. Black arrows indicate direction of unit progradation based on internal reflections. Bathymetric contours are in meters below modern mean sea level. Arrows show directions of progradation of the unit, established thanks seismic reflections interpretation (longer arrow reflects dominant direction).

5. Interpretation of barrier stratigraphy

The unconformity D3, at the top of MU, is the top of the transgressive sequence, and is therefore identified as the maximum flooding surface. The final stage corresponds to the recent period of sea-level highstand, marked by a period of slow RSL rise of ca. 3 m during the last 3000 years (~1 mm/yr). Associated HST deposits include unit UU and the entirety Miquelon-Langlade current Isthmus with its beach/duneridge plain, spit, lagoon, tidal deltas, and dune system (Fig. 10). Marine sedimentary units (DU, LU, MU and UU) are all characterized by unimodal fine sand (mode at 0.125 mm; Fig.1), whereas the barrier is composed of a mixture of sediment ranging from fine sand (mode at 0.2 mm) to cobble-sized gravel (Billy et al., 2014).

5.1. Stratigraphic sequence Stratigraphic units of the Miquelon-Langlade barrier complex can be placed in the context of relative sea-level changes and resulting periods of shoreline regression and transgression, encompassing the four primary sequence stratigraphic system tracts: Falling Stage System Tract (FSST), Lowstand System Tract (LST), Transgressive System Tract (TST) and Highstand System Tract (HST) (e.g. Mitchum et al., 1977; Haq et al., 1987; Posamentier et al., 1988; Van Wagoner et al., 1988) (Table 1, Fig. 2). Regressive unit DU represents the FSST, characterized by rapid RSL fall (> 20 mm/yr) from +40 to −25 m between 13.7 and 10.5 ka (Fig. 8). The LST commonly forms during periods of nearminimum sea level, which occurred here between 10.5 and 8.0 ka. This stage is characterized by an erosional surface: the unconformity D1, interpreted as a maximum regressive surface (e.g., Catuneanu et al., 2009). Following this lowstand, the TST corresponds to a relatively rapid rise in RSL (~4.4 mm/yr) from −25 to −3 m between 8.0 and 3.0 ka. Units LU and MU are both associated with the transgressive sequence of TST, as well as a proto-barrier attached to the south-west of Miquelon Island (Fig. 9). Unconformity D2, which is the boundary between LU and MU, is interpreted as a transgressive ravinement surface.

5.2. Early developmental history – LST and TST: the main role of the flooding history Following retreat of the Laurentide ice sheet at the end of the last glacial period (13.7 ka), rapid crustal rebound and relative sea-level fall (FSST) resulted in subaerial emergence of bedrock between Miquelon and Langlade islands. At lowstand the shoreline was positioned between 3 and 7 km seaward of its modern position. At this time, Miquelon and Langlade islands were connected by a bedrock ridge (Fig. 8) forming a single island with an approximate area of 470 km2. The east and west shores of the island evolved independently, 83

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Fig. 7. Processed (top) and interpreted (bottom) seismic profiles illustrating paleo-channels along the east coast and close the central part of the barrier. Transverse profile (CC′) and longitudinal profiles (DD′, EE′ and FF′) show cut-and-fill structures (paleo-channels) into units UU (orange) and MU (purple). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

modern sea level and submergence of the bedrock ridge that had connected the east and west coasts. This eventually led to the formation of the inlet at southern end of the barrier complex (Fig. 9). This new configuration significantly influenced local hydrodynamics, providing for wave propagation across the shallow bedrock platform and sediment exchange between the two sides of the island. Flooding of the nearshore of Miquelon Island allowed for wave erosion of both subaqueous bottom moraines and subaerial moraines which remain partially exposed around the island periphery (Fig. 3E). This process released sediment, which was transported to the south, leading to the formation of three distinct sedimentary features. Along southwest Miquelon Island, a spit formed and prograded to the southeast, eventually forming a straight, 3.8-km long barrier (Les Buttereaux; Figs. 3A, 9B). This feature was later covered by thick (up to 15–20 m high) parabolic dunes. Elsewhere, sandy sediments were pinned to shallow rocky outcrops in central regions of the isthmus, forming the basal unit (BU) at 3–6 m below modern sea level, as imaged in GPR profiles of Billy et al. (2014). Finally, along the east coast, fine sand was deposited above LU, forming unit MU (Figs. 5, 9). The lobate shape and orientation of internal reflections within the distal part of MU indicate southeast progradation of this system (Figs. 5, 7), reflecting the importance of wave energy and sediment transport from the west (i.e., through the inlet separating Miquelon and Langlade islands) during this period. All such features (spit, BU, and MU) developed synchronously, in concordance with local sediment transport patterns, and the deposition of each may have influenced sediment reworking associated with the development of the others.

influenced by local bedrock morphology and local wave propagation patterns (Billy et al., 2018). The marine transgression (TST) commenced after 8.0 ka and resulted in wide scale flooding of lower elevation regions of the archipelago (Fig. 9A). By the end of this period, the north-south-oriented shallow bedrock ridge still connected the two islands separating the east and west coasts. It is not possible to distinguish a specific seismic unit associated with this period of rapid transgression along the west coast of the island. Sediments were likely driven onshore during the transgression, either as sand sheets or perhaps landward-migrating sand bars; however, no remnants of any such system are identified along the west coast of the isthmus. Along the east coast, fine sand was deposited during this period of rapid transgression atop either DU or exposed bedrock, thereby forming unit LU (Figs. 4, 5 and 9A). Absence of internal reflections within this unit prevents interpretation of paleo transport directions and identification of specific morphologic features. RSL rise rates were higher than those observed elsewhere associated with the formation of subaerial barrier islands (e.g., Timmons et al., 2010; Hein, FitzGerald, Thadeu de Menezes, et al., 2014), and we find here no evidence of any such barrier islands or associated backbarrier lagoon or marsh deposits from this time. However, remnants of any such features may simply have been removed by waves and tidal currents as the transgression proceeded. By the time RSL rise slowed at ~5 ka, unit LU had been fully reworked on the shallow shelf into a large sandy platform situated beneath several meters of water. Continued transgression between 5.0 and 3.0 ka led to progressive flooding of the exposed platform located between 10 and 3 m below 84

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Fig. 8. Conceptual A) plan view and B) west-east cross section showing the exposed bedrock (above the 20 m depth contour) connecting Miquelon and Langlade islands, and lowstand deposit DU. Interpreted paleo-isobaths (shown in 5 m intervals) are derived from onshore-offshore geophysical data. Lowstand sea-level elevation is estimated at 20 to 25 m below present (horizontal blue line in cross section). C) Zoom on the seismic profile (transect #1; Fig. 1) process (top) and interpreted (bottom) data. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

5.3. Change in the RSL rate – TST to HST: barrier development

central portion of the beach-ridge plain, wave refraction around these subaqueous and intertidal shoals locally modified sediment transport patterns, resulting in complex curvature of the beach ridges (Billy et al., 2014). At this same time, the northern and southern sections of the barrier grew independently and converge towards one another, constricting the central channel and forming an inlet. Wave-driven, tidally influenced west-to-east sediment transport during this period led to the development of thick (up to 3 m thick) lobate-shaped deposits of unit UU along the east side of the inlet (Fig. 10A). The location of this central inlet is observed in seismic data along the east coast (Figs. 5, 7). Proximal to this inlet (up to 700 m of the present coast), shallow incisions (1.0–1.5 m depth) into unit UU are observed (seismic profiles CC′ and DD′). From 700 to 1500 m from the present coast, two major incisions are prominent in unit UU extending into the upper section of unit MU (seismic profiles AA′, EE′ and FF′; Figs. 5, 7). These have a distinct northeast orientation, and are ~250 m wide and up to 5 m deep. These structures are interpreted as channel cut-and-fills associated with the former central tidal inlet. It is not clear if two channels existed coincident with one another, or if one closed and another formed during a subsequent breaching event. Northward-dipping reflections within the channel-fill deposits likely indicate northward migration and filling as a result of convergence of sediment transport (Fig. 10B). Deposits of unit UUwest show no such evidence of former channels or inlet closure. Rather, this unit is characterized by a 1-m thick homogenous fine sand sitting atop bedrock (Fig. 4). The relative thickness of unit UUeast deposits, and their complex and well-preserved internal geometry as compared with unit UUwest likely reflects dominant easterly transport of sediment through the inlet and a higher degree of reworking on the western side of the isthmus, both in response to the dominant westerly wave climate.

The shift from TST to HST is marked by a deceleration in relative sea-level rise and attendant slowing of marine transgression and transition to a period of broad progradation and aggradation (Fig. 2). This same shift has been recognized to result in the initiation and development of coastal barrier systems throughout the world (e.g. Davis and FitzGerald (2004); Timmons et al. (2010); Hein, FitzGerald, Thadeu de Menezes, et al. (2014)). At Miquelon-Langlade, a decrease in the rate of RSL rise from +4.4 mm/yr to +1.3 mm/yr (Billy et al., 2015), coupled with sufficient sediment supply from erosion of proximal moraine deposits, is key to the transition from TST to HST. Here, sediment accumulation outpaced creation of accommodation by RSL rise by ca. 3 ka, forcing the system into a period of progradation and subsequent development of the complex barrier/lagoon/dune/beach-ridge-plain system. At the northeast corner of the isthmus, a small set of beach ridges grew from Miquelon Island. Given their morphology and low topography (swales largely flooded at present; Fig. 3B), these features must have been developed early during HST, probably coincident with slowing RSL rise. Connected to this feature, a mainland-attached recurved spit elongated to the south, produced by the southerly longshore transport system. A series of recurved ridges illustrate successive southerly progradation of the spit, and likely attendant southerly migration of Goulet Inlet (Figs. 3, 10). During this period, the remainder of the isthmus was characterized by eastward and westward beach-ridge progradation along two openocean coasts. It is estimated that single ridges took between 65 and 110 years to form (detailed in Billy et al. (2015)). The complex underlying topography associated with the proto-barrier attached to the southwest side of Miquelon Island (TST, Fig. 9) and the central intertidal shoals (basal radar unit, BU, Billy et al. (2018)) provided a platform upon which barrier sediments accumulated (Fig. 10). In the east85

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Fig. 9. Conceptual plan-view map and cross section of Miquelon-Langlade Island during formation of the TST: A) from ca. 8.0 to 5.0 ka, areas up to 10 m below modern sea level are flooded; emerged bedrock separates the east and west coasts; and transgressive unit LU (up to 4 m thick; in red) is deposited; B) from ca. 5.0 to 3.0 ka, areas up to 3 m below modern sea level are flooded, bedrock is completely submerged, and an inlet connects the coastal ocean along the two coasts. Transgressive deposits encompass an emerged barrier above a spit platform, shoals (BU; Billy et al. (2014)) and seismic unit MU (up to 3 m thick; in purple), are connected as a single, complex inter- to supra- tidal feature. Five-meter isobaths are shown. C) West-east cross section (Transect 1) of the barrier illustrates successive subaerial and subaqueous units deposited by 3.0 ka, derived from merged onshore-offshore geophysical data. RSL elevations at 5.0 ka and 3.0 ka are in dashed and continuous blue lines, respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

6. Discussion

the moraines located on and at the periphery of the islands, and likely on the shoreface. Shoreface erosion of these sediments leads to hydrodynamic sorting and the seaward movement of finer sediments, deposited overlying deeper bedrock to form an easterward- dipping unit along the west side of the island (DU), towards the Green Islands Channel (Fig. 4). Moreover, the shallow (or surficially exposed) bedrock of Miquelon, Langlade and Saint-Pierre Islands and the absence of an icemargin stabilization phase linked to ice on the mainland do not allow for the formation of thick delta deposits, such as those found along the coast of Newfoundland. Thus, the basement topography responsible for the presence of the archipelago separate from Newfoundland also played a key role in the sediment depositional processes that explain both the early stratigraphic sequence and the clear differences in deposits with those found on the proximal Newfoundland mainland, which evolved under similar regional conditions.

6.1. Post-glacial deposit at regional scale Despite its proximity to Newfoundland (tens of km), the 140-m deep channels surrounding the Saint-Pierre-et-Miquelon archipelago likely limited the influence of deglaciation processes still acting in Newfoundland during the post-glacial period (up to 8.0 ka; Shaw et al., 2006). As such, glacial and post-glacial sedimentary deposits at SaintPierre-et-Miquelon do not conform to the idealized post-glacial sedimentary sequence common across Newfoundland. Indeed, the archipelago is isolated from a large continental landmass, remote from massive continental sources and therefore, they are less likely to contain glaciomarine clay and outwash plains. For example, neither glaciofluvial deposits (could be related to a stabilization stage of the ice margin, supplied in glacial meltwaters by a large glacial system up-ice) nor glaciomarine mud are observed in seismic profiles and seafloor sediment (neither in samples nor seafloor photographs). The earliest post-glacial deposits here (FSST deposits) appear to directly overlie bedrock/boulders, are not well developed, and – at least in seafloor surface outcrops – are characterized only by fine sand (DU). Sediment delivered from rivers or eroded in situ from earlier unconsolidated deposits (e.g., ice-marginal deposits, fjord-mouth moraines, imaged in Shaw (2003); Shaw et al. (2006); Shaw et al. (2009)), common along the paraglacial coasts of Newfoundland, are limited or absent in Miquelon-Langlade. Here, sediment is sourced primarily from erosion of

6.2. Sea-level lowstand at Miquelon-Langlade Post-glacial lowstand elevations and shoreline features can be investigated through offshore geophysical data. For example, at Port au Port Bay and Saint George's Bay (Fig. 11B), the top of ‘Gilbert’-type deltaic deposits is characterized by a well-defined erosional terrace interpreted as the post-glacial lowstand shoreline (Forbes et al., 1993; Shaw and Courtney, 1997). The lowstand elevation at White Bear Bay and Saint George's Bay (Fig. 11B) is similar, estimated at −25 and − 23 m, respectively, and was reached at 9.4–9.5 ka (Forbes et al., 86

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Fig. 10. Conceptual model for development of the Miquelon-Langlade coastal system during the period of HST deposition. A) From 3.0 ka to the 18th century, highstand deposits are defined by an emerged barrier composed of spits and well-developed beach-ridges systems (Fig. 3), a central channel and seismic unit (UU) on both sides of the barrier (up to 4 m thick along the east side, and ca. 1 m thick on the west side). B) By the mid-18th century the central channel filled and the northern and southern beach-ridge plains merged into a continuous isthmus. Five-meter isobaths are shown. C) West-east cross section of the barrier, derived from merged onshore-offshore geophysical data, illustrates the development of modern features characterizing the island. RSL elevations at 3.0 ka and modern are given in the dashed blue line and continuous blue line, respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Shaw and Forbes (1995); Daly (2002); Shaw et al. (2002)), we observed that lowstand values estimated around Saint-Pierre-et-Miquelon are spatially variable: in a radius of 100 km around the archipelago, lowstand values are estimated from −15 m in the northeast up to −30 m in the northwest (Fig. 11B). Due to its location, Saint-Pierre-et-Miquelon Archipelago presents an opportunity to further explore spatial diversity in shoreline depths at the post-glacial lowstand.

1993; Bell et al., 2003). At Deadman's bay (northeast Newfoundland; Fig. 11B) the lowstand shoreline is defined by wave-cut terraces and is estimated to have been between −17 and − 21 m below present sea level at 9.6 ka (Shaw and Edwardson, 1994). From the synthetic isobase maps at the time of the regional lowstand (9 ka) in south Newfoundland (Fig. 11A, modified from Shaw et al. (2002); and Fig. 11B, refined and based on Liverman (1994);

Fig. 11. A) Isobase (0 line) map at 9 ka for the Gulf of Saint Lawrence and eastern Canada (in meters; modified from Shaw et al. (2002)). B) Lowstand elevation for southern Newfoundland (based on Liverman (1994); Shaw and Forbes (1995); Daly (2002); Shaw et al. (2002)). 87

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The Saint-Pierre-et-Miquelon Archipelago lacks the lowstand shoreline formations commonly found along southern Newfoundland, such shoreline terraces situated above deltaic deposits. However, Kelley et al. (2006) or Martinez-Martos et al. (2016), among others, interpret marine-cut terrace on the bedrock as RSL indicator. Here, seismic data from around Miquelon-Langlade Island do provide some insight into the nature and depth of the lowstand. For example, the top of the deep unit DU on the east coast of the island is defined by a sub-horizontal erosional surface from 20 to 25 m depth (Figs. 4, 9C; D1). Along the west coast, bedrock outcrops are characterized by a smooth and broadly subhorizontal surface between −20 to −25 m (Fig. 9). Together, these contacts are interpreted as a wave-cut surface of unconsolidated sediment, and marine abrasion of bedrock, respectively; both are interpreted as deriving from wave erosion on the upper nearshore during the lowstand. As such, we can estimate the lowstand depth at Saint-Pierreet-Miquelon as between −20 and − 25 m, which is concordant with earlier mapped depths (Fig. 11). Still, the lowstand depth remains poorly constrained. Improved characterization and precise mapping require further investigation through, for example, sediment cores, additional seismic profiles, and dating of key chronological contacts between 20 and 30 m depth along both coasts of Miquelon-Langlade. Nonetheless, this study adds pertinent control points of the lowstand position in southern Newfoundland (Fig. 11) and confirms the estimated depth range of the lowstand. Finally, given the southern location of Miquelon-Langlade with respect to earlier mapping of Newfoundland, these data validate and refine the model of Shaw et al. (2002), or at a larger scale the glacio-isostatic model of the North America of Tarasov and Peltier (2004), through field mapping. 6.3. Comparison of Miquelon-Langlade barrier development with paraglacial barrier evolutionary typology Evolutionary models for gravel barriers along paraglacial coasts have been proposed by Boyd et al. (1987) and Forbes et al. (1995) based on the geomorphic classification of drift- and swash-aligned paraglacial barriers by Forbes et al. (1990), and on the evolutionary framework provided by Orford et al. (1991b). This model proposes the formation of headland-attached barriers which are subsequently breached, broken up, and re-formed in a landward position as transgression proceeds (Boyd et al., 1987; Forbes, 2011). This is envisaged as a repetitive cycle, with landward barrier reestablishment following a phase of barrier destruction (Fig. 12). Individual barriers are initiated through development of swash- or - drift aligned embryos (D0 and S0; Fig. 12), become established as subaerial features (D1, S1-S2), and then undergo a phase of disintegration (D2, S3), followed by either renewed initiation in a landward position (D0, S0) or stranding (D3, S4). Evolution proceeds along different pathways depending upon the allogenic controls (e.g., waves, tides, rate of relative sea-level rise, etc.), the potential for reincorporation of sediment into new embryo structures, and internal morphologic feedback endemic to the system (Forbes et al., 1995). Development of the Miquelon-Langlade Barrier compares well with this earlier model (Fig. 12):

Fig. 12. Evolution of the paraglacial Miquelon-Langlade composite barrier placed in the context of cyclic transgressive paraglacial barrier formation and destruction of Forbes (2011) and Forbes et al. (1995). Areas in orange, green, and red stripping correspond to the morphologic evolution of the NW spit Les Buttereaux from 8 to 3 ka, the central beach-ridge plain from 3 to 0 ka, and the NE spit from 3 to 0 ka, respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

S1-S2) in the last 3000 years (area in green, Fig. 12). This region shows clear evidence of initiation (S0), development, seaward progradation (S1), and flooding of lower and older beach-ridge swales along the lagoon side of each ridge (S2) (Billy et al., 2014, 2015). (4) The relationship of the development of the Langlade attached barrier (southern part; Fig. 3D) with these earlier models is less straightforward, but is likely related to formation of the driftaligned barriers (D0-D1) up to the complete closure of the isthmus by the late 18th century.

(1) Evolution of the northwest spit (Les Buttereaux) from 8 to 3 ka (area in orange Fig. 12), presents an example of the evolution of driftaligned barriers (D0-D1). Presently, due to the retrogradation of the coastline (Fig. 11) and erosion of aeolian dunes (Fig. 3A) this northwest sector appears to be transitioning from between stage D1 (establishment) and stage D2 (breakdown). Our data do not allow us to explore whether previous entire cycles (D0-D1-D2) may also have occurred. (2) At the northeast, the development of the attached recurved spit from 3 ka to present may be linked to the evolution of the driftaligned barrier (D0-D1, area in red stripping Fig. 12); at present, this sector seems still to be at stage D1. (3) The central beach-ridge plain evolved as swash-aligned barrier (S0-

Thus, in total, the post-lowstand development of various sections of the mixed sand-and-gravel Miquelon-Langlade composite barrier can be well placed in the framework of the Forbes et al. (1995) evolutionary model. Given projections for global sea-level rise through 2100 (e.g., Church et al., 2013), combined with a natural depletion of morainal sediment sources, this paraglacial isthmus will likely move towards a phase of breakup, flooding and deterioration, following modes of paraglacial evolution elsewhere, under similar conditions.

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Fig. 13. A) Stratigraphic framework of the Miquelon-Langlade depositional system, illustrating prograded and retrograded regions and their associated shoreface deposits. B) Estimated sediment volume (fine sand to gravel) of the overall Miquelon-Langlade barrier.

which these forces acted (Fig. 13). These data reveal the underlying cause for heterogeneity in modern features: some areas are defined primarily by prograding (regressive) systems such as the mainland-attached recurved spit at the northeast of the isthmus and the prograded southern sector attached to Langlade Island. Others sections of the system, such as the northwest Buttereaux spit (retrograding system in deterioration) and the central beach-ridge plain (distinct shoreface deposits from either sides) are noticeably different than commons stratigraphic models (Fig. 13), as detailed in succeeding sections.

6.4. Stratigraphic framework diversity of a two-open-coast barrier In contrast to many barrier systems in which a barrier fronts a backbarrier consisting of marshes/mangroves, lagoons, and tidal flats, the Miquelon-Langlade isthmus has two open coasts, in which each side of the barrier system is prograding in response to complex feedbacks between sedimentation and accommodation; these are themselves influenced by changes in sediment transport and sediment exchanges within the system, hydrodynamic conditions and RSL over time (Billy et al., 2014, 2015). The merged onshore-offshore geophysical data collected across the island, shoreface, and nearshore reveal the spatial non-uniformity of the stratigraphic framework – across a small area – upon

6.4.1. The NW Buttereaux spit: retrograding system in deterioration This sector is defined by a transgressive barrier (Carter, 1988; Roy 89

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6.5. Insights into the system-wide evolution of the Miquelon-Langlade barrier

et al., 1994; Otvos, 2012), characterized by a retrograding ocean shoreline and overlain by aeolian sand dunes (Fig. 3A). However, unlike common transgressive shorelines, the barriers here contain neither obvious washovers nor evidence of past landward rollover (migration) of the barrier (Fig. 12). This likely reflects the presence of high-elevation (up to 20 m) parabolic sand dunes, which are stabilized by vegetation and which prevent overwash; thus, here, aeolian processes do not influence barrier retrogradation. Moreover, on the eastern side, the lagoon shoreline of the barrier is likewise undergoing erosion by storm waves, likely accelerated because of an increase of the lagoon water level and storminess waves. Connection between shoreface and shoreline behaviors along the west coast – and the role that these play in term of stability or erosion of the barrier – are highlighted by Billy et al. (2013), who demonstrate that: (i) sediment supply via longshore transport is limited, (ii) the shoreface slope (0.6°) is already steepest of the west coast, and (iii) this coast experiences the greatest exposure to high wave energy along the island. Together, these observations explain the thinness and limited spatial extent of the sand reservoir on the adjoining shoreface (UU west; Figs. 6, 10). Hydrodynamic conditions associated with depletion of sand have thus resulted in a retrogradational shoreline. This sector of Miquelon-Langlade is therefore, classified as a retrograded system experiencing active erosion and deterioration. Eventual erosion of the sand dunes will likely result in breaching of the barrier (Stage D2; Fig. 12), which was observed in 2012 during a winter storm (in November 2012). During this time, the Buttereaux sector was segmented in two, which prompted artificial filling. Shoreline retreat of this sector will likely occur through frontal erosion of the aeolian dune, causing a decrease in their elevation, which is likely to promote overwash and a lagoon-ward migration of the barrier (Moore et al., 2010; Duran Vinent and Moore, 2015).

6.5.1. Key components of the barrier developmental history The combined use of seismic and GPR data from across subaqueous and subaerial portions of the Miquelon-Langlade barrier provides for comprehensive insight into development of the system during the period of late-Holocene RSL rise. These data permit mapping of units across the modern shoreline, a critical component in the reconstruction of the early developmental history of the barrier, during which time the attached-spit, shoals (basal radar units), and shoreface deposits (seismic unit MU) developed synchronously and interdependently. Indeed, the initiation and development of the southeasterly prograding spit attached to Miquelon leads to the development of the shoals that influence the direction of progradation of MU. Thus, these entities constitute different parts of a single coastal system (Fig. 10). In this way, this study complements those of Kanbur et al. (2010); Timmons et al. (2010); Cooper et al. (2016); Oliver et al. (2017) among others, in illustrating the importance of investigating both emerged and immerged parts of coastal systems and the necessity to stratigraphically investigate both the emerged barrier and its shoreface. In this manner, this study builds upon those over the last 30 years which demonstrated the use and contribution of GPR technology for the description and understanding of coastal systems (e.g. Jol et al., 1996; Neal and Roberts, 2000; Bristow and Jol, 2003; Buynevich et al., 2004; Neal, 2004; Jol, 2009; Hein et al., 2012). However, studies that combine both GPR and seismic methods to develop a complete view of coastal systems remain rare. Thus, we encouraged the community to develop this dual approach where possible. 6.5.2. Estimation of sediment volume and stocks required in the formation of the barrier In addition to providing for a more comprehensive understanding of the role of changes in sea level, waves, tides, and underlying topography in the formation of the Miquelon-Langlade barrier system, the combined on- and off- shore geophysical mapping of the barrier allows for estimation of sediment volumes within this coastal system, both below modern mean sea level using GPR technology and topographic mapping, and on the shoreface using HR seismic technology and associated bathymetry. From this, the total volume of sediment constituting the barrier is estimated at 235 × 106 m3 (Fig. 13B). Of this, shoreface deposits correspond to 135 × 106 m3, with a large majority (120 × 106 m3) of sediment found along the east coast, and only ~12% (15 × 106 m3) along the west coast. The volume of the barrier platform (between −4.5 to 0 m) and lagoon deposits are estimated at 75 × 106 m3. The strictly subaerial part of the barrier - that located above modern sea-level, corresponding to spits, beach ridges, and aeolian dunes – is estimated at 25 × 106 m3, which is only 10% of the total volume of the barrier (90:10 ratio). Subaerial barrier and barrier platform (Fig. 13B) are together < 30% of the total volume of the barrier. This study also highlights the heterogeneity between the two sides of the isthmus before and after inlet closure. A key component of this insight was the integrated onshore/offshore stratigraphic mapping on subaerial (emerged barrier; 10% of sediment volume) and subaqueous (shoreface and buried barrier platform; 90% of volume) constituents of the barrier system (Figs. 9, 10 and 11). Moreover, from these data, it is possible to estimate past sediment sources, transport patterns, and transport rates responsible for the growth of the barrier system. As in many paraglacial coastal systems (especially those not associated with major rivers), erosion of glacial deposits – and in particular moraines – is the sole source of sediment to the Miquelon-Langlade barrier: gravel to medium sand forms beach ridges and spits; medium to fine sand forms aeolian sand dunes; and fine sand is deposited on shoreface (Fig. 14A). Considering the geometry of the archipelago, we hypotheses that the moraines that currently drape the subaerially exposed parts of

6.4.2. Central beach-ridge plain: bi-directional shoreface progradation and shoreline readjustment This area of the Miquelon-Langlade isthmus is distinct in that beachridge sets built from either side of a former central inlet (Fig. 10). Indeed, beach ridges prograding in opposite directions in the manner observed here is a scenario commonly found at the downdrift end of an island, and have prograded independently from one to the other (e.g. Clemmensen and Nielsen, 2010). At Miquelon-Langlade, development of the system was likely influenced by dynamics associated with the former central inlet (e.g., sediment transport, channels, and currents), the stratigraphic record of which is observed in seismic data through the disparity in the volume of shoreface deposits on either side of the inlet: UUWest is composed of thin sand cover that overlays bedrock (Fig. 10), whereas UUEast is characterized by thick deposits, with a lobate shape and abundant evidence of filled channels, which together overlie the unconsolidated sedimentary deposits of units MU and LU (Fig. 10). Following the closure of the central inlet, the evolution of these two coasts has occurred independently. Despite being largely protected by nearby Newfoundland, which limits fetch and therefore wave energy, and being composed of thick shoreface deposits, the eastern shore has experienced net erosion and a substantial shoreline readjustment in recent centuries (Robin et al., 2013; Billy, 2014). In contrast, no such process is observed along the western shore. Thus, despite their proximity and coupled evolution prior to inlet closure, the regressive beaches on either side of the barrier have distinct synthetic stratigraphic schemes (Fig. 13). Evolution of these two shorelines have been largely controlled by local stratigraphy associated with transgressive shoreface deposits and dynamics of the former inlet, along with contemporaneous local conditions (wave patterns, hydrodynamics, etc.) specific to each coast.

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the Boston Harbor Islands (e.g., Rosen, 1984) and eastern Nova Scotia (e.g., Nichol and Boyd, 1993), the entirety of the Miquelon-Langlade composite barrier formed lacking any fluvial sediment inputs—in situ erosion of glacigenic sediments provide the only source of sand/gravel for the formation of the expansive subaerial/subaqueous coastal system. 7. Conclusions This study describes the formation of a Holocene paraglacial composite barrier, simultaneously prograding along both east- and westfacing open ocean coasts. A key feature of this study is the comprehensive geophysical approach applied, integrating both subaerial and subaqueous sections of the barrier. The comprehensive dataset available from the Saint-Pierre-et-Miquelon Archipelago is used to identify and analyze the range of deposits comprising the barrier. This study enhances knowledge of this area and complements previous studies focused on the central, emerged section of the barrier (Billy et al., 2014, 2015). Our results show that: 1. The complete stratigraphic sequence of Miquelon-Langlade can be placed within a framework of a middle- to late- Holocene shift from shoreline transgression to barrier progradation. Post-glacial deposits observed at this site are distinct from those of the paraglacial sequences commonly observed in mainland-attached systems in southern Newfoundland. Here, sedimentary units reflect the isolated position of the archipelago, disconnected from sediment sources and reworking processes on nearby Newfoundland. Basement morphology and topography are important parameters controlling differences with neighboring systems. 2. Seismic profiles reveal eroded sediment and abraded bedrock surfaces interpreted as a lowstand erosional surface. From this, the depth of sea-level lowstand at the Saint-Pierre-et-Miquelon Archipelago is estimated to be between 25 and 20 m below modern sea level. This study adds a pertinent control point of lowstand elevations in the southern Newfoundland region, and one which correlates well with the lowstand depth estimated through numerical models. 3. The Miquelon-Langlade composite barrier developed in a manner in line with that published in previous studies of paraglacial barrier evolution. This study demonstrates that this system evolved under both drift- and swash- aligned morphologic frameworks, with the central beach-ridge plains and the northwest and northeast spits having formed in a manner dominated by swash- and drift- aligned transport, respectively. 4. Stratigraphic insights provided by merged onshore-offshore geophysical data highlight connections between distinct sedimentary units as emerged, shoal, and shoreface deposits, which are not visible through subaerial or subaqueous mapping alone. The formation and developmental history of the barrier have been reconstructed through this approach. Moreover, comprehensive geophysical mapping is particularly useful for studying complex composite barriers, as it emphasizes different stratigraphic frameworks for each part of the barrier. At Miquelon-Langlade, these include: (i) regressive systems building atop thin (westward) and thick (eastward) shoreface deposits, and (ii) a transgressive barrier undergoing deterioration, in which overwash and barrier rollover are, at present, prevented by the presence of large aeolian parabolic sand dunes. 5. Finally, comprehensive onshore-offshore geophysical mapping allows for an estimation of the total volume of sediment constituting the barrier (235 × 106 m3), which is distributed between the emerged barrier (10%) and shoreface deposits (90%). Given the likely source of these sediments (moraines; 60–80% of sand and 20–40% of gravel (0.8–2.5 mm)) and the geometry of the

Fig. 14. A) Sedimentary composition of moraines at Saint-Pierre-et-Miquelon Archipelago and partitioning of those deposits into sediments available for the development of beach ridges, aeolian sand dunes, and shoreface deposits. B) Location map of modern subaerial moraines and estimates of the required size and possible locations of former moraines which have been eroded to form the barrier system.

the bedrock islands of Miquelon and Langlade were substantially larger immediately following ice retreat. Former moraines were likely located along the seaward extent of the currently visible moraines and oriented along the shrinkage axis (towards north-northwest) of the glacier tongue. The moraines present on Miquelon and Langlade islands are generally 3–5 m thick (Fig. 3E). Assuming a mean thickness of 4 m, we estimate that erosion of at least 60–90 km2 of moraine/till (100 to 150% of the total barrier volume) would have been required to form and nourish the barrier. From this, we determine that the area above the 20 m isobaths which these moraines could have covered was 110–120 km2 (Fig. 14B). Given the much more expansive moraines found atop Miquelon Island today, we assume that these former moraine deposits were primarily located around Miquelon Island. The estimated paleo-moraine volume and area are theoretical and highly dependent upon the exact sedimentary composition and physical separation of sediment in the surf zone (e.g., gravels to fine sand reworked and part of pebbles not transported). However, given the nature and composition of sediments comprising both the moraines and beach, shoreface, beach-ridge, and dune systems in the Saint-Pierre-et-Miquelon Archipelago, these assumptions are realistic. Sediment composition demonstrates that, similar to drumlin-dominated coasts such as 91

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archipelago, we estimated that at least 60 km2 of 4-m thick moraines would have been eroded to form the current barrier. This value is consistent for Saint-Pierre-et-Miquelon and confirms that the entire subaerial and subaqueous parts of the modern sand-and-gravel coastal system could have formed exclusively from glacigenic sediments (moraine/till). Such insights are crucial for coastal management of this, and similar, systems, especially in the context of accelerated RSL rise and the drowning available sedimentary reservoirs in ever deeper water.

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Acknowledgments The authors would like to thank our EGIML (Etude Globale de l'Isthme de Miquelon-Langlade) project partners (le Ministère d'état de l'Outre-Mer, le Conseil Territorial de Saint-Pierre-et-Miquelon, la Direction des Territoires, de l'Alimentation et de la Mer, le Conservatoire du littoral). We are grateful to the Petit Saint-Pierre crew for their help during field campaigns. We would also like to thank Roger Etcheberry for his field assistance. References Aleman, N., Certain, R., Barusseau, J.P., Courp, T., Dia, A., 2014. Post-glacial filling of a semi-enclosed basin: the Arguin Basin (Mauritania). Mar. Geol. 349, 126–135. Allard, J., Bertin, X., Chaumillon, E., Pouget, F., 2008. Sand spit rhythmic development: a potential record of wave climate variations? Arçay Spit, western coast of France. Mar. Geol. 253, 107–131. Aubert de la Rüe, E., 1951. Recherche Géologiques et Minières aux iles de Saint-Pierre et Miquelon. Office de la recherche scientifique outre-mer. Librairie Larose, Paris, pp. 88. Ballantyne, C.K., 2002. Paraglacial geomorphology. Quat. Sci. Rev. 21, 1935–2017. Barusseau, J.P., Certain, R., Vernet, R., Saliège, J.F., 2010. Late Holocene morphodynamics in the littoral zone of the Iwik Peninsula area (Banc d'Arguin — Mauritania). Geomorphology 121, 358–369. Bell, T., Batterson, M.J., Liverman, D.G.E., Shaw, J., 2003. A new late-glacial sea-level record for St. George's Bay, Newfoundland. Can. J. Earth Sci. 40, 1053–1070. Billeaud, I., Tessier, B., Lesueur, P., 2009. Impacts of late Holocene rapid climate changes as recorded in a macrotidal coastal setting (Mont-Saint-Michel Bay, France). Geology 37, 1031–1034. Billy, J., 2014. Morphologie et Architecture d'une Barrière Composite Paraglaciaire: l'isthme de Miquelon-Langlade (NO Atlantique). Département de Sciences de la Terre et de l'Univers. Université de Perpignan Via Domitia, pp. 321. Billy, J., Robin, N., Certain, R., Hein, C., Berné, S., 2013. Barrier shoreline evolution constrained by shoreface sediment reservoir and substrate control: The MiquelonLanglade Barrier, NW Atlantic. J. Coast. Res. SI 65 (2), 2089–2094. Billy, J., Robin, N., Hein, C.J., Certain, R., FitzGerald, D.M., 2014. Internal architecture of mixed sand-and-gravel beach ridges: Miquelon-Langlade Barrier, NW Atlantic. Mar. Geol. 357, 53–71. Billy, J., Robin, N., Hein, C.J., Certain, R., FitzGerald, D.M., 2015. Insight into the late Holocene sea-level changes in the NW Atlantic from a paraglacial beach-ridge plain south of Newfoundland. Geomorphology 248, 134–146. Billy, J., Robin, N., Hein, C., FitzGerald, D., Certain, R., 2018. Dominance of inherited geologic framework on the development of coastal barrier system. In: Shim, J.-S., Chun, I., Lim, H.-S. (Eds.), Proceedings from the International Coastal Symposium (ICS) 2018 (Busan, Republic of Korea). J. Coast. Res. Special Issue No. 85, pp. xxx–xxx. Coconut Creek (Florida), ISSN 0749-0208. Boyd, R., Bowen, A.J., Hall, R.K., 1987. An evolutionary model for transgressive sedimentation on the Eastern Shore of Nova Scotia. In: FitzGerald, D.M., Rosen, P.S. (Eds.), Glaciated Coasts. Academic Press, San Diego, CA, pp. 87–114. Bristow, C.S., Jol, H.M., 2003. In: Bristow, C.S., Jol, H.M. (Eds.), Ground Penetrating Radar in Sediments. Geological Society Special Publications n 211, pp. 339. Brookes, I.A., Stevens, R.K., 1985. Radiocarbon age of rock-boring Hiatella arctica (Linné) and postglacial sea-level change at Cow Head, Newfoundland. Can. J. Earth Sci. 22, 136–140. Brookes, I.A., Scott, D.B., McAndrews, J.H., 1985. Postglacial relative sea-level change, Port au Port area, west Newfoundland. Can. J. Earth Sci. 22, 1039–1047. Buynevich, I.V., FitzGerald, D.M., Heteren, S.V., 2004. Sedimentary records of intense storms in Holocene barrier. Small 210, 135–148. Carter, R.W.G., 1988. Coastal Environments. Academic Press (617 pp). Catuneanu, O., Abreu, V., Bhattacharya, J.P., Blum, M.D., Dalrymple, R.W., Eriksson, P.G., Fielding, C.R., Fisher, W.L., Galloway, W.E., Gibling, M.R., Giles, K.A., Holbrook, J.M., Jordan, R., Kendall, C.G.S.C., Macurda, B., Martinsen, O.J., Miall, A.D., Neal, J.E., Nummedal, D., Pomar, L., Posamentier, H.W., Pratt, B.R., Sarg, J.F., Shanley, K.W., Steel, R.J., Strasser, A., Tucker, M.E., Winker, C., 2009. Towards the standardization of sequence stratigraphy. Earth Sci. Rev. 92, 1–33. Certain, R., Tessier, B., Barusseau, J.P., Courp, T., Pauc, H., 2005. Sedimentary balance and sand stock availability along a littoral system. The case of the western Gulf of Lions littoral prism (France) investigated by very high resolution seismic. Mar. Pet. Geol. 22, 889–900.

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