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Mineralogic evidence for the remnant Sebennitic promontory on the continental shelf off the central Nile delta
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MarineGeology117 (1994) 187-194
ELSEVIER
Mineralogic evidence for the remnant Sebennitic promontory on the continental shelf off the central Nile delta Omran E. Frihy, Morad F. Lotfy Coastal Research Institute, 15 El Pharaana Street, 21514, El Shallalat, Alexandria, Egypt
(Received October 28, 1992; revision accepted November 8, 1993)
Abstract
Heavy mineral compositions of closely spaced bottom samples off the central Nile delta are evaluated to determine provenance, dispersal patterns and sediment sources of a former branch, the Sebennitic. Q-mode factor analysis applied to heavy mineral data of 83 samples yields two factors: factor 1 (augite and hornblende), and factor 2 (opaques, garnet, zircon, monazite, rutile, epidote). The two factors correlate negativelywith each other. The observed mineralogic pattern is attributable to selective transport processes of the sediments related to a former subdelta. They probably originated from extensive progradation of the remnant promontory of the extinct Sebennitic branch which flowed across the subaerially exposed continental shelf during the last Holocene sea-level low stand. Grain sorting patterns of heavy minerals provide additional proof for the presence of a large Sebennitic channel extending across the continental shelf off the central Nile delta.
1. Introduction
The Nile delta shore consists of sandy arcuate beaches, approximately 270 km in length (Fig. 1). The coastline includes the present-day Nile promontories (Rosetta and Damietta). Lying between them is the remnant pre-modern Burullus promontory, which was formed by the discharged sediments from the former Sebennitic branch (Orlova and Zenkovitch, 1974), which was initiated during the Holocene between 7500 and 3000 yr B.P. (Arbouille and Stanley, 1991; Stanley et al., 1992a,b). When that branch was active, it probably formed a promontory similar to those that have developed at the mouths of the Rosetta and Damietta branches. The causes of abandonment of the Sebennitic branch could be related to the reduction of sediment supply and the flow of river water to the coast, resulting from climatic changes over the Nile drainage or due to the avulsion of 0025-3227/94/$7.00 © 1994ElsevierScienceB.V.All fightsreserved SSDI 0025-3227 (93)E0140-2
the Sebennitic channels (channels avulse when their gradient becomes too gentle). Subsequent long-term shoreline erosion resulted from the cut off of the sediment discharge, and the prevailing coastal processes have resulted in retrogradation of this coast to its present position. Similar coastal processes exist today off the Rosetta and Damietta Nile branches due to the cut off of sediment delivery by the High Aswan Dam. The study area is located seaward of the Burullus-Baltim coast off the central delta, fronting a shoreline length of about 27 km and covering a sector that extends seaward for about 12 km to water depths of 20 m (Fig. 2). The coast forms a broad, rounded bulge and is located on a very active shoreline that has experienced widespread erosion (UNDP/q.INESCO, 1978; Abdel Kader, 1982; Stanley et al., 1992b; Lotfy and Frihy, 1993). Heavy mineral assemblages concentrated by selective sorting of minerals due to differences in
O.E Frihy, M.F Lot/~7/Marine Geolog), 117 (1994) 187 194
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size and density have long been used to analyze sources and transport paths of sediments (Rubey, 1933; Rittenhouse, 1943; Trask, 1952; Van Andel, 1959; Komar and Wang, 1984). The Nile River has been identified as the major source of quartzrich sediments and sand-size heavy minerals on the shelves of Egypt and Israel (Ball, 1942; Hilmy, 1951 ). The aim of this study is to examine whether there is evidence for the ancient Sebennitic promontory of the Nile Delta in the grain sizes and
mineralogy of the sediments on the continental shelf. Previous investigations along the Rosetta promontory (Frihy and Komar, 1991) have recognized marked patterns of heavy mineral concentration due to grain-sorting processes as those promontories are eroded. The hypothesis is that similar patterns of heavy minerals formed along the shoreline of the Sebennitic promontory as it eroded for some 2000 years. Such patterns in the shelf sediments would provide evidence for the past extent of the promontory and the position of the river's mouth. There is, however, the possibility that the processes of shelf sediment transport have reworked the sands to such an extent that no remnant pattern exists that can be associated with this ancient promontory.
2. Historical background The Sebennitic branch is one of at least seven branches which crossed the central delta during Middle to Late Holocene times (pre-Dynastic to Roman time)• Five of the Nile distributaries have silted up, leaving two (the Damietta and Rosetta branches) active at present (Fig. 1). Previous studies have recorded archaeological, geographical and sediment•logical evidence for the existence of these former branches. Surficial mapping of the Sebennitic branch has been visually traced by satellite imagery (Abdel Kader, 1982) and aerial photography (UNDP/UNESCO, 1978), based on marked series of point bars and koms (low hills). Relict deposits related to the Sebennitic branch were mapped on the continental shelf by Frihy and Gamai (1991), by examining texture and coarse fraction compositions of bottom samples. Numerous beach pebbles, probably related to the Sebennitic promontory, were dredged from the sea bottom off the Burullus coast (Summerhayes et al., 1978). Recently, subsurface coring analysis have provided strong litho-stratigraphic evidence for the trace of the main Sebennitic channel and its promontory (Coutellier and Stanley, 1987; Chen et al., 1992). Delta lobes comprising marine delta-front and prodelta facies have been cored in the coastal zone of the delta plain. Such lobes forming at the mouths of Nile distributaries, originally accumu-
O.E. Frihy, M.F Lotfy/Marine Geology 117 (1994) 187-194
lating seaward of the present coast. Following their submergence, the distributary sediments have been eroded and reworked by the predominantly easttrending longshore currents (Inman and Jenkins, 1984; Stanley, 1989). The general position of the Sebennitic promontory is believed to have been seaward of the present north-central Nile delta coast, between Burullus inlet and Baltim resort. This channel was particularly important from about 7000 to 3000 yr B.P. (Arbouille and Stanley, 1991; Stanley et al., 1992b).
3. Sampling and methods of analyses Detailed sampling of the middle shelf and adjacent beach off the Burullus-Baltim sector in 1989 was undertaken to seek potential sources of sediments to nourish the Baltim coastal sector. Samples were recovered using hand cores driven by a diver, and vibracores along a grid survey of about 2.0 x 1.0 km (Fig. 2). In this study we use the 20 cm of the upper part of 65 cores as bottom samples, together with 18 beach face samples. Each sample was treated separately for heavy mineral analyses. The samples were sieved to obtain the 63-250 Ixm fractions, the one that contains the highest concentration of heavy minerals. Mineral separations were made using sodium polytungstate which has a density of 2.9 g/cm 3 (Callahan, 1987). Heavy-mineral separates were mounted in Canada balsam, and 400-500 grains were identified and counted using standard petrographic techniques. The number percentages obtained by point counting were converted to weight percentages (cf. Rubey, 1933; Young, 1966). Descriptive statistics are listed in Table 1, including average, minimum, maximum and standard deviation values for heavy mineral data and mean grain sizes of the study samples. These mineral percentages were subjected to factorial analysis. Q-mode factor analysis examines groupings of minerals within the assemblages (cf. Imbrie and Van Andel, 1964) and was performed on 83 bottom cores and beach samples with heavy minerals as variables. We used the CABFACprogram of Klovan and Imbrie ( 1971 ) which defines a small number of end-members by variable combinations
189
Table 1 Heavy minerals weight percentages and mean grain size data for bottom and beach samples of Burullus-Baltim coast Variable
Average Minimum Maximum Standard value value deviation
Heavy minerals Opaques Augite Amphibole Epidote Garnet Zircon Rutile Monazite
35.68 16.54 36.90 5.65 2.97 0.68 0.40 1.19
8.98 4.64 13.22 1.34 0.00 0.00 0.00 0.00
65.66 36.17 63.12 16.42 19.64 3.10 2.85 6.13
13.50 7.20 12.70 2.5 3.97 0.72 0.53 1.50
Offshore sand 2.47 Offshore mud 6.80 Modem beach sand 1.89
0.86 6.20 0.14
3.65 7.63 2.80
0.83 0.60 0.72
Mean grain size (gp)
of different members. The analysis, using the program option to scale all heavy mineral data on a range from 0 to 1, identified two factors which account for 87.8% of the variance. A GIS (Geographic Information System) was used to provide accurate areal distributions for the two factor loadings, taking into account water depths as well as mean grain sizes provided by Frihy et al. (1990). These values are fed into the GIS as individual layer in the form of vector files, and then converted to raster forms (images). This process was executed through modules of the ARCI~,a~o software. The Burullus map depicting the bottom and beach sample locations is first digitized and subsequently overlain by each individual layer (mineral factors, water depths and mean grain sizes). The potential distribution of each variable value is obtained by using the option of distanceweighted averaging interpolation followed by userdefined classification to minimize the number of classes.
4. Results and discussion The mean grain sizes of the bottom sediments are closely comparable to the present-day beach sand (0.14-2.80 ~) (Table 1). Similar coarsegrained sands, concentrated in heavy minerals,
O.E. Frihy, M.F. Lot]~v/Marine Geology 117 (1994) 187-194
190
were recorded along the modern beach east of Baltim (El Fishawi and Molnar, 1983, 1985). Interpolated mean grain sizes using GIS are shown in Fig. 3. Three belts comprising sand-size quartz grains of coarse to very fine sand (0.86-3.65 qb), containing some granules (recording fluvial deposition), beachstones and molluscan shells of littoral origin, occur along two axes. Microscopic examinations of the coarse fraction ( < 4 d~) revealed rounded to subrounded iron-stained quartz grains. The existence of these sediments off the Burullus-Baltim coast is probably related to relict sources, in the sense of being derived and reworked from the underlying substrate. This substrate probably deposited on a subaerially exposed alluvial plain at a time when sea level was much lower and the coastline was located further to the north (cf. Abdel Wahab and Stanley, 1991 ). Previous investigations have indicated that these reworked sandsize sediments are related to fluvial and coastal processes that prevailed when the coastline extended seaward off the central delta to a position close to the mid-shelf edge during Holocene eustatic low stands (Stanley et al., 1992b). At that time the Sebennitic branch discharged large volumes of medium and coarse sands delivered to the coast during floods. Q-mode factor analysis is applied on weight percentages of heavy minerals of the 83 bottom H u n Grain S£se ,q.. . . . . .
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and beach samples: opaques, augite, hornblende, epidote, garnet, zircon, monazite and ruffle. Micas are not included in the counting processes due to their flaky nature. The analysis serves to identify two factors (independent mineral groups), explaining 87.8% of the total mineralogical variations (variance). The derived Varimax factors are plotted on bar graphs in Fig. 4. Factor 1 consists mainly of augite and hornblende, while factor 2 contains opaques, garnet, zircon, monazite, rutile and epidote. An attempt was made to establish the relationship between the mineral factors, and there appears to be an inverse linear relation between them (Fig. 4). This means that minerals of factors 1 and 2 are inversely related, i.e., an increase of factor 1 corresponds to a decrease in factor 2. The sorting patterns inferred from the factor loadings are similar to the grain sorting patterns which have resulted in the development of black sand placers along the present-day Nile delta shoreline. The sorting of individual minerals within the black sand depends on their densities and grain sizes, i.e., the higher the density and smaller the grain diameter, the more concentrated the mineral becomes in the placer (Frihy and Komar, 1991; Komar, 1989). Waves and currents selectively sort and concentrate mineral grains. The less dense and larger grains have a greater probability of being winnowed out and carried away during beach erosion than do the denser and finer grains. Frihy and Komar (1991) found higher concentrations of opaques, zircon, rutile and garnet near the present Nile mouths where the erosion is greatest, while proportions of augite and hornblende increase in the direction of longshore transport to areas of accretion. Similar processes have been noted in India (Rao, 1957), and on Oregon beaches (Komar and Wang, 1984; Komar et al., 1989; Komar and Li, 1991; Li and Komar, 1992). Komar and Wang (1984) documented grain-sorting processes on an Oregon beach where lower density heavy minerals (augite and hornblende) moved offshore and high density minerals (opaques) were concentrated near shore. A similar pattern has been found by Li and Komar (1992) on beaches adjacent to the mouth of the Columbia River, but in the longshore direction. In the cross-shore direc-
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tion, Komar et al. (1989) have applied Q-mode factor analysis on heavy minerals separated from beach profile samples during a period of beach
191
face erosion. They identified two factors "mineral assemblages" correspond closely to selectivesorting patterns on Otter Rock beach as determined by Komar and Wang (1984). The two mineral factors 1 and 2 in the Nile shelf sediments respectively represent the mineral grains most easily entrained and transported versus those that tend to remain as a lag. The loadings of factors 1 and 2 are of great help to interpret heavy mineral dispersal and positions of ancient sediment sources in the study area. The relative loadings of the two factors in the 83 samples obtained in the Q-mode factor analysis are geographically distributed in Fig. 5A and B. A marked compositional pattern exists in the bottom samples around three localized areas deduced by long-term dispersal trends of heavy minerals. These small areas have lower loadings (high dilution) of factor 1 (augite and homblende) and higher loadings (high concentration) of factor 2 (opaques, garnet, zircon, monazite, rutile and epidote). This inverse areal distribution corresponds closely to selective sorting patterns near the modem river mouths: as one mineral suite tends to concentrate near the mouth (densest minerals), the other is transported alongshore away from the mouth. As expected, high loadings of factor 2 exist near the identified river mouths, indicating areas of maximum erosion. Sediment transport divergence, i.e., sediment moves away from the eroded point sources, interpreted river mouths, toward areas of depositional sites that characterize the ancient hydrodynamic regime in this region. The distribution patterns of the identified mineral factors may be related to former distributary branches of the Nile. Mapped areas of marked factor loadings in Fig. 5 are tentatively connected in the form of three dendritic branches, with the main course of the Sebennitic branch located at some point between the Burullus inlet and Baltim. Based on core analyses, the location of the main branch on the landward side of the coast is confirmed by Stanley et al. (1992b). The constructed secondary branches trend in a northwestern direction, and may have extended to the northeast as well. The interpolated position of the Sebennitic branch using heavy mineral species coincides with upper terraces on the inner shelf (Summerhayes
192
O.lz2 Frihy, M~F Lotfy/Marine Geology 117 (1994) 187 194
A.FACTOR 1
B. FACTOR 2
Fig. 5. Areal distribution of the two factor loadings based on heavy mineral data of bottom and beach samples of Burullus-Baltim coast. Divergence trends are shown by arrows oriented in direction of increasing or decreasing values of factor loadings. Major changes occur around three distinct sectors. The dashed dendritic lines depict tentative paths of the former Sebennitic branches. Depth in meters.
et al., 1978), and truncated linear beach ridges and coastal dunes along the Baltim-Gamasa sector (Stanley et al., 1992b). It also has been noted in Fig. 3 that the pattern of localized sand-size sediments almost coincides with the position and trend of the same areas that have the lowest loading of factor 1 and the highest loading of factor 2 (compare with Fig. 5). The relationships between mean grain sizes and the two mineral factors have been statistically tested by applying correlation coefficient analysis. Results show correlation coefficient values of 0.5 and -0.5 for factors 1 and 2, respectively. These results are not the expected relationship as has been found by Frihy and Komar (1991), that is the heavy
minerals are associated with finer sediments on the modern beaches due to selective grain entrainment and transport. In our study the explanation for the existence of heavy minerals in some coarsegrained samples could be attributed to the occurrence of few granules and pebbles included in the sand-size shelf sediments. These granules and pebbles have increased coarsening tendency of the shelf sediments. However, mean grain sizes of these sediments are compared closely to some places along the present-day original beach sand of the delta coast. In addition, Stanley et al. (1992b) have reported that beach sediments of the Sebennitic branch recovered from subsurface coastal cores were formerly coarser, these sediment had been discharged and subsequently delivered to the coast, especially during floods. Marked changes in the shapes of depth contours are noted (Fig. 2). The two broad triangular shapes revealed by bottom contours probably represent current-deformed submarine deltaic lobes related to the former prograded channels. These two contour shapes diverge from the present shoreline. The former river mouths along the ancient headland have probably shifted in time and space due to channel migration and the effect of the prevailing waves and currents.
5. Conclusions
Q-mode factor analysis of samples collected from the Nile Delta shelf yielded two factors. One is dominated by augite plus hornblende, and the second composed of opaques, garnet, zircon, monazite, rutile and epidote. These factors reflect the local patterns of mineral sorting due to contrasting grain densities and sizes during coastal erosion of the ancient Sebennitic promontory. These mineral patterns in conjunction with mean grain size provide information on provenance of recent and relict sediments, including those from the Sebennitic distributary. This latter is the former large Nile distributary which flowed across the shelf area off the central part of the delta, some 2000 years ago. Large-scale transgression and regression induced by sea-level changes as well as prevailing hydrodynamic processes have modified
O.E. Frihy, M.F Lotfy/Marine Geology 117 (1994) 187-194 the u p p e r s e d i m e n t cover on the inner shelf off the Nile delta. F o l l o w i n g d e p o s i t i o n , surficial sedim e n t s have u n d e r g o n e c o n s i d e r a b l e m i n e r a l o g i c a n d textural sorting. T h e m a r k e d v a r i a b i l i t y in h e a v y m i n e r a l a n d grain sorting d i s t r i b u t i o n s is largely a function o f s u b s e q u e n t s e d i m e n t transp o r t trends. T h e p a t t e r n s o f h e a v y m i n e r a l a n d grain sorting on the Nile delta shelf nevertheless r e c o r d relict sediments t h a t are related to the p o s i t i o n o f f o r m e r Nile branches. This m i n e r a l p a t t e r n c o r r e s p o n d s to the sorting p a t t e r n s f o u n d on the m o d e r n R o s e t t a p r o m o n t o r y b y F r i h y a n d K o m a r (1991), a n d p r o v i d e s p a r t i a l c o n f i r m a t i o n o f the hypothesis t h a t the shelf sediments d o reflect the f o r m e r extent o f the Sebennitic p r o m o n t o r y . To summarize, the contribution of petrologic and physiographic data helps to m o r e precisely locate the p r o p o s e d positions o f the s u b m e r g e d p r o m o n t o r y o f the Sebennitic channels. This a p p r o a c h c o u l d also be a p p l i e d for better definining o t h e r relict channels in o t h e r p a r t s o f the Nile delta c o n t i n e n t a l shelf.
Acknowledgements T h e a u t h o r s wish to t h a n k Dr. A h m e d A. K h a f a g y , d i r e c t o r o f the C o a s t a l R e s e a r c h Institute o f E g y p t , for p r o v i d i n g the field assistance o f this study. T h e m a n u s c r i p t was reviewed by Prof. Daniel Stanley, Smithsonian Institution, W a s h i n g t o n D C , Prof. D o n a l d Swift, O l d D o m i n i o n University, N o r f o l k , a n d Prof. Paul K o m a r , O r e g o n State University, U S A .
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MarineGeology117 (1994) 187-194
ELSEVIER
Mineralogic evidence for the remnant Sebennitic promontory on the continental shelf off the central Nile delta Omran E. Frihy, Morad F. Lotfy Coastal Research Institute, 15 El Pharaana Street, 21514, El Shallalat, Alexandria, Egypt
(Received October 28, 1992; revision accepted November 8, 1993)
Abstract
Heavy mineral compositions of closely spaced bottom samples off the central Nile delta are evaluated to determine provenance, dispersal patterns and sediment sources of a former branch, the Sebennitic. Q-mode factor analysis applied to heavy mineral data of 83 samples yields two factors: factor 1 (augite and hornblende), and factor 2 (opaques, garnet, zircon, monazite, rutile, epidote). The two factors correlate negativelywith each other. The observed mineralogic pattern is attributable to selective transport processes of the sediments related to a former subdelta. They probably originated from extensive progradation of the remnant promontory of the extinct Sebennitic branch which flowed across the subaerially exposed continental shelf during the last Holocene sea-level low stand. Grain sorting patterns of heavy minerals provide additional proof for the presence of a large Sebennitic channel extending across the continental shelf off the central Nile delta.
1. Introduction
The Nile delta shore consists of sandy arcuate beaches, approximately 270 km in length (Fig. 1). The coastline includes the present-day Nile promontories (Rosetta and Damietta). Lying between them is the remnant pre-modern Burullus promontory, which was formed by the discharged sediments from the former Sebennitic branch (Orlova and Zenkovitch, 1974), which was initiated during the Holocene between 7500 and 3000 yr B.P. (Arbouille and Stanley, 1991; Stanley et al., 1992a,b). When that branch was active, it probably formed a promontory similar to those that have developed at the mouths of the Rosetta and Damietta branches. The causes of abandonment of the Sebennitic branch could be related to the reduction of sediment supply and the flow of river water to the coast, resulting from climatic changes over the Nile drainage or due to the avulsion of 0025-3227/94/$7.00 © 1994ElsevierScienceB.V.All fightsreserved SSDI 0025-3227 (93)E0140-2
the Sebennitic channels (channels avulse when their gradient becomes too gentle). Subsequent long-term shoreline erosion resulted from the cut off of the sediment discharge, and the prevailing coastal processes have resulted in retrogradation of this coast to its present position. Similar coastal processes exist today off the Rosetta and Damietta Nile branches due to the cut off of sediment delivery by the High Aswan Dam. The study area is located seaward of the Burullus-Baltim coast off the central delta, fronting a shoreline length of about 27 km and covering a sector that extends seaward for about 12 km to water depths of 20 m (Fig. 2). The coast forms a broad, rounded bulge and is located on a very active shoreline that has experienced widespread erosion (UNDP/q.INESCO, 1978; Abdel Kader, 1982; Stanley et al., 1992b; Lotfy and Frihy, 1993). Heavy mineral assemblages concentrated by selective sorting of minerals due to differences in
O.E Frihy, M.F Lot/~7/Marine Geolog), 117 (1994) 187 194
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size and density have long been used to analyze sources and transport paths of sediments (Rubey, 1933; Rittenhouse, 1943; Trask, 1952; Van Andel, 1959; Komar and Wang, 1984). The Nile River has been identified as the major source of quartzrich sediments and sand-size heavy minerals on the shelves of Egypt and Israel (Ball, 1942; Hilmy, 1951 ). The aim of this study is to examine whether there is evidence for the ancient Sebennitic promontory of the Nile Delta in the grain sizes and
mineralogy of the sediments on the continental shelf. Previous investigations along the Rosetta promontory (Frihy and Komar, 1991) have recognized marked patterns of heavy mineral concentration due to grain-sorting processes as those promontories are eroded. The hypothesis is that similar patterns of heavy minerals formed along the shoreline of the Sebennitic promontory as it eroded for some 2000 years. Such patterns in the shelf sediments would provide evidence for the past extent of the promontory and the position of the river's mouth. There is, however, the possibility that the processes of shelf sediment transport have reworked the sands to such an extent that no remnant pattern exists that can be associated with this ancient promontory.
2. Historical background The Sebennitic branch is one of at least seven branches which crossed the central delta during Middle to Late Holocene times (pre-Dynastic to Roman time)• Five of the Nile distributaries have silted up, leaving two (the Damietta and Rosetta branches) active at present (Fig. 1). Previous studies have recorded archaeological, geographical and sediment•logical evidence for the existence of these former branches. Surficial mapping of the Sebennitic branch has been visually traced by satellite imagery (Abdel Kader, 1982) and aerial photography (UNDP/UNESCO, 1978), based on marked series of point bars and koms (low hills). Relict deposits related to the Sebennitic branch were mapped on the continental shelf by Frihy and Gamai (1991), by examining texture and coarse fraction compositions of bottom samples. Numerous beach pebbles, probably related to the Sebennitic promontory, were dredged from the sea bottom off the Burullus coast (Summerhayes et al., 1978). Recently, subsurface coring analysis have provided strong litho-stratigraphic evidence for the trace of the main Sebennitic channel and its promontory (Coutellier and Stanley, 1987; Chen et al., 1992). Delta lobes comprising marine delta-front and prodelta facies have been cored in the coastal zone of the delta plain. Such lobes forming at the mouths of Nile distributaries, originally accumu-
O.E. Frihy, M.F Lotfy/Marine Geology 117 (1994) 187-194
lating seaward of the present coast. Following their submergence, the distributary sediments have been eroded and reworked by the predominantly easttrending longshore currents (Inman and Jenkins, 1984; Stanley, 1989). The general position of the Sebennitic promontory is believed to have been seaward of the present north-central Nile delta coast, between Burullus inlet and Baltim resort. This channel was particularly important from about 7000 to 3000 yr B.P. (Arbouille and Stanley, 1991; Stanley et al., 1992b).
3. Sampling and methods of analyses Detailed sampling of the middle shelf and adjacent beach off the Burullus-Baltim sector in 1989 was undertaken to seek potential sources of sediments to nourish the Baltim coastal sector. Samples were recovered using hand cores driven by a diver, and vibracores along a grid survey of about 2.0 x 1.0 km (Fig. 2). In this study we use the 20 cm of the upper part of 65 cores as bottom samples, together with 18 beach face samples. Each sample was treated separately for heavy mineral analyses. The samples were sieved to obtain the 63-250 Ixm fractions, the one that contains the highest concentration of heavy minerals. Mineral separations were made using sodium polytungstate which has a density of 2.9 g/cm 3 (Callahan, 1987). Heavy-mineral separates were mounted in Canada balsam, and 400-500 grains were identified and counted using standard petrographic techniques. The number percentages obtained by point counting were converted to weight percentages (cf. Rubey, 1933; Young, 1966). Descriptive statistics are listed in Table 1, including average, minimum, maximum and standard deviation values for heavy mineral data and mean grain sizes of the study samples. These mineral percentages were subjected to factorial analysis. Q-mode factor analysis examines groupings of minerals within the assemblages (cf. Imbrie and Van Andel, 1964) and was performed on 83 bottom cores and beach samples with heavy minerals as variables. We used the CABFACprogram of Klovan and Imbrie ( 1971 ) which defines a small number of end-members by variable combinations
189
Table 1 Heavy minerals weight percentages and mean grain size data for bottom and beach samples of Burullus-Baltim coast Variable
Average Minimum Maximum Standard value value deviation
Heavy minerals Opaques Augite Amphibole Epidote Garnet Zircon Rutile Monazite
35.68 16.54 36.90 5.65 2.97 0.68 0.40 1.19
8.98 4.64 13.22 1.34 0.00 0.00 0.00 0.00
65.66 36.17 63.12 16.42 19.64 3.10 2.85 6.13
13.50 7.20 12.70 2.5 3.97 0.72 0.53 1.50
Offshore sand 2.47 Offshore mud 6.80 Modem beach sand 1.89
0.86 6.20 0.14
3.65 7.63 2.80
0.83 0.60 0.72
Mean grain size (gp)
of different members. The analysis, using the program option to scale all heavy mineral data on a range from 0 to 1, identified two factors which account for 87.8% of the variance. A GIS (Geographic Information System) was used to provide accurate areal distributions for the two factor loadings, taking into account water depths as well as mean grain sizes provided by Frihy et al. (1990). These values are fed into the GIS as individual layer in the form of vector files, and then converted to raster forms (images). This process was executed through modules of the ARCI~,a~o software. The Burullus map depicting the bottom and beach sample locations is first digitized and subsequently overlain by each individual layer (mineral factors, water depths and mean grain sizes). The potential distribution of each variable value is obtained by using the option of distanceweighted averaging interpolation followed by userdefined classification to minimize the number of classes.
4. Results and discussion The mean grain sizes of the bottom sediments are closely comparable to the present-day beach sand (0.14-2.80 ~) (Table 1). Similar coarsegrained sands, concentrated in heavy minerals,
O.E. Frihy, M.F. Lot]~v/Marine Geology 117 (1994) 187-194
190
were recorded along the modern beach east of Baltim (El Fishawi and Molnar, 1983, 1985). Interpolated mean grain sizes using GIS are shown in Fig. 3. Three belts comprising sand-size quartz grains of coarse to very fine sand (0.86-3.65 qb), containing some granules (recording fluvial deposition), beachstones and molluscan shells of littoral origin, occur along two axes. Microscopic examinations of the coarse fraction ( < 4 d~) revealed rounded to subrounded iron-stained quartz grains. The existence of these sediments off the Burullus-Baltim coast is probably related to relict sources, in the sense of being derived and reworked from the underlying substrate. This substrate probably deposited on a subaerially exposed alluvial plain at a time when sea level was much lower and the coastline was located further to the north (cf. Abdel Wahab and Stanley, 1991 ). Previous investigations have indicated that these reworked sandsize sediments are related to fluvial and coastal processes that prevailed when the coastline extended seaward off the central delta to a position close to the mid-shelf edge during Holocene eustatic low stands (Stanley et al., 1992b). At that time the Sebennitic branch discharged large volumes of medium and coarse sands delivered to the coast during floods. Q-mode factor analysis is applied on weight percentages of heavy minerals of the 83 bottom H u n Grain S£se ,q.. . . . . .
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and beach samples: opaques, augite, hornblende, epidote, garnet, zircon, monazite and ruffle. Micas are not included in the counting processes due to their flaky nature. The analysis serves to identify two factors (independent mineral groups), explaining 87.8% of the total mineralogical variations (variance). The derived Varimax factors are plotted on bar graphs in Fig. 4. Factor 1 consists mainly of augite and hornblende, while factor 2 contains opaques, garnet, zircon, monazite, rutile and epidote. An attempt was made to establish the relationship between the mineral factors, and there appears to be an inverse linear relation between them (Fig. 4). This means that minerals of factors 1 and 2 are inversely related, i.e., an increase of factor 1 corresponds to a decrease in factor 2. The sorting patterns inferred from the factor loadings are similar to the grain sorting patterns which have resulted in the development of black sand placers along the present-day Nile delta shoreline. The sorting of individual minerals within the black sand depends on their densities and grain sizes, i.e., the higher the density and smaller the grain diameter, the more concentrated the mineral becomes in the placer (Frihy and Komar, 1991; Komar, 1989). Waves and currents selectively sort and concentrate mineral grains. The less dense and larger grains have a greater probability of being winnowed out and carried away during beach erosion than do the denser and finer grains. Frihy and Komar (1991) found higher concentrations of opaques, zircon, rutile and garnet near the present Nile mouths where the erosion is greatest, while proportions of augite and hornblende increase in the direction of longshore transport to areas of accretion. Similar processes have been noted in India (Rao, 1957), and on Oregon beaches (Komar and Wang, 1984; Komar et al., 1989; Komar and Li, 1991; Li and Komar, 1992). Komar and Wang (1984) documented grain-sorting processes on an Oregon beach where lower density heavy minerals (augite and hornblende) moved offshore and high density minerals (opaques) were concentrated near shore. A similar pattern has been found by Li and Komar (1992) on beaches adjacent to the mouth of the Columbia River, but in the longshore direction. In the cross-shore direc-
O.E. Frihy, M.F. Lotfy/Marine Geology 117 (1994) 187-194 1-0
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tion, Komar et al. (1989) have applied Q-mode factor analysis on heavy minerals separated from beach profile samples during a period of beach
191
face erosion. They identified two factors "mineral assemblages" correspond closely to selectivesorting patterns on Otter Rock beach as determined by Komar and Wang (1984). The two mineral factors 1 and 2 in the Nile shelf sediments respectively represent the mineral grains most easily entrained and transported versus those that tend to remain as a lag. The loadings of factors 1 and 2 are of great help to interpret heavy mineral dispersal and positions of ancient sediment sources in the study area. The relative loadings of the two factors in the 83 samples obtained in the Q-mode factor analysis are geographically distributed in Fig. 5A and B. A marked compositional pattern exists in the bottom samples around three localized areas deduced by long-term dispersal trends of heavy minerals. These small areas have lower loadings (high dilution) of factor 1 (augite and homblende) and higher loadings (high concentration) of factor 2 (opaques, garnet, zircon, monazite, rutile and epidote). This inverse areal distribution corresponds closely to selective sorting patterns near the modem river mouths: as one mineral suite tends to concentrate near the mouth (densest minerals), the other is transported alongshore away from the mouth. As expected, high loadings of factor 2 exist near the identified river mouths, indicating areas of maximum erosion. Sediment transport divergence, i.e., sediment moves away from the eroded point sources, interpreted river mouths, toward areas of depositional sites that characterize the ancient hydrodynamic regime in this region. The distribution patterns of the identified mineral factors may be related to former distributary branches of the Nile. Mapped areas of marked factor loadings in Fig. 5 are tentatively connected in the form of three dendritic branches, with the main course of the Sebennitic branch located at some point between the Burullus inlet and Baltim. Based on core analyses, the location of the main branch on the landward side of the coast is confirmed by Stanley et al. (1992b). The constructed secondary branches trend in a northwestern direction, and may have extended to the northeast as well. The interpolated position of the Sebennitic branch using heavy mineral species coincides with upper terraces on the inner shelf (Summerhayes
192
O.lz2 Frihy, M~F Lotfy/Marine Geology 117 (1994) 187 194
A.FACTOR 1
B. FACTOR 2
Fig. 5. Areal distribution of the two factor loadings based on heavy mineral data of bottom and beach samples of Burullus-Baltim coast. Divergence trends are shown by arrows oriented in direction of increasing or decreasing values of factor loadings. Major changes occur around three distinct sectors. The dashed dendritic lines depict tentative paths of the former Sebennitic branches. Depth in meters.
et al., 1978), and truncated linear beach ridges and coastal dunes along the Baltim-Gamasa sector (Stanley et al., 1992b). It also has been noted in Fig. 3 that the pattern of localized sand-size sediments almost coincides with the position and trend of the same areas that have the lowest loading of factor 1 and the highest loading of factor 2 (compare with Fig. 5). The relationships between mean grain sizes and the two mineral factors have been statistically tested by applying correlation coefficient analysis. Results show correlation coefficient values of 0.5 and -0.5 for factors 1 and 2, respectively. These results are not the expected relationship as has been found by Frihy and Komar (1991), that is the heavy
minerals are associated with finer sediments on the modern beaches due to selective grain entrainment and transport. In our study the explanation for the existence of heavy minerals in some coarsegrained samples could be attributed to the occurrence of few granules and pebbles included in the sand-size shelf sediments. These granules and pebbles have increased coarsening tendency of the shelf sediments. However, mean grain sizes of these sediments are compared closely to some places along the present-day original beach sand of the delta coast. In addition, Stanley et al. (1992b) have reported that beach sediments of the Sebennitic branch recovered from subsurface coastal cores were formerly coarser, these sediment had been discharged and subsequently delivered to the coast, especially during floods. Marked changes in the shapes of depth contours are noted (Fig. 2). The two broad triangular shapes revealed by bottom contours probably represent current-deformed submarine deltaic lobes related to the former prograded channels. These two contour shapes diverge from the present shoreline. The former river mouths along the ancient headland have probably shifted in time and space due to channel migration and the effect of the prevailing waves and currents.
5. Conclusions
Q-mode factor analysis of samples collected from the Nile Delta shelf yielded two factors. One is dominated by augite plus hornblende, and the second composed of opaques, garnet, zircon, monazite, rutile and epidote. These factors reflect the local patterns of mineral sorting due to contrasting grain densities and sizes during coastal erosion of the ancient Sebennitic promontory. These mineral patterns in conjunction with mean grain size provide information on provenance of recent and relict sediments, including those from the Sebennitic distributary. This latter is the former large Nile distributary which flowed across the shelf area off the central part of the delta, some 2000 years ago. Large-scale transgression and regression induced by sea-level changes as well as prevailing hydrodynamic processes have modified
O.E. Frihy, M.F Lotfy/Marine Geology 117 (1994) 187-194 the u p p e r s e d i m e n t cover on the inner shelf off the Nile delta. F o l l o w i n g d e p o s i t i o n , surficial sedim e n t s have u n d e r g o n e c o n s i d e r a b l e m i n e r a l o g i c a n d textural sorting. T h e m a r k e d v a r i a b i l i t y in h e a v y m i n e r a l a n d grain sorting d i s t r i b u t i o n s is largely a function o f s u b s e q u e n t s e d i m e n t transp o r t trends. T h e p a t t e r n s o f h e a v y m i n e r a l a n d grain sorting on the Nile delta shelf nevertheless r e c o r d relict sediments t h a t are related to the p o s i t i o n o f f o r m e r Nile branches. This m i n e r a l p a t t e r n c o r r e s p o n d s to the sorting p a t t e r n s f o u n d on the m o d e r n R o s e t t a p r o m o n t o r y b y F r i h y a n d K o m a r (1991), a n d p r o v i d e s p a r t i a l c o n f i r m a t i o n o f the hypothesis t h a t the shelf sediments d o reflect the f o r m e r extent o f the Sebennitic p r o m o n t o r y . To summarize, the contribution of petrologic and physiographic data helps to m o r e precisely locate the p r o p o s e d positions o f the s u b m e r g e d p r o m o n t o r y o f the Sebennitic channels. This a p p r o a c h c o u l d also be a p p l i e d for better definining o t h e r relict channels in o t h e r p a r t s o f the Nile delta c o n t i n e n t a l shelf.
Acknowledgements T h e a u t h o r s wish to t h a n k Dr. A h m e d A. K h a f a g y , d i r e c t o r o f the C o a s t a l R e s e a r c h Institute o f E g y p t , for p r o v i d i n g the field assistance o f this study. T h e m a n u s c r i p t was reviewed by Prof. Daniel Stanley, Smithsonian Institution, W a s h i n g t o n D C , Prof. D o n a l d Swift, O l d D o m i n i o n University, N o r f o l k , a n d Prof. Paul K o m a r , O r e g o n State University, U S A .
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