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The Geology of Six Mile Reef, Eastern Long Island Sound

Discussion and Conclusions

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Title Page
Introduction
Setting
Methods
Results
Discussion
Data Catalog
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The detailed multibeam bathymetry, subbottom, and sediment data presented here yield new geologic perspectives of Long Island Sound's dynamic sea floor (figs. 13, 14, 15, 16). Mathematical simulations of density-driven and tidal-currents in eastern Long Island Sound (Signell and others, 2000) predict net westward bedload transport of medium sand, and clockwise circulation around and convergence over longitudinally directed shoals (fig. 1). Our data support these simulations, and show that sand-wave morphology and directions of dominant tidal current flow are strongly correlated in the vicinity of Six Mile Reef. Orientation and asymmetry of the barchanoid and transverse sand waves and of scour marks around bedrock outcrops, boulders, and shipwrecks indicate that net sediment transport is flood-tide dominated and primarily to the west and southwest. Eastward-facing sand waves northwest of the shoal are evidence for the modeled clockwise circulation; symmetrical trochoidal sand waves on top of the shoal are evidence for flow convergence. Together these processes provide a mechanism for maintenance of the shoal.

To at least some extent, the distribution of sand-wave types is controlled by the availability of sand. Barchanoid wave morphologies typically form in environments where the substrate is firm and the sediment supply is limited (Reineck and Singh, 1980) and they dominate the sediment-limited sea floor around the eastern end of the shoal where thin gravelly sands armor the underlying cohesive glaciolacustrine mud. Transverse wave morphologies dominate atop and off the central and western parts of the shoal, where reworking of the underlying marine deltaic deposit supplies abundant coarser sediment.

The numerous sand waves and obstacle marks reflect the action of strong bottom currents in this part of the Sound. Average maximum near-bottom tidal currents near Six Mile Reef are flood-dominated and exceed 40 cm/s throughout most of survey H11361, 30 cm/s across the eastern and central parts of survey H11252, and 20 cm/s in the western part of this survey (Signell and others, 2000). These current velocities, which correspond to the traction threshold velocities for very coarse sand to fine sand (Nevin, 1946), are strong enough to initiate entrainment and transport and explain the observed surficial sediment distribution. Based on earlier work (Belderson and others, 1982), these current velocities, when related to sediment supply, are also compatible with the sizes, morphologies, and distributions of the bedforms observed in the study area. The presence of current ripples and megaripples on the sand waves and the steepness of the slip faces suggest that this transport is active and that the sand waves are propagating under the present hydraulic regime (Dalrymple and others, 1978; Reineck and Singh, 1980; Allen, 1982). The presence of megaripples with crests oblique to trends of the underlying sand waves is evidence for flow separation and increased turbulence, which can promote increased sediment flux from the development of secondary flows (Allen, 1968). Whether the difference in orientation of sand waves and megaripples translates to variance in sediment transport directions or propagation time scales is uncertain.

Although current ripples may reverse their orientation during semidiurnal tidal cycles because of the small volume of the ripples, no reversal in either the sand-wave or megaripple morphology was observed in the multibeam data of adjacent lines collected at different tidal stages. Whether this tidal independence of large bedform orientation is due to the spatial distribution of residual currents, or to an asymmetry of current velocities is also uncertain, but it does suggest that the larger bedforms are more stable and that they move over longer time scales. While the sand-wave and obstacle-mark asymmetry in the bathymetric data reveal direction of transport, no information on the rate of advance is supplied. A 16-year analysis of giant sand waves located about 2.8 km east of the present study area concluded that those bedforms were migrating to the southwest at about 2.1 m/yr (Fenster and others, 2006). Inasmuch as tidal currents weaken westward in this part of the Sound, we assume a slightly slower migration rate for the sand waves described herein. We also believe that the megaripples, because of their intermediate size, may migrate up the stoss flanks of the sand waves at a much faster rate, perhaps on an annual scale.

Bifurcating crest morphologies within the study area are most common near abrupt edges of sand-wave fields adjacent to flat sea floor and at contacts between fields of different types of sand waves (fig. 26). The occurrence of this crest morphology at these locations should be expected because bifurcation of wave crests indicates transitional flow conditions and the development of secondary flows (Aliotta and Perillo, 1987). The crest of a sand wave may bifurcate if the forces of tidal oscillation do not remain equal along its entire length, especially if the contact between flow cells is relatively sharp or if differences in flow rate or direction are relatively great.

Other interesting bathymetric features we observed are the aligned scour depressions in the troughs of sand waves (fig. 34). They occur in association with both barchanoid and transverse sand waves, along the edges and in the interiors of the wave fields, and are probably related to enhanced turbulence. While bedforms that could provide information on the nature of this turbulence may be present in sizes below the resolution limit of our multibeam data, strong currents alone do not explain the elongate configuration of the depressions. Sedimentary furrows, another elongate erosional bedform, have been identified to the west in surveys H11043, H11255, and the CLIS Dumping Ground (fig. 1; Poppe and others, 2002b). Formation of furrows there was attributed to recurring, directionally stable tidal currents and the development of secondary helical-flow patterns that direct the tidal currents into convergent flow zones. If such flow patterns can develop over sand-wave fields, then they could create narrow currents of greater strength and could explain the alignment of the scour depressions.

Click on figures for larger images.

Figure 1. Map showing location of study area around Six Mile Reef.
Figure 1. Map showing location of study area around Six Mile Reef (open red polygons). Also shown are major onshore moraines (solid black polygons), generalized net bottom sediment transport directions (blue arrows; Signell and others, 2000), and extent of the marine delta produced by the draining of glacial Lake Hitchcock (dashed green line; Lewis and DiGiacomo-Cohen, 2000).

Figure 13. Digital terrain model of the sea floor from NOAA survey H11361 around Six Mile Reef.
Figure 13. Digital terrain model of the sea floor from NOAA survey H11361 around Six Mile Reef. Image is sun-illuminated from the north-northeast and vertically exaggerated 8X. See key for depth ranges.

Figure 14. Digital terrain model of the sea floor from NOAA survey H11252 west of Six Mile Reef.
Figure 14. Digital terrain model of the sea floor from NOAA survey H11252 west of Six Mile Reef. Image is sun-illuminated from the north-northeast and vertically exaggerated 8X. See key for depth ranges.

Figure 15. Geological interpretation of the bathymetry from survey H11361 in the eastern part of the study area around Six Mile Reef.
Figure 15. Geological interpretation of the bathymetry from survey H11361 in the eastern part of the study area around Six Mile Reef. Arrows show directions of net sediment transport based on sand-wave and scour-mark asymmetry. Also shown are locations of the seismic profiles shown in figures 17, 18, 19, and 24, and detailed multibeam views of the sea floor shown in figures 20, 25, 26, 27, 28, 29, 31, 32, 33, and 35. See insets for explanation of symbols.

Figure 16.  Geological interpretation of the sea-floor bathymetry from survey H11252 west of Six Mile Reef.
Figure 16. Geological interpretation of the sea-floor bathymetry from survey H11252 west of Six Mile Reef. Arrows show directions of net sediment transport based on sand-wave and scour-mark asymmetry. Also shown are locations of Chirp seismic profile shown in figure 21, and detailed multibeam views of the sea floor shown in figures 22, 23, 30, and 34. See inset for explanation of symbols.

Figure 26. Detailed planar view of multibeam bathymetry from survey H11361 showing asymmetrical transverse and barchanoid sand waves suggesting net westward transport
Figure 26. Detailed planar view of multibeam bathymetry from survey H11361 showing asymmetrical transverse and barchanoid sand waves suggesting net westward transport. Note sharp contacts between sand waves and surrounding flat sea floor and bifurcation of transverse wave crests near the edge of that field. Location of detailed view is shown in figure 15.

Figure 34. Detailed planar view of multibeam bathymetry from survey H11252 showing the aligned isolated depressions that form in troughs of adjacent sand waves.
Figure 34. Detailed planar view of multibeam bathymetry from survey H11252 showing the aligned isolated depressions that form in troughs of adjacent sand waves. Locations of detailed view is shown in figure 16.


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