Description

The Bay of Islands, western Newfoundland, is a fjord bounded at its seaward end by an arcuate submarine moraine (Fig. 1a) up to 100 m thick (Josenhans et al. 1990; Josenhans & Zevenhuizen 1993; Josenhans & Lehman 1999). The moraine formed after retreat of ice that formerly extended far out on the continental shelf (Shaw et al. 2006). The deepest areas of the fjord (295 m) are immediately landward of the shallow (30 m) moraine. The fjord has three principal arms, the largest of which (Humber Arm, Fig. 1a, c), averages 100 m deep and shallows steeply at its head where the Humber River enters. Humber Arm has many of the attributes of fjords elsewhere; it hosts thick glacimarine sediments (Fig. 1b) overlain by postglacial mud (Shaw et al. 2000; Levac 2003), with high rates of sedimentation prevailing in the late glacial and postglacial periods.

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Fig. 1.

Multibeam bathymetry and seismic data from Bay of Islands, a Newfoundland fjord system. (a) Sun-illuminated image showing the entire fjord and the moraine at its outer threshold. Acquisition system Kongsberg EM120. Frequency 12 kHz. Grid-cell size 5 m. (b) Sparker profile of Humber Arm showing thick glacimarine and postglacial sediments, and a gas-escape chimney whose seafloor location is marked by a circular trench. Profile x–y is dashed line on (c). (c) Humber Arm with inset of irregular mounds on the sea floor. A, V-shaped gullies; B, fluid-escape trenches; C, dredge spoil mounds; D, road construction induced slide. (d) Enlarged view of submarine slides off the city of Corner Brook. A, ring-like trench with methane-rich sediment; B, ridge of wood chip; C and D, submarine slides. (e) Location of study area (red box; map from GEBCO_08).

The fjord sidewalls are notched by V-shaped gullies (A in Fig. 1c) with relief of c. 10 m, formed by late glacial slumping from the fjord sidewalls into the adjacent basin, where stacked lenses of acoustically transparent sediment overlie the acoustically stratified glacimarine mud (Shaw et al. 2000). The approximate age of these events was estimated at 12–11 ka BP, a time when relative sea level was falling rapidly from a high of +120 m in the area (Grant 1994; Shaw et al. 2002b) and earthquake activity was more prevalent than today. Linear trenches on the seafloor (B in Fig. 1c) testify to fluid escape from below the Quaternary sediments, as seen in Figure 1b. A ring-like trench on the seafloor with a diameter of 300 m and depth of 4 m extends downwards through the thick Quaternary sediments off Corner Brook (A in Fig. 1d). A zone of acoustic masking occurs 19 m below the centre of the trench, and sampling revealed methane gas in the sediment. It is likely that the feature is part of a chimney (Fig. 1b) formed by bedrock-sourced gas venting at the seafloor.

However, offshore from Corner Brook much of the seafloor morphology (Fig. 1d) has the signature of planned and inadvertent human activities. These anomalies include a submarine ridge (B) extending seawards from the city. The ridge corresponds to an area of high backscatter strength that masks acoustic records, and is composed of wood chips. Circular patches of debris are present on the seafloor off the modern delta of Humber River (C on Fig. 1c). Also in this area (see inset, Fig. 1c) is a 60 m long bathymetric anomaly that was revealed by Shaw et al. (2000) to be a sunken vessel.

Submarine slides at Corner Brook (Fig. 1d) differ from the late glacial slides further down the fjord, described by Shaw et al. (2000), in that they have a strong expression at the seafloor. The largest slide complex (C on Fig. 1d) consists of 20 m deep gullies incised on the fjord sidewalls and several lobes in deeper water, with rims about 4 m higher than the surrounding seafloor. A smaller slide nearby (D on Fig. 1d) travelled into deeper water and turned to the left, creating a pair of flanking berms approximately 1 m high.

Interpretation

Operation of the pulp and paper mill established in 1923 involved dumping wood chip waste directly into Humber Arm. As the dumping continued over many years, a submarine ridge of waste extended seawards from the shore (B in Fig. 1d). The two large submarine slides (C and D, Fig. 1d) were formed as a result of the extension of the adjacent wharf in the 1990s. The failed material is the glacimarine mud that blankets the fjord sidewalls. It is believed that these slides are analogues for the late glacial slides further down Humber Arm (Shaw et al. 2000) that have resulted in gullied sidewalls and stacked lobes of acoustically incoherent material sandwiched between glacimarine sediments and acoustically transparent postglacial mud.

The small slide at D on Figure 1c was triggered by road construction on land nearby. The area of irregular mounds off Humber River delta (C on Fig. 1c) is interpreted as dredge spoil. On the basis of investigation by submersible, the large bathymetric anomaly previously identified as a shipwreck on side-scan sonograms (Shaw et al. 2000) was revealed to be similar to large barges used for transporting timber to the mill at Corner Brook. Informal questioning of local inhabitants during surveys in the area failed to confirm the identity of this vessel.

The extent of modification of this arm of Bay of Island by industrial activity is unparalleled among the fjords of Newfoundland. By comparison, the adjacent fjord of Bonne Bay (Shaw et al. 2002a), partly located within Gros Morne National Park, is relatively unmodified by anthropogenic activity.