Impact of Relative Sea-Level Changes Since the Last Deglaciation on the Formation of a Composite Paraglacial Barrier

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

Article in Marine Geology · April 2018 DOI: 10.1016/j.margeo.2018.03.009

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

ARTICLE INFO ABSTRACT

Editor: E. Anthony Comprehensive onshore-offshore surficial and sub-surface mapping of a composite barrier (combination of Keywords: prograded, aggraded, and/or transgressive segments) have provided a better understanding of the (i) mechan- Seismic profiling isms responsible for the formation and development of coastal barrier systems, (ii) relationships and interactions Ground-penetrating radar data among individual parts of those systems, and (ii) the overall stratigraphic framework of subaerial and sub- Paraglacial barrier aqueous segments of the barriers. Here, we investigate these facets of barrier evolution through integration of Gulf of Saint Lawrence stratigraphic data from subaqueous high-resolution seismic and subaerial ground-penetrating radar, sedi- Stratigraphic framework mentology (terrestrial cores and seafloor surface samples), and merged topographic and bathymetric mapping of Regressive-transgressive sequences 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.

1. Introduction accommodation creation, respectively (e.g. Galloway and Hobday, 1983; Davis and FitzGerald, 2004; Costas and FitzGerald, 2011; Otvos Coastal systems—consisting of emerged landforms such as barriers, and Carter, 2013): (i) regressive (prograded) barriers defined by a spits, beach ridges, beaches, dunes etc., in addition to their seaward seaward extension of the barrier either under normal (sediment supply terminations (shoreface deposits)—evolved in response to changes in dominated; (e.g. Rodriguez and Meyer, 2006; Barusseau et al., 2010)) the ratio between the rate of sediment accumulation and the rate of or forced (RSL fall dominated; e.g. Tamura et al. (2008); Sanjaume and accommodation creation (space available for sediments to fill, a func- Tolgensbakk (2009)) regression as beach-ridge foredune-ridge, strand- tion of relative sea-level (RSL) variations and shoreface morphology) plain, chenier, or prograded barrier systems; (ii) transgressive (retro- (e.g. Carter, 1988; Roy et al., 1994; Short, 1999; Timmons et al., 2010). graded) barriers defined by landward shoreline translation in response Three main morpho-sequences are identified when the rate of sediment to either erosion or overwash-induced rollover (e.g. Mellett et al., 2012; accumulation exceeds, is less than, and equals than rate of 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. J. Billy et al. Marine Geology 400 (2018) 76–93

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.)

(aggraded) barriers characterized by vertical growth through the histories of post-glacial sea-level change, paraglacial coastal systems stacking of sand layers (e.g. Simms et al., 2006). However, single may record in their subaerial and subaqueous sedimentary archives coastal systems are commonly much more complex than these end evidence of transitions between shoreline transgression and regression. members and often defined by a combination of prograded, aggraded, Examples of such paraglacial sedimentary archives include the Gulf of transgressive segments; these are termed composite or hybrid barriers Saint Lawrence (Boyd et al., 1987; Orford et al., 1991a; Forbes and (Otvos, 1982, 2012). Investigations of such systems contribute to im- Syvitski, 1994), the Gulf of Maine (van Heteren et al., 1998; Hein, proved knowledge of the factors influencing coastal-system develop- Fitzgerald, Buynevich, et al., 2014), the Baltic coast (Hoffmann et al., ment, variations through time, and interactions or connections with 2005; Harff and Meyer, 2011), and the coast of Alaska (Hayes and pre-existing coastal features (underlying geology and antecedent to- Ruby, 1994). pography). Here, we present the results of continuous onshore-offshore geo- Stratigraphy holds the potential to record the history of coastal physical mapping of a paraglacial, composite barrier located south of system formation and evolution, as well as the sedimentation processes, Newfoundland and develop an evolutionary model of its formation time-varying sediment delivery rates (Vespremeanu-Stroe et al., 2016), following the last deglaciation. The morphology and internal archi- role of inherited topography (FitzGerald and Heteren, 1999), and tecture of the subaerial segments of the barrier are derived from pub- changes in RSL (Costas and FitzGerald, 2011; Billy et al., 2015; Costas lished data (Billy et al., 2014, 2015) and are supplemented with new et al., 2016), wave energy (Allard et al., 2008; Hein et al., 2013), tidal seafloor sediment samples and seismic surveys collected along the regimes (Hayes, 1979; Chaumillon et al., 2013), and climate (Billeaud barrier's two open coasts. The proposed stages of barrier formation et al., 2009; Costas and FitzGerald, 2011). Only few studies (Timmons correlate with the overall stratigraphic framework available from this et al., 2010; Oliver et al., 2017) utilize both onshore and offshore da- combined onshore-offshore mapping and are placed in the context of tasets to explore the stratigraphic record as preserved on both sides of the paraglacial environment in which the system developed. Finally, the modern shoreline. However, the combination of both shallow the importance of this kind of comprehensive study is discussed and seismic (offshore; e.g. Certain et al. (2005); Mellett et al. (2012); Billy results are compared to existing stratigraphic models and paraglacial et al. (2013); Aleman et al. (2014)) and ground-penetrating radar barrier evolutionary models. Moreover, this study provides an example (onshore; Rodriguez and Meyer (2006); Hede et al. (2013); Lima et al. of the utility of developing onshore-offshore geophysical datasets to: (i) (2013)) technologies presents a powerful tool with which to investigate investigate barrier stratigraphic complexity, (ii) highlight correlations the evolutionary history of, in particular, composite barrier systems. between sedimentary units and contacts to better establish the devel- Paraglacial coastal systems are those whose characteristics reflect: opmental framework of the barrier, and (iii) to estimate barrier sedi- (i) glacio-isostatic and eustatic sea-level changes, (ii) reworking of ment volumes and quantity of sediment deposits needed to form this generally glacigenic sediment representing a range of grain sizes, and coastal feature. (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 J. Billy et al. Marine Geology 400 (2018) 76–93

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 Laur- entide 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 −3m 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 The emergent part of Miquelon-Langlade Barrier (Figs. 1, 3)is (Fig. 1). This paraglacial system formed from the reworking of glacial dominated by a 12-km long, 50–2500-m wide, Y-shaped isthmus with a deposits by waves, tides, and wind during subsequent changes in RSL dominant north-south orientation (Billy et al., 2014). Longshore cur- – (Billy et al., 2014, 2015). Following ice retreat (13 12 ka BP, Fig. 2) rents drive convergence of sediment transport both towards the narrow and crust variation in Canada, mainly due to postglacial rebound (Dyke central region, along the west side of the barrier, and towards the inlet and Peltier, 2000; Tarasov and Peltier, 2004), these RSL variations of the Grand Barachois lagoon at the northeast (Robin, 2007; see lo- southwest Newfoundland encompassed several periods of change in- cation Fig. 1). The northwestern section of the barrier (Fig. 3A) consists itially emergent then subsiding (type B sea-level curve; Quinlan and of a narrow (50–200 m wide) and high (up to 15–20 m) dune system, Beaumont (1981)). Four distinct periods are distinguished (e.g., Daly, called ‘Les Buttereaux’. This area formed during the early development 2002; Shaw et al., 2006): 1) a period of rapid crustal rebound and of the subaerial barrier (Billy, 2014), and is termed the “proto-barrier” − – falling RSL (+40 to 25 m) at 13.7 10.5 ka; 2) a period of relative sea- in this study. The northeastern section is composed of a sandy, main- − level stability at ca. 25 m between 10.5 and 8.0 ka; 3) a period of RSL land-attached recurved spit that terminates at its southern end at the − − – rise between 25 and 3 m at 8.0 3.0 ka; and 4) slow RSL rise during active Goulet tidal inlet (Fig. 3B). This inlet connects the coastal ocean the last 3000 years (Brookes and Stevens, 1985; Forbes et al., 1993; to the 12 km2 Grand Barachois Lagoon. The inlet is characterized by a Forbes and Syvitski, 1994; Shaw et al., 1997; Bell et al., 2003; Daly well-developed flood-tidal delta in the lagoon and a smaller ebb-tidal et al., 2007). During the period of ice retreat, a large volume of sedi- delta on the ocean side. South of the lagoon, the central and southern ment (till and outwash) was deposited on the shelf and coastal zone sections of the barrier consist of a well-developed sand-and-gravel (Syvitski, 1989; Forbes et al., 1993; Billy et al., 2014). The paraglacial beach-ridge plain and are fronted by a modern foredune ridge (Fig. 3C, coastal systems of southwest Newfoundland share similar regional D). The historical map of Fortin (1782) shows an active tidal inlet stratigraphy. Basal units are composed of glacial deposits associated connecting the eastern and western coasts of the barrier. It closed by the fl with ice-sheet contact, include (1) till and (2) glacio- uvial deposits, late 18th century (Aubert de la Rüe, 1951), due to elongation and – and by (3) thick (20 45 m) glaciomarine mud (Syvitski, 1989; Forbes convergence of the northern and southern sections of the isthmus et al., 1993; Forbes and Syvitski, 1994; Bell et al., 2003). Post-glacial (Robin, 2007). deposits are characterized by mud and deltaic deposits (sand to gravel, Previous work has primarily concentrated on the emergent part of fl ff at tops and steep foreset beds), which prograded o shore during the the barrier (Robin, 2007; Billy, 2014), notably the 5 km2 central beach- period of falling RSL (falling-state systems tract; FSST) associated with ridge plain, which has been mapped and studied using a combination of post-glacial isostatic rebound (Shaw and Forbes, 1992). The thickness ground-penetrating radar (Mala ProEx GPR system with a 100, 250 and of each of these units is spatially variable, but the presence of each is 500 MHz antennae coupled with a survey wheel and a Magellan-Ash- generally a consistent feature along the southwest coast of Newfound- tech RTK-GPS), optically stimulated luminescence (OSL) dating, and land. At the Saint-Pierre-et-Miquelon archipelago, moraines (generally sediment cores and surface samples (Billy et al., 2014, 2015). The plain – 3 5 m thick; Figs. 1 and 3E; Robin (2007); Billy et al. (2015)) border is formed from two distinct, wave-constructed beach and dune-ridge Miquelon and Langlade Islands (forms locally a land relief up to 10 m systems which have built along a bi-directionally prograding coast; four thick along the west coast of Langlade Island). They are composed of a ridges sets with concave shape and two ridges sets with fan-shaped fi combination of very ne (< 0.05 mm) to coarse (pebbles up to tens of define the eastward- and westward-prograding systems, respectively – – centimeters) sediments (80 60% sand; and 20 40% gravel (Billy et al., 2014). Each ridge set marks a successive phase of plain – (0.8 2.5 mm); Billy (2014)). Erosion and reworking of these sediments progradation, and records local changes in sediment delivery and re- by coastal processes led to the development of the modern sand-and- gional variations in RSL rise rates over the last 3000 years. Progradation gravel barriers, beaches, spits, beach-ridge systems, and shoreface de- of the eastward-building system occurred at a rate of 65 ± 45 years per posits that comprise and surround the Saint-Pierre-et-Miquelon Archi- ridge (75 ± 50 m per 100 years) for the two older ridge sets (21 pelago (Robin, 2007; Billy, 2014). The wave regime of the region is ridges), and decreased over the past ca. 600 years to 100 ± 35 years dominated at present by regular, high-energy Atlantic swells from the per ridge (30 ± 10 m of progradation per 100 years) (Billy et al., west to south; 10% of waves are > 5 m in height and 3% are > 7 m in 2015). height (maximum recorded height is 8.4 m). The western and south- The internal architecture of the subaerial isthmus system is de- western shores of the archipelago receive the dominant wave energy, scribed in detail by Billy et al. (2014). Beach ridges overlie a deeper

78 J. Billy et al. Marine Geology 400 (2018) 76–93

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.

Holocene sedimentary unit (BU) that disrupted incoming wave energy both side of the barrier. and altered the planform morphology of the prograding plain. In- dividual dune/beach ridges commonly display a well-developed (3.0–7.5 m thick) sigmoidal internal configuration with seaward-dip- 3.1. Very high resolution seismic data ping internal reflections (Table 1). At a finer scale, lens-shaped forms and overtopping features can be identified at the top of the wave-built Architecture of the shoreface was investigated from very high-re- ridges (Billy et al., 2014). Ridges formed along the most exposed solution (VHR) seismic data and was acquired using a 4–12 kHz bi- westward-facing coast have internal reflectors that are 1–2° steeper frequency INNOMAR seismic sounder, fixed at 6 kHz. This sounder is than those formed along the more sheltered eastward-facing coast. specifically adapted to shallow-water environments (e.g., Aleman et al., Wave-built ridges are overlain by peat or aeolian sand deposits, which 2014; Nutz et al., 2014), with a maximum theoretical resolution of can form dune ridges. The topography of the beach-ridge plain, and the 5 cm. Approximately 330 km of seismic-reflection data (Fig. 1), 115 km interface between wave-built facies and overlying aeolian deposits in- and 215 km along the west and east sides of the barrier respectively, crease in elevation by ca. 2.4 m in a seaward direction (towards the were acquired during three field campaigns in 2011 and 2012 aboard Le modern shoreline). Mapping of the elevation of this interface, combined petit Saint-Pierre (DTAM's ship: Direction des territoires de l'alimentation et with a series of 13 OSL ages from across the plain, have been used to de la mer). Along the west side of the barrier, surveys extended offshore develop the first late Holocene (last 2400 years) sea-level curve for the to the limit of sedimentary cover (up to 2.5 km of the coast and 25 m Saint-Pierre-et-Miquelon Archipelago (Fig. 2)(Billy et al., 2015). De- water depth). Along the east side of the barrier, surveys extended off- spite these advances, previous studies have been limited to the onshore shore up to 12 km from the coast, to the flank of a deep (bottom 110 m Miquelon-Langlade system, and specifically the beach-ridge plain. below mean sea level) channel which separates the archipelago from Here, we extend this database to the nearshore and shallow shelf, Newfoundland. Seismic profiles were processed (tide correction, wave mapping sedimentary contacts, and placing development of various filter, and amplitude correction) using ISE V.2.92 software and the key segments of this complex system into context of past relative sea-level contacts were digitized (seafloor, contact between unconsolidated se- changes and underlying geology. diment 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 3. Methods depth of penetration in unconsolidated sediment was 10 m below the sea floor. Seismic data interpretation (reflection geometries and unit Investigation of the subaqueous part of Miquelon-Langlade barrier configuration) follows the principles of seismic stratigraphy, allowing includes seismic data and seafloor sediment samples collected along the for the identification of seismic units (after Mitchum et al., 1977), and

79 J. Billy et al. Marine Geology 400 (2018) 76–93

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. 8000 10,500 – – 5000 (?) 0 – – 3.2. Seaﬂoor sediment samples 3000 ? ? Chronological framework (estimated in BP) – –

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)

20 m 10,500 and 11 additional samples were collected during seismic surveys in 10 m 8000 3 m 5000 (?) - 3000 − 25 m 13,700 − −

− 2012 and 2013. Sixty-four and 159 surface samples were collected

25 to along the west and east sides of the barrier, respectively (Fig. 1). All 3to0m 20 to 10 to − 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 Rate: 1.3 mm/yrs Stable: change trend (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) ooding surface; change in sediment supply fl 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 HST: High stand deposits (emerged M-L Barrier) Rise: LST: Low stand periodregressive Erosional surface surface; maximum supply and slow RSL Rise rate and slow RSL Rise rate FSST: Regressive sequence Fall: +40 to 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 mor- : irregular: Top Polygenic, of bedrock (basin, outcrop) : Sub-horizontal Transgressive ravinement surface; change in sediment : Sub-horizontal Maximum

0 1 2 3 phology with numerous outcrops (e.g., the 10 m high peak imaged on shore of the Miquelon-Langlade barrier, and interpretations, estimated chronology, and associated changes in relative sea level during the D D sub-horizontal Discontinuity Interpretation Associated RSL depth (m msl) and D D

ff Fig. 4). Moreover, photographs of the seafloor (obtained in 2011 by IFREMER, Goulletquer et al. (2011)) confirm this unit is consolidated ed o fi 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), ) 3 and (4) the uppermost surficial unit, UU. On the west side of the m 6 isthmus, only a single seismic unit, corresponding with unit UU, is 15 (10 40.5 38 TST: Transgressive sequence Rise: – 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. ections (Slope: ections (Slope:

ﬂ 4.1. DU: deep unit ﬂ

The basal discontinuity (D ) corresponds to the top of the bedrock.

ections Estimated volume 0 ﬂ It contains large spatial irregularities and is characterized by both a

0.5%) basin, with a central depression estimated to be at least 25 m below ections (Slope: 0.7%) – ﬂ ﬂ ~0.7%) North-eastward re Transparent 0.3 –– re modern sea level (Fig. 5), and sea oor 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

est towards the Green Islands Channel, where it reaches depths of up to East 150 m. The upper boundary of DU is a smooth erosional surface (D1). D1 Seismic units Facies/re UU w LU Transparent 40 TST: Transgressive sequence Rise: MUUU South-eastward re Bedrock DU Transparent or some eastward Table 1 Synthesis of seismic units (discontinuities, facies, and volumes) identi period of formation of each unit. is interpreted to represent the long term (> 2500 years) wave

80 .Blye al. et Billy J. 81

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 Marine Geology400(2018)76–93 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.) J. Billy et al. Marine Geology 400 (2018) 76–93

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.)

reworking of DU during sea-level lowstand between 10.5 and 8.0 ka center of the barrier (Figs. 5A, 7). (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 4.4. UU: upper unit 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). 4.4.1. Unit UU along the eastern shore: UUEast This unit was not imaged on the west side of the archipelago, but may Seismic unit UUEast (upper unit) overlays MU and contains deposits nonetheless be present, especially to the west, proximal to the Hermitage which are > 4 m thick (Figs. 4, 5) along the coast, and a well-defined 2 Channel (ca. 20 km westward). lobate shape towards its center (Fig. 6D). UU East is a 17.5 km area and contains an estimated 40.5 × 106 m3 of sediment. On distal parts of the 4.2. LU: lower unit 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

Seismic unit LU (lower unit) is acoustically transparent and overlies high-amplitude internal reflections, across UUEast and the top of MU either bedrock or unit DU. LU is characterized by a gently eastward- (Fig. 5A). Proximal to the shore, cut-and-fill structures of 1.0–1.5 m dipping, fan-shaped, 24 km2 deposit. It reaches up to 4 m in thickness thick are observed as high amplitude reflections visible up to 1.0 km proximal the shore and thins seaward, extending to the east into −20 m seaward (Fig. 7). More distally (1.5 km from the coast), two incisions (A water depth (Figs. 6B, 4). Surface exposures of unit LU are composed of and B) are recorded and identified by 5–6 m thick cut-and-fill structures fine sand (mode of surficial sediment samples: 0.125 mm) and its vo- (Figs. 5A, 7). Together, these structures are interpreted as paleo-chan- 6 3 lume is estimated at 40 × 10 m . The upper boundary of LU, D2, dips nels associated with a northward-migrating channel (Fig. 7). gently and smoothly seaward at a slope of ca. 0.1°.

4.3. MU: middle unit 4.4.2. Unit UU along the western shore: UUWest Unit UUWest overlies bedrock along its entire length (Fig. 4). It is Seismic unit MU (middle unit) overlies LU. It is characterized by acoustically transparent (Fig. 4) and 1-2 m thick (Billy et al., 2013). The acoustic transparency close to the shore (Figs. 4, 5A) and southeast- northern part of UUWest contains high-relief (2 m high) rocky outcrops dipping internal reflections throughout distal sections. Along north- within 100–200 m of the shore. Here, the sand reservoir extends to south profiles (Fig. 5B) this unit is marked by high-amplitude reflec- 1.0–1.5 km offshore (to the 13 m depth contour) and a shoreface (upper tions dipping 0.2–0.3°, reflecting paleo sediment transport directions contact of the unit) slope 0.6°. The central and south parts of UUWest and progradation. MU covers a lobate area of 22 km2, with a NW-SE extend to 2.0–2.5 km offshore (corresponding to a maximum depth orientation and direction of progradation. It reaches up to 3 m in contour of −17 m) and a shoreface slope of 0.35–0.40°. The overall thickness and thins seaward (Figs. 4, 5 and 6C). MU is composed of fine volume of this unit is estimated at 15 × 106 m3 and its surface exposure sand (grain size mode at 0.125 mm; Fig.1) and its volume is estimated is composed of fine to very fine sand deposits (mode: 0.125 mm). 6 3 at 38 × 10 m . The upper boundary of MU, D3, is smooth, sub-horizontal, and contains local cut-and-fill (erosional) structures near the

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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) 0–4m 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. 5.1. Stratigraphic sequence 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 Stratigraphic units of the Miquelon-Langlade barrier complex can be 3000 years (~1 mm/yr). Associated HST deposits include unit UU and placed in the context of relative sea-level changes and resulting periods the entirety Miquelon-Langlade current Isthmus with its beach/dune- of shoreline regression and transgression, encompassing the four pri- ridge plain, spit, lagoon, tidal deltas, and dune system (Fig. 10). Marine mary sequence stratigraphic system tracts: Falling Stage System Tract sedimentary units (DU, LU, MU and UU) are all characterized by un- (FSST), Lowstand System Tract (LST), Transgressive System Tract (TST) imodal fine sand (mode at 0.125 mm; Fig.1), whereas the barrier is and Highstand System Tract (HST) (e.g. Mitchum et al., 1977; Haq composed of a mixture of sediment ranging from fine sand (mode at et al., 1987; Posamentier et al., 1988; Van Wagoner et al., 1988) 0.2 mm) to cobble-sized gravel (Billy et al., 2014). (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 5.2. Early developmental history – LST and TST: the main role of the 10.5 ka (Fig. 8). The LST commonly forms during periods of near- flooding history minimum sea level, which occurred here between 10.5 and 8.0 ka. This stage is characterized by an erosional surface: the unconformity D , 1 Following retreat of the Laurentide ice sheet at the end of the last interpreted as a maximum regressive surface (e.g., Catuneanu et al., glacial period (13.7 ka), rapid crustal rebound and relative sea-level fall 2009). Following this lowstand, the TST corresponds to a relatively (FSST) resulted in subaerial emergence of bedrock between Miquelon rapid rise in RSL (~4.4 mm/yr) from −25 to −3 m between 8.0 and and Langlade islands. At lowstand the shoreline was positioned be- 3.0 ka. Units LU and MU are both associated with the transgressive tween 3 and 7 km seaward of its modern position. At this time, sequence of TST, as well as a proto-barrier attached to the south-west of Miquelon and Langlade islands were connected by a bedrock ridge Miquelon Island (Fig. 9). Unconformity D , which is the boundary be- 2 (Fig. 8) forming a single island with an approximate area of 470 km2. tween LU and MU, is interpreted as a transgressive ravinement surface. The east and west shores of the island evolved independently,

83 J. Billy et al. Marine Geology 400 (2018) 76–93

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.) influenced by local bedrock morphology and local wave propagation modern sea level and submergence of the bedrock ridge that had con- patterns (Billy et al., 2018). The marine transgression (TST) com- nected the east and west coasts. This eventually led to the formation of menced after 8.0 ka and resulted in wide scale flooding of lower ele- the inlet at southern end of the barrier complex (Fig. 9). This new vation regions of the archipelago (Fig. 9A). By the end of this period, configuration significantly influenced local hydrodynamics, providing the north-south-oriented shallow bedrock ridge still connected the two for wave propagation across the shallow bedrock platform and sedi- islands separating the east and west coasts. It is not possible to distin- ment exchange between the two sides of the island. Flooding of the guish a specific seismic unit associated with this period of rapid nearshore of Miquelon Island allowed for wave erosion of both sub- transgression along the west coast of the island. Sediments were likely aqueous bottom moraines and subaerial moraines which remain par- driven onshore during the transgression, either as sand sheets or per- tially exposed around the island periphery (Fig. 3E). This process re- haps landward-migrating sand bars; however, no remnants of any such leased sediment, which was transported to the south, leading to the system are identified along the west coast of the isthmus. Along the east formation of three distinct sedimentary features. Along southwest Mi- coast, fine sand was deposited during this period of rapid transgression quelon Island, a spit formed and prograded to the southeast, eventually atop either DU or exposed bedrock, thereby forming unit LU (Figs. 4, 5 forming a straight, 3.8-km long barrier (Les Buttereaux; Figs. 3A, 9B). and 9A). Absence of internal reflections within this unit prevents in- This feature was later covered by thick (up to 15–20 m high) parabolic terpretation of paleo transport directions and identification of specific dunes. Elsewhere, sandy sediments were pinned to shallow rocky out- morphologic features. RSL rise rates were higher than those observed crops in central regions of the isthmus, forming the basal unit (BU) at elsewhere associated with the formation of subaerial barrier islands 3–6 m below modern sea level, as imaged in GPR profiles of Billy et al. (e.g., Timmons et al., 2010; Hein, FitzGerald, Thadeu de Menezes, et al., (2014). Finally, along the east coast, fine sand was deposited above LU, 2014), and we find here no evidence of any such barrier islands or forming unit MU (Figs. 5, 9). The lobate shape and orientation of in- associated backbarrier lagoon or marsh deposits from this time. How- ternal reflections within the distal part of MU indicate southeast pro- ever, remnants of any such features may simply have been removed by gradation of this system (Figs. 5, 7), reflecting the importance of wave waves and tidal currents as the transgression proceeded. By the time energy and sediment transport from the west (i.e., through the inlet RSL rise slowed at ~5 ka, unit LU had been fully reworked on the separating Miquelon and Langlade islands) during this period. All such shallow shelf into a large sandy platform situated beneath several me- features (spit, BU, and MU) developed synchronously, in concordance ters of water. with local sediment transport patterns, and the deposition of each may Continued transgression between 5.0 and 3.0 ka led to progressive have influenced sediment reworking associated with the development flooding of the exposed platform located between 10 and 3 m below of the others.

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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 The shift from TST to HST is marked by a deceleration in relative patterns, resulting in complex curvature of the beach ridges (Billy et al., sea-level rise and attendant slowing of marine transgression and tran- 2014). sition to a period of broad progradation and aggradation (Fig. 2). This At this same time, the northern and southern sections of the barrier same shift has been recognized to result in the initiation and develop- grew independently and converge towards one another, constricting the ment of coastal barrier systems throughout the world (e.g. Davis and central channel and forming an inlet. Wave-driven, tidally influenced FitzGerald (2004); Timmons et al. (2010); Hein, FitzGerald, Thadeu de west-to-east sediment transport during this period led to the develop- Menezes, et al. (2014)). At Miquelon-Langlade, a decrease in the rate of ment of thick (up to 3 m thick) lobate-shaped deposits of unit UU along RSL rise from +4.4 mm/yr to +1.3 mm/yr (Billy et al., 2015), coupled the east side of the inlet (Fig. 10A). The location of this central inlet is with sufficient sediment supply from erosion of proximal moraine de- observed in seismic data along the east coast (Figs. 5, 7). Proximal to posits, is key to the transition from TST to HST. Here, sediment accu- this inlet (up to 700 m of the present coast), shallow incisions mulation outpaced creation of accommodation by RSL rise by ca. 3 ka, (1.0–1.5 m depth) into unit UU are observed (seismic profiles CC′ and forcing the system into a period of progradation and subsequent de- DD′). From 700 to 1500 m from the present coast, two major incisions velopment of the complex barrier/lagoon/dune/beach-ridge-plain are prominent in unit UU extending into the upper section of unit MU system. At the northeast corner of the isthmus, a small set of beach (seismic profiles AA′,EE′ and FF′; Figs. 5, 7). These have a distinct ridges grew from Miquelon Island. Given their morphology and low northeast orientation, and are ~250 m wide and up to 5 m deep. These topography (swales largely flooded at present; Fig. 3B), these features structures are interpreted as channel cut-and-fills associated with the must have been developed early during HST, probably coincident with former central tidal inlet. It is not clear if two channels existed coin- slowing RSL rise. Connected to this feature, a mainland-attached re- cident with one another, or if one closed and another formed during a curved spit elongated to the south, produced by the southerly longshore subsequent breaching event. Northward-dipping reflections within the transport system. A series of recurved ridges illustrate successive channel-fill deposits likely indicate northward migration and filling as a southerly progradation of the spit, and likely attendant southerly mi- result of convergence of sediment transport (Fig. 10B). Deposits of unit gration of Goulet Inlet (Figs. 3, 10). UUwest show no such evidence of former channels or inlet closure. Ra- During this period, the remainder of the isthmus was characterized ther, this unit is characterized by a 1-m thick homogenous fine sand by eastward and westward beach-ridge progradation along two open- sitting atop bedrock (Fig. 4). The relative thickness of unit UUeast de- ocean coasts. It is estimated that single ridges took between 65 and posits, and their complex and well-preserved internal geometry as

110 years to form (detailed in Billy et al. (2015)). The complex un- compared with unit UUwest likely reﬂects dominant easterly transport of derlying topography associated with the proto-barrier attached to the sediment through the inlet and a higher degree of reworking on the southwest side of Miquelon Island (TST, Fig. 9) and the central inter- western side of the isthmus, both in response to the dominant westerly tidal shoals (basal radar unit, BU, Billy et al. (2018)) provided a plat- wave climate. form upon which barrier sediments accumulated (Fig. 10). In the east-

85 J. Billy et al. Marine Geology 400 (2018) 76–93

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 hydro- 6.1. Post-glacial deposit at regional scale dynamic sorting and the seaward movement of finer sediments, deposited overlying deeper bedrock to form an easterward- dipping unit Despite its proximity to Newfoundland (tens of km), the 140-m deep along the west side of the island (DU), towards the Green Islands Channel channels surrounding the Saint-Pierre-et-Miquelon archipelago likely (Fig. 4). Moreover, the shallow (or surficially exposed) bedrock of Mi- limited the influence of deglaciation processes still acting in quelon, Langlade and Saint-Pierre Islands and the absence of an ice- Newfoundland during the post-glacial period (up to 8.0 ka; Shaw et al., margin stabilization phase linked to ice on the mainland do not allow 2006). As such, glacial and post-glacial sedimentary deposits at Saint- for the formation of thick delta deposits, such as those found along the Pierre-et-Miquelon do not conform to the idealized post-glacial sedi- coast of Newfoundland. Thus, the basement topography responsible for mentary sequence common across Newfoundland. Indeed, the archi- the presence of the archipelago separate from Newfoundland also pelago is isolated from a large continental landmass, remote from played a key role in the sediment depositional processes that explain massive continental sources and therefore, they are less likely to con- both the early stratigraphic sequence and the clear differences in de- tain glaciomarine clay and outwash plains. For example, neither glacio- posits with those found on the proximal Newfoundland mainland, fluvial deposits (could be related to a stabilization stage of the ice which evolved under similar regional conditions. 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 6.2. Sea-level lowstand at Miquelon-Langlade post-glacial deposits here (FSST deposits) appear to directly overlie bedrock/boulders, are not well developed, and – at least in seafloor Post-glacial lowstand elevations and shoreline features can be in- surface outcrops – are characterized only by fine sand (DU). Sediment vestigated through offshore geophysical data. For example, at Port au delivered from rivers or eroded in situ from earlier unconsolidated Port Bay and Saint George's Bay (Fig. 11B), the top of ‘Gilbert’-type deposits (e.g., ice-marginal deposits, fjord-mouth moraines, imaged in deltaic deposits is characterized by a well-defined erosional terrace Shaw (2003); Shaw et al. (2006); Shaw et al. (2009)), common along interpreted as the post-glacial lowstand shoreline (Forbes et al., 1993; the paraglacial coasts of Newfoundland, are limited or absent in Mi- Shaw and Courtney, 1997). The lowstand elevation at White Bear Bay quelon-Langlade. Here, sediment is sourced primarily from erosion of 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.,

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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.)

1993; Bell et al., 2003). At Deadman's bay (northeast Newfoundland; Shaw and Forbes (1995); Daly (2002); Shaw et al. (2002)), we observed Fig. 11B) the lowstand shoreline is defined by wave-cut terraces and is that lowstand values estimated around Saint-Pierre-et-Miquelon are estimated to have been between −17 and − 21 m below present sea spatially variable: in a radius of 100 km around the archipelago, low- level at 9.6 ka (Shaw and Edwardson, 1994). stand values are estimated from −15 m in the northeast up to −30 m in From the synthetic isobase maps at the time of the regional low- the northwest (Fig. 11B). Due to its location, Saint-Pierre-et-Miquelon stand (9 ka) in south Newfoundland (Fig. 11A, modified from Shaw Archipelago presents an opportunity to further explore spatial diversity et al. (2002); and Fig. 11B, refined and based on Liverman (1994); in shoreline depths at the post-glacial lowstand.

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)).

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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 sub- horizontal 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-Pierre- et-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 Newfound- land, 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 Fig. 12. Evolution of the paraglacial Miquelon-Langlade composite barrier framework provided by Orford et al. (1991b). This model proposes the placed in the context of cyclic transgressive paraglacial barrier formation and formation of headland-attached barriers which are subsequently brea- destruction of Forbes (2011) and Forbes et al. (1995). Areas in orange, green, ched, broken up, and re-formed in a landward position as transgression and red stripping correspond to the morphologic evolution of the NW spit Les proceeds (Boyd et al., 1987; Forbes, 2011). This is envisaged as a re- Buttereaux from 8 to 3 ka, the central beach-ridge plain from 3 to 0 ka, and the petitive cycle, with landward barrier reestablishment following a phase NE spit from 3 to 0 ka, respectively. (For interpretation of the references to of barrier destruction (Fig. 12). Individual barriers are initiated through colour in this figure legend, the reader is referred to the web version of this development of swash- or - drift aligned embryos (D0 and S0; Fig. 12), article.) 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 S1-S2) in the last 3000 years (area in green, Fig. 12). This region (e.g., waves, tides, rate of relative sea-level rise, etc.), the potential for shows clear evidence of initiation (S0), development, seaward reincorporation of sediment into new embryo structures, and internal progradation (S1), and flooding of lower and older beach-ridge morphologic feedback endemic to the system (Forbes et al., 1995). swales along the lagoon side of each ridge (S2) (Billy et al., 2014, Development of the Miquelon-Langlade Barrier compares well with 2015). this earlier model (Fig. 12): (4) The relationship of the development of the Langlade attached barrier (southern part; Fig. 3D) with these earlier models is less (1) Evolution of the northwest spit (Les Buttereaux) from 8 to 3 ka (area straightforward, but is likely related to formation of the drift- in orange Fig. 12), presents an example of the evolution of drift- aligned barriers (D0-D1) up to the complete closure of the isthmus aligned barriers (D0-D1). Presently, due to the retrogradation of the by the late 18th century. coastline (Fig. 11) and erosion of aeolian dunes (Fig. 3A) this northwest sector appears to be transitioning from between stage D1 Thus, in total, the post-lowstand development of various sections of (establishment) and stage D2 (breakdown). Our data do not allow the mixed sand-and-gravel Miquelon-Langlade composite barrier can be us to explore whether previous entire cycles (D0-D1-D2) may also well placed in the framework of the Forbes et al. (1995) evolutionary have occurred. model. Given projections for global sea-level rise through 2100 (e.g., (2) At the northeast, the development of the attached recurved spit Church et al., 2013), combined with a natural depletion of morainal from 3 ka to present may be linked to the evolution of the drift- sediment sources, this paraglacial isthmus will likely move towards a aligned barrier (D0-D1, area in red stripping Fig. 12); at present, phase of breakup, flooding and deterioration, following modes of this sector seems still to be at stage D1. paraglacial evolution elsewhere, under similar conditions. (3) The central beach-ridge plain evolved as swash-aligned barrier (S0-

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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.

6.4. Stratigraphic framework diversity of a two-open-coast barrier which these forces acted (Fig. 13). These data reveal the underlying cause for heterogeneity in modern features: some areas are defined In contrast to many barrier systems in which a barrier fronts a back- primarily by prograding (regressive) systems such as the mainland-at- barrier consisting of marshes/mangroves, lagoons, and tidal flats, the tached recurved spit at the northeast of the isthmus and the prograded Miquelon-Langlade isthmus has two open coasts, in which each side of southern sector attached to Langlade Island. Others sections of the the barrier system is prograding in response to complex feedbacks be- system, such as the northwest Buttereaux spit (retrograding system in tween sedimentation and accommodation; these are themselves influ- deterioration) and the central beach-ridge plain (distinct shoreface enced by changes in sediment transport and sediment exchanges within deposits from either sides) are noticeably different than commons the system, hydrodynamic conditions and RSL over time (Billy et al., stratigraphic models (Fig. 13), as detailed in succeeding sections. 2014, 2015). The merged onshore-offshore geophysical data collected across the island, shoreface, and nearshore reveal the spatial non-uni- formity 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 J. Billy et al. Marine Geology 400 (2018) 76–93 et al., 1994; Otvos, 2012), characterized by a retrograding ocean 6.5. Insights into the system-wide evolution of the Miquelon-Langlade shoreline and overlain by aeolian sand dunes (Fig. 3A). However, un- barrier like common transgressive shorelines, the barriers here contain neither obvious washovers nor evidence of past landward rollover (migration) 6.5.1. Key components of the barrier developmental history of the barrier (Fig. 12). This likely reflects the presence of high-eleva- The combined use of seismic and GPR data from across subaqueous tion (up to 20 m) parabolic sand dunes, which are stabilized by vege- and subaerial portions of the Miquelon-Langlade barrier provides for tation and which prevent overwash; thus, here, aeolian processes do not comprehensive insight into development of the system during the influence barrier retrogradation. Moreover, on the eastern side, the period of late-Holocene RSL rise. These data permit mapping of units lagoon shoreline of the barrier is likewise undergoing erosion by storm across the modern shoreline, a critical component in the reconstruction waves, likely accelerated because of an increase of the lagoon water of the early developmental history of the barrier, during which time the level and storminess waves. Connection between shoreface and shore- attached-spit, shoals (basal radar units), and shoreface deposits (seismic line behaviors along the west coast – and the role that these play in term unit MU) developed synchronously and interdependently. Indeed, the of stability or erosion of the barrier – are highlighted by Billy et al. initiation and development of the southeasterly prograding spit at- (2013), who demonstrate that: (i) sediment supply via longshore tached to Miquelon leads to the development of the shoals that influ- transport is limited, (ii) the shoreface slope (0.6°) is already steepest of ence the direction of progradation of MU. Thus, these entities constitute the west coast, and (iii) this coast experiences the greatest exposure to different parts of a single coastal system (Fig. 10). In this way, this study high wave energy along the island. Together, these observations explain complements those of Kanbur et al. (2010); Timmons et al. (2010); the thinness and limited spatial extent of the sand reservoir on the Cooper et al. (2016); Oliver et al. (2017) among others, in illustrating adjoining shoreface (UU west; Figs. 6, 10). Hydrodynamic conditions the importance of investigating both emerged and immerged parts of associated with depletion of sand have thus resulted in a retro- coastal systems and the necessity to stratigraphically investigate both gradational shoreline. This sector of Miquelon-Langlade is therefore, the emerged barrier and its shoreface. In this manner, this study builds classified as a retrograded system experiencing active erosion and de- upon those over the last 30 years which demonstrated the use and terioration. Eventual erosion of the sand dunes will likely result in contribution of GPR technology for the description and understanding breaching of the barrier (Stage D2; Fig. 12), which was observed in of coastal systems (e.g. Jol et al., 1996; Neal and Roberts, 2000; Bristow 2012 during a winter storm (in November 2012). During this time, the and Jol, 2003; Buynevich et al., 2004; Neal, 2004; Jol, 2009; Hein et al., Buttereaux sector was segmented in two, which prompted artificial 2012). However, studies that combine both GPR and seismic methods filling. Shoreline retreat of this sector will likely occur through frontal to develop a complete view of coastal systems remain rare. Thus, we erosion of the aeolian dune, causing a decrease in their elevation, which encouraged the community to develop this dual approach where pos- is likely to promote overwash and a lagoon-ward migration of the sible. barrier (Moore et al., 2010; Duran Vinent and Moore, 2015). 6.5.2. Estimation of sediment volume and stocks required in the formation of the barrier 6.4.2. Central beach-ridge plain: bi-directional shoreface progradation and In addition to providing for a more comprehensive understanding of shoreline readjustment the role of changes in sea level, waves, tides, and underlying topo- This area of the Miquelon-Langlade isthmus is distinct in that beach- graphy in the formation of the Miquelon-Langlade barrier system, the ridge sets built from either side of a former central inlet (Fig. 10). In- combined on- and offshore geophysical mapping of the barrier allows deed, beach ridges prograding in opposite directions in the manner for estimation of sediment volumes within this coastal system, both observed here is a scenario commonly found at the downdrift end of an below modern mean sea level using GPR technology and topographic island, and have prograded independently from one to the other (e.g. mapping, and on the shoreface using HR seismic technology and asso- Clemmensen and Nielsen, 2010). At Miquelon-Langlade, development ciated bathymetry. From this, the total volume of sediment constituting of the system was likely influenced by dynamics associated with the the barrier is estimated at 235 × 106 m3 (Fig. 13B). Of this, shoreface former central inlet (e.g., sediment transport, channels, and currents), deposits correspond to 135 × 106 m3, with a large majority the stratigraphic record of which is observed in seismic data through (120 × 106 m3) of sediment found along the east coast, and only ~12% the disparity in the volume of shoreface deposits on either side of the (15 × 106 m3) along the west coast. The volume of the barrier platform inlet: UUWest is composed of thin sand cover that overlays bedrock (between −4.5 to 0 m) and lagoon deposits are estimated at 6 3 (Fig. 10), whereas UUEast is characterized by thick deposits, with a lo- 75 × 10 m . The strictly subaerial part of the barrier - that located bate shape and abundant evidence of filled channels, which together above modern sea-level, corresponding to spits, beach ridges, and overlie the unconsolidated sedimentary deposits of units MU and LU aeolian dunes – is estimated at 25 × 106 m3, which is only 10% of the (Fig. 10). Following the closure of the central inlet, the evolution of total volume of the barrier (90:10 ratio). Subaerial barrier and barrier these two coasts has occurred independently. Despite being largely platform (Fig. 13B) are together < 30% of the total volume of the protected by nearby Newfoundland, which limits fetch and therefore barrier. wave energy, and being composed of thick shoreface deposits, the This study also highlights the heterogeneity between the two sides eastern shore has experienced net erosion and a substantial shoreline of the isthmus before and after inlet closure. A key component of this readjustment in recent centuries (Robin et al., 2013; Billy, 2014). In insight was the integrated onshore/offshore stratigraphic mapping on contrast, no such process is observed along the western shore. Thus, subaerial (emerged barrier; 10% of sediment volume) and subaqueous despite their proximity and coupled evolution prior to inlet closure, the (shoreface and buried barrier platform; 90% of volume) constituents of regressive beaches on either side of the barrier have distinct synthetic the barrier system (Figs. 9, 10 and 11). Moreover, from these data, it is stratigraphic schemes (Fig. 13). Evolution of these two shorelines have possible to estimate past sediment sources, transport patterns, and been largely controlled by local stratigraphy associated with trans- transport rates responsible for the growth of the barrier system. As in gressive shoreface deposits and dynamics of the former inlet, along with many paraglacial coastal systems (especially those not associated with contemporaneous local conditions (wave patterns, hydrodynamics, major rivers), erosion of glacial deposits – and in particular moraines – etc.) specific to each coast. 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

90 J. Billy et al. Marine Geology 400 (2018) 76–93

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 west- facing 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 numer- Fig. 14. A) Sedimentary composition of moraines at Saint-Pierre-et-Miquelon ical models. Archipelago and partitioning of those deposits into sediments available for the 3. The Miquelon-Langlade composite barrier developed in a manner in development of beach ridges, aeolian sand dunes, and shoreface deposits. B) Location map of modern subaerial moraines and estimates of the required size line with that published in previous studies of paraglacial barrier and possible locations of former moraines which have been eroded to form the evolution. This study demonstrates that this system evolved under barrier system. 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 the bedrock islands of Miquelon and Langlade were substantially larger transport, respectively. immediately following ice retreat. Former moraines were likely located 4. Stratigraphic insights provided by merged onshore-offshore geo- along the seaward extent of the currently visible moraines and oriented physical data highlight connections between distinct sedimentary along the shrinkage axis (towards north-northwest) of the glacier units as emerged, shoal, and shoreface deposits, which are not tongue. The moraines present on Miquelon and Langlade islands are visible through subaerial or subaqueous mapping alone. The for- generally 3–5 m thick (Fig. 3E). Assuming a mean thickness of 4 m, we mation and developmental history of the barrier have been re- estimate that erosion of at least 60–90 km2 of moraine/till (100 to constructed through this approach. Moreover, comprehensive geo- 150% of the total barrier volume) would have been required to form physical mapping is particularly useful for studying complex and nourish the barrier. From this, we determine that the area above composite barriers, as it emphasizes different stratigraphic frame- the 20 m isobaths which these moraines could have covered was works for each part of the barrier. At Miquelon-Langlade, these in- 110–120 km2 (Fig. 14B). Given the much more expansive moraines clude: (i) regressive systems building atop thin (westward) and thick found atop Miquelon Island today, we assume that these former mor- (eastward) shoreface deposits, and (ii) a transgressive barrier un- aine deposits were primarily located around Miquelon Island. The es- dergoing deterioration, in which overwash and barrier rollover are, timated paleo-moraine volume and area are theoretical and highly at present, prevented by the presence of large aeolian parabolic sand dependent upon the exact sedimentary composition and physical se- dunes. paration of sediment in the surf zone (e.g., gravels to fine sand re- 5. Finally, comprehensive onshore-offshore geophysical mapping al- worked and part of pebbles not transported). However, given the nature lows for an estimation of the total volume of sediment constituting and composition of sediments comprising both the moraines and beach, the barrier (235 × 106 m3), which is distributed between the shoreface, beach-ridge, and dune systems in the Saint-Pierre-et-Mi- emerged barrier (10%) and shoreface deposits (90%). Given the quelon Archipelago, these assumptions are realistic. Sediment compo- likely source of these sediments (moraines; 60–80% of sand and sition demonstrates that, similar to drumlin-dominated coasts such as 20–40% of gravel (0.8–2.5 mm)) and the geometry of the

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2 archipelago, we estimated that at least 60 km of 4-m thick moraines Chaumillon, E., Féniès, H., Billy, J., Breilh, J.F., Richetti, H., 2013. Tidal and fluvial would have been eroded to form the current barrier. This value is controls on the internal architecture and sedimentary facies of a lobate estuarine tidal – fi bar (the Plassac Tidal Bar in the Gironde Estuary, France). Mar. Geol. 346, 58 72. consistent for Saint-Pierre-et-Miquelon and con rms that the entire Church, M., Ryder, J.M., 1972. Paraglacial sedimentation: consideration of fl uvial pro- subaerial and subaqueous parts of the modern sand-and-gravel cesses conditioned by glaciation. Geol. Soc. Am. Bull. 83, 3059–3072. coastal system could have formed exclusively from glacigenic sedi- Church, J.A., Clark, P.U., Cazenave, A., Gregory, J.M., Jevrejeva, S., Levermann, A., Merrifield, M.A., Milne, G.A., Nerem, R.S., Nunn, P.D., Payne, A.J., Pfeffer, W.T., ments (moraine/till). Such insights are crucial for coastal manage- Stammer, D., Unnikrishnan, A.S., 2013. In: Stocker, T.F., Qin, D., Plattner, G.-K., ment of this, and similar, systems, especially in the context of ac- Tignor, M., Allen, S.K., Boschung, J., Nauels, A., Xia, Y., Bex, V., Midgley, P.M. (Eds.), celerated RSL rise and the drowning available sedimentary Sea Level Change. Cambridge University Press, Cambridge, United Kingdom and New reservoirs in ever deeper water. York, NY, USA. Clemmensen, L.B., Nielsen, L., 2010. Internal architecture of a raised beach ridge system (Anholt, Denmark) resolved by ground-penetrating radar investigations. Sediment. Acknowledgments Geol. 223, 281–290. Cooper, J.A.G., Green, A.N., Meireles, R.P., Klein, A.H.F., Souza, J., Toldo, E.E., 2016. Sandy barrier overstepping and preservation linked to rapid sea level rise and geo- The authors would like to thank our EGIML (Etude Globale de l'Isthme logical setting. Mar. Geol. 382, 80–91. de Miquelon-Langlade) project partners (le Ministère d'état de l'Outre-Mer, Costas, S., FitzGerald, D., 2011. Sedimentary architecture of a spit-end (Salisbury Beach, le Conseil Territorial de Saint-Pierre-et-Miquelon, la Direction des Massachusetts): the imprints of sea-level rise and inlet dynamics. Mar. Geol. 284, 203–216. Territoires, de l'Alimentation et de la Mer, le Conservatoire du littoral). We Costas, S., Ferreira, Ó., Plomaritis, T.A., Leorri, E., 2016. Coastal barrier stratigraphy for are grateful to the Petit Saint-Pierre crew for their help during field Holocene high-resolution sea-level reconstruction. Sci. Rep. 6, 38726. campaigns. We would also like to thank Roger Etcheberry for his field Daly, J., 2002. Late Holocene Sea-Level Change Around Newfoundland. M.S. University of Delaware, pp. 220. assistance. Daly, J.F., Belknap, D.F., Kelley, J.T., Bell, T., 2007. Late Holocene sea-level change around Newfoundland. Can. J. Earth Sci. 44, 1453–1465. 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Embouchures Tidales de l'Archipel de St Pierre et Miquelon et de la Côte Ouest du

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