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Technical Report Series
Number 77-1
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SEDIMENTARY FRAMEWORK
OF A CHANNEL MARGIN SHOAL
OF AN EBB DELTA,
WASSAW SOUND, GEORGIA
Wassaw Island Erosion Study
Part I
by
George F. Oertel
31
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Georgia Marine Science Center
University System of Georgia
Skidaway Island, Georgia
81
SEDIMENTARY
FRAM~AORK
OF A CHANNEL MARGIN SHOAL
OF AN EBB DELTA, WASSAW SOUND, GEORGIA
WASSAW ISLAND EROSION STUDY
PART I
BY
George F. Oertel
Skidaway Institute of Oceanography
P.O. Box 13687
Savannah, Georgia 31406
JANUARY 1976
The Technical Report Series of the Georgia Marine Science
Center is issued by the Georgia Sea Grant Program and the Marine
Extension Service of the University of Georgia on Skidaway Island
(P.O. Box 13687, Savannah, Georgia 31406). It was established
to provide dissemination of technical information and progress reports resulting from marine studies and investigations mainly by
staff and faculty of the University System of Georgia. In addition,
it is intended for the presentation of techniques and methods reduced data and general information of interest to industry, local,
regional, and state governments and the public. Information contained in these reports is in the public domain. If this prepublication copy is cited, it should be cited as an unpublished manuscript.
INTRODUCTION
The Wassaw Sound is located on the northern margin of Wassaw
Island, 10 miles south of the Savannah River entrance.
Although
used by the majority of the Savannah shrimping fleet, it does not
require maintenance dredging.
(Hoyt, Weimer and Henry, 1964).
Wassaw Island i s a Holocene barrier
Historically the northern end of this
island has illustrated alternating periods of erosion and accretion
(Oertel and Chamberlain, 1975).
These patterns of shore development
are natural since there is no private or commercial development on
Wassaw Island which is a Federal Wildlife Refuge.
Patterns of fore-
shore sedimentation are believed to be controlled by the changes in
shape of a large sand shoal adjacent to the north end of the Island
(Oertel, in Press).
This report is a detailed description of the
sediment carpet on the shoal based on the analysis of 36 box cores
taken from 2 x 4 mile grid (Fig. 1).
The coring device took relatively
undisturbed samples that could be analyzed for primary and biogenic
sedimentary structures, grain size and textures.
The cores sampled a
surface 10 x 17 centimeters and penetrated the sea bed to a depth of
22 centimeters.
Sediment samples were extruded from the core cans
and trimmed to wafers 2.5 centimeters thick which were x-ray radiographed.
Surface and bottom plugs were taken from each core for grain
size analysis.
The constituent portions of very coarse, coarse, medium,
fine and very fine sand and mud (silt and clay } were determined by
sieving.
The relative stability of sediment on the shoal surface was
also estimated by comparing the areal-distribution percentages of
primary sedimentary structures and bioturbated sedimentary structures.
3
SEDIMENT DISTRIBUTION
Sediments of the shoal were composed of varying grain-size
concentrations, which were plotted on maps in order to illustrate
their spatial distribution.
Very coarse and coarse sand
Cores with high percentages of very coarse (Fig. 2) and coarse
sand (Fig. 3) generally also contained large amounts of shells and
shell fragments (Fig. 4).
Shells and shell fragments of Mulinia
lateralis and Donax variabilis were quite common; however, numerous
fragments of gastropods and other bivalves were also present.
The
distribution pattern of coarse and very coarse sand had a shore normal
orientation which corresponded to the channel bottom entering Wassaw
Sound.
The highest concentrations of coarse material occurred in the
deepest part of the scour channel.
Cores from the marginal shoal gen-
erally contained less than 5% very coarse sand and less than 10% coarse
sand.
The coarse and very coarse sand on the shoal was found only on
the channelward side.
Moderate concentrations of coarse grained shell
fragments were also occasionally found on the distal and leeward side
of the shoal.
The coarse material in the bottom of the scour channel was
apparently a lag deposit.
Foresets of coarse material were generally
indicative of megaripple type deposition.
Bedload transport of this
material was apparently only possible during maximum tidal current
velocities.
The percentage of coarse sand and shell fragments also increased
approximate ly 8
mil~s
offshore in a shore parallel orientation.
This
Figure 2.
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Map illustrating the percentage distribution of the very coarse sand
( > lrr~l, 0.0~) fraction in box core samples from the shoal and shoreface
at the north end of Wassaw Island
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( 0.0¢)
Percentage Very Coarse Sand
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Figure 3.
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Map illustrating the percentage d·i stribution of the coarse sand
(.5-l.Omnt, 1.0- 0.0~) fraction in box core samples from the shoal
and shoreface at the north end of Wassaw Island.
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( 1.0 ¢ )
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Percentage Coarse Sand
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Fig. 4.
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5
)5
25 10
5
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Map illustrating the percentage distribution of shell fragments in
box core samples fr~n the shoal and shoreface at the north end of
Wassaw Island.
~
10
Percentage Shell
en
7
may in fact be the landward limit of the relict surface described by
Gorsline (1963) and Pilkey and Frankenberg (1964).
Medium sand
High percentages of medium sand occurred in cores from several
different areas of the sample grid (Fig . 5) .
Occurrences in the scour
channel and the offshore zone were sim ilar to the distributions found
for the coarse and very coarse deposits.
Several other isolated
occurrences were on the leeward side of the shoal.
These areas apparent-
ly corresponded to large complex sand waves that were produced by
reversing tidal currents (Oertel, 1974).
Cores from the shoal surface
generally contained less than 20% and often less than 10% medium sand.
High concentrati ons of medium sand were associated with shells and
shell fragments and were deposited in foresets of megaripples.
Fine or very fine sand
Cores from the shoal area conta ined between 40 and 80% fine and
very fine sand (Fi g. 6).
On the channelward side of the shoal, the
concentration of fine and very fine sand decreased to less than 20%.
On
the leeward side of the shoal, the concentration of fine and very fine
sand increased to greater than 80%.
The percentage of fine and very
fine sand al so decreased in the offshore area.
Heavy minera l s in the
fine size fractions were generally more abundant on the leeward than
the channelward sides of the shoal.
trated wavey surfaces of ripples.
Heavy mineral laminations illusThe cores containing large quantities
of fine and very fine sand were generally thinly laminated and physical
sedi mentary structures were often interrupted by dwelling tubes of
polychaetes.
10
Mud
For the purposes of this project grain diameters less than 4phi
(silt and clay) were designated as mud.
Samples from the study area contained from 0.2 to 4.5% mud.
Concentrations of less than one percent were confined to the sediment
of the shoal.
Apparently high wave ,activity on the shoal surface
would not permit the deposition of silt and clay material. On the
leeward side of the shoal, a relatively broad apron of the shoreface
contained less than two percent mud (Fig. 7).
Core samples from the
channel margin that contained high concentrations of coarse material
(see above), also contained relatively high percentages of mud.
The
inner shoreface contained less than 3% mud; however, offshore the
percent of mud increased to 4.5%. The increase approximately corresponds to the increase in the percentage of coarse material noted
above and previously described as relict (Pilkey and Frankenberg,
1964).
The mud in the offshore area was often intensely mixed in a
highly bioturbated matrix.
BIOTURBATION
In many cores the primary sedimentary structures and textures
were secondarily disturbed by the benthic infauna.
Genera l ly the
upper 8 centimeters were less intensely bioturbated than the lower
8 centimeters (Figs. 8 and 9).
The upper 8 centimeters of the sediment prism generally illustrated foresets of ri pples and megaripp l es.
Secondary mixing by
benthic infauna was particularly limited directly over and south of
the shallowest portion of the channel-margin shoal.
Bioturbation
13
gradually increased away from the shoal.
The most intense secondary
mixing occurred in channels adjacent to the shoal and in the offshore
area.
These areas also contained a relatively high weight percentage
(3%) of mud.
In the main channel, a portion of the secondary mixing
may be more apparent than real, because it is difficult to observe
primary structures in sediments having large concentrations of shells
and shell fragments.
served in these cores.
However, some bioturbation was definitely obIn the offshore area the minor occurrences of
shells and fragments did not affect analysis procedures and bioturbation
was observed .
Bioturbation was generally more intense in the lower parts (-8
to -25 em) of cores (Fig. 9).
The distal and seaward end of the shoal
illustrated less than 10% bioturbation as did the upper most shoreface
adjacent to the shore.
The intensely bioturbated areas had the same
areal distributions as the distributions for the upper parts of the
·core; howeveGthe area of the zone with less than 10% bioturbation
decreased considerably in size.
A zone of intense bioturbation which
was not present in the upper layers was observed parallel to the beach
in a quarter mile wide zone approximate ly 0.5 miles from mean low
water.
This area is shielded by the shoal from intense wave activity,
and had a higher mud content than the surrounding areas.
Degrees of bioturbation may be used as a tool for approximating
the relative stability of the bottom (Oertel, 1973).
Cores with
secondary mixing illustrate the ex i stence of a substrate that was sufficiently
stable for organisms to occupy and mix the primary sedi mentary structures.
Intense bioturbation indicates rapid mixing by organisms or
sufficient periods of stability to permit coverage of an area by s low
15
benthic reworking.
Sharp contacts between layers of bioturbated sedi-
ment and layers having primary sedimentary structures may indicate
the periodic interruption of previously stable substrates by storms.
Bioturbation was minimal in the marginal shoal area. and structures
indicative of current and wave deposition were predominant.
Thus the
sediments at the shoals were being reworked by physical processes at
a rate sufficient to erase any biologic mixing.
In the offshore area, which experiences minor current flow and
only periodic wave surge, the sediments were intensely bioturbated and
relatively stable.
In the nearshore zone, wave and current energy is
generally more intense than in the offshore areas.
Thus, bioturbated
areas of the nearshore zone must undergo intense biologic mixing in
order to stay ahead of reworking by physical processes.
On the floor
of the channel to Wassaw Sound, the shells in sandy layers illustrated
primary sedimentary structures; however, primary sedimentary structures
were often camoflaged in concentrated deposits of shelly material.
The
illusion of secondary mixing in the channel deposits may partially be
an artifact of core processing and observation technique.
CONCLUSIO NS
The fine and very-fine grained sands on the marginal shoa l have
low threshold velocities.
These well sorted, fine sediments are very
susceptible to rapid transit with increases in physical energy.
The
general lack of secondary mixing in shoal sed iments suggests that these
sediments undergo continuous reworking and apparently t he shoal is in
dynamic equilibrium with physical forces.
The coarse sed iment on the channel margin is a "lag" deposit
and is also in dynamic equilibrium with reversing inlet t i dal currents.
16
The offshore area and the area
11
Shielded
11
by the shoal have rela-
tively stable sediment carpets and were less susceptible to ambient
energy conditions.
These relatively stable substrates were periodically
reworked by large storm waves.
17
LIST OF REFERENCES
Gorsline, D.S. 1963.
Bottom sediments of the Atlantic shelf and
I
slope off the southern United States.
Jour. Geol. 71:422-440.
Hoyt, J.H., R.J. Weimer, and V.J. Henry. 1964.
Late Pleistocene and
Recent sedimentation, central Georgia coast, U.S.A. p. 170-176,
In:
Straaten, Van
Deposits.
L.t~ .•
J.V . (ed), De1taic and Shallow Marine
Elsevier, Ams terdarn.
Oertel, G.F. 1973.
Sedimentary framework of the eroding beach-shoreface
system adjacent to Tybee Island, Georgia, Georgia Marine Science
Center Tech. Report Series 73-5, 27 p.
Oertel, G.F. 1974.
Residual currents and sediment exchange between
estuary margins and the inner shelf, southeast coast of the
United States. p. 135-143, In: Memoires de l'Institut de Geologie
du Bassin d'Aquitaine No . 7.
Oertel, G.F. and C.F. Chamberlain. 1975.
Differential rates of shore-
line advance and retreat at coastal barriers of Chatham and
Liberty Counties, Georgia.
Tran. Gulf Coast Assoc. Geol. Soc.
XXV:383-390.
Oertel, G. F., Geomorphic cycles in ebb deltas and related patterns of
shore erosion and accretion. Jour. Ind. Petrology.
Pilkey, O.H. and D. Frankenberg. 1964.
The relict-recent sediment
boundary on theGeorgia continental shelf.
Bull. 22:37-40.
Georgia Acad. Sci.
13
ACKNOWLEDGMENTS
This project was sponsored (in part) by the Georgia Sea Grant
Program, supported by N. O.A.A., Office of Sea Grant, Department of
Commerce, under grant no. 10-32-RR-273 -077.
The U.S. Government is
aut horized to produce and distribute reprints for governmental purposes
not withstanding any copyright notation that may appear hereon.