Morphology

Offshore of Seven Mile Island, the shoreface and inner shelf have alongshore-variable morphologies and slopes. Bathymetric maps reveal that the subaerial barrier island and shoreface widen at the northern and southern ends of the island (fig. 2A). The width of the barrier island and shoreface, seafloor morphology, and seismic stratigraphy were used to classify the Seven Mile Island margin into three alongshore zones: northern, central, and southern (fig. 2B). The shoreface in southern Seven Mile Island is indented by several eastward-trending depressions that are about 2 m deep (fig. 2A). South of these depressions are eastward-trending bedforms. In north-central Seven Mile Island, a narrow depression follows the northern shoal and curves landward. Backscatter intensity maps reveal that depressions in northern and southern Seven Mile Island have higher reflectance than the surrounding sediment. The highest backscatter intensities are recorded at depths of 13 m offshore of northern Seven Mile Island (fig. 2B).

This study further used bathymetric profiles taken along shore-normal chirp lines to subdivide the shoreface into upper and lower cross-shore domains (fig. 3). Upper shorefaces are the most landward portions of the shoreface and shelf, and their distinguishing feature is a steep gradient ranging from 0.67 to 0.87 degrees (°) (fig. 3). The transition between the steeper upper shoreface and gradual slopes of the lower shoreface (0.21 to 0.24°) occurs at depths greater than (>) 5 mbsl in central and southern Seven Mile Island (fig. 3). The bathymetric profiles in southern Seven Mile Island show the highest variability alongshore (high standard deviation; fig. 3C), which coincides with the Hereford Inlet ebb-tidal delta and bedforms. The bathymetric profiles in northern Seven Mile Island have a steep upper shoreface until depths of 5 mbsl, a flat middle shoreface at depths between 5 and 5.3 mbsl, and a steep convex-upward lower shoreface that dips at 0.5° to depths of 13.5 mbsl (figs. 3A and D).

Seismic Stratigraphy

Three stratigraphic units were identified in all seismic profiles (figs. 4–7), and one additional unit was observed only offshore of northern Seven Mile Island (fig. 4). Units are separated by regional unconformities that represent vertical and horizontal shifts in acoustic character, and the unconformity may appear as a reflector with medium to high acoustic amplitudes (Mitchum and others, 1977). The most common type of regional unconformity observed in the study area is a transgressive unconformity, which dips gently seaward and displays low relief. In some areas, subaerial unconformities were observed, which are undulatory surfaces that display greater relief and commonly truncate reflections in underlying units (Mitchum and others, 1977). Within units, some internal reflections were interpreted as paleochannel bases if they satisfied these criteria: (1) internal reflections are concave-upward or undulatory, (2) internal reflections display relief from <1 to 5 m. Additionally, some of the reflections that were interpreted as paleochannel bases were undulatory and truncated underlying flat-lying reflections (figs. 5B and 6A).

The basal stratigraphic unit, unit A, is mostly acoustically transparent with some concave-upward or undulatory discontinuous internal reflections that are truncated at the top of the unit. The lower boundary of unit A could not be detected because of a lack of penetration at depth by the chirp system. Within unit A, some internal reflectors were interpreted as paleochannel bases because they are concave-upward and paleochannel heights range from <1 to 5 m, where channel height is defined as the vertical distance between the channel top and thalweg.

The paleochannels within unit A are divided into two categories based on their depth within the unit. Broad A_Low paleochannels are 15–20 mbsl, with widths of 0.2–0.5 km and heights of about 5 m. Paleochannels in the upper portion of the unit, A_Upp, are 10–15 mbsl and are smaller, with widths of 0.1–0.4 km and heights of 2–5 m (figs. 4–6). Reflections at the base of A_Low paleochannels are lower in amplitude and appear to be less continuous than reflections at shallower A_Upp paleochannels (figs. 4–6).

Unit A1 overlies unit A and is observed only in profiles 35–49 offshore of northern Seven Mile Island (fig. 4). Unlike unit A, which is mostly acoustically transparent, unit A1 contains multiple, uniformly spaced, conformable, medium-amplitude reflections (fig. 4). In profile 35, the base of unit A1 thins landward, and internal reflections onlap onto laterally adjacent deposits in unit A. Surficial sediment sample 31 from profile 45 was composed of well-sorted sediment from unit A1 that consisted of rounded gravel clasts, 61 percent coarse sand, 26 percent medium sand, and 4 percent mud, and had a median grain size (D₅₀) of 579 micrometers (µm; fig. 4A; Everhart and others, 2022). The seaward portion of line 45, where unit A1 outcrops at the seafloor, is spatially coincident with a feature containing the highest backscatter reflectance in the study area (fig. 2B).

Unit B, which onlaps units A or A1, is distinguished by its discontinuous, chaotic, high-amplitude internal reflections that are most prominent in central and southern-central Seven Mile Island (profiles 31 and 11; figs. 5A and 6A; ~8-km mark on line 53, fig. 7B). Several of the seismic profiles (45, 35, 25, and 03) contain a subunit, B1, which is composed of packages of acoustically transparent sediment in between the chaotic, high-amplitude reflections in the upper and lower boundaries of the unit (figs. 4, 6B, and 7). B1 deposits are more continuous and slightly thinner in south-central Seven Mile Island and less continuous and deeper in northern Seven Mile Island (fig. 7). In profile 25 in the central geomorphic zone, a B1 deposit is wedge-shaped and located within the landward side of unit B (fig. 5B); however, in profiles 35 and 45 in the northern geomorphic zone, B1 deposits appear as beds at more seaward locations and at depths of about 10 mbsl (fig. 4).

The thickness of unit B varies with the elevation of its upper surface (fig. 8A), which dips towards the south, and the presence or absence of underlying unit A1. The upper boundary of unit B dips seaward at a steeper gradient than the lower boundary, which results in unit B thickening landward and tapering seaward. Erosional incisions truncate unit B reflections at the upper boundary in southern Seven Mile Island (at 0.5 and 1.5 km in fig. 6A; at 1–1.5 km in fig. 6B; at 4, 6, and 10 km in fig. 7). Sediment sample 30 contained sediment from unit B. The sample consisted of 53 percent mud and 47 percent sand, had a D₅₀ of 53 µm, and was finer than samples collected landward of it (fig. 4A; Everhart and others, 2022).

Unit C overlies unit B and is mostly acoustically transparent but has occasional internal reflectors (figs. 4–7). The base of unit C is delineated by the truncation of dense high-amplitude reflections in underlying unit B. Offshore of southern Seven Mile Island (the southern 7 km in line 53), unit C thickens and contains southward-dipping, medium-amplitude internal reflections (fig. 7). The upper portion of unit C offshore of southern Seven Mile Island (kilometers 3–6 in line 53) contains a high-amplitude internal reflection at depths of 5–6 mbsl that truncates underlying dipping reflectors (fig. 7). Most of the sediment samples from unit C had D₅₀ values ranging from 135 to 189 µm and consisted of 54–75 percent fine sand (figs. 4A and 5A; Everhart and others, 2022). Within the upper shoreface, small changes in depth can cause great variability in grain size (figs. 4 and 5). Sediment sample 22 from central Seven Mile Island contained slightly coarser sand, with a D₅₀ of 441 µm, and consisted of 37 percent coarse sand, 50 percent medium sand, and 9 percent fine sand (Everhart and others, 2022). The slightly coarser grain size may be related to the presence of bedforms or reworking of coarser A1 deposits located to the north.

Seismic dip profiles and isopach maps reveal that unit C thins offshore and has variable thickness alongshore (fig. 8B). Unit C in central Seven Mile Island is thin and extends past the survey area, but unit C deposits in southern and northern Seven Mile Island thicken landward and pinch out 1.5–2 km seaward of the shoreline (figs. 4, 6, 8B, and 9B). In northern Seven Mile Island, the seaward extension of unit C likely contributes to its anomalous, convex morphology in bathymetric profiles (figs. 3A and 4). This also results in the greatest average thickness of unit C in northern Seven Mile Island (table 1). Seaward extension and thickening of unit C are also observed in profile 03 adjacent to the Hereford Inlet ebb-tidal delta (fig. 6B). Southern Seven Mile Island has the greatest unit C volume (table 1). Another possible contributor to this larger volume in the southern part of the study area is the deepening of the unit B surface, which provides greater accommodation for unit C sediment (fig. 7). Strike line 53 reveals southward-dipping reflectors in unit C where accommodation increases (at 2–4 km; fig. 7A). An isopach map (fig. 8B) and sediment volumes calculated along the margin (fig. 9A) indicate that unit C has the smallest volume between profiles 11-25 at the transition between southern and central Seven Mile Island. The greatest unit C volumes are adjacent to the Hereford Inlet and just south of the transition between central and northern Seven Mile Island (fig. 9A).

Seismic Unit Interpretation

Seismic units in this study were tied to units interpreted by the New Jersey Geological and Water Survey in Cape May County (Sugarman and others, 2016) and sediment cores and interpreted seismic surveys offshore of Seven Mile Island and Sea Isle City (Smith, 1996). Units B and C were interpreted as Holocene deposits that correspond to Q m and Q b s, respectively, from Sugarman and others (2016). Unit A was interpreted as a Pleistocene deposit that corresponds to the Cape May Formation interpreted by Sugarman and others (2016). Further age constraints are required to determine the age of subunit A1. Of the Holocene deposits, unit C is identified as sediment available for barrier island replenishment because unit B may contain a higher concentration of fine-grained sediment. Ideally, sediment that replenishes Cape May County beaches has mean grain sizes that are equal to or slightly coarser than 100–200 µm (Uptegrove and others, 1994).

The oldest deposits in this study, unit A, coincide spatially with deposits interpreted by Sugarman and others (2016) as the Cape May Formation. Previous studies have established that the Cape May Formation was initially emplaced during a Pleistocene highstand and was subaerially exposed, subsequently incised by tidal channels, and finally infilled by transgressive deposits (Smith, 1996; Sugarman and others, 2016). The lowermost deposits of unit A observed in this study are acoustically transparent substrate at depths of 20–30 mbsl, and the depths of these deposits are coincident with Sangamonian (MIS 5) highstand deposits (Knebel and Circé, 1988). Therefore, the lower portion of unit A observed on seismic profiles could be Sangamonian or younger.

Several lines of evidence were used to interpret unit A1 as an infilled paleochannel. First, unit A1 is a localized deposit, and its laminated infill onlaps laterally adjacent unit A (fig. 4B). The seismic signatures of unit A1 and A resemble those observed offshore of Kitty Hawk, North Carolina. There, an acoustically transparent sediment unit is laterally adjacent to laminated infill and was interpreted as channels that were infilled by laminated pebbly sand and acoustically transparent silty sand (Browder and McNinch, 2006). In this study, the depth of unit A1, which ranges from 14 to 19 mbsl, is similar to that of nearby incised valleys and drainages of the ancestral Delaware River, which have depths between 10 and 40 mbsl (Knebel and others, 1988). The channel heights of A1, which range from 4 to 5 m, are consistent with other paleovalley systems that typically were incised no deeper than 10 m during the late Pleistocene (Mattheus and others, 2020). This study proposes that the A1 paleochannel was formed during the MIS 2, rather than the MIS 6 lowstand, because the ancestral MIS 6 Delaware River paleovalley, which coursed through present-day Wildwood, N.J., had channel thalwegs at depths >47 mbsl (Knebel and Circé, 1988), which is deeper than the lower limit of this study’s seismic profiles. This study cannot definitively constrain unit A1’s formation processes and timescale of formation without sediment cores. Future collection of a seismic strike line at the seaward end of the survey could better support the interpretation of A1 as a paleochannel formed during MIS 2.

Interpretation of unit A1 as fluvial in origin is supported by the coarse grain size from sediment sample 31. The A1 paleochannel is located offshore of Townsends Inlet, and it may have been formed initially by a branch of a fluvial drainage and reoccupied by tidal channels, which is a phenomenon commonly observed at tide-dominated and mixed-energy barrier islands (Belknap and Kraft, 1985). Ponar sample 31 from this unit is similar in composition to fluvial paleochannel infill observed by Smith (1996). These fluvial paleochannel deposits may have been reworked into a lag deposit that could be represented by the north-south-trending band of high backscatter at the northeast extent of the survey area. Lag deposits may be commonly observed at other nearby paleovalleys, such as at the mouth of Delaware Bay (Knebel and Circé, 1988). The sand within the unit A1 deposits that outcrop at depths of 14–19 mbsl could potentially be mobilized onshore during energetic wave events, but its potential for supplying sand to the shoreface and beach cannot be evaluated without further analysis and hydrodynamic modeling.

Units B and C are interpreted as Holocene-age units based on terrestrial mapping efforts (Sugarman and others, 2016). Unit B is tied to the unit Q m, which was interpreted by Sugarman and others (2016) as a Holocene-age salt marsh and estuarine deposit. Surficial sediment samples with high concentrations of silt and very fine sand support this interpretation. Acoustically transparent packages of B1 may represent flood-tidal delta lobes or overwash deposits because they appear as beds that are laterally continuous in the along- and cross-shore directions. These flood-tidal delta deposits could have been deposited if there was a breach in the barrier island, similar to deposits observed adjacent to breaches at Cedar Island, Virginia (Hein and others, 2021). Although the acoustically transparent nature of unit B1 suggests that it has a higher sand content than adjacent unit B deposits, this could not be confirmed because unit B does not outcrop at the seafloor within the survey area.

Unit C is interpreted as barrier island and shoreface deposits Q b s from Sugarman and others (2016). This interpretation is supported by surficial sediment samples from this unit that contain mostly fine to very fine sand. Furthermore, unit C is acoustically transparent in seismic profiles and has relatively high backscatter. Of these two Holocene units, unit C is likely the only one that can provide sand to the shoreface and beach once it is reworked because unit B deposits may be too fine grained to be retained in nearshore environments.

The alongshore variability in the acoustic character and thickness of unit C may reveal insights into tidal channel formation and rates of barrier island elongation. Elongation of Seven Mile Island is supported by southward dipping reflectors within unit C offshore of southern Seven Mile Island (0–6 km in line 53; fig. 7A). Another indicator of Seven Mile Island’s southerly growth is the deepening of the base of unit C at 3–7 km in line 53 that is interpreted as a paleochannel. This paleochannel may have formed at the end of the barrier island as a tidal inlet channel, similar to those observed at Fire Island (Liu and Goff, 2018) and offshore of Rockaway Beach, New York (Wei and Miselis, 2022). At 6–7 km of line 53 in central Seven Mile Island, the paleochannel has a height of about 4 m and is spatially coincident with a wide tidal creek channel through the backbarrier marsh (fig. 1). The relatively deep incision suggests that rates of barrier island elongation may have slowed, which could have provided more time for deeper channel incision at the southern end of the barrier island (fig. 7A). Thereafter, this paleochannel was infilled by unit C sediment with southward-dipping and seaward-dipping reflections, which suggests that southward elongation was initiated by an increase in sediment supply. The speed of barrier island elongation may have increased; paleochannels observed at 3–6 km are shallower than the paleochannel at 6-7 km, suggesting a shorter duration of incision at the barrier terminus (fig. 7). Alternatively, tidal channel morphology could be asymmetric. Even further south, the lack of deep tidal paleochannels observed at 0-3 km in line 53 suggests that barrier island elongation occurred rapidly. Once Hereford Inlet reached its present position, the ebb-tidal delta grew and promoted seaward progradation of unit C that is observed on line 03 (fig. 6B). The rates of Seven Mile Island’s elongation cannot be constrained by historical navigation charts because the earliest charts of Seven Mile Island suggest that the island has been in its current configuration since before the mid-eighteenth century. Thus, constraining the timing of southward elongation and formation of flood-tidal deltas would require subsampling for age constraints from sediment cores.

Holocene Sand Availability

While the thickness and spatial extent of unit C sediment are variable throughout the northern, central, and southern geomorphic zones of the island, the deposit volumes calculated within each zone are similar (table 1). Nevertheless, the shape of the shoreface and the geometry of the unit C deposits suggest that several different processes contributed to unit C formation. In northern Seven Mile Island, volumes of unit C sediment thicken in the upper shoreface adjacent to the Townsends Inlet ebb-tidal delta (fig. 9A); however, this deposit has a limited cross-shore extent which helps explain why zonally averaged unit C volumes are smallest in northern Seven Mile Island (fig. 9B). One process that likely contributed sediment to the northern Seven Mile Island upper shoreface is inlet sediment bypass, a process in which waves break over the ebb-tidal delta and produce swash bars at its periphery. These swash bars migrate downdrift and eventually weld to the beach of the downdrift barrier island, causing the updrift end of the drumstick barrier island to accrete sediment (Fitzgerald and others, 1984). Thus, the northern Seven Mile Island shoreface thickens landward and dramatically tapers seaward because sediment accumulation patterns are controlled by wave breaking and swash bar processes, which are typically confined to nearshore environments.

Central Seven Mile Island has the second highest volume despite having the thinnest deposits (table 1), likely because unit C deposits do not pinch out offshore of central Seven Mile Island, remain uniformly thin, and extend beyond the end of the seismic profiles (fig. 9B). Thin sediment deposits in central Seven Mile Island are adjacent to the narrow portion of the barrier island, which was likely formed by spit accretion (Fitzgerald and others, 1984). Since bar welding processes may be minimal, most of the sediment may instead be sourced from along- and cross-shore sediment transport. As a result, much of the sediment accumulation along the central Seven Mile Island shoreface may occur at deeper parts of the shoreface, compared to northern Seven Mile Island.

Southern Seven Mile Island has the greatest unit C deposit volumes and the second greatest zonally averaged sediment thickness (table 1). These large volumes are likely influenced by incision of underlying unit B deposits by tidal channels, which were later infilled by sandy unit C sediment as Seven Mile Island elongated southward and the ebb-tidal delta adjacent to Hereford Inlet grew seaward. Even though the geometry of unit C is highly variable along the island, unit volumes in each zone are relatively uniform, with each zone containing 31.05–36.48 percent of the total Holocene sand volumes (table 1).