GRAIN SIZE VARIATION ON A MODERN FLUVIAL-ESTUARINE SPIT IN WEEKS BAY ALABAMA

Weeks Bay is a small (7 km²) estuary attached to the eastern side of Mobile Bay in southwestern Alabama. The bay is dominated by fine-grained siliciclastic sediment (clayey silt), but is fringed by sand along much of its shoreline and along a spit at the mouth of the Fish River, the largest influx of fresh water to the estuary. In 1998, six months after Hurricane Danny made landfall at Weeks Bay, a surface sediment study suggested that the Fish River spit was dominated by fine quartz sand. Subsequent surface sampling in 1999, 2000, 2005 and 2006 revealed that the spit was becoming less well defined. The inference was that the spit was “silting up.” In the winter of 2010, an undergraduate research piston coring project obtained 5 sediment cores along a north – south line along the longitudinal axis of the spit. Cores were spaced approximately 75 to 100 m apart. The northern most core was extracted from very near to the shoreline where marsh vegetation flourished. The final core was extracted approximately 400 m from the first in an area that was identified in earlier studies as near the southern termination of the spit. Each core was split before being examined and one side was processed for grain size analysis. One to 2 cm thick slices were extracted from each split either at a specific interval (3-5 cm), or at distinctive sedimentary layers. On average, each core yielded approximately 35 to 40 samples. Once dried, these samples were analyzed for grain size variation using the pipette and sieve method. Our results suggest that the Fish River spit is mostly silt and clay rather than sand near its terminus and that the original interpretation that the spit was dominantly sandy well into the bay was incorrect. A more likely scenario is that a relatively thin sand storm layer associated with Hurricane Danny was deposited atop the Fish River spit and that subsequent years of fair-weather processes have returned the sediment to its normal fine-grained disposition.