Mattituck sill

The present configuration of the surface of the glacial drift in Long Island Sound is a deep central basin bounded by sills on the east and west. The submergence history of the Sound depends on the elevations of the lowest points on these sills relative to the sea level curve. On the Mattituck sill (to the east) this elevation is now -25 m. Sand is now being transported from east to west across the Mattituck sill and it is possible that the sill is now at a higher elevation than it was immediately after retreat of the ice. (More detailed acoustic reflection profile studies of the internal structure of the sill may answer this question.) The lowest point on the sill to the west (which has not yet been surveyed in as much detail) is higher than -20 m. (The locations of the saddle points on the eastern... 

Fig. 1. Map of Long Island Sound showing contours of water depth, the Mattituck Sill (MS), Falkner Island (FI), Cable and Anchor Reef (CAR), Eatons Neck (EN), and the Race (R). 

The crest advanced 6.7 m in 148 days and the sand flux was 0.7 m / sec. The westward flux of sand over the Mattituck sill is due to the excitation of the sand by the tidal stream and advection by the estuarine circulation (Bokuniewicz et al., 1977). According to this model, the sand... 

Fig. 12. Observed displacement of a large sand wave on the Mattituck sill over 148 days.

The initial conditions at the start of the marine regime in the Sound at 8000-yr bp are shown in Fig. 15. Lacustrine mud had accumulated on part of the bottom of the lake formed behind the Mattituck sill. The sill itself, and the lake floor surrounding the lacustrine mud, was formed of deposits of glacial outwash. Most of this outwash, already submerged in fresh water, was not subject to reworking under the advancing surf zone. Small... 

Fig. 15. Boundaries of the lake that occupied the basin of Long Island Sound before the sea reached the elevation of the sadle on the Mattituck sill. The Connecticut River (CR) did not flow into this lake.

Sand covers about 44% of the Sound floor and is the dominant component over about 75%. The major feature of the sand distribution is the transition from the sand floor of the eastern Sound to the nearly sand-free silts of the central Sound. The transition zone occurs over a distance of about 20 km. Within the transition zone the sand content increases uniformly to the east. Contours of constant sand fraction run approximately north and south in this region. The eastern boundary of the transition zone is a sand ridge (Mattituck Sill, Fig. 1) that crosses the Sound from north to south. The U. S. National Marine Fisheries Service (1974) has... 

Fig. 1. Long Island Sound. The bathymetry is contoured in meters and the location of Mattituck Sill is shown by the shaded area. 

When the sand content of the sediment rises above 90%, sand waves appear on the Sound floor (Bokuniewicz et al., 1977). Sand waves that cover Mattituck Sill are composed of fine sand with a mean grain diameter of about 0.3 mm. Divers have observed sand grains moving over the bottom in this area and have found ripples superimposed upon the sand waves. The rate of migration of the sand waves has been documented at two locations on the sill by Bokuniewicz et al. (1977) and Karen Zim-... 

The boundary conditions are chosen to approximate the present situation where sand is supplied to the central Sound from Mattituck Sill which remains mud free c = 1 at x = 0 and c remains finite as x becomes large. When the coefficients , k, and g are constants, the solution is... 

If the distribution of sand in the transition zone has indeed persisted unchanged, then the total amount of sand contained there may be calculated from the surficial distribution of sand and the thickness of marine sediments. This value comes out to be about 4 x 10 m (J. Gebert, personal communication), which is only about half of the estimated volume that has been supplied by the floor of the eastern Sound. The deficit may represent that volume of sand that has been deposited on Mattituck Sill. It seems likely, therefore, that the elevation of the Sill has been raised several meters. 

It was found that the sand-mud transition zone could be adequately represented by allowing the sedimentary processes to proceed at constant rates. When this was done, the calculated sand flux agrees well with the sand flux that has been measured over the sand-wave field in the eastern Sound. Since the tides and the estuarine circulation control the sand fluxes, the resulting distribution of sand approaches a steady state very quickly. It is likely that the sand-mud transition was established soon after the Sound became an arm of the sea and has persisted unchanged to the present day. As a corollary to this hypothesis, it would seem that Mattituck Sill has been accreting over the lifetime of the Sound. Small variations in the sand content preserved in short cores are probably due to the perturbation of the sand distribution by a series of winter storms rather than individual storm events. 

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