J.K.A. U.: Mar. Sci., Vol. 4, pp. 115-131 (1413 A.H./1993 A.D.

Geomorphological Features and Primary SedimentaryStructures of the Deltaic Coastal Plain Along Burullus

Lake Area, Egypt

,F.

EL-FAYOUMY, A.M. GHEITH*, M.A. ABD-ALLA and N.G. TOUBARGeology Department, Faculty of Science,

Mansoura University, Egypt; and* Faculty of Marine Science, King Abdulaziz University,

Jeddah, Saudi Arabia

ABSTRACT. The Burullus coastal area can be geomorphologically dividedI

into: beach, backshore plain, barrier, tidal inlet and deltaic zones. It showsseveral landforms characteristic for marine, riverine and aeolian processes.These include:

I. Shore landforms: represented by foreshore slop, backshore flats andsand barrier.

2. Lake landforms: represented by shallow lake or lagoon, islands, inletsand shorelines.

3. Aeolian landforms: represented by sand sheets and sand dunes.4. Riverine landforms: represented by interdistributary margin deposits,

these include; subaqueous and subaerial natural levees, paleochannels andcrevasse splays.

Furthermore, the primary sedimentary structures are studied in thebeach, inlet, sand barrier, sand dunes and southern coast of the lake. Thehydrodynamic conditions responsible for the origin and distribution of var-ious internal structures in the above environments are discussed.

Introduction

Burullus, is the second largest Delta lakes in Egypt. It is situated at the nothern partof the Nile Delta and stretches between Rosetta and DamiettaNile branches and oc-cupies a part of the coastal zone which bends to the north. The lake lies between lon-

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FIG. 2. Steep beach (foreshore and backshore) oc.cupied with cusps and well develop ridge(Photo at Abu Hossein, western AI-Burg inlet).

FIG. 3. Flat to very gently sloping foreshore. (Photo atEl-Khoshoey eastern inlet).

FIG. 4. Sand barrier separating the lake from theMediterranean Sea.

FIG. 5. Salt crust (1 cm) on the sand barrier.

important for fish production as well as is often inhabited by mixed stands of sub-merged aquatic vascular plants. The lake floor is covered with fine-grained sedi-ments (mud).

The eastern and southern borders of the lake are characterized by their irregularityand surrounded by agricultural land and fish cultures. However, the southern shoreof the lake is almostly low, flat, and mar\ced by some marshes and swamps. It hasconsiderably changed since 1800, having moved 10-12 km northwards (Sestini 1976).

121Geomorphologica! Features and Primary Sedimentary Structures.

dunes are accumulated where sand is in short supply and the direction of wind is con-stant or varies seasonally. However, Read and Watson (1962) men~joned that ~eifdune results from the combined action of persistent gentle winds that supply the sandwhich is then trimmed into long ridges by strong winds from another quarter.

East of AI-Burg village and along the eastern peninsula separating the lake fromthe Mediterranean, high coastal dunes ~haractetjz;ed by a lack of anchoring vegeta-tion, move generally landward in response to the prevailing winds. In general, thesemigrating dunes are taking the form of isolated barchan dunes that migrate inland atrates of 10 m to 30 m annually. However, the migration of these dunes landward isshortly fixed, using brushwood fences and palm trees (Fig. 9). Furthermore, these

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Abu Rageh.

Geomorphological Features and Primar)' Sedimentary Structure, 123

FI(;. II. High harchan dunl: I:xtends perpl:ndil:ular to.the wind direction. The slip-face is concave tothe downwind direction. The ripples run trans-vcrsely to thl: wind direction.

10. Precipitation dunes. The palmtrees are slowlycovered by wind blown sand on the coastalback shore (EI-Khoshoey Locality).

FIG. 12. Lagoonal mud sequence down t05 m interbeu- FIG. 13. Thinly bedded sandstones with ferric nodulesdedwith two shallow marine shell zones. represent subaerial natural levees result from

overbank flow. (Photo at Drain Nasser).

due to slight difference in size and/or mineral composition as well as colour. How-ever, the differences in mineral composition and grain size are related to differencesin competence of the fast landward rush of wave swash followed by the slower sea-ward return of the water (Emery and Gale 1951, Clifton 1969).

1. Internal Structure of Beach Ridge

A beach is a continuous linear mound of rather coarser sediments near the high

Geomorphological Features and Primary Sedimentary Structures 125

water line (Reineck and Singh 1973). The ridges develop mainly during storms andhigh waters. .

Several phases can be recognize in the development of beach ridges, a beach ridgemigrates toward land. In a strong progradational coast a series of beach ridges maydevelops, however, during slackening of waves and retreat of water level a lowerbeach ridge is produced with a concave sea-ward slope. In general, aeolian layers andripple-bedded units are sandwiched within the beach layers.

Internal structures of the ridge in the western coastal stretch of AI-Burg inlet (Fig.15) show plan beds in the upper portion of the ridge that are nearly horizontal orslightly dipping seaward. These can be formed as the tide rises and falls. The thinlayers of water moving over the ridge causes upper flow regime conditions and planbed deposition. Generally, as the ridge begins to weld to the beach face and buildinto a berm configuration, there is a reworking of the ridge sediments by swash ac-tion and this causes accumulation of seaward-dipping stratification at inclinations of100 to 150 (Davis eta/. 1972). Detailed study of internal structure of sand beach ridgesis found in the work of Werner (1963).

2. Internal Structure of the Foreshore

At EI-Khoshoey east inlet, the foreshore stratification (Fig. 16) includes layers andlaminae of black sand rich with heavy minerals that developed as lag concentrationduring beach erosion. In general, sand laminae have been deposited from suspensionclouds, brought by incoming waves. The grains of heavy minerals in the beach sandwill be concentrated while the lighter particles are removed during high energyperiods. Thus, the relative sorting by water storms produces heavy mineral placers.However, the swash and backwash produces a characteristic sorting in individuallaminae.

3. Flaser, Wavy and Lenticular Bedding

The process of alteration of bed load and suspension sedimentation gives rise tovariety of flaser, wavy and lenticular bedding. The differentiation of these types ofstructures is controlled however; by the relative volume of sand and mud, by the re-lative duration of both the bedload and suspension and by the current velocities.

A ripple bedding in which mud streaks are preserved completely in the troughs andpartly on the crests is known as flaser bedding (Fig. 17). Flaser bedding arises whenboth sand and mud are available. During periods of current activity, the sand is trans-ported and deposited as ripples, while mud is held in suspension. When the currentpauses, the mud in suspension are deposited mainly in troughs or cover the ripples.Thus, flaser bedding is produced in environments in which conditions for depositionand preservation of sand are relatively more favourable than for the mud (Reineckand Singh 1973).

If the mud layers almost completely fill the ripple troughs and make a thin C'-, verover the ripple crests wavy bedding is formed (Fig. 18). In contrast to flaser bedding,

Geomorphological Features and Primary Sedimentary St,

duced under conditions more favourable for the deposition and preservation of mudthan for sand. .

4. Water Level Marks

The sand barrier to the west at Sidi Youssif Abu Rageh is characterized by a lowdune field. However, ground water rises in this locality, so that it become easy to ob-tain water through man-made channel in the barrier. However, at the contact bet-ween water level and a sloping sediment surface, a marking is engraved (Fig. 20).With the falling water level, at different levels a series of such marking known aswater level marks are formed.

5. Stratification and Discordant Ripple

Stratification is the common primary structure observed in this region. Fig. 21shows nearly horizontal laminae results from migration of linear ripples. On theother hand, a change in stratification type from horizontal lamination alternatingwith discordant ripple layers is illustrated by trench cut in the sand barrier at SidiYoussifAbu Rageh (Fig. 22).

6. Mud Cracks

The most commonly observed features on the southern part of Burullus Lake aremud cracks (Fig. 23). Mud cracks result from desiccation and concomitant shrinkageof a surface sediments. Their depth, length, and width are apparently due to innum-erable factors such as bedding thickness, salinity, rate of water loss and waterchemistry. An increase in salinity can generate shrinkage cracks in the mud layers(Burst 1965). This process is important in coastal lagoons and inland sabkhas wheresalinity of water increases markedly during certain period. However, cracks widthsand depth up to several centimeters and have a tendency to take quadrangles andtriangles shape.

Generally, mud cracks are widespread on the area surrounding the lake, and theirsediments refers to sabkha deposits formed by periodic flooding of sea water on gen-tle depressions. However, these deposits are now under reclamation.

7. Wind Sand Ripples

Aeolian ripples are quite common on all parts of the dunes and beaches. Wind rip-ples generally have straight long, parallel crests and are asymmetrical with high rip-ple indices. They may show well developed bifurcating crests.

Sand ripples, composed of well sorted medium to fine grained sand, often occur onthe sides of dunes, w1th their axes parallel to the slope. There is a tendency for thecoarsest grain and/or heavy mineral grains to accumulate on the crests of the sandripples rather than on the troughs. In the coastal zone, sand ripples provide a goodindicator of the local wind direction and the complexity of the wind current aroundthe dune. They commonly formed in wind shadows on the lee side of the dune, wherewind and sand are funneled through a low in the dune crest.

129Geomorphological Features and Primary Sedimentary Structures.

small fluctuation in the wind direction. However, when wind direction changesgreatly, a new set orripples is initiated and superimposed on the old one with a diffe-rent trend.

F:IG. 23. Mud cracks in a wet soil, at west Baltim (east FIG. 24. Wind impact ripples with rather continuousBurullus Lake). crestlines and a slight asymmetry. Wind from

top right to bottom left.

FIG. 25. Asymmetrical wind ripples show well de- FIG. 26. Interference pattern in asymmetrical wind rip-veloped undulatory bifurcation of crests. pies resulting from at least coexisting wind

sets. Note, old ripples superimposed by nearset of ripples.

References

Abdel-AaI, M.A. (1987) Geomorphological and Sedimentological Studies on the Mediterranean CoastalPlain along the Nile Delta, Egypt. M.Sc. Thesis, Fac. ofSci., Mans. Univ.

Geomorphological Features and Primary Sedimentary Structures.

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