Mediterranean Basin Program U.S. National Museum of Natural History Smithsonian Institution Washington, DC 20560, U.S.A. ABSTRACT

Transcription

1 Journal of Coastal Research Fort Lauderdale, Florida Distinguishing Sand Facies in the Nile Delta, Egypt, by Stained Grain and Compositional Component Analyses Daniel Jean Stanley and Zhongyuan Chen Mediterranean Basin Program U.S. National Museum of Natural History Smithsonian Institution Washington, DC 256, U.S.A. ABSTRACT _.tflllllllt.. wi1" -+ &-- STANLEY, D.J. and CHEN, Z., Distinguishing sand facies in thenile Delta, Egypt, by stained grain and compositional component analyses. Journal of Coastal Research, 7(3), Fort Lauderdale (Elorida). ISSN Counts of stained grains and of compositional components serve to simply and rapidly, yet reliably, distinguish sand facies in different modern environments of the Nile delta in Egypt. River, lagoon, beach, and marine nearshore sands are characterized by distinct petrographic attributes, and each facies is readily differentiated from each other and from modern desert sands. Less clearly defined are coastal dune sands which present compositional characteristics that overlap with those of marine nearshore and beach facies. Additional measurements, including textural analyses, proportions of specific heavy minerals and/or identification of surface textures on quartz grains can help discriminate coastal dune, nearshore, and beach sands. The stained grain and compositional component method is applied to the study of subsurface sand facies. In this investigation, it is used to help determine the origin of sands of late Pleistocene to Holocene age recovered in Nile delta core sections. This approach is particularly helpful to interpret sand strata whose primary structures and original textures have been modified and/or altered during drilling, and/or whose chronostratigraphic position in core sections is unknown or poorly defined. Improved definition of sand facies enhances stratigraphic correlations and, in turn, our understanding of how the evolution of fluvio-delt.aic sequences relates to migration of distributary branches of the River Nile, to progradation and/or erosion of the coast, and to eustatic changes in sea level and subsidence. ADDITIONAL INDEX WORDS, Beach [acies, coastline evolution. coastal dune. lagoonal sand, nearshore sand, petrography of sand, River Nile sand, sea level change. INTRODUCTION This investigation identifies some petrographic attributes which can serve to distinguish different sands in modern depositional environments and in late Pleistocene to Holocene core sections in the Nile delta of Egypt (Figure 1). The study was undertaken as part of a regional investigation to interpret the late Quaternary evolution of this delta, a research program based largely on analysis of a series of about 1 cores collected across the northern Nile delta (STANLEY, 199). Earlier generalized interpretations of sand facies in Nile delta cores have been made largely on the basis of their stratigraphic position and compositional and textural attributes (EL FISHAWI, 1985; EL ASKARY and FRIHY, 1986; COUTELLIER and STANLEY, 1987; FRIHY and STANLEY, 1988; SES TINI, 1989; ARBOUILLE and STANLEY, 1991) received 3 March 1991: accepted in revision 18 April The present paper focuses on river, lagoon, beach, marine nearshore, and coastal dune facies which account for most sand bodies of late Quaternary age in this depocenter (cr. UNDPIUNESCO, 1978; FRIHY et al., 1988; SES TINI, 1989)_ Modern desert sands are also considered in this study since the River Nile, winding its way across hundreds of kilometers of desert (BALL, 1942; HURST, 1944; SAID, 1981), has transported an important eolian component to the delta and beyond to the Mediterranean Sea. Earlier work has shown that eolian attributes of sediments derived from desert settings are preserved in some delta sandy facies (FRIHY and STANLEY, 1987). Interpretation of sand types in subsurface sections can be enhanced by defining the primary structures, age, and stratigraphic position of sand sequences relative to other deltaic facies with which they are associated (cf. COLE MAN, 1982). Sediment texture is also used to

2 864 Stanley and Chen 3' 31' 3,?" M E o T E R R A N E A N S E A OTel Farama w e (") Modern Environments o Desert River + Lagoon 6 Coastal Dune Beach Nearshore M - Maryut Lake O5Mi 1- Idku Lake B - Burullus Lake 25Km MZ - Manzala Lake RP - Rosetta Promontory DP - Damietta Promontory w Smithsonian Cores Mi 25Km o 7 15 Figure 1. Maps showing: (upper) the locations of surficial (modern) samples from the desert and five Nile delta environments; and (lower) the positions of 16 Smithsonian cores in the northern Nile delta. characterize river, beach, and dune sands (cf. MASON and FOLK, 1958; FRIEDMAN, 1961, 1979). In modern Nile delta coastal environments, discrimination of sand facies can be made by size distribution patterns (EL FISHAWI, 1985), shape analyses (roundness and sphericity) of particles (EL FISHAWI, 1984), and grain size versus heavy mineral content (EL FISHAWl and MOLNAR, 1985). However, the textural approach cannot be used in the case of the Smithsonian cores because most of the thicker subsurface sand units were recovered by water circulation, rather than in undisturbed core tubes. This water injection method disrupts the internal structure of strata and modifies texture by artificial removal and/or addition of some size fractions. A previous study of the texture and composition of subsurface sands in Nile delta cores was only moderately successful (FRIHY and

3 Sand Facies in the Nile Delta 865 STANLEY, 1988). Statistical (Q-mode factor) analysis to discriminate sand facies was made using 375 samples of late Pleistocene and Holocene age from 14 Nile delta borings. In that study, 3 textural and 14 mineralogical, faunal and floral components were considered for each of three discrete sand units that are present in many of the cores. Factorial analysis was unable to discriminate among the three sand units in the cores. The explanation for this is that (a) the texture of the sediment was altered during drilling, and (b) each of the three sand units actually comprises sand derived from two or more depositional settings. It is not surprising, therefore, that quantitative treatment of data which groups overlapping sediment environments would preclude discrimination of the three sand units. An additional difficulty to clearly differentiate among Nile delta sand types on the basis of texture alone results because sediment has been reworked and displaced from one environment to another. In consequence, sand particles record the effects of this multiple sediment transport (both subaqueous and eolian) mode and of the complex migratory shifts between different depositional environments (EL FISHAWI and MOLNAR, 1984; FRIHY and STANLEY, 1987). The present study considers petrographic attributes which can readily distinguish modern Nile delta sand facies and subsurface sand sequences. Any method selected for this should be applicable to core samples whose primary structures are disrupted during drilling, i.e. the emphasis needs to be on compositional rather than textural characters. Special consideration is given to a simple and rapid petrographic technique which can easily be used in the field. The approach, requiring only a binocular microscope, can be applied to all sand types recovered, whether surficial/modern or core samples. The method proposed involves counts of two sets of readily measured parameters: staining on light mineral particles and sand grain composition. METHODOLOGY A total of 36 samples of sand-rich deposits (Table 1) were selected from six modern environments (locations are shown in Figure 1): river (n = 5); lagoon (n = 5); coastal dune (n =1); beach (n = 7); nearshore marine (n = 5); and desert close to the River Nile and delta (n = 4). Some samples used in this investigation are the same as those used in a previous study of quartz grain surface textures (FRIHY and STANLEY, 1987, their Figure 1). General observations of surficial desert sands in the vicinity of the Nile delta and River Nile (EL-BAZ, 1978) and petrographic study of about 25 sand-fraction samples from more than 7 cores recovered in the Nile delta proper (MEDIBA, 199) have revealed different types and extents of iron oxide coatings on sand grains. It was determined that an analysis of the relative degree of staining of light mineral grain particle surfaces could help to discriminate different sand facies. Counts of 3 or more grains (primarily quartz) were made for each of the 36 modern sand samples using a binocular microscope. Three categories were recognized: transparent (clear grains without any oxide coating); partially stained (grains that are only partly coated); and completely stained (translucent to opaque particles, usually yellowish to reddish brown). The relative percentages of the three stain groups for each of the 36 samples are listed in Table 1 and plotted in a ternary diagram (Figure 2a). The different origins of iron oxide coating, discussed in investigations of Egyptian deserts (EL-BAZ, 1978) and numerous other settings (cf. NORRIS, 1969; VAN HOUTEN, 1973; FOLK, 1976), are not discussed in the present study. A separate compositional analysis was then made of each of the 36 modern sand samples. Three subset cuts from each sample were placed in separate gridded grain-mount slides for a total of 18 analyzed samples. Sand grains were identified with a binocular microscope. For each of the 18 sample cuts, 3 or more sand grains (>63 u.) were counted and the relative percentages of compositional components were determined. Counts were made of 3 or more particles lying along the grids printed on the microscope slides. Measurements were made of 13 components (Table 1): light minerals, heavy minerals, glauconite (often associated with verdine, cf. PIMMEL and STANLEY, 1989), gypsum, foraminifera, shell fragments, unidentified carbonate particles, ostracods, mica, pyrite, plant debris, lithic fragments and diatoms. Grain count data is stored in the MEDIBA (1991) database file at the Smithsonian's National Journal of Coastal Research. Vol. 7. No

4 866 Stanley and Chen Table 1. Relative percentages, from two separate analyses, ofpartially stained grains and ofcompositional components for 36 samples from desert and five modern Nile delta environments. Sample locations are shown in Figure 1 (upper). Facies Sample -Stain- Major in2 Components Associated Components No. Part. Light Heavy Glaucon Gypsum Forams Shell Unident Ostracods Mica Pyrite Plant Diatoms stained minerals minerals % % % Frags. Carbon- % % % Debris % % % % % ales % % DS Desert DS Facies DS Tel Farama R River R Facies Darnietta Mansoura Cairo MZ Lagoon MZ Facies MZ MZ MZ Baltirn-I Baltim-Z BaJtim Coastal Baltim Dune Ganasa Facies S-33.Wl S-34.Wl S-35.surf S-35.Wl S-36.Wl Baltim Baltim-S Bea:h B Facies B B B B NR-I Nearshore NR Facies NR-14-a 17.] NR-14-b NR Museum of Natural History in Washington, D.C. The relative percentages calculated for the 13 components in the three sub-samples were then averaged for each of the 36 modern sand samples (data in Table 1). From these data, we averaged the relative percentages of the two dominant or major parameters (light minerals, heavy minerals) and all other components grouped together (these latter are named herein associated components). These three major data groups are depicted graphically for each of the six major sand facies (Figure 3). The relative percentages of the associated components (all except light and heavy minerals) have been averaged for each sample. The averaged value for 1 of the associated components in each of the six sand facies are depicted graphically in Figure 4. For each of the six modern sand facies, the range of relative percentages of partially stained grains and the ranges of relati ve percentages for 11 of the 13 major and associated components are summarized in Table 2. Gypsum and lithic fragments, do not appear to be diagnostic components to discriminate among sand facies, and thus are not shown in this table. This stained grain-compositional component method was also used to help identify the depositional origin of 18 sand-rich subsurface samples from 16 borings collected in the northern Dile delta (Figure 1), The depth and age (late Pleistocene to Holocene) of these core samples are listed in Table 3. Petrographic analysis of the 18 samples was identical to that of the 36 modern sand samples. Measurements were made for three subset samples for each of the 18 core samples, for a total of 54 grain counts. Data averaged from the three subset counts for these 18 subsurface core samples are listed in Table 3. The results of stained grain analysis of these subsurface samples is shown in Figure 2b.

5 Modern Environments Core Samples o Desert Facies River Facies -t- Ulloun Fades 6. Coastal Dune Facies Btach Facies Nearshore Facies 8 River Itlvuth Facies Transparent /1\, a / \!.+*, \..,... \.. / Transparent. I.. -. " ,., {j.'. D. / Completely Stained ----,, O---'-"(l..J)'-)L ,\ o Figure 2, Ternary diagrams showing relative percentages of transparent, partially stained and comp 36 samples from modern environments, and (b) 18 subsurface samples (A-RI from 16 Smithsonian core on the ternary triangle in (b) is interpreted to have accumulated in the depositional environment as s

6 868 Stanley and Chen Modern Depositional Facies Grouping Percentage Desert Facies n=4 Coastal Dune Facies n=lo River Facies n=5 Beach Facies n=7 Lagoon Facies n=5 Nearshore Facies n= Light nerals [] Heavy Minerals Other Components Figure 3. Pie diagrams showing averaged relative percentages of light minerals, heavy minerals, and associated components for the desert and five modern Nile delta environments. Data is derived from Table I. MODERN NILE DELTA SAND FACIES CHARACTERISTICS The following sections summarize the major compositional and grain stain attributes of the sand facies in five modern Nile delta environments (river, lagoon, beach, marine nearshore and coastal dune), and also characteristics of some modern desert sands. Sample locations are shown in Figure 1. Salient petrographic

7 Sand Facies in the Nile Delta 869 Desert Facies River Facies Lagoon Facies OJ Ol ro C e OJ CL Nearshore Facies 3 Beach Facies Coastal Dune Facies 2 ro c OJ OJ CL 2 o Figure 4. Histograms showing averaged relative percentages of 1 associated components (excludes light and heavy minerals and lithic fragments) for each of the desert and 5 modern Nile delta environments. Data is derived from Table 1. characteristics of the six modern facies, derived from data listed in Table 1, are highlighted in Table 2 and illustrated in Figures 2 to 4. River Nile Sands Of the five deltaic facies, River Nile sands are characterized by the highest proportions of light minerals and partially stained grains. This fluvial facies often contains mica, pyrite and plant debris. Marine biogenic components are generally absent, and the glauconite/verdine content IS the lowest of the five delta facies. Lagoon Sands Sands of lagoonal origin usually contain the lowest proportions of both light and heavy minerals, and the highest percentages of associated components, usually those of biogenic origin. From the data in Table 1, two lagoonal subtypes are recognized. The first (type-l) comprises a high proportion of biogenic components, includ-

8 87 Stanley and Chen Table 2. Salient petrographic characteristics (relative percentages vf partially stained grains and compositional components) for the desert and five modern Nile delta environments. Numerical data from Table 1. MODERN FACIES CRITERIA Desert River Lagoon Coastal Beach Nearshore Dune Light minerals Heavy minerals Glaucon Mica Shell Frags Pyrite Diatoms Unident. Carbonate Particles Plant Debris ing ostracods, foraminifera, molluscan shell fragments and diatoms. The proportions of partially stained grains is not diagnostic (values are lower than those of river sands, but higher than those of nearshore marine samples). The second (type-s), somewhat more terrigenous, comprises a higher light and heavy mineral and plant debris content, but lower proportions of other components. These latter usually include some foraminifera (mostly Ammonia tepidai and ostracods (commonly Cyprideis torose) recording at least some marine influence. The percentages of stained grains in type 2 lagoonal sands is not diagnostic. Journal of Coastal Research. Vol. 7, No.3, 1991

9 Sand Facies in the Nile Delta 871 Table 3. Relative percentages, from two separate analyses, ofpartially stained grains and ofcompositional components for 18 samples from 16 Smithsonian cores recovered in the northern Nile delta. These can be compared to ranges of values for samples from modern environments (see Table 1). These subsurface samples are dated late Pleistocene (LPJ, late Pleistocene to Holocene (P-H), and HoLocene (H). Locations of borings are shown in Figure 1 (lower).... I"') Nv.- e! -Ju.. j.. iii '-q o OOO N S -- or-: r- iii V ""; C c: 6 9 ;} N (".l- "";""';N""; c.. S U cigd N I"') "..;c cig d9 v\o 69 'u "2 s ""; ;:) c:s V) V) =:tii < e o v o - u.. N \ (".l -:.or-: v r- 1"')'1 1Ii\O\ONvv) O... f"l -- "'- "'''''; """; 6 ci 9:; (".l \ f"lnf"ln--f"l u, t? oq f"l- \ -:. o'-q N - 11"'I f"l v: or-: f"lf"l oqv: N'<:tr-\O... " ;...;,...;..,:... - O"'MO 's ' N v f"""t "'l:t '1\ "'l:t r;o MOO o -(".lm-r-\o """; a-v) ""': -... M 6"";""";""": C o ciq V'lN \11"'I- 9 g ::g:! ""; 9 N\OO'If"lMqOO s... - N...1Iir-r- t'f"lv '1 N r--:..o-or--:o\:::oo oc 2& S.c V) '1 r--ii"'i \ r-- (".l"'l:tv) N r-- N o \...,.-r-omoo.g 9v) gggg gg&i 6Jgg r--: g,.,.; o\9r--: r- N \ g;;gg U l" 9 -o cs sc e- "o:t COl ;c -;li"i I.DOONM i:g (".lrrlo'l'v MOo\o\ 9""; ;92 '1,.,.;...; N V) r-:r;qo-:oo V) v"<f""'it rjj.- N N r--::: f"lnn - "<f" NM C ig oov:q f"l N OV)V)OO\OMOO Nf")-r- NM...,. r-..:..o... o\ o :! E L. CN :-::::,. «a1ucl Ul u..o::r: Jzoo..acr. c2 u,.5 z :J :r: ::r:::r:7:r: :J <:..> ) c.. :I:...J II II U :r: II II II II II II eu V) a. II II '1. II II V) II r--q.. " E - N lrl O'IIrION"<f"MV) q- OO-lrl N-l"""l--r")("'l V),- I ooo.. J..J, J..--6,+OOob.nN CFJ V)r--r- Vl Vl C/) C/) \D Vl \O U') '5 c I Q,) v: ' u h c -s c28 j,-,. eu ] c;;;;;)) c E c. 'u J] t l] f- ", u Beach Sands Of all delta sand samples, those collected on beaches (includes the associated breaker, swash, and backshore zones) comprise the highest proportions of heavy minerals. With respect to associated components, glauconite/verdine and gypsum contents are relatively important and, unlike river deposits, beach sands usually contain some reworked marine biogenic com-

10 872 Stanley and Chen ponents. The proportions of partially stained grains is not diagnostic (values are intermediate between river and nearshore samples). Nearshore Sands The most characteristic feature of shallow marine (inner shelf to coast) sands is the high proportion of transparent grains and lowest percentages of partially stained grains. When compared to beach and coastal dune facies, the proportions of heavy minerals is somewhat lower, and those of total associated components (particularly glauconite/verdine, foraminifera and gypsum) are higher. Coastal Dune Sands Of the five sand facies in the Nile delta, those of coastal dunes are the least clearly distinguished using the petrographic parameters considered in this study. Proportions of light and heavy mineral contents are intermediate and overlap with values for beach and nearshore sand facies. The total associated components content, however, is usually lower than those two other facies. Moreover, the proportions of partially stained grains in coastal dune sands is in most cases higher than in nearshore sands. Desert Sands Desert sands are clearly distinguished from all five Nile delta sand facies in that these eolian deposits consistently contain (by far) the highest proportion of partially stained grains and, also, the highest relative percentage of light minerals. Moreover, desert facies comprise the lowest percentages of heavy minerals and associated components. Unidentified carbonate grains tend to be the most common associated particles (almost always present in low quantity). SUMMARY OF OBSERVATIONS ON MODERN SAND FACIES From the above results, it appears that stained grain and compositional component analyses serve to (1) readily distinguish among four of the five Nile delta sand facies, and to (2) show that all five of these deltaic sands are markedly different from desert sands. The information listed in Tables 1 and 2 and depicted graphically in Figures 2 and 3 indicate that River Nile sands are more similar to desert sands than to the other four delta facies. It is recalled that the River Nile crosses hundreds of kilometers of desert between the African ri ver headlands and the delta of the Mediterranean coast (BALL, 1942; HURST, 1944; BUTZER and HANSEN, 1968). Thus, it is not surprising that Nile sands contain such high proportions of stained and partially stained grains (cf. EL BAZ, 1978) and also high percentages of light minerals. The two lagoonal sub-facies recognized in this study are related to two different depositional environments: type-l, influenced by higher marine input, contains a larger proportion of associated biogenic components; type-2, more directly influenced by river flow, comprises a high terrigenous particle content. A recent study of Holocene lagoonal facies in the Nile delta (ARBOUILLE and STANLEY, 1991) has shown that different depositional sequences in lagoons can be recognized on the basis of their geographic position: proximal to river mouths (type-2), to lagoonal outlets (type-l), and to coastal sand ridges and sites of sediment washover (type-l). The data compiled in the present study show that the nature and proportions of compositional components of sands from the marine nearshore, beach and coastal dune are more similar to each other than to the sands from the lagoon, river or desert. However, the very low proportion of partially stained particles in nearshore sands serves to clearly distinguish this facies from the beach and coastal dune sands. Distinguishing beach facies from coastal dune sands is not so obvious. That the two facies are gradational with regards to compositional components and grain staining is not surprising: coastal dunes are derived primarily from beaches. First, sediment is moved along the coast by the dominant east-trending coastal currents and, once on the beach flats, some sand particles are driven landward by prevailing southeast-directed wind patterns (UNDP/ UNESCO, 1978; EL FISHAWI and EL ASKARY, 1981; INMAN and JENKINS, 1984; FRIHY et al., 1988). It is helpful to consider the results of additional petrological treatment to reliably distinguish coastal dune sands from those of

11 Sand Facies in the Nile Delta 873 beaches and nearshore sands, i.e. grain size parameters, particle shape, and proportions of different heavy mineral types (cf EL FISHAWI, 1984, 1985; EL FISHAWI and MOLNAR, 1985). Earlier studies also showed that identification by Scanning Electron Microscope of specific chemical features on quartz surfaces can be used to help distinguish coastal dunes from beaches and nearshore samples (cf EL FrSHAWI and MOLNAR, 1984; FRIHY and STANLEY, 1987). As an additional independent test of the stained grain-compositional method, the relative percentages of partially stained grains and of major and associated components for the 36 modern sand samples (Table 1) were treated using the discriminant analysis procedure of the SAS System run on an IBM Results of this analysis indicate that 29 ofthe 36 data sets were correctly classified as to the six depositional environments considered. The seven misclassified samples, as expected, were primarily from the beach and coastal dune group. STAINED GRAIN-COMPOSITIONAL COMPONENT METHOD APPLIED TO DELTA CORE SAMPLES General It was deemed useful to apply the combined stained grain- compositional component method to study 18 core samples recovered in 16 Smithsonian borings in the northern Nile delta (core locations are shown in Figure 1). All 18 sand and sand-rich samples examined are 'washings', i.e. collected by water circulation rather than in undisturbed cohesive core sections. Detailed lithologic logs, and petrologic and stratigraphic information for each of the 16 cores considered are stored in the Smithsonian's MEDIBA (1991) database file. The samples selected for study here are from sands of late Pleistocene, latest Pleistocene to early Holocene, and Holocene age. The lack of preserved primary structures and the artificially altered textures makes it difficult, if not impossible, to assign these samples to specific depositional environments by visual observation only. Each core sample was treated petrographically by the same operator and in exactly the same manner as the modern samples. Numerical data from the examination of the 18 samples are listed in Table 3, where relative percentage values of compositional components can be compared to the range of such values calculated for modern sand facies of known origin. The proportions of stained, partially stained, and transparent grains of the 18 unknown Nile delta core samples are plotted in a ternary diagram (Figure 2b). Determinations of the depositional environment of the 18 core samples, based on stained grain content, is shown in Figure 2c. These interpretations are based on comparison with stained grain data of modern sands as highlighted in Table 2 and graphically depicted in Figure 2a. Depositional Environments of Late Pleistocene Sands Although Nile delta cores have been collected and described for well over a century (cf FOUR TEAU, 1915; ATTIA, 1954; ZEINELABDINE et al., 1966), little is known about the origin of thick, tan to yellow-brown sands that lie below the grey sands (of late Pleistocene-early Holocene age) and grey progradational muds (of Holocene age). This late Pleistocene stratigraphic sequence is thick and formed of medium- to coarse-grained, poorly sorted sands. This facies constitutes a large part of subsurface sequences, especially in cores of the northern delta west of the Damietta branch. In this latter region, the sands occur at progressively shallower depths because Holocene mud and the immediately underlying grey sand units are of reduced thickness. Radiocarbon dating of stiff muds interbedded with the yellow-brown sands indicates that this facies is generally older than 12, years before present (MEDIBA, 1991). Recent studies that emphasize stratigraphic position and general composition attribute this facies to a fluvial (COUTELLIER and STANLEY, 1987) and to a fluvial and/or shallow marine to coastal origin (ARBOUILLE and STANLEY, 1991, their Figure 3). In the present study, five samples of late Pleistocene age were selected from the lower parts of five cores (coded B to D, Q, and R in Table 3). On the basis of compositional components and stained grain content, two distinct groups are identified: one is fluvial (samples B, C and D); the other is nearshore marine (samples Q and R). The fluvial attributes of late Pleistocene samples B, C, and D include high proportions ofpar-

12 874 Stanley and Chen tially stained grains and light minerals, and a low content of heavy minerals and non-marine biogenic components. These sands were likely deposited on a subaerially exposed alluvial plain at a time when eustatic sea level was much lower and the Nile delta coastline was located further to the north (cf. ABDEL WAHAB and STANLEY, 1991). In contrast, the nearshore marine attributes of samples Q and R include much lower proportions of partially stained grains and somewhat higher proportions of glauconite/verdine and marine biogenic components. These marine sands probably accumulated just seaward of the delta coast, in a delta-front environment. The late Pleistocene sands in the northern delta cores usually contain insufficient carbonate material to provide reliable radiocarbon dates, so their chronostratigraphy is difficult to establish. Thus, the ability to distinguish the two facies on the basis of petrography is most helpful. Discrimination of fluvial from shallow marine sands now provides a means to relate the stratigraphic position of pre-holocene sand units to eustatic sea level fluctuations. Depositional Environment of Late Pleistocene to Early Holocene Sands In many cores, the medium- to coarse-grained yellow-brown sands (considered above) and interbedded indurated brown and grey-green muds of late Pleistocene age are overlain by medium-grained grey sands. These grey sands constitute a discrete stratigraphic unit that almost always underlie the fluvio-marine deltaic sequences of Holocene age in the northern delta. This grey sand unit is dated from the latest Pleistocene to earliest Holocene (COUTEL LIER and STANLEY, 1987) and is interpreted as a retrograding shoreline deposit (EL ASKARY and FRIHY, 1986). These Nile delta grey sands occupy a similar stratigraphic position as those depicted in the generalized delta model ofscru TON (196) for the Mississippi delta. On the basis of ongoing studies, this facies appears to have been derived from a thorough reworking of older (alluvial, nearshore and other deltaic) deposits, with a removal of fines and concentration of sands near former coastlines. Accumulation of this sand unit took place during the last major marine transgression (EL FISHAWI, 1985; EL ASKARY and FRIHY, 1986; ARBOUILLE and STANLEY, 1991). Five sand samples (L-P in Table 3) attributed to the transgressive sand unit were selected from five cores in widely different sectors of the nile delta (core positions in Figure 1; logs in MEDIBA, 1991). These samples were selected for this study in order to determine if the sands underlying the Holocene deltaic sequence are, in fact, of nearshore marine origin, i.e. having formed along a retrograding shoreline. The five sand samples, as a group, are characterized by low proportions of partially stained grains (Figure 2b) and high percentages of light minerals. Moreover, these sands contain a low heavy mineral content, but relatively high proportions of glauconite/verdine and marine biogenic components. On the basis of all attributes, these sands most closely resemble nearshore coastal facies (compare with examples of modern nearshore sands in Figures 2a, 3 and 4). SEM analysis of quartz grain surfaces (cf. KRINSLEY and DOORNKAMP, 1973) would likely further substantiate that this facies comprises a large proportion of sediments that have been reworked from older environments. Distinguishing River from Lagoonal Sands in Holocene Sections Surveys indicate that most of the recent sand in the modern Nile delta plain south of the coast was deposited by active River Nile distributary channels (the Damietta and Rosetta branches) and also accumulated, locally, in some of the lagoons (EL-WAKEEL and WAHBY, 197). On the modern delta plain, it is possible to readily distinguish ri ver from lagoonal facies as was demonstrated in an earlier section of this study. In the case of subsurface sections, however, differentiating river channel sands from lagoonal sands is more ambiguous. In this study, five sand samples (A, F-I in Table 3) of undefined origin were selected from Holocene sections in five cores recovered in the north-central delta (Figure 1). These sands are interbedded with soft grey muds that are identified as lagoonal and marsh deposits (ARBOUILLE and STANLEY, 1991; HOWA and STANLEY, 1991). The petrology of one of the five samples (A in Table 3) most closely resembles that of modern River Nile sands, particularly with regards to high proportions of partially stained grains

13 Sand Facies in the Nile Delta 875 (Figure 2b) and light minerals. Moreover, this sample is characterized by a relatively low content of heavy minerals and associated components. The associated components do not include marine biogenic particles. These observations suggest that this sample (from a core collected south of Burullus lagoon) was likely deposited by a former distributary branch of the River Nile, perhaps the Saitic channel system (cf. TOUSSOUN, 1922; SAID, 1981). The compositional attributes of four Holocene sand samples (F-I in Table 3) more closely resemble those of modern lagoonal sands. They contain higher proportions of marine biogenic components (Table 3) than in river sands, and are of type-2 lagoonal facies as defined earlier. Moreover, the proportions of partially stained grains (Figure 2b) and light minerals are generally intermediate between those of river and nearshore samples. Thus, the compositional attributes of the four Holocene sand samples most closely resemble modern terrigenous-rich lagoonal facies that are influenced by fluvial input. These core sand samples perhaps record input of river sand on lagoon margins at times of high Nile discharge. Distinguishing Facies in a Thick All-Sand Core Core S-65, collected on the western part of the Rosetta promontory near the present mouth of the Rosetta Branch, recovered nearly 5 meters of medium to coarse sand as 'washings'. Sedimentary structures are not preserved and texture has been altered. There is insufficient carbonate material for radiocarbon dating, so that it is not possible to determine if the sands are of late Pleistocene or of Holocene age. An initial petrographic analysis of 33 samples collected between the base and top of core S-65 showed an essentially constant clastic composition dominated by light and heavy minerals, and a low proportion of marine biogenic components (data in MEDIBA, 1991). This preliminary compositional component analysis, however, does not provide a clear indication of whether the sands in core S-65 are of marine or of nonmarine origin. In the present study, a petrological reexamination was made of selected samples. Sand at the base of the core (sample E in Table 3) is characterized, like most river samples, by relatively high proportions of partially stained grains (Figure 2b) and light minerals. However, unlike most river sands, this sample displays a higher heavy mineral content and also the presence of a minor proportion of associated marine biogenic components. It is of note that the dominant river a ttrib utes are combi ned with those of some coastal facies. On this basis, this sample is interpreted as having been deposited at or near a river distributary channel mouth. Sand samples in the middle and upper parts of the core (respectively K and J in Table 3) also indicate accumulation at or close to a river mouth, but in a setting more strongly affected by beach processes. Samples K and J more closely resemble modern beach deposits than does basal core sample E with its more pronounced fluvial characteristics. Analyses of additional samples from this boring show a subtle but nearly constant change upward from the base to the top of the core. Of note is the decrease upward in proportion of partially stained grains and concurrent increase in heavy mineral content provide a basis for a general interpretation. It appears that the core site, with time, has occupied a position progressively closer to a beach setting, i.e. to the present mouth of the Rosetta Branch of the Nile. This evolution is probably a response to changes in (1) Rosetta promontory position and configuration, the result of channel migration and coastal erosion (BLODGET et al., 1991) and also to (2) rising sea level and subsidence (STANLEY, 199). We propose that the sands at the base of core S-65 are derived from the Bolbitine (pre-rosetta distributary) Branch (cf. TOUSSOUN, 1922; SAID, 1981), which may be as young as about 25 2 years B.P. Thus, the core site is one of very rapid accumulation (>2 em/year of river channel and coastal sands) in a sector of rapid subsidence (to em/year, cf. STANLEY, 199). CONCLUSIONS A petrographic method that involves counts of stained grains and of compositional components serves as a simple and rapid, yet reliable, means to readily distinguish dominant sand facies in the modern Nile delta. River, lagoon, beach, and nearshore marine facies are each characterized by distinct petrographic attri-

14 876 Stanley and Chen butes. Coastal dune sands, with characteristics intermediate and overlapping between nearshore and beach facies, are less clearly identifiable. Identification of coastal dune sands may require additional measurements, such as textural analyses, proportions of specific heavy minerals, and/or identification of surface textures on quartz grains. The petrographic method used to discriminate facies in modern environments can also help to interpret the origin of various subsurface sand deposits such as those of late Pleistocene to Holocene age in Nile delta cores. The approach is particularly helpful to interpret sand strata whose structures and textures have been altered during drilling operations and whose chronostratigraphic position in the core sections is poorly defined. Definition of sand facies helps to improve the quality of stratigraphic correlations and better interpret the evolution of the Nile delta coastline. Sand-rich lithofacies can then be more precisely related to such factors as changes in distributary mouth position and configuration resulting from channel migration and changes induced by eustatic sea-level oscillations and subsidence. It is suggested that the stained grain-compositional component method could be applied to the study of modern and ancient sand facies in other delta settings. ACKNOWLEDGEMENTS We thank Professor A. Bassiouni, Dean of Faculty at the Ain Shams University and Dr. B. Issawi, Undersecretary for State, Cairo, for facilitating field surveys and sediment core collection in Egypt and for their strong support of the Nile Delta Project. Dr. D.E. Frihy, Coastal Research Institute, Alexandria, generously provided some of the modern Nile delta samples examined here. Assistance in the Smithsonian's NMNH Sedimentology Laboratory was provided by Mr. S. Miller. We thank Dr. A.G. Warne, Dr. G. Randazzo and Mr. S.J. Miller for their constructive reviews of this paper. Funding for this study was provided by awards from the Smithsonian Scholarly Studies Program, National Geographic Society and I.E.O.C. Egypt (to D.J.S.), and by a Smithsonian Post Doctoral Fellowship (to z.e.). LITERATURE CITED ABDEL WAHAB, H.S. and STANLEY, D.J., Clay Mineralogy and the recent evolution of the northcentral Nile delta, Egypt. Journal of Coastal Research, 7(2), ARBOUILLE, D. and STANLEY, D.J., Late Quaternary evolution of the Burullus Lagoon region, north-central Nile delta, Egypt. Marine Geology, (in press). ATTJA, M.I., Deposits in the Nile Valley and the Delta. Geological Survey ofegypt, Cairo, 356p. BALL, J., Egypt in the Classical Geographer. Cairo: Government Press, 23p. BLODGET, H.W.; TAYLOR, P.T., and ROARK, J.H., Shoreline changes along Rosetta-Nile Promontory: Monitoring with satellite observations. Marine Geology, (in press). BUTZER, K.W. and HANSEN, C.L., Desert and River in Nubia: Geomorphology and Prehistoric Environments at the Aswan Reservoir. Madison, Wisconsin: University of Wisconsin Press, 526p. COLEMAN, J.M., Deltas: Processes of Depositional and Models for Exploration. Boston: International Human Resources Development Corporation, 124p. COUTELLIER, V. and STANLEY, D.J., Late Quaternary stratigraphy and paleogeography of the eastern Nile delta, Egypt. Marine Geology, 77, EL ASKARY, M.A. and FRIHY, a.e., Depositional phases of Rosetta and Damietta promontories on the Nile delta coast. Journal ofafrican Earth Sciences, 5, EL BAZ, F., The meaning of desert color in Earth orbital photographs. Photogrammetric Engineering and Remote Sensing, 44, EL FISHAWI, N.M., Roundness and sphericity of the Nile Delta coastal sands. Acta Mineralogica Petrographica, Szeged, 26, EL FISHAWI, N.M., Textural characteristics of the Nile Delta coastal sands: An application in reconstructing the depositional environments. Acta Mineralogica-Petrographica, Szeged. 27, EL FISHAWI, N.M. and EL ASKARY, M.A., Characteristic features of coastal sand dunes along Burullus-Gamasa stretch, Egypt. Acta Mineralogica-Petrographica, Szeged, 23, EL FISHAWI, N.M. and MOLNAR, B., Distinction of the Nile Delta coastal environments by scanning electron microscopy: A statistical evaluation. Acta Mineralogica-Petrographica, Szeged, 26, EL FISHAWI, N.M. and MOLNAR, B., Mineralogical relationships between the Nile Delta coastal sands. Acta Mineralogica-Petrographica, Szeged, 27,89-1. EL-WAKEEL, S.K. and WAHBY, S.D., 197. Bottom sediments of Lake Manzala. Journal of Sedimentary Petrology, 4, FOLK, R.L., Reddening of desert sands: Simpson Desert, N.T., Australia. Journal of Sedimentary Petrology, 46, FOURTEAU, R., Contribution it I'etude des depots Nilotiques. Memoire de l'ircstitut d'egypte, 8,57-94.

15 Sand Facies in the Nile Delta 877 FRIEDMAN, G.M., Distinction between dune, beach, and river sands from their textural characteristics. Journal of Sedimentary Petrology, 31, FRIEDMAN, G.M., Address of the retiring President of the International Association of Sedirnentologists: Differences in size distributions of populations of particles among sands of various origins. Sedimentology, 26, FRIHY,O.E.; EL FISHAWI, N.M., and EL ASKARY, M.A., Geomorphological features of the Nile Delta coastal plain: A review. Acta Adriatica, 29, FRIHY, a.e. and STANLEY, D.J., Quartz grain surface textures and depositional interpretations, Nile delta region, Egypt. Marine Geology, 77, FRIHY, O.E. and STANLEY, D.J., Texture and coarse fraction composition of Nile Delta deposits: facies analysis and stratigraphic correlation. Journal African Earth Sciences, 7, HOWA, H.L. and STANLEY, D.J., Plant-rich Holocene sequences in the northern Nile delta plain, Egypt: Petrology, distribution and depositional environments. Journal of Coastal Research (in press), 33p. HURST, H.E., A Short Account ofthe Nile Basin. Physical Department, Ministry of Public Works, Egypt, Paper 45, 77p. INMAN, D.L. and JENKINS, S.A., The Nile littoral cell and man's impact on the coastal zone of the southeastern Mediterranean. Scripps Institute Oceanographic Reference Series, 31, KRINSLEY, D.H. and DOORNKAMP, J.C., Atlas of Quartz Sand Surface Textures, Cambridge: Cambridge University Press, 91p. MASON, C.C. and FOLK, R.L., Differentiation of beach, dune and eolian flat environments by size analysis, Mustang Island, Texas. Journal of Sedimentary Petrology, 28, MEDIBA (MEDITERRANEAN BASIN PROGRAM), Nile Delta Project Data-base Listings. Records (unpublished), U.S. National Museum ofnatural History, Washington, D.C. NORRIS, R.M., Dune reddening and time. Journal of Sedimentary Petrology, 39, PIMMEL, A. and STANLEY, D.J., Verdinized fecal pellets as indicators of prodelta and delta front deposits in the Nile delta, Egypt. Marine Geology, 86, SAID, R., The Geological Evolution of the River Nile. New York: Springer, 151p. SCRUTON, p.e., 196. Delta building and the deltaic sequence. In: SHEPARD, F.P., PHLEGER, F.B. and VAN ANDEL, T.H. (eds.), Recent Sediments, Northwest Gulf of Mexico. Tulsa, Oklahoma: American Association of Petroleum Geologists, pp SESTINI, G., Nile delta: A review of depositional environments and geological history. In: WHATE LEY, M.G.K. and PICKERING, K.T., (eds.), Deltas: Sites and Traps for Fossil Fuels. Geological Society of London Special Publication 41, STANLEY, D.J., 199. Recent subsidence and northeast tilting of the Nile delta, Egypt. Marine Geology, 94, TOUSSOUN,., Me rno ires sur les anciennes branches du Nil-Epoque Ancienne. Memoire de l'institut d'egypte, 4, 212p. UNDP/UNESCO, Coastal Protection Studies, Project Findings and Recommendations. Report UNDP/EGY/73/63, Paris, 483p. VAN HOUTEN, F.B., Origin of red beds: A review. Annual Review ofearth and Planetary Sciences, 1, ZEINELABDINE, A.; FATHI, A.H. and EL ARKAN, M.Y.S., Study of the subdeltaic sandy formations and their effect on the adjacent soils of the Nile Delta, U.A.R. Journal ofsoil Science, 6,