Maryland Coastal Bays Sediment Distribution Map

C&EG

Maryland's Coastal Bays Sediment Distribution Map

Sediments serve as both sinks and sources for pollutants. Many pollutants and nutrients introduced into the bays tend to accumulate and remain in the sediment. The amount of pollutants contained in a sediment is largely influenced by sediment texture. Pollutants, such as toxic metals and organics, usually are associated with fine grained, or muddy, sediments. The sediments can act as a source of pollutants, either through remobilization of these pollutants by way of natural processes (i.e., diagenetic reactions), or by physical disturbance or mixing. Because the coastal bays are shallow, with water depths averaging less than 2 meters, wind generated waves easily cause mixing of bottom sediments with overlying water. Likewise, human activities, such as dredging and boating, can also resuspend sediments. Nutrients and pollutants in sediments may be released into the water column during resuspension, thus affecting overlying water chemistry.

In the coastal bay, bottom sediments include eight of the ten Shepard's (1954) classifications. Most of the samples fall in the sand and the clayey silt classifications. The third most common sediment type represented is silty sand. Table I presents a summary of the classification of the surficial sediments.

**Table I.** Summary of sediment classification and areal extent for each classification mapped in Maryland’s Coastal Bays.
Shepard's (1954) Classification	Number of Samples	% of Total Samples	Areal extent of class mapped (km²)	% of Total Area
Sand	417	42.2	122.9	44.5
Silty Sand	152	15.4	38.7	14.0
Clayey Sand	2	0.2	0.1	--^a
Silt	3	0.3	0.4	0.2
Sandy Silt	57	5.8	16.2	5.9
Clayey Silt	249	25.2	72.8	26.3
Silty Clay	13	1.3	3.8	1.4
Sand Silt Clay	95	9.6	21.4	7.8
Total	988	100.0	276.2^b	100.0
^a Areal extent was not calculated; less than 0.1 km². ^bTotals may not add due to rounding.

The surficial sediments were mapped according to Shepard’s (1954) Classification, producing a sediment distribution map. The areal distribution of the different sediment types reflects basin geometry, energy conditions and proximity to sediment source. The trend is a westward (landward) decrease in grain size of bottom sediments.

Sandy sediments (i.e. sand > 75%), which cover approximately 45% of the bottom of the coastal bays, are found primarily along the eastern side of the bays. Sandy sediments represent material carried across the barrier island (Fenwick and Assateague Islands) as overwash or eolian deposits, or carried through the inlet. These sediments cover areas that are shallower and exposed to a relatively large fetch. The bottom in these areas are subject to higher wave energy. Fine grained sediments either are not being deposited or are actively being winnowed from these higher energy areas.

Clayey silts, which cover approximately 26% of the study area, are found in the tributaries and along the western shore of the bays. Clayey silts dominate the bottom of Newport Bay and along the western shore from Public Landing to Johnson Bay. Silty clays are restricted to upstream areas of the tributaries in Isle of Wight and Assawoman Bays. Sources of these fine-grained sediments (clayey silts and silty clays) include sediments contained in surface run-off and from shoreline erosion. The study area is underlain by the Sinepuxent Formation which was described by Owens and Denny (1979) as being sandy with layers of black clay and peat beds. During shore erosion processes, the finer grained material is selectively removed, suspended, and deposited in areas where wave action is minimal, such as in the protected marshy areas (i.e.- areas of limited fetch) and in deeper mid-channel areas (i.e.- below wave base).

Silty sands, sandy silts and sand-silt-clays are found in pockets along marshy shorelines and along the boundaries between sands and clayey silts. The boundary areas represent zones of mixing between the coarse (sand) and fine grained end members (silty clay and clayey silt) of the sediment distribution. The transition between mud and sand dominated areas is quite abrupt in Assawoman and Isle of Wight Bays. These transitional sediments are more common in Chincoteague Bay where the bay is wider and subjected to higher waves, contributing to the reworking and mixing of bottom sediments. Also, in Johnson Bay and Scott Hammock Neck, the pockets of sand-silt-clays, silty sand, sandy silt and sand are associated with the numerous islands. The source of these sediments is most likely material eroded from the islands' shoreline.

References

Owens, J.P., and Denny, C.S., 1979, Upper Cenozoic deposits of the Central Delmarva Peninsula, Maryland and Delaware: U.S. Geol. Survey. Prof. Paper 1067-A, 28 p.

Shepard, F.P., 1954, Nomenclature based on sand-silt-clay ratios: Journal of Sedimentary Petrology, v. 24, p. 151-158.